At the end of a sunny day, a popular recreational beach can resemble a plowed field, the surface churned up by thousands of human footprints. But if we saw that beach for the first time after the crowd had left, could we tell what had roughed up the surface? Up in the wrack line there might be a concentration of cigarette butts and beverage containers that would give a few clues that humans were involved, and on some wet, hard-beach stretches, footprints may be preserved.
It is not only humans that can churn up a beach surface, however. In addition to a myriad of marks and bedforms created by wind, waves, and currents, most beaches reveal that they are alive with the activity of animals that leave a variety of tracks, trails, and traces as evidence of their presence.
The traces animals and plants produce in the beach and nearby tidal flats and dunes are as fascinating and informative as the physical structures formed by waves and currents. Such tracks, trails, and burrows are seen more frequently than the animals that produced them. Perhaps we see the same birds that left their footprints, but the crab burrow holes, their pellets, and the web of trails that run from one hole to another are certainly a more common sight than the shy animals that left these traces. Lucky observers may see the morning’s trail of a turtle’s crawl across the beach from the night before, but fewer people have actually witnessed such a critter’s beach visit. Other large creatures such as seals, walruses, and sea lions leave characteristic crawl tracks that persist until erased by a high tide or a storm. Relating these sometimes mysterious, sometimes obvious features to their seen and unseen parents is part of reading the beach. This chapter highlights the organisms that live on and within the beach. The following chapter examines shells and skeletal material that become an important part of beach sediment.
Crab excavation (not fecal) pellets and claw prints around a burrow opening, Cape Tribulation, Queensland, Australia. The Australian fifty-cent coin is shown for scale.
An elephant seal crawl track on an Antarctica beach with a penguin shown for scale. Photo courtesy of Norma Longo.
The beach is a hostile environment in which to make a home. To live on the beach, an animal must adapt to changing wave energy, rapid changes in the shape of the beach (taking place in a matter of hours), swift and variable currents, frequent movement of the sand and gravel, alternate wetting and drying, strong winds, changing water salinity, changing temperatures, and rapid burial or exposure. Yet the beach is alive. It is not the jungle, but the place where you spread your beach towel is likely to be in the midst of millions of animals. It is true, however, that most of these animals are at or near the microscopic level, living between the sand grains of the beach, and only a few are in the size range that we are likely to see.
Biologists divide these beach animals into microfauna, which are of microscopic size; meiofauna, which are the size of small sand grains, barely visible to the naked eye; and macrofauna, which are big enough to be seen readily, although many of these also are very small (the size of coarse sand) and usually escape our attention. Another term, psammon, refers to both very small plants and animals that grow on, grow in, or move through the sand of the beach.
The psammon and meiofauna generally leave no visible traces or evidence of their presence; however, the abundant presence of algae or diatoms (usually considered to be a type of plant) may give a greenish or slightly yellowish tinge to the wet sand. The meiofauna are so small that most can live between the sand grains, moving freely through the pore spaces without displacing the grains. Their abundance in the sand is often mind-boggling; words like millions are needed to describe population densities. Yet if you scoop up a handful of sand, you may not see a single one. Some of these organisms are phosphorescent, however, and if you stroll on the beach at night, try stomping on the surface or dragging a stick through the sand; it may stimulate the critters to glow and reveal their presence, if only briefly.
Widely spaced kangaroo tracks on an Australian beach.
The tiny organisms of the beach are also very diverse in the numbers of different species present and the variety of roles they play in their home environment of the pores between the sand grains. Nematodes are often the most abundant, typically accounting for about 85 percent of the meiofauna. They feed on algae, bacteria, and organic detritus and serve the natural role of beach cleaning. They are, however, part of a larger food chain; the meiofauna are the food source for a range of larger animals from invertebrates to fish to birds. This dual role of cleaning the beach and being near the base of the food chain is why the abundance and diversity of the meiofauna are often used as a measure of the beach’s health. Artificial beaches constructed from dredged sand certainly lack such fauna in their initial construction, and beach bulldozing and daily manicuring severely impact or destroy the meiofauna, as well.
An idealized illustration of the meiofauna that live between the sand grains of a beach. Illustration courtesy of the Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution.
The beach macrofauna include the visible animals that live in the beach as well as those that visit the beach regularly, either from the land or from the sea. These creatures leave evidence of their presence and their activities in the form of footprints, tracks, trails, burrows, or fecal material.
The mollusk group accounts for most of the shells and the calcareous sand on the beach (see chapter 10), but most live offshore, and the traces of clams and snails are not particularly common on the beach. Perhaps the two most globally common clams in beaches are the surf clams (Spisula sp.), sometimes called pipi, and the numerous species of the coquina clam (genus Donax sp.), also known as bean clams. These beach clams are found from temperate to tropical climatic zones. (The designation of [Name sp.] for an organism refers to its generic name and indicates that there is more than one species in the genus.) Commonly found in the swash zone in groups, these small, colorful clams burrow just below the surface, forming a tiny pit in the beach.
Donax clams live within the beach in a vertical orientation, obtaining food by filtering the water that the waves move back and forth. The clams protect themselves by burrowing deep within the beach when they sense that storm waves are on the way. While alive, they seem to congregate in patches, some of which may have up to ninety individuals per square foot (about 10 cm2) of beach. After their death, their shells can make up a major portion of the seashells on the beach.
Groups of small clams living just below the beach surface become obstacles to backwash flow, creating rhomboidal and obstacle-mark patterns. A U.S. quarter is shown for scale.
The clams extend their siphons just out of the sand to form two small holes, and the holes and pits are somewhat like the nail holes produced by air escaping from the beach (see chapter 7). These micro-irregularities formed by Donax siphons may interfere with swash flow and generate a pattern somewhat like rhomboidal ripple marks. Breaking swash may briefly expose these clams, but they quickly burrow back into the sand. Larger clams (e.g., soft-shell clams and razor clams) may be found as burrowers in tidal flats or the beach at low tide.
Small snails sometimes leave a continuous trace on the beach surface as they move in what appears to be a random pattern. At low tide such snails may be numerous on tidal flats as they graze over the ripple marks. Some snails (e.g., the North American moon snail [Polinices sp.] and the lettered olive [Olivia sp.]) burrow just under the surface and then laterally as they feed, leaving a feature that looks like a sharp slash mark in the sand or a convoluted trail of disturbed sand. If you wish to find out who the trail maker is, follow the trail to its end and pluck out the shell! Just be sure to put it back in place once you’ve viewed it!
A mollusk burrow trail just below the surface of a Gold Coast, Australia, beach. The Australian twenty-cent coin is shown for scale.
Crustacea is a subphylum of the Arthropoda, a group whose tracks are well represented on the beach. Crabs are the most familiar; worldwide, crabs include numerous genera that appear on beaches and their associated environments, both offshore and onshore. Their most common traces on the beach are burrow openings, footprint trails between burrow holes, feeding trails, and fecal pellets. Often these feeding trails involve a systematic pattern of scraping food from the surface of the beach and leaving the remnants as small pellets. On the Queensland coast of Australia, beaches that are initially covered in antidunes as the tide falls become the scene of frantic feeding activity by crabs that emerge at low tide. Soon the whole beach surface can be covered in feeding traces radiating out from the central dwelling holes. Crabs vary in behavior; they may be shy creatures, such as the ghost crab (Ocypode sp.), or seemingly fearless crabs that gather in large groups, such as fiddler crabs (Uca sp.). Similarly, crabs range in color from almost colorless, to camouflage colors that blend with the sand, to vivid bluish, green, orange, and red. Crabs are common on the upper beach and in closely associated environments such as salt marshes, mangroves, and mudflats (e.g., fiddler crabs, with their one large fiddlelike claw, and hermit crabs make these their home). Their burrow structures can be complex and may extend for several feet below the surface of the beach.
A snail’s travel trail on Playa de la Griega Colunga, northern Spain, with a curious loop as if he had temporarily forgotten where he was going. The snail is at the lower right edge of the photo, near the ten-cent euro coin (shown for scale)
A snail trail that looks as if the creature was trying to write a message. The straight portions of the trace are travel trails, made as the snail went from one point to another, whereas the pattern of small loops is probably the animal’s feeding or grazing trail. The U.S. penny is shown for scale.
A concentration of small periwinkle snails grazing on a beach.
A moon snail beginning its burrow into the beach.
Sand hoppers, which belong to the groups known as amphipods and isopods, are also common on many beaches along the wrack line or just at the high-tide line. These small nocturnal crustaceans move onto the wet beach at night to feed and by day retreat up the beach to burrow into the sand or use the shade of wrack to protect them from the heat of the sun. Large groups of hoppers may be found beneath piles of seaweed, which is one of their main food sources; they release nutrients back into the food chain or are themselves eaten by birds. Although they often dig their own burrows, sand hoppers utilize the existing air escape holes in beaches for their daytime hideouts. Where they do burrow, they may leave telltale excavation patterns on the surface of the beach. Smaller species generate burrow holes about 0.08 in (2 mm) in diameter and only 0.4 or 0.8 in (1 or 2 cm) deep, leaving the excavated sand in a tiny anthill-like mound.
A crab feeding trail and fecal pellets pattern, Hillsborough, Australia. The Australian fifty-cent coin is shown for scale.
Larger species, the size of jelly beans, dig their holes by powerful backward kicks that throw the sand back onto the beach, then alternately dig on each side of the hole, producing a two-rayed pattern on the beach surface. Because there are usually several of these animals living together, the abundance of holes and the characteristic ray pattern, which looks like “chicken tracks,” are giveaways of their presence. These holes are elliptical in shape, on the order of about 0.5 in (5 to 12 mm) at their widest, and up to 10 in (25 cm) deep.
The greater size of these holes and the associated ray pattern make them distinguishable from holes produced by the physical escape of air or water from the beach (see chapter 7). However, the presence of the holes has been associated with the formation of sand blisters or domes. Such blisters may be initiated when the tide line rewets the burrowed beach and a wave seals the top of the burrow with wet sand. Water seeping into the beach forces air up the former burrow hole, but the wet sand at the surface of the beach effectively traps the air, which then lifts the overlying sand layer into a blister. As with other similar blisters, these can be truncated by waves to form pits and rings (see chapter 7).
Upper A mysterious pattern of mounds on a beach in Marbat, Oman.
Center A close examination reveals that the mounds are the sand excavated by a type of ghost crab from their burrows. This close-up of one of the pinnacled excavation mounds associated with a burrow opening shows the crab’s fecal pellets on the front side of the mound and its claw traces from the burrow to the right side of the mound. Photos courtesy of Miles Hayes.
A dense cluster of sand hoppers feeding on seaweed flotsam.
Sand-hopper holes on a Japanese beach. Note that the holes are irregular in diameter, unlike many nail holes and air holes. Some of the openings are surrounded by a rim of excavated sand. The U.S. penny is shown for scale.
Similar visitors from nearby dunes or other environments can come down onto the beach to feed. In South Africa, curious traces spotted on the upper beach turned out to be made by tiny crickets from the adjacent forest, feeding on the beach after dark.
Mole crabs (including such genera as Emerita sp. and Mulina sp.) are another common group of crustaceans in the beach; however, they are found in the edge of the breaking surf and lower swash zones. Although they do not leave common traces on the beach, you may see them when you are wading along the water’s edge. These suspension feeders live in groups, burrowing just below the sand surface with their antennae exposed to capture zoo-plankton. When a breaking wave or backwash exposes the mole crabs, they quickly rebury themselves. They are fairly easy to capture as the swash returns to the sea.
This pattern was made by sand hopper–type organisms on the beach surface, but one does not always know which specific organism is producing such a pattern. Photo courtesy of Dave Schoeman.
Sand hoppers and mole crabs sometimes show up under other common names such as sand fleas; however, they are neither insects nor fleas. The term sand flea suggests a small, biting creature, and the term is commonly used for tiny mosquitoes and “no-see-ums.” A true flea, the chigoe, is sometimes common on beaches in tropical and subtropical Africa and the Americas.
The forest cricket Tridactylidae comes out at night onto the adjacent beach at Cape Vidal, South Africa, leaving its traces behind. Photo courtesy of Lynette Perissinotto.
Another Crustacean that leaves traces on the intertidal beach is the genus Callianassa, which has numerous species. These shrimplike animals belong to a group known as the decapods and are deposit feeders that burrow deep into the sediment. Sometimes called mud shrimp or ghost shrimp (although other genera go by the latter name also), Callianassa species are found worldwide. Depending on the species, they range in habitat from intertidal beaches and mudflats to subtidal depths. Their burrow openings are on the order of 0.25 in (about 0.5 cm), and the burrows are intricate and extend to depths of as much as 6 ft (about 2 m). As they burrow, sand and water may be extruded from the burrow opening, forming miniature sand volcanoes and flows. They also extrude fecal pellets that surround the burrow opening, having the appearance of tiny cylindrical chocolate sprinkles. Because they are deposit feeders that strip microorganisms from the sediment as they burrow, they tend to prefer fine sand to muddy sediment, so the burrow holes are more likely to be found on fine sandy beaches and tidal flats at low tide.
These burrows are more than just holes in the sand. The animal builds a slightly cemented wall to give strength to the burrow opening, one strong enough to resist the inflow and outflow of water during a tidal cycle. When waves or swash erode sand from around the burrow opening, the wall may be left standing as a tube or a chimney structure.
Callianassa major, a species common to beaches along the southeastern U.S. coast, can be found from the intertidal zone to the midtide line and no higher. This fact has been used to determine the level of the sea in fossil sands from ancient shorelines left behind by higher sea levels on coastal plains.
Horseshoe crabs are arthropods that are not crabs but are more closely related to the arachnids (e.g., scorpions and spiders) and have their own taxonomic class, the Merostomata, in the classification of the Arthropoda. A close relative of the long-extinct trilobites, which existed hundreds of millions of years ago, horseshoe crabs are said to be “living fossils,” and their carcasses are commonly found washed up on beaches. During mating season they come into the shallow waters adjacent to the beach (e.g., off Delaware Bay, on the Atlantic Coast of the United States), and they are sometimes seen in association with their tracks in the sand. The group is represented by three genera: Limulus sp., along the North American Atlantic and Gulf of Mexico coasts; Carcinoscorpius sp. and Tachypleus sp., on the east coast of India; and species of the latter genus in some areas of the Japanese coast.
Sharing beach space with worms does not sound too appetizing; however, the polychaetes are a class of annelid worms that includes numerous species found in nearly all intertidal zones, some leaving unique traces. Although they are too numerous to outline in detail, the following are good examples. The common lugworm (Arenicola sp.) lives in U-shaped burrows on tidal flats and excretes a distinctive coiled casting that has the appearance of brown paste squeezed from a tube. Numerous piles of the tubular mud mark burrow openings where the worms live in close proximity to each other. The common genus Balanglossus lives in similar-shaped burrows and also produces ribbons of excreted sand with the same appearance. Some of the polychaetes produce burrows that are lined with material that persists when the surrounding sand is washed away. For example, some secrete a chitinous material to paste together shell fragments and plant debris to line their tubes, which stand as little pipes when exposed by erosion. Some burrow tubes are filled with sediment and shell debris that become cemented, thus preserving the shape of the burrow, even when the burrowing organism is unknown. Sometimes tubes may remain intact when the surrounding sand is washed away, or the cemented fill is eroded and washed up on the beach. The Onuphis sp. lines its burrow with a parchmentlike substance that sometimes is exposed at low tide, appearing as limp tubes that feel slimy underfoot.
Upper A toothpastelike mound of lugworm excrement on a rippled tidal-flat surface. Photo courtesy of Miles Hayes.
Upper left A Callianassa sp. burrow opening from which the animal has extruded a sand-water mixture to form a sand volcano. The pattern around the little mound is where water flowed away from the opening. The U.S. nickel is shown for scale.
Upper right Callianassa sp. fecal pellets around a burrow opening. The pellets have a characteristic appearance of chocolate sprinkles. The U.S. penny is shown for scale.
Center left A small horseshoe crab leaving a very faint trace on a rippled tidal flat. The sun is reflecting off the ripple crests and the burrow mound to the right.
Lower Sometimes burrow tubes are filled with sediment and naturally cemented by water percolating down through the sediment preserving the shape of the burrow. This example preserves small snail shells in the fill as well. The animal that produced this burrow is unknown. Did it selectively bring the snails into its burrow, or were they simply washed into the opening at random? The U.S. quarter is shown for scale.
One way to search for these creatures that we know are there, but rarely see, is to screen the beach or tidal-flat sediment. Use a piece of window screen or a kitchen strainer and wash the beach sand or tidal-flat sediment through your sieve. You may find some very curious-looking animals.
A characteristic trail of a sand dollar on South Mission Beach, Australia, at low tide. The animal has a covering of tiny spines that allow it to move—slowly—and to burrow as seen here, just beneath the sand surface. The Australian fifty-cent coin is shown for scale.
Fly larvae traces (white patches) are burrow structures just below the surface of the beach. This beach surface has a roughened appearance due to raindrop impressions. The U.S. penny is shown for scale.
Sand dollars (Mellita sp., Dendraster sp., Echinarachnius sp.) normally live offshore but on occasion are found at low tide, burrowing just below the beach surface and leaving a trail the width of the shell. Perhaps these are individuals that survived being washed into shallow water by storm waves.
Other burrowers include a variety of flies, beetles, and other insects or their larvae, which leave unique patterns in the sands of both beaches and associated dunes. Sometimes, when an insect burrows just below the surface, its trace may be followed by a bird that pecks at the trace in search of food, thereby enhancing the appearance of the trace. The same is true for dead creatures washed up on the beach. They can be surrounded by the trails of a variety of beach creatures that emerge to feed on them. Similar traces in the dunes also include those created by ants, beetles, and bees, as well as the characteristic semicircular to circular scribe marks, where the tips of grass blades or ends of plant stems etch lines in the sand as they move in the wind (see chapter 8).
Birds are common beach visitors that come to feed on the abundant microbes and small invertebrates that inhabit the beach. The association of specific birds and their footprints is one of the more obvious examples of identifying and interpreting tracks and associating patterns with behavior (e.g., feeding or resting). Some species nest on beaches (e.g., various plovers, terns, and penguins) or in the nearby dunes, marshes, or adjacent sea cliffs. Piping plovers and least terns are listed as endangered species in the northeastern United States, and their presence has stopped beach development in Maine.
A bird’s foraging trail. Indistinct bird footprints come into the photo from the right, and then the bird pecked at a food source that was just below the surface, producing a line of tiny bumps and depressions (beak marks). After lunch, the bird went on its way as the tracks show, toward the bottom of the photo.
A scribe mark etched as a perfect circle by the end of a single windblown dune-grass frond, South Stradbroke Island, Australia.
Depending on the climatic zone, several types of other large animals visit beaches and tidal flats. Sea turtles visit beaches to lay their eggs or simply to lie on the beach and bask in the sun. From the sea come seals and sea lions, which leave their trails of movement or resting troughs, while from the land come such animals as deer, horses, cattle, foxes, raccoons, rabbits, crocodiles, lizards, kangaroos, lions, leopards, camels, elephants, and smaller animals such as mice, depending on the location. Even snakes visit the beach, and especially the dunes.
All are recognizable because of unique tracks or trails. One of the authors was surprised to see three elephants on a beach in Northern Ireland but later discovered they were from a traveling zoo and were being exercised, not unlike the Irish racehorses that are exercised on beaches. Along the shores of Maputo Bay, Mozambique, in West Africa, two of the authors saw a tree on the beach that had been knocked down by an elephant the previous day.
The trails of these and domestic animals give clues as to what other creatures use the beach. We wondered why and how cow footprints covered the beach of an isolated barrier island we visited in Turkey until we saw a herd swimming across a tidal inlet, the herder holding on to the tail of one of his swimming charges.
A small stingray pit on a rippled tidal flat.
Sea turtles regularly come ashore on the black sands of Punalu’u Beach, Hawaii, to rest and bask on the beach. Photo courtesy of Norma Longo.
Just as thousands of human footprints rework the sand surface of recreational beaches, the animals that burrow and till the sand below the surface are the natural reworkers of the beach sediment. Bioturbation is the term given to this natural mixing of sediment by animals. Just as the surface sedimentary structures formed by the waves and wind can be destroyed by animal traces, in the subsurface, beach bedding and laminations can be crosscut by streaks of burrow backfill or entirely eliminated as a result of the churning, reworking, and ingesting of sediment. Fecal pellets are evidence of such feeding habits altering the sediment. Other organisms, such as some snails, bore into other invertebrate shells and skeletal parts to produce sand grains. Similarly, offshore, boring animals (e.g., starfish, snails, some sponges) break down shell material into sand-size and finer particles (see chapter 10).
Upper left Bioturbation exposed in a trench at the back of a beach. Note that some horizontal laminations have been completely destroyed by lateral burrowers, while other layers show vertical crosscutting patterns of sand-filled burrows.
Upper right Marine plants anchored to cobbles and pebbles may be washed ashore during storms, rafting the attached rock to the beach. Such rafted material is a good example of the “unexpected” find in beachcombing.
Lower The bored clamshell (note numerous holes in the shell) in the center of the photo reflects another type of feeding, in which an organism attaches itself to the clam and rasps or bores a hole to get at the soft parts. Such borings help break down the strength of the shell, allowing it to be broken more easily by wave action or by turbulence during the pumping of sediment onto the beach for artificial nourishment. The U.S. penny is shown for scale.
Burrowers are also natural aerators of the beach, both while burrowing and through their open burrows. Deep burrows associated with some crabs and decapods are important pathways for air and water to infiltrate well below the beach surface, providing for respiration of other organisms and creating oxidizing conditions. At the same time, such open passages can be detrimental in the case of oil spills, allowing the oil to find its way into the depths of the beach quickly.
Some beach discoveries are best left alone. Snakes frequent dunes and beaches, and this North Carolina copperhead (poisonous) has found an ideal spot to bask in the sun! Photo courtesy of Elizabeth Ray-Schroeder.
The tracks, trails, and burrows of beaches are the undeniable evidence that a huge and highly varied ecosystem lives in this, the most dynamic environment on earth. Anyone who is curious about the plants and animals of the beach, how the beach works, and what the components are need only spend time on the beach looking at tracks and trails, digging, screening, and observing. It is a fascinating exercise for anyone at any age.
Other organisms found on the beach may not be from the beach, but rather have been washed ashore, particularly during storms. Some of these are unusual occurrences—and some are organisms that should be approached with caution and not handled! Chapter 10 includes a more extensive survey of beach organisms, and particularly those organisms whose remains may wash ashore to be found by beach strollers.
