Do sharks migrate?
Although some sharks, such as Bull and Bonnetheads, are comparative stay-at-homes (see “Are any sharks territorial?” in chapter 4), others travel long distances on a regular basis. It isn’t surprising that many long-distance movers (Whale Sharks, Basking Sharks) are large, but even some relatively small sharks such as Spiny Dogfish will cross entire ocean basins. Migratory behavior is in fact a prime reason that many sharks are overfished.
Migrations can be both horizontal and vertical. Horizontal movements are generally much greater and can be along coastlines or across oceans. Coastal species stay near continental shelves, over depths of generally less than 200 m (650 ft), but can still travel great distances. Coastal sharks include threshers, Sandbar, Blacktip, and Dusky sharks. Bigeye Threshers (Alopias superciliosus) have been tracked from waters off New York to the eastern Gulf of Mexico, a distance of 2,767 km (1,719 miles). Sandbar Sharks move between the northeastern United States and the Yucatan Peninsula of Mexico, a distance of about 5,600 km (3,480 miles). Great Hammerheads are found along coastlines and also far at sea over seamounts. One “coastal” individual was followed for 62 days. It left South Florida and traveled north to New Jersey, sometimes drifting with the Gulf Stream. It traveled a minimum distance of 1,200 km (745 miles). Scalloped Hammerheads tagged at Malpelo Island, Colombia, have been followed to the Cocos Islands off Costa Rica (627 km, or 389 miles); and one shark then moved to the Galapagos Islands, another 710 km (440 miles), for a minimum distance of 1,340 km. Because Malpelo, Cocos Island, and the Galapagos are among the few places where Scalloped Hammerheads are known to congregate (see “How can I see sharks in the wild?” in chapter 8), it is likely that some of these are the same sharks, meaning hammerhead populations are smaller than if we were looking at entirely separate groups.
A Scalloped Hammerhead at Cocos Island, off the coast of Costa Rica. Scalloped Hammerheads migrate between Cocos Island, the Galapagos Islands, and Malpelo Island off Colombia, forming large aggregations. To protect this globally endangered species, fishing in all three areas needs to be stopped. Photo by Barry Peters, Wikimedia Commons, http://en.wikipedia.org/wiki/File:Scalloped_hammerhead_cocos.jpg
Even comparatively small coastal sharks can cover large distances during their lifetimes, and the distinction between “coastal” and “oceanic” is far from hard and fast. Some coastal species will cross deep ocean areas to get to another coast. A School Shark less than 1.5 m (5 ft) long that was tagged off New Zealand’s South Island was caught 4,940 km (3,069 miles) away off South Australia, 3.5 years later. To get to Australia, it had to cross the stormy Tasman Sea, which is more than 5,180 m (17,000 feet) deep. Spiny Dogfish are most common along coastlines and generally move up and down a coast, but some fish don’t know this. One tagged off the coast of Newfoundland crossed the Atlantic Ocean and was captured near the Shetland Islands 11 years later, a minimum distance of more than 3,200 km (2,000 miles). Spiny Dogfish tagged off the west coast of Canada moved long distances along the coast, traveling north in spring and summer and south in fall and winter. Some were recaptured years later up in Alaska and south in Washington State, Oregon, California, and Mexico. But several traveled to Japan, across the North Pacific Ocean.
The champion horizontal migrators are species that cross ocean basins or move north and south between hemispheres. Blue Sharks make round trips across the Atlantic between North America and Europe, a straight-line journey that exceeds 16,000 km (9,900 miles), although the sharks undoubtedly cover more area because it’s unlikely they swim in a straight line. Some Blue Sharks move north and south; a Blue Shark that was tagged off New York was later recaptured off Brazil, a one-way straight-line journey of 5,980 km (3,715 miles). Some other long distances traveled by oceanic species in the North Atlantic include 6,700 km (Tiger Shark), 4,500 km (mako), and 2,800 km (Oceanic Whitetip)—but again, these distances are underestimates because they record only starting and ending points for sharks tagged at one place and recaptured at another.
White Sharks are now known to migrate between central California and Hawaii, a minimum one-way distance of 3,800 kilometers (2,360 miles); the return trip makes the journey almost 8,000 km (4,970 miles). Some “Californian” White Sharks move back and forth along a fairly narrow corridor to a place in the Pacific about 2,500 km (1,550 miles) west-southwest, taking only three to four weeks to make the journey. They spend fall and part of the winter in California and most of spring and summer in the offshore area, now called the White Shark Cafe. White Sharks tagged off South Africa go to western Australia and back. One 4-m (13-ft) female nicknamed Nicole (after Australian actress Nicole Kidman, an advocate of shark conservation) was followed by satellites across the Indian Ocean and then back, traveling between South Africa and Australia. The round trip took nine months, covering a minimum distance of 22,000 km (13,670 miles).
Whale Sharks move around and across oceans, arriving at particular places to feed on eggs spawned by reef fishes and invertebrates. A Whale Shark tagged off Mexico traveled 13,000 km (8,000 miles) across the North Pacific Ocean. Another tagged in the Philippines traveled to Vietnam, a distance of 4,567 km (2,830 miles), in only 2.5 months, while another tagged off Malaysia moved 8,025 km (4,987 miles) over a 4-month period. Many of these movements involve migrations to areas of briefly abundant food; after the food is gone, the animals may return to their starting point or go to places that we don’t know about yet. Despite all this moving around, genetic analysis of sharks sampled from the eastern Pacific (Mexico, Costa Rica, Galapagos Islands), the Caribbean and the western Atlantic (Honduras, Florida), and South Africa, western Australia, and the Indian Ocean (Seychelles, Maldives, Djibouti, India) indicates that sharks intermingle often when breeding. Basically, there is one world population of Whale Sharks totaling between only 250,000 and 500,000 individuals, a finding with strong conservation implications. (For comparison, the Spiny Dogfish population off Canada’s east coast may number over 500 million sharks; see “Which sharks are most or least abundant?” below.)
Basking Sharks, second in size only to Whale Sharks, also migrate across oceans. Baskers tagged off the United Kingdom have been found three months later off Newfoundland, Canada, on the other side of the North Atlantic. Sharks tagged off the Cape Cod area during summer headed south as water temperatures fell in autumn. Some went to the Bahamas, others to the Caribbean, and a few traveled as far south as Brazil, a straight-line distance of 6,500 km (4,030 miles); the actual track of one shark indicated it had moved nearly 9,000 km (5,580 miles). While in productive northern areas, Basking Sharks feed at the surface, but when in the tropics, they descend to open ocean depths as great as 1,000 m (3,300 ft), where they remain active for months, probably feeding.
The migration track of Nicole, a 4-m female White Shark who swam from South Africa to western Australia and back over a nine-month period. Redrawn from R. Bonfil et al., “Transoceanic migration, spatial dynamics, and population linkages of white sharks,” Science 310 (2005): 100–103; used with permission of Wiley-Blackwell
These findings solve one of shark biology’s great mysteries, namely where Basking Sharks go in the winter. Because they “disappeared,” it was guessed that they hibernated on the bottom somewhere. The new information is based on “pop-up” tags that periodically record depth, temperature, and location: these tags break free from the shark, float to the surface, and transmit their stored data via satellite to a laboratory (see “How do scientists study the movements of large sharks?” in chapter 12). We don’t know what the sharks do after their tags are shed. It is assumed that they return to northern areas the following spring because that is when they start to reappear in surface waters off New England. Learning that Basking Sharks probably feed in deep, cool waters for months at a time also calls into question their name. They are called “Basking” Sharks because it was assumed that their slow movement at the surface was for sunbathing (Irish fishermen used to also call them “sunfish”). It now seems more likely that they are at the surface because that’s where their food is located at high latitudes. We are open to suggestions for a new name for this plankton-eating, migrating giant.
We know less about ray migrations, especially among smaller species. Atlantic Stingrays migrate seasonally, apparently avoiding cold water. During summer and fall they occur in areas such as the Chesapeake Bay, but when surface waters cool, they move south or offshore to warmer water. Spotted Eagle Rays in the Bahamas move offshore in early summer and return to shallow reef areas by late summer. Manta ray behavior is slowly being revealed; they too migrate. For example, Giant Mantas migrate across the Indian Ocean from Mozambique to the Maldives, a distance of 1,120 km (700 miles) in two months. Cownose Rays are among the better-known species because they migrate in large numbers, often near the surface, and are therefore visible from boats and planes. Cownose Rays move seasonally along the U.S. Atlantic coast, going north in the late spring and south in late fall between Virginia and Florida and even as far as between the Chesapeake Bay and Brazil.
A few studies have looked at whether skates migrate. Because they are small animals that lie on the bottom in deep, cool water, we wouldn’t expect them to be highly mobile, and many aren’t. A 20-year study of Thorny Skates off Newfoundland found that most skates didn’t undergo obvious seasonal migrations in any particular direction but instead moved slowly from their tagging location. Many skates moved less than 30 miles (48 km), and most less than 60 miles (96 km), even after many years. One moved less than 5 miles (8 km) in 14 years. Among five skate species that occur along the Atlantic coast of the United States (Thorny, Smooth, Clearnose, Little, and Winter), Thorny and Clearnose skates did not migrate, whereas Clearnose, Little, and Winter skates undertook apparent seasonal migrations. Clearnose Skates move both north and south as much as 580 km (360 miles), and inshore and offshore as much as 120 km (74 miles). The inshore movement in early spring is probably a spawning migration. Some Little Skates move inshore in the spring and offshore in autumn and winter. Winter Skates are the strongest migrators, although again not all individuals move. In winter and spring, Winter Skates move south or offshore from an area in the Gulf of Maine, some animals traveling as much as 940 km (584 miles). But compared with their shark and ray relatives, these skates are weak migrators.
The movements of chimaeras are poorly understood. The Australian Ghostfish of Australia and New Zealand normally occurs in fairly deep water, down to 200 m (656 ft). Reproductively active animals move into shallow bays to lay eggs during their summer and autumn spawning season. They move back offshore to deep water in winter and spring. Egg cases of the Elephantfish, Callorhinchus callorynchus, have also been found in shallow water, suggesting that this chimaera also moves inshore to spawn. The Spotted Ratfish is a vertical migrator, moving up from deeper water in the day to shallower water at night. Aside from these three, we can’t say much about chimaera movements.
Many sharks move up and down periodically or irregularly. Although the distances traveled are less spectacular than transoceanic movements, the vertical migrations of some sharks are impressive. Vertical migrators experience greater changes in temperature and pressure over a few hours than do sharks moving the length of a coastline or across an ocean over many days. The diving profiles of these sharks, as revealed by ultrasonic telemetry and archival tags, show a yo-yo pattern: animals move up and down several hundred meters repeatedly during the day and night or over the course of several days. Tiger Sharks have been described as “bounce divers,” moving almost continuously between the surface and 90 m (300 ft) down. White Sharks may move up and down between the surface and 200-m (660-ft) depths as often as 96 times in 24 hours; others may spend days at the surface and then more days at depth. Female Gray Reef Sharks observed in Palau exhibited a distinctive, complex up-and-down movement pattern that extended over only 15 m (50 ft) each day. They tended to move to shallower water (30 m, or 100 ft, deep) at dawn and dusk but spent daylight hours at 45-m (150-ft) depths. The depths at which they stayed at night were usually shallower than daytime depths, except on full moon nights, when they descended to 60 m (200 ft).
Bigeye Threshers, Megamouths, Shortfin Makos, and School Sharks spend daytime at depths of several hundred meters and then move to within 10 to 100 m (33–330 ft) of the surface for the night, probably tracking food. Whale Sharks engage in both irregular and day versus night vertical movements, moving between the surface and 1,000 m (3,300 ft). They spend most of their time near the surface regardless of light, especially when feeding. Their attraction to the surface is probably because their food— recently spawned fish and invertebrate eggs—is concentrated in shallow water. Basking Sharks around the British Isles move up and down in the classic pattern of going deeper (to 70–80 m, 225–265 ft) by day and staying shallower (10–30 m, 33–100 ft) at night when they are in deep water that is colder at depth. But in shallower areas where there is no temperature difference with depth, these sharks stay in shallower water by day and deeper water at night. In both the classical daytime-deep / nighttime-shallow and reversed patterns, it appears the Basking Sharks are following the movement of their zooplankton prey.
A few exceptional sharks undertake a daily vertical migration clearly linked to food availability. Many bony fishes (lanternfishes, lampfishes, bristlemouths, lightfishes) live at mesopelagic (middle-ocean) depths between 200 and 1,000 m (660 and 3,300 ft) during daylight hours. They move up near the surface in the evening to feed under the cover of darkness on the zooplankton that is more abundant in the shallows. They are met there by a number of predators, such as dolphins, billfishes, tunas, and Megamouth Sharks. These predators find their prey in the dark because mesopelagic fishes are often bioluminescent, giving off a green glow from light organs on their sides and bellies. Capitalizing on this predator-prey interaction are Cookiecutter Sharks. Cookiecutters are also bioluminescent, but for devious reasons. The Cookiecutters hide among the mesopelagic fishes, apparently blending into the bioluminescent background, and ambush their prey. Cookiecutters aren’t feeding on zooplankton or mesopelagic fishes but on the predators of the vertical migrators. Unlike most sharks that chop up their prey and swallow large mouthfuls, Cookiecutters feed one small bite at a time, taking a single, circular, half-dollar-size plug out of their victims’ sides. (See “What kind of teeth do sharks have?” in chapter 2 and “Do sharks chew their food?” in chapter 7.)
Because many sharks are highly migratory species, moving around and through entire ocean basins, they encounter fishing boats from many countries. This exposes them to capture, repeatedly. A shark may be protected by local or national fishing regulations when near the shoreline, but the high seas are largely unregulated, and few police exist to enforce any laws on the books. As a result, pelagic (open-ocean) sharks are heavily over-fished (see chapter 10).
How do sharks navigate?
Moving across an ocean and back again, returning to the exact same locale after a nine-month absence, as was the case with Nicole the White Shark, requires a highly sophisticated navigational system. So does arriving at the right place and time to feed on prey that is available for only a short period. Tiger Sharks show up in the northwestern Hawaiian islands just as young Blackfooted and Laysan albatross are taking their first, sometimes unsuccessful and therefore fatal, flights. Tiger Sharks do the same thing at Raine Island, Great Barrier Reef, Australia, just as green sea turtles appear by the thousands to lay eggs. At Raine Island, the Tigers scavenge heavily on dead turtles that had been stranded at low tide on exposed reefs. Some Tiger Sharks occur year-round at both locations, suggesting that Tiger Shark populations consist of resident and transient or migratory individuals. Whale Sharks are also famous for showing up at different locales around the tropics just when reef fishes or invertebrates are about to spawn. Manta rays do the same. White Sharks arrive at the Farallon Islands off San Francisco in late autumn to feed on young elephant seals that arrive there at about that time.
How sharks accomplish these and other precise long-distance navigational feats remains a mystery. Our best guess is that they rely on their acute electric sense, navigating via geomagnetic cues related to the earth’s magnetic field and its anomalies and gradients (see “How do sharks detect electric fields?” in chapter 2). This kind of navigation means that sharks have an internal (genetic or learned) map, compass, and clock. They compare where they are and in what direction they are moving with that mental map. The details of this ability remain one of the great mysteries of shark biology. Someday, a shark biologist, perhaps a reader of this book, will discover the answer.
How many shark species live in rivers and lakes?
We think of sharks as marine animals, and this is mostly true. Of the 1,250 or so known species of sharks, skates, and rays, fewer than 50 live all or most of their lives in fresh water. (Chimaeras live only in the ocean, usually far from coastlines.) Among the “freshwater” elasmobranchs are one guitarfish, 6 sawfishes, 8 sharks, and 31 stingrays.
Stingrays make up the largest group of freshwater elasmobranchs. Two stingray families enter or live full-time in rivers and lakes. The largest family, members of which are entirely freshwater inhabitants, consists of the potamotrygonid river stingrays, with 20 species, 18 of which are in the genus Potamotrygon. All live in South America, particularly in the Amazon River. Many are popular aquarium pets, despite the strength of the poisons in their barbs. The other group consists of 11 species in the large stingray family Dasyatidae. Most freshwater dasyatids occur in Africa, Southeast Asia, India, Myanmar, northern Australia, and New Guinea. This family includes what may be the world’s largest stingray, the Giant Freshwater Stingray of Southeast Asia. These rays can be 4.6 m (15 ft) long and 1.9 m (6.2 ft) across the back and can weigh more than 500 kg (1,100 lb). One population of Atlantic Stingrays lives in the St. Johns River of Florida, more than 350 km (217 miles) from the coast. This makes it the only elasmobranch in North America that spends its entire life cycle in fresh water. These rays are frequently observed by snorkelers in the crystal clear springs that flow into the St. Johns River.
Lemon Sharks occasionally enter brackish river mouths, but we don’t normally think of them as “freshwater” sharks. Five other species of carcharhinids, all in the genus Glyphis and all residents of Asian or Australian rivers, are permanent freshwater or estuary dwellers. Their names suggest their habitats: the Ganges Shark of India (Glyphis gangeticus), the Northern River Shark of New Guinea (G. garricki), and the Irawaddy River Shark of Myanmar (G. siamensis). Undoubtedly the most infamous freshwater shark is the Bull Shark. Most Bull Sharks live along coastlines and are perfectly good marine inhabitants, aside from their well-deserved reputation for attacking humans (see “Are sharks dangerous to people or pets?” in chapter 9).
Bull Sharks, however, commonly swim up rivers, often quite far, and are known to have attacked people in rivers in Iran, India, and South Africa. In 1937, two commercial fishermen in Alton, Illinois, were surprised to find a 1.5-m-long (5-ft) Bull Shark in a trap baited with chicken guts. Alton lies about 1,160 river miles (1,850 km) from the mouth of the Mississippi and 15 miles (24 km) north of St. Louis, near where the Mississippi, Illinois, and Missouri rivers come together. Alton declares itself “the most haunted city in America” because of the many ghosts that are claimed to live there (including one Bull Shark?). Today, a shark trying to get to Alton would have to traverse a lock and dam built in 1953, so it’s unlikely any have made the trip since then (three juvenile Bull Sharks reportedly caught in Wisconsin and Minnesota in 2006 were part of an April Fools hoax).
The Alton, Illinois, Bull Shark. In 1937, something was tearing up the wooden fish traps set by these two fishermen, Herbert Cope and Dudge Collins, so they built a large trap out of wire mesh and baited it with chicken guts. To their surprise, the culprit was a 1.5-m (5-ft), 38-kg (84-lb) Bull Shark. The shark would have traveled a minimum of 1,160 river miles from the mouth of the Mississippi to get to Alton. Photo by Ralph Manns
Bull Sharks have also been found as far as 4,200 km (2,610 miles) up the Amazon, and they regularly traverse the 175-km-long (105-mile) Rio San Juan of Nicaragua, moving between the Caribbean Sea and Lake Nicaragua, where they are called Lake Nicaragua sharks. They also move up other rivers in Mexico, and Central and South America. Bull Sharks, along with Largetooth Sawfish (Pristis microdon), have been heavily overfished in Lake Nicaragua and are now protected by Nicaraguan law. Bull Sharks occasionally swim up a river and then get stranded in water bodies after a flood. This is the probable explanation for how several Bull Sharks got into a water hazard at the Carbrook Golf Club in Brisbane, Australia. It’s prudent to just take a penalty stroke if you find yourself in the lake on the 15th fairway at Carbrook.
Sawfishes were once common in tropical nearshore areas around the world. They are now among the world’s most imperiled marine and estuarine fishes. Shown here are saws taken from sawfish during a 1920 fishing tournament in Key West, Florida. Photo property of Matthew McDavitt; photographer unknown
All six (or maybe seven) species of sawfishes inhabit estuaries and rivers. The Largetooth Sawfish has been found as far up the Amazon as Santarem, Brazil, more than 800 km (500 miles) from the sea. Largetooth Sawfish established a genetically distinct reproducing population in Lake Nicaragua but have become increasingly rare as a result of overfishing and habitat destruction. Because of their large size, dangerous tooth-studded snouts, and tremendous strength, sawfishes are the stuff of legend and respect in many countries (see “What roles do sharks play in religion, mythology, and legends?” in chapter 11). Unfortunately, they are also among the most endangered elasmobranch species in the world.
Several sharks and sawfishes move freely between marine and fresh water, prompting the question of how they deal with such drastically changing salinities. Unlike the full-time freshwater species, these estuarine migrants can adjust the molecular concentrations of their blood. A Bull Shark in fresh water reduces the salt concentration of its blood by about 20% and the urea concentration by about 50%. It also compensates for the extra water that comes into its body by increasing its urine production 20-fold. These and other physiological adjustments are quickly reversed when the shark returns to the ocean.
People live, work, fish, play, and dump their trash in rivers. As a result, freshwater elasmobranchs are more threatened than are chondrichthyans in general (see “Are any sharks endangered?” in chapter 10).
How far down in the ocean do sharks live?
Sharks do not live as deep in the ocean as bony fishes. The deepest living bony fish is a cusk eel, Abyssobrotula galatheae, found at 8,370 m (27,460 ft). The depth records for sharks are held by a Great Lanternshark (Etmopterus princeps) at 4,500 m (14,763 ft), a Portuguese Dogfish at 3,675 m (12,057 ft), and a Leafscale Gulper Shark (Centrophorus squamosus) at 3,280 m (10,761 ft), suggesting that sharks go only half as deep as bony fishes. The record for skates is held by Bigelow’s Skate (Rajella bigelowi), at 4,156 m (13,635 ft), and the Pallid Skate (Bathyraja pallida), at 3,280 m (10,761 ft). Holocephalans appear to bottom out around 3,000 m (9,900 ft), represented by the Atlantic Chimaera (Hydrolagus affinis) at 2,909 m (9,544 ft) and an unidentified member of the genus Harriota at 3,010 m (9,875 ft).
Chondrichthyan fishes as a group may avoid greater depths because of their comparatively large size, which would be hard to maintain given the general lack of food in the huge expanse of the ocean’s deepest waters.
This is not to say that sharks don’t live in fairly deep water. In fact, about half of all shark, ray, and chimaera species occur predominantly in water deeper than 200 m (660 ft), which is near the cutoff depth that marine biologists use to define the deep sea. Sharks occur on and just above the bottom as well as in open water. Breaking deepwater sharks down by groups, 254 sharks (53% of all sharks), 236 batoids (38%), and 40 holocephalans (89%) occur below 200 m. Twenty-three families of sharks appear in deep water, but of these, only five families contain more than 10 species: scyliorhinid catsharks (102 species), etmopterid lanternsharks (42 species), squalid dogfish sharks (25 species), centrophorid gulper sharks (18 species), and somniosid sleeper sharks (16 species). The remaining families contain only one or a few deep-sea members. Some of the world’s strangest sharks are deep-sea denizens, with several monotypic (only one species in the family) groups represented. These include the Frilled Shark (Chlamydoselachus anguineus), the Crocodile Shark (Pseudocarcharias kamoharai), the Goblin Shark (Mitsukurina owstoni), and Bramble and Prickly sharks (Echinorhinus brucus and E. cookei). Carcharhinid sharks dominate shallow seas, but only two species in this family, the Bignose Shark (Carcharhinus altimus) and the Night Shark (C. signatus), spend much time in the deep.
Skates live on the sea floor, with three families—rajid hardnose skates, arhynchobatid softnose skates, and anacanthobatid legskates—making up 90% of the 236 deep-sea skate and ray species. Only five species in four families of stingrays occur in the deep sea, of which two are monotypic: the Sixgill Stingray (Hexatrygon bickelli) and the Deepwater Stingray (Plesiobatis daviesi). As with the carcharhinid sharks, the whiptail stingray family Dasyatidae, so successful in warm, shallow waters, has only one deep-sea member, the Shorttail Stingray, which also occurs in shallow water. Holocephalans are well represented in the deep sea; 40 of the world’s 46 chimaera species are deep-sea animals. As a group, chimaeras generally swim just above the bottom.
A few of the deep-sea sharks are large: somniosid sleeper sharks can be 6 m (20 ft) long, and hexanchid sevengill and sixgill sharks grow to 3 m and 5 m (10 ft and 16.5 ft), respectively. Otherwise, deep-sea sharks are generally small and include the world’s smallest sharks, maturing at only 17 to 25 cm (7–11 in) in length, much smaller than the vast majority of shallow-water sharks. These microsharks include the etmopterid lanternsharks, dalatiid pygmy sharks, and proscyliid finback catsharks. (See “What are the largest and smallest sharks alive today?” in chapter 1.)
Which geographic regions have the most species of sharks?
Zoologists divide the world’s oceans into four major zones, depending mostly on temperature: tropical, warm temperate, cold temperate, and polar. Each zone is further divided by oceans and continents, and oceans are again divided according to depth.
Overall, the warm temperate regions house the greatest combined diversity of chondrichthyans (720 species). The tropics come next, with 654 combined species. Then come cold temperate regions (296 species); and last are polar seas, with only six species, all in the north polar (Arctic) zone. Many species, such as Tiger Sharks and Spiny Dogfish, occur in several areas and are thus counted more than once in this distribution, so the numbers far exceed the world’s overall shark diversity of 520 or so species.
Exploring further, the North and South Pacific warm temperate regions combined have more species than the North and South Atlantic regions (340 versus 292 species), with a small region of high diversity around South Africa (88 species). The Indo-West Pacific tropical area (which encompasses the western tropical Pacific plus Indian oceans) is home to about 1.5 times more species than the tropical eastern Pacific and tropical Atlantic areas combined (423 versus 231 species). In the cold temperate seas, the highest diversity is around Australia and New Zealand (128 species), followed by the North and South Atlantic (92) and the North and South Pacific (26). Another hotspot of 40 cold temperate sharks is found around the southern tip of South America, but true sharks are absent farther south around Antarctica.
The diversity of the major taxonomic groups varies by geographic region. The greater overall shark diversity in warm temperate regions is in part due to the large number of squaliform dogfish shark species there (200 species); squaliform sharks are also abundant in the tropics (143 species) and in cold temperate waters (100 species) but not in the Arctic (5 species). Scyliorhinid catsharks are likewise (slightly) more diverse in warm temperate than tropical oceans (97 versus 83 species), falling off in cold temperate waters with only 33 species, and no polar species. Carcharhinid requiem sharks, in contrast, are most diverse in the tropics (184 species) and the warm temperate regions (169 species), with a remaining 48 cold temperate carcharinids. Orectolobiform carpet, bamboo, and Nurse Sharks are another “majority tropical” group, with 44 tropical species, 33 warm temperate species, and 10 cold temperate representatives.
Comparing distribution by depths, squaliforms are most diverse in the deep sea, carcharhinids are coastal and oceanic, scyliorhinids are coastal and deep sea, and orectolobiforms are sharks of coastal shallows.
The batoids (electric rays, sawfishes, guitarfishes, skates, and stingrays) are primarily tropical and warm temperate elasmobranchs that live on the bottom in relatively shallow water—though with many exceptions. Torpediniform electric rays occur worldwide in shallow warm temperate and tropical oceans, with a few found as deep as 1,100 m (3,630 ft). Pristid sawfishes are nearshore, shallow warm temperate and tropical fishes, some of which enter fresh water. Rhinobatid guitarfishes are also worldwide in primarily shallow warm temperate and tropical bottom regions. The rajid skates have a much wider geographic and depth distribution, ranging from the poles to the equator and as deep as 3,000 m (9,900 ft). As a group, they tend to live in deeper, cooler water and at higher latitudes than the other batoids, including a species that occurs in Antarctica and nowhere else. The myliobatiform rays are again found worldwide in tropical to warm temperate seas, primarily in shallow water, with a number of tropical freshwater dasyatoid stingrays such as the potamotrygonid freshwater rays of South America. Overall, the batoids are most diverse in the Indo-West Pacific region. North and South America are second in batoid diversity.
The holocephalan chimaeras inhabit all but polar seas. Most are deep-water dwellers at 500 m (1,640 ft) and deeper. One family, the Callorhynchidae, occurs only in the Southern Hemisphere (South America, South Africa, and Australia / New Zealand). Members of the other two families (Rhinochimaeridae and Chimaeridae) are found in all the world’s oceans. Some species occur throughout an ocean basin; this is the case with the Pacific Spookfish (Rhinochimaera pacifica) in the Pacific Ocean and the Broad-nose Chimaera (R. atlantica) in the Atlantic. The Narrownose Chimaera (Harriotta raleighana) occurs in both the Pacific and the Atlantic. Australia and New Zealand may have the highest diversity of chimaeras, with at least nine species recorded from that region.
Which sharks are most or least abundant?
Probably the most abundant shark in the world is the Spiny Dogfish (also known as the Spur Dog or Piked Dogfish), Squalus acanthias, and its close relative, Squalus suckleyi. S. acanthias occurs off the Atlantic coasts of the United States, Canada, and Europe, and also off southern South America, Africa, Australia, and New Zealand. The closely related S. suckleyi, until recently considered the same species, inhabits the North Pacific. Commercial landings of the latter off the west coasts of the United States and Canada exceeded 50,000 metric tons (100 million pounds) per year in the mid-1940s. At about 3 pounds average per fish, that is over 33 million sharks! Some recent estimates of the abundance of the Atlantic species off the coast of Canada run as high as 535 million sharks, and the number off the combined Pacific coasts of Canada and the United States is put at almost 2 billion individuals. As abundant as those numbers appear, these are still sharks with low reproduction rates (see chapter 6). Some dogfish stocks have been fished down as much as 95%, leading to restricted catches and a “Do Right by Dogfish” campaign to reduce overexploitation.
(Top) A Spiny Dogfish, probably the world’s most abundant shark, off the coast of New England. The typical color is bronze-gray with white spots. (Bottom) Geographic ranges, shown in black, of the Spiny Dogfishes Squalus acanthias and S. suckleyi. The latter occupies the North Pacific. Top, photo by Doug Costa, National Oceanic and Atmospheric Agency, https://marinelife.noaa.gov/media_lib/preview.aspx?ID=7299&p=img; bottom, map from http://en.wikipedia.org/wiki/File:Squalus_acanthias_distmap.png
Some other abundant sharks include Blue, Silky (Carcharhinus falciformis), School or Tope, and Gummy (Mustelus antarcticus) sharks—all of which are targets of major fisheries. Among batoids, Bat and Cownose rays occur in schools of thousands and even millions of individuals (see “Do sharks form schools?” in chapter 4). Spotted Ratfish may be the most abundant chimaera, with an estimated 200 million in Washington State’s Puget Sound, an area of about 2,600 km2 (1,000 square miles). That’s roughly 77,000 ratfish per square kilometer, or about one ratfish every 10 m2 (equivalent to an area of 30 by 30 ft).
It’s hard to say what the rarest shark, skate, ray, or chimaera might be. The rarer species are certain small sharks and rays that are endemics (confined to a very small geographic region). Many elasmobranchs are threatened from overfishing and have declined greatly in numbers (see chapter 10). Sawfishes as a group are among the most depleted because they live near shores and in estuaries, habitats that put them in contact with people. Even when they are not targeted by fisheries, their saw can become entangled in any kind of net, and they drown or are killed so that someone can claim the saw as a trophy (even though possession of one is illegal).
Some of the least known sharks, such as small deepwater species, may or may not be rare; we don’t know for sure because they are difficult to capture or live in places that are poorly explored or hard to get to. These include some surprisingly large species, such as the Goblin Shark, which can grow to 3.3 m (11 ft) long and weigh 159 kg (350 lb); fewer than 50 specimens are housed in museum collections. Among rare large sharks, the 5.5-m-long (18-ft) Megamouth Shark gets the most attention because it wasn’t discovered until 1976 and, as of 2012, only 54 individuals have been seen.
Several sharks are known from only one or a few specimens that are housed in museum collections. The Irrawaddy River Shark, a carcharhinid, is known from a single museum specimen that was caught in the late 1890s at the mouth of the Irrawaddy River in Myanmar. A close relative, the Northern River Shark, may have a total global population of only 250 mature individuals. The Pondicherry Shark (Carcharhinus hemiodon) of India is known from only 20 specimens and hasn’t been seen since 1979. Haploblepharus kistnasamyi is a rare shark known only from three adult specimens, all collected from a small area near Durban, South Africa. Among skates, the Pita Skate (Okamejei pita) is also known from only a single female specimen collected in 1992 from the northernmost corner of the Persian/ Arabian Gulf. The Ornate Sleeper Ray is known from two museum specimens plus visual sightings by divers of four animals along 300 km of the eastern coastline of South Africa. The Java Stingaree (Urolophus javanicus) is another species known from a single specimen. It was collected near Java, Indonesia, 150 years ago and hasn’t been found since.
When during the day are sharks most active?
Sharks as a general rule are more active at night than by day, although their opportunistic nature means they will take advantage of an easy meal, such as a dead fish or baited hook, almost anytime. A common pattern, then, is resting by day—alone or in groups, on the bottom or swimming slowly—then moving at dusk to a nighttime feeding area (see “Are sharks social?” in chapter 4). In many species, activity times are determined as much by food availability as by light levels. Plankton-eating sharks such as Whale and Basking sharks feed during daylight or we wouldn’t have so many great photos of them. But their activity patterns as revealed by depth-recording tags suggest they also feed at night, following moving prey up and down in the water.
Manta rays—also plankton eaters—appear to feed both during the day and the night; some of the most dramatic videos of feeding mantas have been filmed at night. Many stingrays and skates forage actively during the day, but torpedo rays and angel sharks are more active at night. One exception is the Ornate Sleeper Ray, which vacuums up worms by day. White Sharks can feed anytime, but they tend to hunt seals during daylight and especially at dawn and dusk because that is when seals leave and return to their rookeries and when White Sharks can take advantage of backlighting to strike upward at a seal at the surface preparing for or recovering from a foraging dive.
How do sharks survive the winter?
Because they are oceanic animals and live mostly away from the shoreline, winter survival isn’t a big issue for most sharks, especially tropical species. Some species that live at higher latitudes move seasonally but mainly to follow their food, migrating to places where food will be more abundant (see “Do Sharks Migrate?” above). Basking Sharks leave northern regions in the fall, moving south to warmer areas, but they then swim in cool depths, so they aren’t gaining any great temperature advantage. Spiny Dogfish spend spring and summer off the west coast of Canada and move south in fall and winter.
Like many sharks and bony fishes, some rays and skates in the Northern Hemisphere move offshore or south to warmer waters in autumn, suggesting avoidance of colder winter conditions. Atlantic Stingrays leave the Chesapeake Bay during the fall when surface waters cool and head south or offshore to warmer water. Cownose Rays move up and down the Atlantic coast of the United States, going north in the late spring and south in late fall. Clearnose and Winter skates move inshore in the spring and offshore in autumn.
How do sharks survive droughts?
Many bony fishes are “intertidal,” meaning they live in tide pools or along shorelines that dry up once or twice daily at low tide. These fishes have many behavioral and physiological adaptations to help them through these relatively dry spells. Few sharks live in water so shallow that it goes dry on a daily or seasonal basis. An exception is the Epaulette Shark of Australia and New Guinea, which has the amazing ability to survive low oxygen conditions by lowering its blood pressure and heart and breathing rates, and switching off non-essential brain functions. Under laboratory conditions, Epaulette Sharks have survived up to an hour without oxygen at 30°C (86°F). (Recall that warmer water holds less oxygen at the same time that metabolic rates usually increase with increasing temperature.) This is apparently an adaptation for hunting in tidal flats at night when oxygen levels fall.
Some bony fishes such as lungfishes and Salamanderfishes dig into the mud when ponds lose their water in the dry season. We don’t know of any freshwater sharks or rays that do this (on freshwater species, see “How many shark species live in rivers and lakes?” above). However, some sharks do live in places where rivers dry up seasonally, as in the Northern Territory of Australia. There have been instances in this region of Bull Sharks becoming stranded in pools as the former river dried up. An Australian fish biologist almost fell victim to a Bull Shark when he went to cool off in such a pool after a hot day of fish collecting. This fairly large Bull Shark had survived by eating animals that came to the pool to drink; dismembered kangaroo and camel skeletons on the shoreline were a clue that told him to exercise caution.
Are there sharks in the desert?
Deserts by definition are short on water, especially salt water. Where water occurs, it usually emerges from the ground at springs, which besides being too fresh for sharks also lack a necessary connection to the ocean that would allow invasion by sharks. So desert sharks are unlikely (although the movie Sand Sharks might lead you to think otherwise).
Do sharks have any enemies other than humans?
Sharks’ greatest enemies, after humans, are other sharks. Several of the larger species—Tiger, Bull, White, and sleeper sharks—are known sharkivores. Cannibalism within a species is also common, as shown by Lemon Sharks in the Bahamas. One species of shark may even feed preferentially on another, as is true with the Broadnose Sevengill Shark (Notorynchus cepedianus), whose main prey off Tasmania, Australia, is the Gummy Shark. Some seemingly lethargic sharks are also successful predators on sharks; Tasselled Wobbegongs (Eucrossorhinus dasypogon), for instance, are able to ambush Brown-banded Bamboo Sharks on the Great Barrier Reef of Australia.
The shark-eat-shark nature of the ocean is probably the reason that females of many species go to particular nearshore “pupping grounds” to give birth to their young. These shallow inshore areas are infrequently visited by the males of the pupping species or by most large, predatory oceanic species.
Rays are another favorite prey of several shark species. Some hammerheads appear to be stingray specialists. Individual Great Hammerheads have been found with as many as 96 stingray barbs embedded in their mouth, throat, and tongue. Divers in the Bahamas watched a 3-m (10-ft) Great Hammerhead knock a Southern Stingray down with its flattened head, pin the ray to the bottom, then rotate on the ray and take bites out of the ray’s wing.
Although we don’t have such spectacular firsthand reports of sharks feeding on other rays, we do have circumstantial evidence. Manta rays observed at Tofo, Mozambique, have huge scars on their backs that indicate they were chomped on by large sharks and survived. Injured mantas, with fresh wounds, hover at “cleaning stations” on the Tofo reef, where various cleaner fish nibble at the wound to remove dead tissue and presumably prevent infection (see “Do sharks socialize with other kinds of animals?” in chapter 4). Torpedo rays show up in the stomach contents of Broad-nose Sevengills and Sixgill (Hexanchus spp.) sharks. Some torpedos have bite patterns on their backs indicating unsuccessful attacks by Sixgills, possibly because the torpedo was able to discharge its electricity before being swallowed by the shark. Despite possessing toxic barbs, young stingrays fall prey to other predators: Great Blue Herons have been observed spearing and swallowing small Atlantic Stingrays in Florida and Mississippi.
A Broadnose Sevengill Shark cruises a kelp bed off South Africa. Seven-gills occur in temperate Pacific and Atlantic shallow-water locales and feed on other sharks, rays, and chimaeras. Photo by Austin Gallagher, http://austingallagher.com; used with permission
A Great Blue Heron captures a small Atlantic Stingray in shallow water in the Florida Keys. Moments later, the heron swallowed the ray whole. Photo by Kaye DeHays; used with permission
The prominent fin spines of chimaeras and Spiny Dogfish may offer some protection from predation. But chimaeras occasionally show up in the stomach contents of other ratfish as well as some sharks and seals. Even adult chimaeras are victimized, as shown by the presence of adult Australian Ghost Sharks in the stomachs of New Zealand Carpet Sharks (Cephaloscyllium isabellum) and School Sharks. White and Sevengill sharks happily munch on Spiny Dogfish.
A number of mammals kill and eat sharks of all sizes. Smaller shark species such as Horn Sharks are the prey of northern elephant seals (Mirounga angustirostris), the seals feasting on adults, juveniles, and egg cases. Juvenile Cape fur seals in South Africa have been seen capturing Puffadder Shy-sharks and tossing them repeatedly in the air, slowly dismembering them. The main object of this behavior doesn’t appear to be feeding so much as playing with the shark, but the outcome is still usually fatal for the shark. Even if the seal doesn’t eat the shark but drops it, Black-backed Kelp Gulls are always around to finish the shark off. Adult Cape fur seals have been filmed capturing and eating small Blue Sharks, also in South Africa.
Some of the biggest sharks fall victim to some of the ocean’s biggest mammals. Sperm whales are best known for feeding on giant squid, but whaling records indicate they also eat small deep-sea sharks. Sperm whales were also seen attacking a Megamouth Shark off North Sulawesi, Indonesia, the only known record of anything other than a Cookiecutter Shark attacking a Megamouth.
On an even larger scale, whale researchers have recently discovered that orcas (killer whales) feed on sharks around Vancouver Island, British Columbia, and farther north near Alaska. Packs of orcas gang up on sleeper sharks (probably Pacific Sleeper Sharks) and tear them apart. One feeding frenzy produced a large slick of floating shark meat and liver; the identity of the shark was confirmed by DNA analysis. Other observations suggest that orcas attack Blue Sharks farther south. Orcas have also been seen eating White Sharks off the California coast, with one often-seen female orca feasting on White Sharks at the Farallon Islands, offshore from San Francisco. Orcas have also been filmed attacking Whale Sharks, Common Threshers, Shortfin Makos, mantas, and stingrays.
Incidents in public aquariums suggest that sharks have other enemies, including some without backbones. Spiny Dogfish were disappearing mysteriously from a large exhibit at the Seattle Aquarium. Workers at the facility stayed late one night to see what was happening. They saw (and videotaped) a large giant Pacific octopus capturing and consuming a Spiny Dogfish. Whether such predation occurs in nature, and if so, how often, is unknown, but octopuses (not “octopi”) are intelligent animals capable of capturing and eating a variety of prey, so it would not be surprising if the Seattle octopus was showing its natural predatory behavior.
Not all attacks on sharks are for the purposes of feeding. Dolphins and porpoises are reputed to ram sharks, apparently in defense of their young, although actual observations of this are hard to find. An incident at the Miami Seaquarium in which bottlenose dolphins attacked and killed a Sand bar Shark during the birth of a baby dolphin may have inspired the “conventional wisdom” that such events are common. In Hemingway’s The Old Man and the Sea, the fisherman Santiago wonders how many sharks his marlin had killed when it was alive. What Hemingway based this idea on is unknown. A marlin being attacked by false killer whales in Hawaii speared a diver who tried to film the event, and fishermen off Nova Scotia and New York have observed Broadbill Swordfish turning on attacking sharks, so it isn’t beyond reason to think that billfish may attack sharks, at least as a defensive maneuver.
Better documentation exists of other fishes ramming sharks. For reasons that remain mysterious, Giant Trevally (Caranx ignobilis), a relative of pompanos and jacks, attack and kill reef sharks much larger than themselves but don’t eat them. Observations from the tropical Pacific (Saipan, Mariana Islands; Palau, Western Caroline Islands; Marshall Islands) have documented large trevally singly or in pairs repeatedly head-butting reef species such as Blacktip Sharks and even Tiger Sharks. The trevally focus their ramming attacks on the sides and gill region of the shark. The shark often defends itself by trying—unsuccessfully—to bite its more agile attacker. Sometimes, a shark will attempt to flee to deeper water but be forced back into the shallows by the attacking trevally. The trevally ignore their own injuries, such as multiple cuts on their heads where they contact the sharks, and keep ramming. After repeated high-speed rammings, the shark, bleeding from its gills, sinks to the bottom and dies. Dissections of victimized sharks indicate damage to several internal organs that were probably the result of the rammings. Interestingly, trevally may ram more than just sharks. A Hawaiian spear fisherman was rammed by a large Giant Trevally, suffering three broken ribs.
How do sharks avoid predators?
Sharks avoid or deter predators in four major ways: (1) by running away; (2) by hiding, either in a hole or in plain view but with camouflage coloration; (3) by defending themselves with teeth, spines, toxins, or other structures and chemicals; or (4) by being hard to chew or swallow. Chondrichthyan fishes do all four.
BY RUNNING AWAY. Running away can mean just trying to outswim your predator, or it can mean moving to a safer locale. It all depends on whether you are faster or more maneuverable than your pursuer. Juvenile Lemon Sharks in the Bahamas take advantage of an incoming tide to move into shallow areas around mangroves that larger, faster Lemon Sharks— one of their main predators—avoid. Young Largetooth Sawfishes spend their early lives in similar habitat.
Knowing when to run away is obviously important because getting a head start can make all the difference. Some stingrays improve their chances of detecting a predator and fleeing by associating with other stingray species that are better at keeping a watchful eye for predators. Cowtail Stingrays (Pastinachus sephen) in western Australia rest during the day on sandy bottoms. When doing so, they prefer to rest near another species of dasyatid ray, the Reticulate Whipray, rather than with other Cowtail Stingrays. Reticulate Whiprays have a quicker flight response than Cow-tails when a predator or a boat with researchers approaches.
It’s not just small sharks that run away from predators. Killer whales are known to attack and eat White Sharks. Researchers studying White Sharks at the Farallon Islands off California noticed that their study animals became scarce after one shark was eaten by a killer whale. One tagged shark immediately left the area and went to Hawaii. How other sharks were aware of the attack is something of a mystery, although the commotion and splashing during the attack and the copious amount of White Shark blood and body fluids in the water could have been detected by other sharks.
BY HIDING. Many smaller sharks hide from predators. Some small night-active sharks spend their daytime resting in caves and crevices, often in the company of other members of their species (see “Are sharks social?” in chapter 4). Other sharks, as well as skates and rays, rely on blotchy, camouflage coloration to make themselves hard to see when they are lying motionless on the bottom (see “Are any sharks colorful?” in chapter 3). Wobbegongs also have tassles and skin growths that help them blend in better with the variable colors and structures of an algae-covered reef. On sandy bottoms, angel sharks, skates, and stingrays cover themselves with sand with just their eyes and maybe their spiracles showing, allowing them to see and breathe while being hard to be seen.
BY DEFENDING THEMSELVES. Defending yourself with teeth and spines is effective, although it means you actually have to battle a predator that is in all likelihood bigger than you. All but the biggest sharks (Whale, Basking, Megamouth) have sharp teeth and strong jaws that can deliver an injury-causing bite. Some species, such as Nurse Sharks, are even flexible enough to stick their tails in their mouths, which makes pulling the tail of a Nurse Shark lying on the bottom a very bad idea. Torpedo rays have the unusual option of defending themselves with their electric shocking equipment.
Many prey species that have good defensive mechanisms advertise the fact to predators. This makes sense because it’s better to not be attacked or swallowed, if at all possible. Several stingrays, such as Crossback Stingarees (Urolophus cruciatus), Southern Stingrays, and Shorttail Stingrays raise their tails and stings above their heads, scorpionlike, when approached by a diver. We assume that they do the same when approached by a potential predator. Some torpedo rays advertise their defensive armament by arching their back and tumbling and twisting in the water, a behavior that may be a warning display, or may just present the strongest discharge surface to an attacker. The most colorful electric ray, the Ornate Sleeper Ray of South Africa, arches its back and raises its tail in what might be a warning to a predator that it can defend itself with an electric shock.
Skates aren’t as well-equipped as torpedo or sting rays, and their defenses appear to be more passive. A few divers have seen skates roll themselves up into balls, curling their pectoral fins down to protect their more vulnerable bellies and exposing their backs, which are often adorned with hard thorns. Southern Thorny Skates (Amblyraja doellojuradoi) do this when caught in nets.
Divers in Norway have made intriguing observations of the defensive behavior of skates. Divers who bump into or prod juvenile Thorny Skates have been surprised to see the skates quickly roll up hedgehog-style with their pectoral fins curled around their bellies, essentially lying on their backs with their heads up and their thorny tails presented to the intruder. What makes this behavior intriguing is that the surface presented to a would-be predator is the part where the electrogenic tissue of skates is concentrated. One of the nagging questions in elasmobranch anatomy and behavior is the function of this tissue in skates. Skates emit a relatively weak electric charge, nothing like that of a torpedo ray. The function(s) of skate electric discharge remains something of a mystery, although mating skates discharge electricity, implying some reproductive purpose. The major predators of Starry Skates are two species of sharks, the Greenland Shark and Tope. One idea proposed is that a skate that is discharging its electric organs might somehow jam or overwhelm the extreme electric sensitivity of a predatory shark. It’s an idea worth testing.
Most chimaeras have first dorsal fin spines with a sawtooth back edge, covered with a venomous toxin. Researchers and fishers unlucky enough to get stuck by a ratfish report that it is a painful experience, the wound remaining red and swollen for several days. Presumably, natural predators have the same experience. The toxic spines of Spiny Dogfish may function similarly.
BY BEING HARD TO EAT. Anything a prey animal can do to make itself harder to swallow works to its advantage. Tough skin and stiff spines, especially long spines with toxins, deliver a message of inedibility. Stiff spines also increase an animal’s overall size, assuming the predator must swallow its prey whole (this works better as a defense against predatory bony fish that can’t chop up their prey the way that most predatory sharks can). Bramble sharks (Echinorhinus, Echinorhinidae) live as deep as 900 m (2,970 ft). Their unique large thornlike denticles might give them physical protection from deepwater predators such as somniosid sleeper sharks and sperm whales. The tough skin and spines of Horn Sharks apparently work to deter predators. A Pacific Angelshark (Squatina californica) was filmed inhaling a juvenile Horn Shark, only to spit it out shortly afterward, probably because of the boxlike body and stiff spines that give the shark its name.
The idea that making yourself bigger prevents a predator from swallowing you underlies the anatomy and behavior of several shark species. Puff-adder shysharks (Haploblepharus spp.) and Pajama Catsharks (Poroderma africanum) curl into a circle and cover their eyes with their tail when threatened; this is why they are known as shysharks or even doughnut sharks. This behavior may make these small (60-cm, 24-in), slender sharks bigger and therefore harder to swallow.
Swellsharks (Cephaloscyllium) go a step further in being able to increase their size when threatened. Swellsharks are unique among sharks because they can swell their entire body by filling their stomachs with water, a behavior better known in bony fishes such as pufferfishes. When threatened, a swellshark bends its body into a U shape, grabs its tail fin in its mouth, and swallows water, swelling its body to twice its normal size. This makes the shark harder to grab, and if the shark happens to be tucked away in a crevice, it would also make it difficult to extract. So, inflatable sharks are more than pool toys.
Do sharks get sick?
Very few captured sharks are found to be diseased. Sharks have strong, efficient immune systems. Their immune reaction to disease may be faster than it is in other vertebrate groups such as mammals. In sharks, immune cells are produced in the spleen, thymus, and other places, but not in bone marrow as they are in mammals (remember: sharks don’t have bones). Immune cells are already in the shark’s blood and can multiply quickly in response to a challenge from a disease-causing substance or organism. But this doesn’t mean sharks are disease-free. They suffer from a variety of ailments and illnesses, including bacterial infections, liver disease, meningitis, tumors, and the effects of a huge variety of internal and external parasites. Parasites are discussed in the next question.
It was long thought that sharks were immune to cancer. As a result, sharks were killed to extract potential cancer-curing remedies. Shark cartilage was promoted as an anticancer drug because laboratory studies showed that cartilage extracts could slow the growth of blood vessels that often occurs when cancer cells proliferate. Shark cartilage powder has sold for as much as $145 a gram ($65,685 a pound), and at one time a single plant in Costa Rica processed 235,000 sharks per month to make cartilage pills. But sharks do get several kinds of cancer, including cartilage cancers. Also, real trials with real people showed no beneficial effect from cartilage pills. Regardless, sharks are still processed for their cartilage, and cartilage capsules can still be purchased via the Internet to treat cancer and joint problems (see “Are any medicines made from sharks?” in chapter 8).
We like to think of big sharks as residing at the top of the food pyramid, surpassed only by an occasional killer or sperm whale. But in reality, each shark is a tasty, swimming platform for a zooful of smaller parasitic animals that feast on its blood and body tissues. Shark parasitology is a popular field of study because of the intimate evolved responses between a vast diversity of parasites and their shark hosts. Some parasites live in or on only one type of shark and attach only in a particular place inside or outside the shark, and only during a particular part of their life cycle. (Earlier life stages first parasitize invertebrates, and then the small fishes that eat the invertebrates, linking parasite life cycles to the entire food web.) About 1,500 different parasites are known from sharks, skates, and rays, with each elasmobranch species host to between 3 and 14 unique species; no doubt, many more will be discovered.
Remarkably, according to shark parasitologists Janine Caira and Claire Healy, “no organ system of elasmobranchs has escaped the attention of one or more groups of parasites.” If we performed a CT scan across a shark, from head to tail and including internal organs, we would find parasites in the following shark organs:
Skin——One type of copepod crustacean (a relative of shrimp and crabs) lives only on the pelvic fins of Blue Sharks, and one monogenean flatworm lives only on the undersides of certain skates. Other skin parasites include a snail that lives on the dorsal side of torpedo rays, 20 species of leeches, 275 species of copepods, 200 other monogenean flatworms (8 of which occur on chimaeras), three types of nematode roundworms, barnacles that attach to fin spines and flesh, amphipods (another shrimp-like crustacean), branchiuran fish lice, isopods (same group as pill bugs), and a few cestode tapeworms and digenean flukes.
Eyeballs—Eyeball parasites include several copepods (see below regarding Ommatokoita on Greenland Sharks), a leech on Spiny Dogfish, and a fluke on Whitetip Sharks.
Olfactory bulbs (the complex foldings inside the nose that lead via the olfactory nerve to the brain)—Among nasal parasites are ostracod seed shrimps, isopods, copepods, roundworms, leeches, and flatworms. One copepod infects the Barbeled Houndshark (Leptocharias smithii), the head of the parasite being lodged in the olfactory lobe of the brain and the tail resting in the olfactory bulbs. A roundworm has also been found wrapped around the optic nerves and much of the brain of a Spotted Eagle Ray.
Lateral line—A copepod lives inside the lateral line canals of the Porbeagle.
Spiracles—Three families of copepods and some leeches live inside these small breathing holes on the top of the head of some elasmobranchs.
Gills and gill chambers—Gills are infested with leeches, roundworms, ostracods, Argulus fish lice, isopods, digenean flukes, monogenean flatworms, an occasional tapeworm, and a host of copepods from 11 different families. In Blue Sharks, some copepod species live only on the walls between the gills, others on small gill filaments, and others in the channels of outflowing water; still others draw blood from specific blood vessels in the gills.
Digestive system——The different regions of the digestive system, from mouth to rectum, each have their own catalogue of specific parasites. The mouth and esophagus are home to leeches (some of which occur in only one shark or ray species), copepods, flukes, flatworms, roundworms, and barnacles. The stomach, despite its hostile chemical nature, houses flatworms, roundworms, spiny-headed worms, isopods, and numerous tapeworms. The spiral valve intestine has its own group of parasites, including spiny-headed worms, flukes, flatworms, 50 species of roundworms, and several hundred tapeworm species, some of which live only in specific regions of the spiral valves. One family of tapeworms lives only in chimaeras. One genus of spiny-headed worms is specific to the potamotrygonid freshwater stingrays of South America. Some batoid skates and rays can house as many as 10 species of tapeworms in the spiral valve. The rectum and cloaca at the posterior end have their own list of flukes and flatworms, copepods, leeches, and isopods.
Gall bladder, bile ducts, pancreatic ducts, liver, etc.—A few flukes, tape-worms, and nematodes occur only in these organs and tissues. One nematode also lives in the spleen of the Big Skate (Raja microocellata), and roundworms have been found in shark kidneys. The body muscles of sharks are relatively parasite free, except for an occasional roundworm, unlike the situation in bony fishes.
Heart and blood vessels—The circulatory system is occupied by roundworms, flukes, flatworms, mites, isopods, and an occasional eel (see below).
Reproductive tracts—Male gonads are relatively parasite free (an occasional tapeworm has been found); females house nematodes, tape-worms, flatworms, flukes, leeches, copepods, and isopods variously in their ovaries, oviducts, and uterus. Developing embryos are already parasitized externally by leeches and copepods.
Eyeball from a Greenland Shark infected with a parasitic Ommatokoita elongata copepod. The photo shows a female Ommatokoita anchored in the shark’s eyeball via a pair of modified legs. Her egg cases are the white bulb-like structures protruding from the eyeball to the right. An infected shark may be essentially blinded by the copepod but is still able to feed successfully. Photo courtesy of George Benz
Most internal parasites are small, wormlike animals. But others are large and have backbones. For example, when researchers cut open a 395-kg (870-lb) Shortfin Mako Shark caught off New York, they found two live Snubnose Parasitic Eels (Simenchelys parasiticus) curled up happily inside the shark’s heart. The eels, which are true bony fishes, were about 20 cm (8 in) long, took up most of the space in the heart, and had shark blood in their guts. It was difficult to say how long they had lived there, but scar tissue in the heart indicated that the eels had been there quite a while.
Most of the external parasites on sharks can move across the skin and around the scales. They may find the hard dermal denticles and thick skin of sharks a challenge for attachment. But others seek smoother, softer body parts where they can anchor themselves. One example is a particular type of copepod, Ommatokoita elongata, which targets the eyeballs of Greenland Sharks. Ommatokoita is a good example of a habitat-specific parasite; it is a 5-cm (2-in) copepod that penetrates the shark’s eyeballs and feeds on the surface of the cornea. The parasite isn’t fatal; sharks with parasites embedded in both eyes are functionally blind but still grow to large sizes and feed successfully.
Reflecting on all this information about internal and external parasites can change how we look at sharks. When you see a shark swim by in the wild or at a public aquarium, try to realize that what you are seeing is not just a single animal but more like an entire, highly evolved, mobile ecosystem.
How can you tell if a shark is sick?
A healthy shark swims normally. A sick shark would appear disoriented, swimming erratically or lying on the bottom on its side or back. Such an animal would be eaten quickly by other sharks or by a variety of scavengers in the wild. Public aquariums have specially trained veterinarians to deal with sick animals, but there is little the average home aquarium keeper can do. This is one of the reasons we advise against keeping sharks in fish tanks at home.
Are sharks good for the environment?
In some areas, rays serve as “ecosystem engineers” by digging up clams, crushing their shells and turning them into sand, and moving sediment around in the process. Their digging often pushes soft sediment aside, exposing hard bottom and objects that become settling habitat for a variety of invertebrates. A study in western Australia estimated that three ray species reworked 42% of the sediment in a bay each year.
Rays are an example of the different roles played by different species in natural ecosystems. When species are eliminated, relationships and processes are affected. Predators play important ecological and evolutionary roles, preventing overpopulation by reducing prey numbers and removing weak or sick individuals. Predators therefore maintain healthier prey populations with less disease and fewer genetic defects. When predators are removed, prey populations become overpopulated. Deer populations throughout the United States have reached unnaturally high numbers and have become pests because their natural predators (wolves, mountain lions) have been “controlled.” Deer then outstrip their food resources and starve, contract fatal epidemic diseases, and cross highways too often. Rabbit populations skyrocket and strip vegetation bare in the absence of predators.
The same rules hold for marine systems. Coral reefs that are overfished become covered with coral-killing algae because the fishes that ate the algae have been removed. When sea otters were hunted almost to extinction in the North Pacific, the sea urchins that they used to eat increased. Urchins eat algae such as kelp and consequently create “urchin barrens,” which are rocky areas devoid of plants. Kelp beds provide habitat for a vast array of nearshore fishes and invertebrates. Without kelp, these animals are homeless, and their populations diminish.
Sharks are at the top of the food chain in many systems (ignoring their parasites for the moment). When they are overfished, their absence affects the different levels of the food chain below them.
