Taking the 31,000-plus fish species worldwide, you could say that fishes eat everything. Some fish species are in fact omnivorous generalists, eating vegetation, insects, other fishes, detritus, zooplankton, you name it. Such generalist feeding habits help explain the amazing success of a few species that have been introduced in many places. Common Carp (Cyprinus carpio, Cyprinidae) and bullhead catfish (Ameiurus, Ictaluridae) are two freshwater examples. Many other fishes are opportunistic feeders, preferring certain foods but able to take advantage of what is available. Bluegill and Pump-kinseed Sunfish (Lepomis, Centrarchidae) eat zooplankton, aquatic insects, larval fish, snails, and just about any other animal that they can fit in their mouths.
In contrast, many if not most fishes are more particular about what they can and will eat, specializing on certain food types. Their anatomies reflect these specializations. The freshwater cichlids of South America and Africa, numbering perhaps 2,000 species, include species that specialize on sponges, sediments, algae, leaves, mollusks, insects, phytoplankton, zooplankton, fish scales and fins, fish eyes, fish eggs and embryos, and other fishes. Their lips, gill rakers, and jaw and pharyngeal (throat) teeth are especially suited for their preferred food type.
Across the 500-plus families of fishes, the catalog of unusual food types is long, with many surprising entries:
• One feeding type attacks parts of other fishes, such as scales and fins. Fin biters have sharp-edged teeth, whereas the teeth of scale eaters are often more file- or rasplike. Fin biters include young piranhas (Serrasalmus, Characidae) in the Amazon, saber-tooth blennies (Aspidontus, Plagiotremus; Blenniidae) on coral reefs, many African cichlids, some Asian barbs (Puntius, Cyprinidae), freshwater pufferfishes (Tetraodon, Tetraodontidae), and bumblebee gobies (Brachygobius, Gobiidae). Scale eaters include several African cichlids and leatherjackets (Oligoplites, Carangidae) on coral reefs. Cookie-cutter sharks (Isistius) sneak up on tuna, marlin, and dolphins and take a single, circular, half-dollar size plug out of their victim’s sides. Unfortunately, several of these so-called “partial consumers” are also popular aquarium fishes, causing no end of headaches for aquarists.
Madagascar cichlids are considered to be ancestral to the incredibly diverse cichlids of continental Africa, with their equally diverse feeding habits. Unfortunately, the Pinstripe Damba cichlid (Paretroplus menarambo), such as this one. are extinct in the wild, but the species is kept going through captive breeding.
• A number of fishes both in marine and fresh water pick parasites from the body, mouth, and gills of other fishes. Specialist cleaners include the cleaner wrasses (Labroides, Labridae), remoras (Echeneidae), and some butterflyfishes (Chaetodon) in the tropical Pacific, and neon gobies (Gobiosoma) in the tropical Atlantic (see “Are fishes social?” in chapter 4). Many other fishes pick parasites part-time, otherwise feeding more generally on small invertebrates (juvenile damselfishes, angelfishes, butterflyfishes, surfperches, wrasses, and centrarchid sunfishes).
• Fishes in many habitats feed on the feces of other fishes, an activity called “scatophagy” or “coprophagy.” Scats (Scatophagus), a popular Asian aquarium fish, get their scientific name from this behavior. Large Labeo minnows in Africa accompany hippos, using their excrement as a constantly supplied food source. Coprophagy is common on coral reefs. A study in Palau, Western Caroline Islands, found 45 different species of reef fishes from eight families (mostly sea chubs, damselfishes, wrasses, rabbitfishes, surgeonfishes, and triggerfishes) eating feces from 64 fish species in 11 different families, preferring the feces of planktivores over waste products from herbivores.
• A few fishes specialize on, or at least supplement their diet with, the eyes of other fishes. A narrow-bodied cichlid in Lake Malawi, called the Malawi Eyebiter (Dimidiochromis) does not make a good aquarium fish because of its eye-popping activities. A North American cyprinid, the Cutlip Minnow (Exoglossum maxillingua) will also remove eyes from its tankmates.
• Some African cichlids specialize in attacking female cichlids that are carrying young in their mouths (see “Do fishes care for their young?” in chapter 6). These babynappers repeatedly ram the head of the mother until she spits out a few young, which are then snapped up by the predator.
More usual food types include small plants or animals in the water column (phytoplanktivores and zooplanktivores), other fishes (piscivores), plants (herbivores), and detritus (detritivores). Many fishes that normally feed on live prey will not hesitate to scavenge on dead things.
Fishes that swim above the bottom and feed on zooplankton have a mouth that can shoot forward and thereby greatly enlarge. This increase in mouth volume creates strong suction pressures that cause small, swimming prey to be vacuumed into the round tube of the jaws. Such “pipette” mouths occur in both freshwater and marine zooplanktivores, including Bluegill Sunfish, herrings, damselfishes, fusiliers, bogas, wrasses, and cichlids, to name a few.
Piscivores come in three general types. Lie-in-wait predators rest on the bottom and are usually well-camouflaged or even bury themselves with just their eyes exposed (flatfishes, stonefish, scorpionfishes, lizardfishes, weeverfishes). These ambush predators wait for a prey fish to swim by close enough to be captured with an explosive rush. Frogfishes and anglerfishes speed up the process by waving a small lure just above their mouths (see “How do fishes find food?” below). Ambush predators generally inhale their prey with large mouths or impale prey on needle-like teeth, in the case of lizardfish.
Most other piscivores pursue their prey. Pursuers fall into two groups, burst swimmers and active chasers. Burst swimmers come from a wide variety of fish groups but all have evolved a similar body shape. They are elongate and have long, tooth-studded jaws, with their fins placed far back on their bodies. They hover in the water and dash at prey with a single burst of speed, impaling their prey with sharp teeth. These are the classic predatory fishes—barracuda, pike, gar—but an amazing variety of unrelated fishes in both marine and fresh water have converged on this body type and feeding method. Their common names often refer to their resemblance to a pike (Esocidae), including pike minnows (Cyprinidae), pike characins (Ctenoluciidae), pike killifish (Poeciliidae, the guppy family), pikehead (Osphronemidae), Australian Long-finned Pike (Dinolestes lewini, Dinolestidae), as well as needlefishes (Belonidae) and some cichlids.
Burst-swimming predators tend toward similar shapes and behaviors, regardless of their ancestry and size. These fishes catch their prey with a single burst of speed, impaling their victims with sharp teeth. They include (A) gars (Lepisosteidae), (B) pike-characins (Ctenoluciidae), (C) pikes and pickerels (Esocidae), (D) needlefishes (Belonidae), (E) Pike Killifish (Poeciliidae), and (F) barracudas (Sphyraenidae). From Helfman et al. (2009); used with permission of Wiley-Blackwell
The remaining group of predators actively chases down their prey, pursuing it for more than an initial burst of speed. Body shapes vary but most are fairly robust and streamlined, and capture usually involves overtaking prey by swimming rapidly, inhaling prey with an expandable mouth, or immobilizing prey with a weapon. Many are also popular game fishes. Examples include the Largemouth and Smallmouth Black Bass (Micropterus salmoides and M. dolomieu, Centrarchidae); snooks (Centropomidae); numerous cichlids that look very much like a black bass or snook (e.g., Peacock Cichlids in South America, Cichla ocellaris, also called Peacock Bass) and Rhamphochromis cichlids in Africa; many seabasses (Serranidae); tunas (Scombridae); and mahi-mahis (Coryphaenidae), to name just a few. The weapon wielders are the various billfishes and are probably the fastest fishes in the sea (e.g., marlins, Sailfish, spearfishes, Istiophoridae; and Swordfish, Xiphiidae).
Regardless of attack mode, many piscivores rely on an extremely expandable pipette mouth that creates strong suction. Frogfishes (Antennariidae), Stonefish (Synanceia), black basses (Micropterus), dories (Zeus), slingjaw wrasses (Epibulus), and cornetfish and trumpetfish (Fistularia, Aulostomus) are examples. Both suction pressure and forward shooting of the mouth aid in capture. During an attack, the mouth becomes an extension of the body that can be shot out faster than the fish can swim through the water. Mouth volume may be increased by 15 to as much as 40-fold at the critical moment.
A major factor determining what a carnivorous fish can eat is simply food size. Most fishes cannot eat anything they cannot swallow. Swallowing involves first getting something into the mouth and second getting it down the throat. Hence fishes are “gape-limited.” Gape limitation is less a challenge for the few exceptional fishes that can actually tear up or chop up their food into swallowable bites (Tigerfish, piranhas, Bluefish, barracudas, and sharks). But most fishes do not have strong enough jaws and large, sharp teeth with cutting edges. If a fish miscalculates prey size and attacks a prey item too large to swallow, it can choke to death. It is not unusual to find a Largemouth Bass with a Bluegill Sunfish sticking halfway out its mouth or a pickerel with a stickleback caught in its throat, floating at the surface of a lake, dead.
As a result, and although larger fishes can eat larger prey because they have larger mouths, many very large fish eat remarkably small items. Yellowfin Tuna (Thunnus albacares, Scombridae) weighing 23 kilograms (50 pounds) or more may have their stomachs packed with larval fishes, each measuring a few millimeters (a fraction of an inch) in length. This is why an angler can often catch large gamefish with what seem to be very small lures. The notable exceptions to the “smaller-than-your-mouth” size rule are found in the deep sea, where many fishes have hinged jaws and expandable stomachs and can swallow prey longer than themselves. Feeding opportunities in the deep sea are few and far between, so fishes there must be able to take advantage of anything that comes along.
Many fishes that lack cutting teeth still overcome gape limitation by breaking up prey, but they do it in an unexpected manner. They shake, twist, and spin their food until it comes apart, much as you would twist taffy or separate a chicken leg from the bird’s body. The best spin-feeders are “eels,” including the freshwater eels (Anguillidae) but also many other eel-like fishes closely or distantly related to the anguillids (moray eels, Muraenidae; snake eels, Ophichthidae; conger eels, Congridae; swamp eels, Synbranchidae; pricklebacks, Stichaeidae; gunnels, Pholidae).
Even large predators are gape-limited. Most fishes can only eat things they can swallow whole. Large groupers, such as this Giant Grouper (Epinephelus lanceolatus), that grows as large as 2.7 meters (9 feet) long and weighs up to 600 kilograms (1,320 pounds), must swallow prey whole and hence cannot attack and dismember large prey. It is the exceptional predators with sharp, cutting teeth that are able to chop up their prey.
An American Eel (Anguilla rostrata) spins while feeding. The 50 centimeter (19 inch) eel has grasped the bait, a fish filet attached to a lead weight, in its mouth and has begun to rotate. Twist in the body is evident: the light underside of the head and the dark back both face the camera at the same time that the light belly is facing upward.
These and other “rotational feeders” have one thing in common, namely a long, thin body that lacks hard fin spines or large paired fins such as pectorals or pelvics. This body type allows them to twist and spin rapidly, up to 14 rotations per second, while holding onto prey (the best human ice skaters cannot rotate faster than about five or six rotations per second). Other long-bodied or eel-like animals also spin to dismember prey, including some reptiles (i.e., crocodiles engaged in their famous death roll, some snakes) and a few amphibians (sirens and caecilians). The cookie cutter sharks mentioned earlier use the same spinning behavior to remove divots of flesh from their much larger, living prey. These small (50 centimeter, 20 inch) predators are also called cigar sharks because of their relatively long bodies and small fins.
Moray eels (Muraenidae) are among the best spinners and add another tactic also made possible by having an eel-like form. While holding onto a prey fish with strong jaws and sharp teeth, they tie a knot in their tail and pass the knot forward until it presses against the prey fish. As the knot presses and the jaw holds, the prey is torn apart. Hagfishes, also elongate, use this knot tying technique to remove chunks of skin and flesh from their already dead meal. Hagfishes (Myxinidae) lack true jaws and teeth and instead hold onto their prey with the sucking disc at the front of their body.
Plant-eating herbivores do not have to fool their prey or chase it down, but they do have to overcome mechanical and chemical defenses. Bony fishes can eat microscopic algae (shads, herrings, Milkfish, Blackfish, mullets, cichlids, minnows), scrape or nibble algae off rocks (Chinese Algae Eater, minnows, catfishes, cichlids, sea chubs, blennies, pupfishes, damselfishes, parrotfishes, surgeonfishes), or eat whole plants or take pieces out of leaves or digest seeds (Asian minnows, cichlids, characins, halfbeaks). However, plants give up their nutrients begrudgingly due to their hard, leathery, or rubbery exteriors; extremely tough cell walls; placement of more nutritious parts inside inedible packages; or noxious chemicals throughout their tissues. Herbivorous fishes rely on vision, taste, and digestive capabilities to separate the readily edible from the tough and sometimes toxic. Herbivores are almost entirely daytime feeders, pointing out the importance of vision in food choice.
Tooth type (see “Do all fishes have teeth?” in chapter 2) and intestinal tract anatomy are adapted to plant type. Leaf eaters generally have broad, flattened jaw teeth suited for nipping off the ends of algal fronds or leaf tips. Herbivorous fishes often have grinding stomachs and very long and convoluted intestinal tracts (animal flesh is relatively easy to digest and predators consequently have relatively short intestines). Herbivores use pharyngeal mills or highly acidic stomachs to break down the cell walls of plants. Unlike insects and many herbivorous mammals, fishes generally lack intestinal bacteria that help digest plant matter via fermentation. The few exceptions include surgeonfishes (Acanthuridae) and sea chubs (Kyphosidae). Sea chubs feed heavily on brown algae that are avoided by most other herbivores.
An extreme among herbivores that points out the challenges presented by eating algae involves the parrotfishes (Labridae), which feed on the algae that grow on and often inside corals. To get at the algae, parrotfishes have massive beaklike teeth and equally massive pharyngeal jaws. Their jaw teeth scrape or bite off chunks of coral rock. Coral fragments are then passed back to the pharyngeal jaws where the coral is ground up. The fish is not after the coral animals but the algae that grow inside the coral skeleton. The pharyngeal grinding breaks up the cell walls of the algae. In the process, coral is ground down into sand. As parrotfishes swim about the reef, they excrete clouds of sand along with feces. On some reefs, abundant parrotfish are significant producers and movers of sand.
The ability to eat a wide variety of foods was a major step in fish evolution. The living members of the more primitive bony fish groups (lungfishes, coelacanths, sturgeons, paddlefishes, bichirs, gars, Bowfin) are all carnivores, as are all sharks and the jawless hagfishes and lampreys. Most are primarily fish eaters, habits that undoubtedly reflect the feeding behavior of their ancestors (lungfishes eat mollusks and North American paddle-fish feed on zooplankton). It is only in the more recently evolved teleosts (modern bony fishes) that make up 99% of living fish groups that we see a diversified diet that includes plant life in its various forms. This ability greatly expanded the habitats in which fishes could live as well as moving fishes down to the most productive levels of the food chain.
Unlike reptiles, fishes can chew their food. Unlike mammals, most fishes chew not with their jaw teeth but instead use throat teeth. The teeth in these pharyngeal jaws are well adapted to handling the kind of food a fish normally eats. Mollusk feeders have large, flattened or rounded pharyngeal teeth best suited for crushing hard-bodied prey. Fish eaters have long, curved pharyngeal teeth that pierce the prey and also prevent it from escaping back out the mouth. Insect feeders have smaller, pointed pharyngeal teeth.
The mouths of most fishes are therefore primarily the “capture” tool of fishes, chewing and other processing of prey occurring in the pharyngeal jaws or even in a gizzard-like structure in the stomach. Gill rakers (bony, fingerlike projections on the inside of each gill arch) also help to keep prey from escaping out the gill openings while food is being moved to the stomach. Plankton feeders have fine, closely spaced gill rakers that help in the capture of tiny animals, whereas piscivores often have stout, prickly rakers that keep larger prey from escaping.
Although most of the teeth on the various bones are pretty much the same, a few exceptional fishes have different shaped teeth in different parts of the mouth. For example, wolffishes (Anarhichadidae) feed on hard-bodied prey such as sea urchins. Wolffishes are more like mammals than like other fishes in having biting and grasping teeth in the front of their jaws and crushing teeth toward the back, analogous to our canines and molars.
Some bony fishes replace their teeth as they grow. Mammals have two sets of teeth, baby or milk teeth when young, and adult or permanent teeth later. Many fishes, by contrast, replace their teeth throughout the lives. Functional teeth sit on the jawbones while replacement teeth lie under them, embedded deep in the jaws. Replacement teeth erupt when the outer teeth are lost or shed. Sharks are best known for the multiple rows of replacement dentition, but many bony fishes also have teeth-in-waiting (e.g., Bowfin, characins such as piranhas, salmons and trouts, surgeonfishes, gobies).
Parrotfishes bite coral and scrape the surface of coral heads, then grind coral bits into sand in the process of digesting the algae inside. (A) Bite marks on a coral head from parrot-fish grazing. (B) A model showing how coral fragments are ground into sand as they pass through the mouth, pharyngeal mill, and gut of a parrotfish.
Some large predators such as seabasses, Largemouth Bass, snook, and marlin that feed on active swimming prey actually have relatively small teeth. Their teeth feel like sandpaper and would seem incapable of capturing much. Instead, these fishes rely on a large mouth that when opened rapidly creates strong suction pressures. Hence many fishes capture their prey by vacuuming them in, close their mouth once the prey are inside, use their various mouth teeth to manipulate prey so they can be swallowed head first, and then pass the prey back to their pharyngeal jaws where the real action takes place (billfishes skip much of this process by whacking or spearing prey with their lethal swords and swallowing the helpless victim).
Fishes use all their senses, including one we do not have, to find food. Vision plays an important role in food finding, but many fishes forage at night or live in permanent darkness (the deep sea, caves, very murky water) and must rely on other means of locating food.
Even fishes that initially locate food via nonvisual means rely on vision in the final attack phase of feeding. Most zooplanktivores locate and track their prey visually, even though their prey is very small. Daytime feeders can see and follow extremely small objects because of the high density of visual receptor cells (retinal cones) in their eyes. Having many small cones also makes their eyes exceedingly sensitive to movement. Their eyes therefore have high acuity (point-to-point resolving power) and high motion detection. Predators such as tarpon, trout, black bass, barracuda, pompano, and tuna rely on motion detection and bright flashes of light reflecting off the silvery sides of many bait fishes to tell them where prey are. This behavior explains why fishing lures that are cast and retrieved or trolled often have silvery, reflective surfaces.
Surprisingly, many night-feeding fishes rely on vision, especially fishes that swim in open water and feed on zooplankton. Their visual tasks are made somewhat easier because the invertebrates and fish larvae they feed on are larger than the plankton animals that swim about by day. Fish groups includes sweepers (Pempheridae), squirrelfishes (Holocentridae), cardinalfishes (Apogonidae), and bigeyes (Priacanthidae) on coral reefs; some surf-perches (Embiotocidae) in kelpbeds; and Black Crappie (Pomoxis nigromaculatus, Centrarchidae) in temperate lakes.
Nocturnal feeders have very large eyes. Many also have a reflective layer at the back of their eyeballs, the “tapetum lucidum” (from the Latin, meaning “bright tapestry”),” which causes light to pass through the eye twice, thereby increasing light sensitivity. A tapetum is the same structure that causes eye shine in cats, dogs, crocodiles, raccoons, and deer. In addition, the receptor cells (retinal rods) in their eyes are very large and thus able to capture any available light. Such light sensitivity comes at a cost because large rods sacrifice acuity and motion detection for sensitivity. Several predators feed at night and also have very large eyes (e.g., most sharks and chimaeras; coelacanths; sturgeons; Walleye and Sauger, Percidae).
Special situations require special adaptations. A few fishes spend a good deal of their foraging time out of water, such as mudskippers (Periopthalmus, Gobiidae), which move across exposed mudflats and climb up plant roots to find food. They have a strongly curved cornea and slightly flattened lens that allows them to focus on objects in air. The Four-eyed Fish (Anableps, Anablepidae) swims with half of its eye out of the water, searching for insects. Its eyes are split in half horizontally, each having two pupils and a retina that is divided into top and bottom sections. Cavefishes and deep-sea fishes that live in perpetual darkness have very small eyes or have lost their eyes completely over the course of evolution. Plants do not grow in the dark and prey animals are scarce in both caves and the deep sea. Fishes in these habitats save energy by not having eyes, but they instead use sound and smell to find food.
Water is an excellent transmitter of sound; sound travels 4.5 times faster through water than through air (1,500 meters / 5,000 feet per second in water versus 330 meters / 1,100 feet per second in air). As a result, sound travels great distances and is a good source of information. Fishes are very sensitive to sounds, especially at the low frequencies that travel farthest. Few of the sounds important to fishes occur at higher frequencies. Most fishes cannot hear sounds above 500 Hz or cycles per second, whereas human hearing goes up to 20,000 Hz. The exception is some herring relatives (Clupeidae) that are able to hear sonar clicks at 100,000–180,000 Hz (100,000–180,000 cycles per second) produced by the dolphins that feed on them.
Although fishes cannot hear frequencies as high as humans can, fishes can tell which direction a sound came from, a useful ability for finding food and avoiding predators. Anyone who has snorkeled or scuba dived knows that humans are unable to tell the direction a sound comes from. Try it the next time you are in a pool.
Fishes hear with both their lateral line and their inner ear. The lateral line is a series of nerve cells along the side and head of a fish that are sensitive to water movement. Sound waves traveling through water, such as vibrations caused by a swimming prey fish, move the water and stimulate the lateral line receptor cells or the “otoliths” (ear bones) of the inner ear. Many fishes (minnows, suckers, characins, catfishes, herrings, cods, squirrelfishes) use their gas bladder as a sounding board that vibrates and amplifies sounds, sending a sensory signal to the inner ear and increasing sensitivity to sound. Minnows and their relatives (otophysan fishes) possess a series of small bones derived from modified vertebrae called “Weberian Ossicles” that connect the anterior end of the gas bladder to the inner ear. These bones pivot end-to-end, turning a small vibration at the gas bladder end into a greater vibration at the head end. These fishes have the highest sensitivity and greatest frequency range of hearing among non-clupeid fishes.
Fishes use hearing to find prey, turning toward the source of a sound and moving in that direction until they find the sound producer. A swimming fish actually leaves a sonic trail of disturbed water behind it. Lake Trout (Salvelinus namaycush, Salmonidae) can detect and follow prey fishes in total darkness by listening to the water disturbance noises left behind by their swimming prey. Several predators, especially sharks, are attracted to the squeaks, squeals, grunts, thumps, and buzzes produced by injured fishes. Many fishing lures attract gamefish because they make a sound like swimming or injured fish.
Four-eyed Fish (Anableps) frequently swim at the water’s surface with the “terrestrial” half of their eyes poking out of the water as they search for insects. Wikimedia Commons, http://en.wikipedia.org/wiki/Anableps_anableps
Smell and taste are related senses that rely on detection of chemicals. Fishes are sensitive to a wide variety of chemicals but respond most strongly to proteins and amino acids. Most fishes have nostrils to detect odors. The sense of smell (olfaction) is used in finding mates, recognizing predators, directing migrations, and finding food. Taste buds are used more often for foraging. Taste buds occur on the tongue, as in mammals, but many fishes, especially catfishes, have taste buds all over their body, fins, and especially on their whisker-like barbels. Goatfishes (Mullidae) have muscular barbels with which they probe the sand in search of invertebrates. Sea robins (Triglidae) have taste buds at the tips of their pectoral fins.
Fishes have a sixth sense that humans lack. Fishes are sensitive to bioelectric fields. Many fishes can find prey by simply detecting the electric field the prey creates due to muscular contractions, nerve impulses, or just because the salt concentration of a fish’s body fluids differs from the salt concentration of the water around it. Sharks, lungfishes, coelacanths, paddlefishes, sturgeons, eels, salmon, trout, catfishes, tunas, and many other fishes are electrosensitive. Electrosensory organs are scattered across the skin of these fishes and are often concentrated in the head region. Paddlefish (Polyodon spathula, Polyodontidae), which feed on zooplankton, use their electric sense to detect planktonic animals, an ability that was discovered just recently. The elongate and flattened snout of this fish contains many electric receptors. It acts as an antenna that allows Paddlefish to detect individual zooplankters 9 centimeters (almost 4 inches) away.
Goatfishes occur in coral reef areas around the world. They are frequently seen over sandy areas probing for invertebrates with their chin barbels (muscular whisker-like appendages) and then push into the sand to snatch up a prey item. Inset photo (upper right) shows the prominent barbels of the Caribbean Yellow Goatfish (Mulloidichthys martinicus, Mullidae) foraging in the larger photo. Main photo by Doug Bronski; inset photo courtesy of Cecil Berry
Sharks and rays are so sensitive to electric fields that they could detect the 1.5 volt output of a D-cell flashlight battery several miles away if it were not for electrical interference from the earth’s geomagnetic field. Sharks find prey buried in the sand by homing in on the bioelectric output of the prey. Sharks also dive into the sand after buried, active electrodes. Sharks can sense the electrical output from a swimming human 2 meters (6 feet) away.
Some amphibians (and duckbill platypuses) are also sensitive to electric fields. However, one thing that fishes do that no other group of organisms can do is create an electric field and then detect and identify objects that enter into the field. Special electricity-generating cells made from modified muscle cells produce the field. Electricity-sensing cells are made from modified lateral line cells. Three families of tropical freshwater fishes possess this complete electrolocalization sense: gymnotid knife fishes of South America, elephantfishes (Mormyridae) of Africa, and upside-down catfishes of Africa (Malapteruridae, Mochokidae; see “What are electric fishes?” in chapter 2).
Many fishes simplify the task of finding food by bringing it to them instead. Luring is most highly evolved in the anglerfishes and their relatives (goosefishes, frogfishes, batfishes; order Lophiiformes). Many of these bizarre-looking animals live in the deep sea, where food is scarce and energy conservation is crucial. It is far preferable to lure prey to you rather than having to find and chase it.
Anglerfishes attract prey by dangling a lure just above and in front of their mouth. The lure, called an “esca,” is the tip of their first dorsal spine, which is modified to look remarkably like a small fish, shrimp, or worm. The lure is wriggled in a lifelike manner, and may even produce a chemical attractant. The predator is well camouflaged to resemble the bottom or the dark surrounding waters. Small fishes approach the lure and are quickly inhaled by a large mouth. Escape is often prevented by long, backward-facing teeth.
The North American Paddlefish (Polyodon spathula) lives in large rivers and filters zooplankton with the aid of its paddle-like snout. The paddle contains numerous electricity receptors that help it find zooplankton swarms.
Luring is such a successful tactic that it has evolved in many fish groups other than anglerfishes, but especially in other deep-sea fishes. The lure may involve the dorsal spine (as in a scorpionfish, Scorpaenidae) or other body parts. Deep-sea hatchetfishes (Sternoptychidae), lanternfishes (Myctophidae), and stargazers (Uranoscopidae) have lures inside their mouths; barbeled plunderfishes (Artedidraconidae) wave chin barbels (whiskers); chacid catfishes use barbels attached to their upper jaw; snake eels (Ophichthidae) use their tongue as a lure; and in gulper eels (Eurypharyngidae) the tip of the tail glows to attract prey.
An unusual variation on luring and ambush predation involves a predatory cichlid from Lake Malawi, Africa. The Kalingo (Nimbochromis living-stonii) is a moderate size cichlid (25 centimeters, 10 inches) that takes advantage of the habit of many small cichlids to scavenge on recently dead fishes. The predator lies on its side on the bottom and assumes a blotchy coloration typical of a dead fish. When scavengers come to investigate and even pick at its body, the predator erupts from the bottom and inhales them. This is the only fish that is known to play possum, except the possum is more like a panther.
Many fishes besides some cichlids will scavenge. Some feed primarily on dead things, but just about any fish—including many herbivores and predators—will take advantage of the easy meal presented by a dead or dying animal lying on the bottom. If this were not the case, bait fishing would not be so successful.
Species that make a living primarily by scavenging are sharks, cat-fishes, freshwater eels (Anguilla), and especially the hagfishes (order Myxiniformes). Hagfishes are primitive boneless, jawless, eel-like marine fishes that aggregate in large numbers on anything lying dead on the seafloor. They use their rasping “teeth” to tear away at flesh and organs, sometimes holding on with their mouth and running an overhand knot from tail to head, levering against the body of the food item to tear chunks away. A few hundred hagfish working over a large dead fish can turn it into skin and bones in a matter of hours. The efficiency of scavenging hagfish might cause one to think twice about being buried at sea.
Many fishes eat hard-bodied prey such as mollusks (snails, clams) and sea urchins. These “durophages” (as in the phrase “durable goods”) usually have very strong jaws with rounded, crushing teeth, often backed up with similar teeth on their pharyngeal jaws. Freshwater Drum (Aplodinotus grunniens, Sciaenidae), large suckers (Moxostoma, Catostomidae), Shell-cracker Sunfish (Lepomis, Centrarchidae), and even small stream fishes such as the federally protected Snail Darter (Percina tanasi, Percidae) are part of this group. Marine species include many stingrays, Sheepshead (Archosargus probatocephalus, Sparidae), and wolffishes (Anarhichadidae) in temperate oceans. On coral reefs, mollusk crushers include wrasses such as the giant Humphead Wrasse (Cheilinus undulatus, Labridae) that eats cowries as well as crown of thorns starfish. Not surprisingly, several African cichlids are molluskivores and possess molar-like pharyngeal teeth necessary to crush shells.
Apparently not. Water is a poor medium for food storage because it carries odors so well. It is difficult to hide a food item from other scavengers, which means just about all other fishes.
Not very often. Tool use involves manipulating something in the environment to achieve a purpose. Although many mammals and birds use tools such as sticks and rocks to probe for, catch, or crush prey, tool-using fishes are unusual. The best-known examples come from the incredibly diverse tropical family of wrasses (Labridae: Halichoeres, Cheilinus, Coris, Thalassoma). Many wrasses feed on hard-bodied invertebrates such as clams, scallops, snails, and urchins. These fishes have strong pharyngeal jaws, discussed earlier, to crush their prey after it is swallowed. However, some make the task easier by first holding the mollusk in their mouth and bashing it against a rock as if the rock were an anvil.
Archerfishes (Toxotidae) are well-known for their ability to shoot down objects above the water, both insects on branches and pieces of hamburger held above the surface by someone. Experiments have shown that one archerfish can watch another and learn how best to aim at a target. Photo by R. Wampers
A few fishes use water jets to acquire food, which is a form of tool use. Archerfish (Toxotes, Toxotidae) are the best studied. Archerfish shoot droplets of water by pressing their tongue against a groove in the roof of their mouth. The water bullets can travel 150 centimeters (4.5 feet) and knock insects off branches. Archerfish not only allow for the bending of light rays at the water’s surface (refraction) but can also track and shoot down moving prey. In an underwater variation on this tactic, triggerfishes (Balistidae) blow water out their mouths repeatedly at the base of a sea urchin, tumbling the urchin over and exposing its undefended lower half. The triggerfish then tears the urchin apart with its powerful jaws and incisor-like teeth.
