Beaches and seashells are as inseparable as bread and butter or bricks and mortar. Seashells always fascinate, both because of their amazing beauty and because of the wondrous role they play in the life of the sea. Some less beautiful, like the oysters, are delicacies that have sustained humans for thousands of years. Seashells are also important components of the ecosystem of beaches, and the remnants of their skeletons often make up a large part of the sand that is moved about in nature’s most dynamic environment.
Almost all seashells are made up of calcium carbonate (CaC03) that has been extracted from seawater. There are two mineral forms with the same chemical composition but slightly different crystal arrangements: aragonite, which most snails precipitate, and calcite, produced by most clams. Shells made of calcium carbonate are referred to as calcareous, and the portion of the beach sediment that is calcareous is referred to as the carbonate fraction. Beaches that are made up mostly of calcareous materials are called carbonate beaches.
A great variety of organisms make up the carbonate fraction of beaches, and some beaches have a larger variety of species than others. For example, the beachcomber strolling along a tropical beach is liable to find a much greater number of species than would be found on temperate or Arctic beaches. Beaches that are particularly active and are affected by frequent storms usually have fewer species than those in calmer climates.
A beautiful white carbonate beach in the Maldives, the classic beach of everyone’s dreams on a wintry snowy day in the northern latitudes.
Often the carbonate fraction is responsible for beach color. The pink shade of some Bermuda and Bahamas beaches is caused by tiny pink foraminifera (protozoans) that are washed ashore. The brown color of beaches is caused by postmortem staining of shells by an iron oxide (discussed later in this chapter), as seen along the shorelines of U.S. East Coast and Gulf Coast barrier islands and some east and north African coasts.
The carbonate fraction of beaches is usually made up of some combination of the following invertebrate animal shells along with the remains of calcareous algae: mollusks, barnacles, arthropods, echinoderms, brachiopods, serpulid worms, and the microfauna that live within the beach, often between grains.
What most people think of when the subject of seashells comes up are mollusks, of which there are four main types likely to be found on beaches. These are clams and mussels (bivalves or pelecypods), snails (gastropods), cuttlefish (cephalopods), and the less common tusk shells (scaphopods). Much of the calcium carbonate that is deposited on the world’s continental shelves is from mollusks. Most shells on the beach did not live there but were washed ashore from the continental shelf or washed off nearby rocks.
Clams range in size from the giant trident clams, sometimes weighing more than 400 lb (180 kg) and found near coral reefs in the tropics, to micromollusks that require a magnifying glass to observe.
Cuttlebone, the white, oval, flat, and porous internal skeleton of cuttlefish, is a very common constituent of beaches ranging from the tropics in Southeast Asia to South Australia to the North African Mediterranean beaches. Cuttlebones are usually 2 to 4 inches (5 to 10 cm) long and were once ground up for use as a component of toothpaste. They are commonly seen in parakeet cages, where their function is to provide entertainment for the birds and calcium for the bird’s diet.
Perhaps the most beautiful and sought-after shells are any of the six species of the genus Nautilus. These open-ocean cephalopods are rare on beaches. Among the four authors of this book, only one of us (Pilkey) has ever found a Nautilus shell on a beach, and that was on the west coast of Taiwan.
Widespread clams in beaches and sediments below the low-tide line include the quahog (Mercenaria sp., formerly Venus), razor and jackknife clams (genera of the family Solenidae), geoduck clams (Panopea sp.), and giant clams (Tridacna sp.). Other common edible clams include mussels (Mya sp.), oysters (species of genera including Crassostrea and Ostrea), and scallops (Pecten sp.). Snails include several general categories: conchs (Strombus is the true conch genus, but other genera go by the same name), whelks (Busycon and Cabestama spp.), cones (Conus sp.), cowries (Cypraea sp.), periwinkles (Vinca and Catharanthus spp.), abalones (Haliotis sp.), and limpets (Scurria sp.). A side note: Any snail shell that seems to be moving suspiciously fast may be occupied by a hermit crab that is “wearing” an empty shell as its mobile home.
Barnacles are a type of arthropod (as are crabs) and include genera belonging to the subgroup Crustacea. Barnacles are encrusting organisms, like mussels, very commonly found on the beach (dead) attached to pieces of wood, rope, bottles, or other objects that floated ashore. These organisms secrete shells of calcium carbonate consisting of six plates connected by organic matter. The plates become disarticulated after death and are difficult to distinguish from broken-up mollusks in the beach. Such fragments are usually assumed to be mollusks. As a consequence, barnacle fragments are probably much more common in beach sands than usually recognized.
One unusual type, called the gooseneck barnacle (Lepas sp.), is found worldwide on beaches, hanging from driftwood. They attach to their substrate by a thick muscle (the neck) that is considered a culinary delicacy in Spain and Portugal. In medieval times, it was thought that these barnacles became geese—the flying variety—hence the name.
A beach in Laguna Madre, Mexico, made up almost entirely of small clamshells. In part, this shell concentration may have been caused by past commercial exploitation of this particular clam (i.e., this may be a shell dump from past commercial operations, although this could not be verified.)
An unusual beach on the Salton Sea, California, that consists almost entirely of whole barnacles and barnacle fragments (white, purple, and pink). Some thin, elongate fish bones also are apparent. Although this example is from a saline lake, barnacle fragments are a common component of carbonate beaches everywhere. However, because barnacle plates tend to disaggregate on beaches, they are difficult to distinguish from mollusk fragments. The U.S. penny is shown for scale.
Spiny sea urchins and sand dollars (Echinoidea) are common components of beaches at all latitudes, but because they are quite delicate compared to snails and clams, they are usually broken up in the surf zone and, like barnacles, are not readily recognizable in the carbonate fraction of beaches. Their delicate nature is due to the fact that their tests, as their calcareous skeletons are called, consist of plates that are held together by degradable organic matter. The plates are made of magnesium-rich calcium carbonate, known as high-magnesium calcite. The occasional sand dollar is found whole on beaches and is much sought after by beachcombers.
Another widely distributed echinoderm is the starfish (class Asteroidea), which can occasionally be found on beaches. These too disarticulate into individual plates when buffeted about in the surf but are nonetheless more robust and more likely to be found whole than are sand dollars and sea urchins. In any case, make sure your starfish or sand dollar is long dead before you put it into your shell-collecting bag. The smell will tell you in the next day or two if you’ve misjudged.
Tropical beaches behind offshore reefs may receive a significant amount of carbonate sediment from the reef. Broken fragments of various corals (a group of reef-forming organisms within the phylum Cnidaria, formerly Coel-enterata) are common, and large chunks of coral may be found on such beaches after storms. More delicate calcareous skeletal remains of lacelike Bryozoa also are common to tropical beaches but are often broken up beyond recognition.
British Virgin Island conch shells in the hands of coauthor Andrew Cooper. The slit in the top of the conch in Andrew’s left hand was made by a local fisherman in order to remove the meat from the shell.
Hundreds of millions of years ago, in the Paleozoic era, brachiopods were the most common shelled animals in the shallow seas. At first glance they resemble mollusks, which have replaced them in the modern environment as a dominant marine invertebrate. The two valves of brachiopods have different shapes, as opposed to clams, which usually have symmetrical valves (i.e., valves that are mirror images of each other). The brachiopod shells found on beaches are likely to be small, less than 1 in (2.5 cm) in length. Like mollusk shells, for which they are frequently mistaken, brachiopod shells are found on beaches ranging from the tropics to the Arctic. One relatively rare brachiopod (Lingula reevii) was common in Kaneohe Bay, Hawaii, but ironically began disappearing when sewage was diverted away from the bay.
Upper Coral Beach, Maldives, shows a size-sorted carbonate beach with an abundance of stick-shaped coral fragments concentrated at the last storm line. The rest of the beach is fine carbonate sand, except for small cobbles of dark-colored rock that is probably from volcanic bedrock.
Lower left This close-up of a gravel beach in the British Virgin Islands reveals an interesting mix of pebbles, including abundant, very well-rounded coral fragments (white) as well as fragments of coralline algae, mollusk shells, and rounded noncarbonate pebbles (dark colored). The rounding indicates that these pebbles have been on the beach for some time and have been subjected to abrasion. The credit card is shown for scale.
Lower right Starfish on a rippled beach. The starfish was flipped over, revealing the impression it left in the beach sand. Starfish have relatively delicate skeletons and are often broken up in the surf zone.
Tiny foraminifera, calcareous protozoans, seldom live in beaches but are commonly washed ashore from nearby ocean waters. These are one of the more common components of the microscopic carbonate fraction of beaches, especially along generally rocky beaches in Western Ireland.
These marine polychaete worms secrete strawlike calcium carbonate tubes that form intertwining masses known as worm reefs. They attach themselves to rocks or large shells and often, after storms, wind up in the beach as pebbles or larger fragments.
This very large and loosely defined group of microscopic organisms lives within the beach, usually between the sand grains, and are mostly noncalcareous (e.g., copepods, nematodes, bacteria, polychaetes; see Chapter 9).
The plant kingdom also is a major contributor to the carbonate fraction of beaches in tropical areas, primarily in the form of calcareous algae. Plants are often thought of as lacking hard parts, but the humble single-celled algae often have hard parts as a component of their colony. In particular, the marine green algae genera Halimeda, Penicillus (shaving-brush algae), and Acetabularia (mermaid’s wineglass) and the coralline algae (red algae) are important calcium-carbonate producers. Halimeda grows as branching colonies, with individual segments composed of the mineral aragonite, while the Penicillus “bristles” of the shaving brush are composed of the same mineral. When the organic matter deteriorates, tiny needle-shaped sand grains result. The internal, spherical hard part of the Acetabularia at its joint between the stemlike structure and the cup is also aragonite.
When these plants die or are ripped up by storm waves, the individual hard parts separate into sand-size aragonite grains that may be washed up onto beaches. With time, these particles break down into finer-than-sand-size sediment that is usually washed away from the beach. The breakup of algae and other calcareous fragments, which are much softer than quartz grains, is why the surf zone of carbonate beaches, containing carbonate mud in suspension, is often white or milky in appearance. Cancún Beach, Mexico, and various Red Sea beaches are typical examples.
The coralline algae (red algae) commonly associated with coral reefs are composed of calcite and are more resistant than other algae to breakdown into sizes smaller than sand grains. Island and mainland beaches behind reefs usually have carbonate beaches that are often dominated by algal sands.
Upper Close-up of a British Virgin Islands beach reveals that it is made up almost entirely of coralline algae fragments, derived from offshore. Although the particles have been rounded slightly, this beach is subjected to smaller waves than the beach shown in the Maldives photo (page 190). The U.S. nickel is shown for scale.
Lower left Microscopic view of a Bahamian fine sand composed almost entirely of ooids, showing their typical spherical shape and polished appearance. Ooids form when thin, concentric layers of calcium carbonate form around a seed crystal, such as a shell fragment, by precipitation out of seawater in warm, shallow, wave-agitated environments. The line scale is 1 mm (1,000 microns).
Lower right Microscopic view of a thin section of ooids showing the layering formed as the grains were precipitated, layer by layer. Photo courtesy of Ronald Perkins.
Other types of carbonate sediment, including nonskeletal carbonate remains and inorganically produced carbonate grains, are important as beach materials. Aragonite mud can also form on beaches when coarser grains of aragonitic material break down into mud-size material. The sand-size grains include pellets and ooids.
Pellets are sand-size bits of animal fecal material and may be calcareous if the animal is ingesting carbonate mud or the carbonate skeletal material of other calcareous organisms. Usually pellets will be reduced to mud-size sediment by wave activity, but they are known to persist in some low-wave-energy environments and are reported from some carbonate beaches. Over time some of these pellets become cemented and hard, resistant to breakdown into mud.
Ooids, sometimes called oolites, are hard, spherical, calcareous sand grains that are inorganic in origin. These concentrically layered (like the cross section of an onion), spherical grains form when calcium carbonate is precipitated from seawater around a small seed grain, such as a tiny shell fragment, and the grains are rolled back and forth by wave or tidal current activity in tropical waters. The spheres resemble fish roe and are carried onto beaches from their nearby points of origin, typically warm, shallow, agitated waters of tropical shoals such as in the Bahama Banks, the Red Sea, and the Persian Gulf and along some Caribbean shores.
Several groups of animals were noted in the previous chapter in terms of the tracks, trails, and traces that they leave on beaches. Some of these animals are also important in producing sedimentary particles that make up the biogenic (nonterrigenous) portion of the beach (see chapter 3).
Crab and lobster carapaces (hard body parts) are frequently found on beaches. Often these carapaces are molts shed by a still-living organism. These are not calcium carbonate but rather are made of an organic material known as chitin. Horseshoe crabs (Limulus sp.), relatives of the long-extinct trilobites, are the most spectacular crablike skeletons on beaches. As noted in chapter 9, horseshoe “crabs” are arachnids (not true crabs), a class of arthropods, up to 2 ft (more than 0.5 m) in length.
This beach on West Caicos Island, Bahamas, is made entirely of ooids. The grains form offshore and are transported to the beach by wave action. Photo courtesy of Ronald Perkins.
Life-and-death drama on the beach. A small crab (with attached green algae) is attempting to excavate a moon snail for its dinner on this New Brunswick, Canada, beach. This event is taking place in the intertidal zone, where ripple marks are present, and the mound in the center of the photo is an expression of the moon snail’s burrow.
A few other noncalcareous organisms are found on beaches, particularly after storms, including sponges, sea cucumbers, and jellyfish. Look along the wrack line, and particularly in patches of seaweed, for these soft-bodied remains.
Seaweed can be the most abundant organic component on many of the world’s beaches; it is particularly abundant in the wrack line. Sometimes seaweed layers are buried in the beach and become important to the beach’s food chain and in forming a chemical-reducing, low-oxygen-content environment that may alter sediment properties such as shell color (discussed later in this chapter).
Drift seeds are another plant component found on beaches. These long-floating seeds, typically from tropical plants, have been used by oceanographers to chart ocean currents. Some drift seeds can float for a couple of years and travel very long distances. Coconuts from tropical sources, for example, have been found as far north as Norway.
Some important drift seeds and their points of origin include coconuts and coral beans from the tropics, anchovy pear, sea heart, and crabwood from the New World tropics, box fruit from Polynesia, the sea bean from the Indian Ocean tropics, coco-demer from the Seychelles, and puzzle fruit from Southeast Asia. Among some beach aficionados, drift seeds are as important as shells.
Seaweed that has accumulated along the shoreline at Ballywalter, Northern Ireland, obviously makes beach access, strolling, and other forms of recreation difficult.
Although not direct producers of sediment, sea grasses play an important role in trapping carbonate sands and muds in the nearshore zones of tropical beaches. Meadows of sea grass grow in shallow water on the submerged part of the beach, often extending right up to the low-tide line. These sea grasses bind the sand grains together and impart cohesiveness to the seabed. In some instances, the grasses can even form a kind of turf on the seabed. During storms, sea grasses are washed ashore and often accumulate in thick deposits at the wrack line, even covering the beach (see Chapter 3).
As anyone who has strolled on a lot of beaches knows, the shell content of beach sand can be highly variable. Some natural beaches are so shelly they are impossible to walk on with bare feet. Others are so “unshelly” that one has to search diligently for the occasional shell. Beach shells come from many sources.
There are some instances where beaches on adjacent barrier islands, in seemingly similar environmental conditions, exhibit large differences in their shelliness. Such is the case for the beaches of Shackleford Banks and adjacent Bogue Banks, North Carolina. Shackleford has the region’s most shelly beach, while Bogue’s beaches contained few shells (prior to an artificial dredge-and-fill beach-nourishment project that brought in an oyster-shell hash).
There are two principal reasons why the shell content (the carbonate fraction) of beaches can vary so much: productivity and dilution.
Shell productivity, the variety of species and abundance of individuals, is determined by water temperature, the most important control on shell abundance globally. All other things being equal, the warmer the water, the higher the beach’s shell content. Calcareous marine organisms are more abundant and more productive in warm water, which explains the frequent occurrence of pure carbonate beaches in the tropics and in warm waters such as on atolls, the beaches of the Caribbean and the Red Sea, and on some Southeast Asian shores. Thus, beaches in Morocco are dependably more shelly than beaches in Norway, as are beaches in Florida compared to those in Labrador.
A large accumulation of mangrove seeds at the high-tide line on one of the Brazilian Gurupi Islands south of the Amazon River. These seeds from mangrove forests along estuarine shorelines floated out to sea through inlets. Mangrove seeds are sometimes transported hundreds of miles before being deposited on a beach.
The most prolific calcium carbonate producers are coral reefs. It is common for beaches that are sheltered by coral reefs (e.g., in the Caribbean and the Red Sea) to consist of pure calcium carbonate and contain abundant fragments of corals, shells, and calcareous algae. But there are many exceptions to this rule of water temperature and shell productivity. Purely calcareous beaches exist in some far-northern areas (including Labrador, Norway, and Southeast Alaska). These occur where the rocky seafloor or nearby cliffs are covered with attached mussels, barnacles, sea urchins, algae, and foraminifera, which are torn off by storm waves and piled on the beach. Such beaches are free of abundant pebbles or boulders; if these hard rock components are present, they will crush and grind the much softer calcareous fraction to powder in the erosive surf.
An example of this effect is Sand Beach, Mount Desert, Maine, a pocket beach between granite headlands. Storms tear the mussels, barnacles, limpets, and sea urchins off the nearby rocks and toss them onto the beach, where wave action quickly grinds the shell debris into sand-size material. In this fashion, a calcareous beach is born in cold water.
Dilution by nonshelly sediment, usually at river mouths, is another one of the controls on carbonate content in beach sand. In the Virgin Islands and along the north shore of Puerto Rico, for example, there are glittering white and light-brown pure-shell beaches only a short distance from blackish gray-sand beaches, derived from volcanic bedrock, that contain few seashells. The dark-sand beaches on these islands are found at river or stream mouths, where fresh sand from the interior is provided to the nearby beaches during every flood. This noncarbonate fraction dilutes the otherwise high carbonate background content.
Collection of tropical seeds found on the beach of one of the Gurupi barrier islands in Brazil. To the right is a mangrove seed that already has a sprouting leaf. The toe of the shoe is shown for scale. Photo courtesy of Allen Archer.
A sea-grass bed exposed at low tide on a carbonate beach on Great Camanoe, British Virgin Islands. Sea grass causes sediment to accumulate and also holds sediment in place, offering resistance to sand transport by waves. In the background, multiple wrack lines are visible at the back of the beach, marking storm-tide or high spring-tide levels. Most of the wrack material is sea grass. Note also the well-developed swash marks and ripple marks.
This carbonate beach in Hamerfest, Norway, above the Arctic Circle at 71 degrees north latitude, is an exception to the rule. Carbonate beaches are found mostly at low latitudes in warm water, but in the absence of noncarbonate sand in high latitudes, and in the presence of shelled organisms to produce sediment, these unusual beaches can form. The shells on this beach were derived from offshore and from nearby rocks, from which storm waves tore them away.
On a larger scale, the beaches on the west coasts of North and South America tend to be less shelly than those of the east coasts of these continents. This difference is because of the close proximity of west coast mountain ranges, from which fast-moving streams carry much sand directly to beaches; this sand contains no shell material. Even at the equator, where carbonate productivity is high, the Pacific Coast beaches of Colombia and northern Ecuador contain relatively few seashells. The nearby Andes Mountains furnish massive amounts of sand through numerous streams, large and small, spurred on by the locally high rainfall of the tropical rain forests.
On the Atlantic side of North and South America, mountain ranges are well inland from the ocean, and streams tend to dump their sand loads at the upper ends of estuaries, far from the beaches. These estuaries are river valleys drowned by the last sea-level rise. Thus, dilution of the carbonate fraction of beach sand by river sand is relatively unimportant because the river sand does not make it to the beach. Exceptions are the beaches near the mouths of large rivers such as the Mississippi, Magdalena, and Oronoco.
A shell lag on a beach near Kashima, Japan. This same accumulation of shells on a southern U.S. beach would be extensively brown stained, but colder waters such as those off Japan do not favor brown staining. Note the predominant concave-downward orientation of the shells on this beach. As explained in the text, this is the hydrodynamically stable orientation of shells on open-ocean beaches. Unfortunately, a plastic bottle is visible for scale.
On most shores, the shells we hunt for on the beach are derived from organisms that lived offshore, near the beach; after they died, the shells were carried ashore by waves and currents. Some organisms live in the beach itself, and you may find the occasional snail or happy clam, which is best left alone unless it is for the stew. For the most part, however, the shells we collect no longer have an organism living inside. Years or decades may have passed while the shell journeyed to where it is found. For some beaches, a shell’s history is even longer and more complex. This is especially true on barrier islands and trangressive coasts, where the rising sea is rolling over earlier marine sediments and reworking the contents, including buried shells. Dating of some shells on southeastern U.S. beaches has given ages in thousands of years, old enough to be considered fossils, even though the shell may appear as if it were lived in yesterday!
By definition, a fossil is some remains of an animal that lived in the geologic or prehistoric past. These shells on barrier island beaches that are hundreds and even thousands of years old, in part, tell the story of the immediate geologic past. The organisms lived at a time when the level of the sea and the temperature of the ocean water were slightly different from what they are at present. Now that beach nourishment is so common for tourist beaches, some of the shells found on such beaches may have been dredged from sediments that are a few million years old.
Seashells on a particular beach may come from any of the following sources:
• Animals that lived within the beach are an important source of shells on some beaches but not on others. The surf zone is a very harsh environment for living things. Besides the pounding waves, there are also wide ranges of temperatures and alternate wetting and drying as tides move up and down. Typical of harsh environments, the numbers of shell species that live within the beach tend to be small, but the number of individuals is often large. Purchase of a local shell book will provide information about which shelled animals actually live within your beach. One of the most familiar is the coquina clam (Donax sp.), a thumb-size edible clam (also known as cockles and pipi). There are nineteen Donax species around the world, and they live on just about every beach in the world where people are likely to swim.
An accumulation of razor clam shells from the Sefton Coast of England. The black objects among the shells are chunks of peat derived from nearby peat outcrops on the beach. A shell accumulation consisting mostly of a single species is caused either by the animals’ great abundance near the beach or by selective physical sorting of the shells in the swash zone based on their shape and buoyancy (or by humans, in the case of middens).
• Shells from animals that lived offshore are driven ashore by the waves. A wide variety of clams, snails, sand dollars, and sea urchins, discussed earlier in this chapter, live on the inner continental shelf. Similarly, reefs and meadows of sea grasses are offshore environments that produce an abundance of shelly fauna that are likely to be washed up onto adjacent beaches. Those who visit beaches at different times of the year can observe that different species of shells come ashore with the changing seasons, in part because of varying wave conditions. Pen shells (Pinna sp.), of which there are 250 related species around the world, are large fan-shaped clams that partially bury themselves in the sand on the seafloor. During storms they are readily torn away and wind up on beaches, especially in the winter.
• Shells that have been peeled off nearby pilings and rocks by incoming storm waves can be a major component of pocket beaches on rocky coasts. Common constituents of a beach fauna derived from rocks would include barnacles, limpets, mussels, and other shells (as discussed earlier in this chapter). On sandy coasts without naturally occurring rocks, such shells may be contributed to the beach from rock seawalls and jetties and even from pilings on nearby piers and docks.
• Fossil shells derived from ancient rocks that outcrop on the beach or offshore can be very important locally. Many continental shelves are not completely covered with sand, especially those that are far from any major river mouths, and old rock outcrops are frequently exposed to seafloor weathering and wave activity. The fossils that are pre–Ice Age (3 million years or older) generally are very old in appearance and often are from species that no longer exist. An example of this shell source is the occurrence of 20-million-year-old shells of Ostrea gigantissima, an oyster that is almost 2 ft (60 cm) long, on North Topsail Beach, North Carolina.
A beautiful white-sand beach with abundant coral fragments in Kenya that is protected by an offshore coral reef marked by the white line of breaking waves. The sand is white because most of it is calcareous material derived from the coral reef and its associated organisms.
A view of misnamed Coral Beach at Galway, Ireland; there are no coral fragments in the sand and no corals offshore in the cold water off Ireland. However, the beach is made up of an unusual concentration of almost pure calcareous sediment (coralline algae and mollusk shells), hence its name.
• Lagoon shells in lagoonal sediments that were overrun by a migrating barrier island are a source of relatively young (a few thousand years old) fossils. These old lagoonal deposits are a particularly important shell source on barrier island beaches. Barrier islands can migrate in a landward direction in response to a sea-level rise. When this happens, the islands overrun mud from mangroves or marshes and sand deposits from the bay or lagoon originally behind the island. This migration is evident from the mangrove or salt-marsh mud deposits on the open-ocean beach in places as varied as Ossabaw Island, in the U.S. state of Georgia; Santa Barbara Island, Colombia; and Xefina Island, Mozambique. Very common examples of this type of shell on barrier island beaches are various species of oysters that grow exclusively in quiet, lower-salinity bay waters and not in the open ocean. Such shells on beaches can be almost any age, but most are a few thousand or tens of thousands of years old, not counting those left behind from oyster roasts and clambakes.
• Shells from artificial beach fill are commonly fossils, hundreds to thousands of years old and from any environment depending on where the truck or dredge has obtained sand to construct the artificial beach. Beach sand in Waikiki, Hawaii, was once brought in by freighter from California and Australia. Shelly beach sand with Gulf of Mexico shells was once brought from a Gulf of Mexico beach across the Florida peninsula to Miami Beach, where a hotel owner wanted to improve the shell hunting! Ordinarily, a seashell species assemblage on a beach that is very different from nearby local beaches (that are still in a natural state) can readily identify an artificial beach. For example, some artificial beaches are derived from dredging lagoon sediments and thus have abundant lagoon shells, such as oysters, on the open-ocean beach, where they do not belong. In addition, shell coloration may provide a clue as to the origin of the shell. If most shells are black or gray, the beach may be artificial. Shell color is discussed later in this chapter.
Shells on beaches can be broken up for a number of reasons, some natural and some due to human activities. The relative importance of the two will vary widely from beach to beach. In the human-activity category, driving vehicles on the beach and dredging are the usual suspects, whereas shell-cracking animals and crashing waves are natural causes of shell breakage.
Vehicular traffic on beaches causes shell breakage and sediment compaction and impacts burrowing and nesting animals. This example from Portstewart, Northern Ireland, illustrates the conflicting problems arising from multiple beach uses.
Shells display a wide range of vulnerability to breaking. Animals that live in environments exposed to large waves tend to have robust shells that may survive on a beach surface. Animals that live in more sheltered habitats, such as sea urchins, sand dollars, and other echinoids, are usually far more delicate (as mentioned previously) than most clams and snails, which is why they are not often found whole on a beach.
Shell-breaking processes include the following:
• Beach driving. Obviously the continuous back-and-forth movement of wheeled vehicles can break up the more delicate shells and sometimes the hardy shells as well. Many of the world’s developed beaches allow some sort of vehicular traffic for sightseers and fishers—even in national parks. Some beaches, like Daytona Beach, Florida, were once the site of car races. An exception is South Africa, where in 2006 virtually all driving on beaches in the entire country was prohibited. Unfortunately, on some U.S. National Seashores’ wilderness areas where driving is otherwise prohibited, the National Park Service insists on sending a daily four-wheeler patrol up and down the beach—crunching shells all the way and leaving unsightly tire tracks. Wilderness areas need not be patrolled in this fashion. On many developed beaches, the daily or weekly beach cleanup consists of some sort of mechanical rake that is dragged along behind a tractor, which can readily break up shells.
• Dredges. Sometimes a cluster of shell hunters can be seen gathered in a semicircle around the end of a pipe that is spewing a slurry of offshore sand and water onto a beach. Every once in a while someone darts in and triumphantly grabs a shell. In some dredging projects, however, few shells survive the long path from the offshore dredge through the pipe to the beach, and most shells come ashore in fragments. One can hear the clanging and banging of shells as they move along the pipe. Probably the degree of breakage depends on the distance piped and the concentration of the slurry as it rushes through the pipe. We observed one nourished beach project on Emerald Isle, North Carolina, where even the thick shells of the up to 4 in (10 cm)-long hard clam or Northern Quahog “Mercenaria” clams were fragmented. On a beach where almost all the shells are broken, the usual suspect should be dredging.
Upper left This calcareous beach on Barbuda in the Caribbean shows the famous pink-to-red color that is attributable to a form of calcareous algae that washes up onto the beach. The sand is highly fragmental but is slightly more rounded than the carbonate beach sand shown from Galway, Ireland.
Lower left Black-stained shells on a nourished beach on Bogue Banks, North Carolina. In most beaches, an accumulation of pure black shells is almost certainly evidence of a nourished beach. Black sand comes from an environment that is without oxygen, and often that environment is at a burial depth of 3 ft (1 m) or more in continental shelf sediment. In some communities, engineers have gone to a lot of extra expense to find sand that is the same color as the original beach sand.
Upper right This beach shell lag has shells that are highly fragmented, and many of them are brown stained and show no evidence of abrasion from wave activity. The small clamshells are mostly “correctly” oriented, with their concave sides down.
Lower right This close-up of a shell lag on Core Banks, North Carolina, shows the highly rounded shells that are typical of high-wave-energy beaches. The reworking and abrasion of the shells gives some of them a natural polish, and although the calcium carbonate is soft, these rounded beach shell fragments are sometimes used to make jewelry. Photo courtesy of Rob Greenberg.
• Predators. If a lot of fragmented shells are found on beaches with generally small waves and no driving, it is a sure indication that something else is responsible for the breakage. That something could be a wide range of animals, vertebrate and invertebrate predators, that are busy every day cracking shells to get at the meat inside. A variety of crabs and rays cruise the seafloor grabbing any shelled animal unwary enough to come out of its burrow at the wrong moment. Crabs, especially those with one oversized claw such as the Florida stone crab (Menippe sp.) and the European green crab (Carcinus sp.), specialize in attacking snails. Rays can crack large, thick shells between two hard plates in their jaws. Some fish species also have jaws strong enough to crack shells. Parrot fish contribute to the carbonate fraction of beach sediment where coral reefs are offshore. These fish scrape algae off coral heads with their parrotlike beaks and in the process create a lot of coral-fragment sand grains scraped from the coral heads. Then there are the seagulls that have learned to drop shells on concrete sidewalks and pavements to break them open. When gulls drop shells on rocky beaches to break them, the fragments will become part of the beach sediment.
This highly degraded conch shell from the beach on Ocracoke Island, North Carolina, is likely to be many years old, as evidenced by the numerous boreholes in its surface. A variety of organisms, including a type of sponge, bore into shells to obtain nutrition from the organic matter, or into empty shells to seek shelter. This boring contributes to the weakening of the shell, and it will someday be reduced to sand-size fragments.
• Waves. Breaking storm waves can resuspend beach sediment, causing a mixture of sand and shells to churn violently about in the turbulent surf zone. On rocky coasts, the shells torn off the cliff by storm waves and washed ashore to the nearby beach sometimes crash into rocks and break up along the way.
As shells break and tumble about in the surf zone, their corners and edges slowly become rounded and less sharp. The more wave energy there is and the longer a shell is on a beach (and not buried or brought onto land by storm overwash or carried offshore by a storm), the rounder the shell becomes.
Sometimes the rounded shell fragments display beautiful patterns of color that are used for jewelry. Unfortunately, the relative softness of shell material makes the jewelry a bit delicate. Beautiful rounded fragments of shell can be found on just about any sandy beach that has frequent high waves. Examples include the Outer Banks beaches near Cape Hatteras, North Carolina, beaches on the north side of Puerto Rico, and the beaches of western Morocco and northern Tunisia.
This roundness scale for the carbonate fraction can be used to compare shell fragments on a beach to the chart silhouettes to quantify their stage of rounding. Box 1 represents highly angular fragments at initial breakage. As abrasion occurs, shell fragments become subangular (box 2) to subrounded (box 3). For the most part, highly rounded shell fragments (box 4) are found on beaches with high waves. Beaches with vehicular traffic may have artificially induced poor rounding of shell fragments by breakage that increases angularity.
Shells on beaches often exhibit preferred orientations; the preferred orientation refers to the position attained on a beach by the majority of the shells. The shell position or orientation can refer to the alignment of a shell relative to the shoreline, the waves, or the wind direction. On a beach with significant wave activity, shells will orient in the most stable position—that is, a position most likely not to change with time, a position that the shell maintains because it offers the most resistance to the forces of waves and wind. The long axis of elongated shells (those that are longer than they are wide), such as oysters and razor clams, can clearly show a preferred orientation. Snails and larger shells such as conchs have a narrow end and a wide end, which can be the basis of a preferred orientation.
Sometimes wind creates the preferred orientation. In strong winds, small shells may be picked up and redeposited. Also in strong winds, large conchs and whelks may be turned to a preferred orientation as the wind excavates sand along the shell margin, slowly causing the shell to move. It is likely that under the right circumstances, wind and water may act in concert to orient shells.
Clamshells lying on a beach can show a preferred orientation with respect to which side is up. A clamshell has a concave side, where the animal once lived, and a convex or curved outward surface, which faced the outside world when the animal was alive. Such a shell lying on a beach can be oriented one of two ways: concave side up or concave side down. On most beaches, there is a strong preferred concave-down orientation of clamshells. Typically 80 to 90 percent of such shells will lie on the beach with the cavity that once protected the living animal facing down. The reason for this orientation can be quickly demonstrated by placing a clamshell in the concave-up orientation within the swash zone. The shell will be quickly turned over by the swash and reoriented with the concave side down. Subsequent swashes may move the shell about but usually will not turn it over. The concave-down orientation is known as the hydrodynamically stable orientation.
With the aid of a mask and snorkel, one can wade out and watch the process occurring in hip-deep water (providing the surf is not too large) but still within the surf zone. Releasing a clamshell at the surface of the water column reveals that the shell will immediately attain a concave-up orientation as it rocks back and forth on its way through the water column to the seafloor. Concave up is the hydrodynamically stable orientation for falling shells. They land on the bottom still concave up. But if the surf is strong enough, the shells will be quickly turned over to their more stable concave-down orientation.
Roundness of sand-size material also reflects the degree of abrasion to which the sediment has been subjected, as in this carbonate beach sand from the island of St. Maarten in the Caribbean. Carbonate sand grains often show glossy, polished surfaces, as in this example. Several of these grains are foraminifera (e.g., lower right margin). The line scale is imm (1,000 microns).
A brown-stained scallop shell is residing in a stable concave down position. Notice the current scour around the shell.
The clamshells on this beach adjacent to a jetty at Oregon Inlet, North Carolina, are all oriented “correctly” in the concave-down position. The shells seen here on this open-ocean beach are primarily lagoonal species, not normal open-ocean forms, suggesting that they were dredged from an offshore exposure of old lagoon deposits that were left behind as the barrier island moved landward in response to rising sea level. In addition, note that most of the shells are white and a few are black, but there are very few brown shells. The species makeup and shell colors strongly indicate that this is a nourished beach. Note also that the beach surface is pitted with raindrop impressions.
Now go farther offshore to the realm of scuba gear, to a depth of 30 ft (9 m) or more. A shell released in the water column here will settle down concave up, just as it would in the surf zone. But once the shell reaches the seafloor, it remains concave up because the waves aren’t generally strong enough to turn over the shell. In fact, early studies of bottom photographs of the surfaces of continental shelves revealed that most shells offshore are oriented concave side up. Even though storms can occasionally kick up the shells, they tend to resettle in concave-up orientation.
This diagram shows how the hydrodynamics of a shell determines its orientation under different circumstances. A shell sinking in quiet water often takes a concave-up orientation (depending on its shape) but may be oriented to a convex-up position when turned over by a wave or current. Drawing by Charles Pilkey.
One way that geologists who study ancient sandstones can distinguish between a beach deposit and a continental shelf deposit is by observing the orientation of the fossil clamshells (if they are present). In fact, geologists’ interest in interpreting the paleoenvironments in which sandstones and limestones formed is one of the reasons why we know so much about the surface features of beaches that we discuss in this book.
Many of the shells on the beach are not the original color that they were when the animal that lived in them was alive. Such secondary colors are imprinted in the shell after the animal has died. Brown staining of shells, or the common light yellow-brown stain on shells, is probably caused by the formation of limonite, an iron oxide, in the microscopic interstices of shells. Apparently the brown coloration is given to the shell while it resides exposed to the air on or near the surface of the beach. The iron that combines with the oxygen from the atmosphere to form limonite may come from the degradation of iron-bearing heavy minerals (the black sand) in the beach sand.
Although brown staining does not occur offshore on the continental shelf off the southern United States, apparently it does on the shelf off Puerto Rico. The evidence for this is pretty overwhelming, since, down to a depth of 150 ft (more than 45 m) or so, the entire north shelf of Puerto Rico is paved with brown-stained mollusk shells, while the continental shelf off the southeastern United States has only a few patches of brown shells. Perhaps the warmer and clearer water and the more energetic wave climate in Puerto Rico are responsible for this phenomenon. Whatever the cause, it is clear that brown-stained shells are created in different ways on different coasts. It is also an illustration of an old principle of beaches: Be very careful when transferring generalizations about beach processes across state lines, and even more so across oceans!
Where brown staining occurs in widespread fashion, it is often responsible for the overall light-brown color of beaches that is particularly characteristic of southern U. S. beaches. Brown-stained shells are probably a common global occurrence on beaches in all climates except perhaps Arctic beaches, but no studies have been made to quantify this.
The brown staining seems to be durable as long as a shell resides on the beach. If it is washed onto the upland by storm overwash, the brown staining slowly disappears, and after a few years of residence on land, the shell is bleached to gray. Bleached shells are common on overwash fans and also along the margins of flower gardens and on outdoor railings where avid shell collectors have placed them. With time, usually within two or three years, all shell coloration, original or secondary, disappears when the shell is exposed to the elements without occasional inundation by seawater.
Black-stained shells are at least as abundant and as widespread as brown shells, perhaps more so. The black staining forms in an entirely different fashion from brown staining. The black color is probably due to microscopic crystals of iron sulfide that formed within the microscopic interstices of shells, and the crystals form in the absence of free oxygen. Burial in mud or deeply in sand 3 to 6 ft (1 to 2 m) provides the requisite oxygen-free environment, so instead of oxides forming, as in the case of the brown shells exposed to the air, sulfides form.
One can easily demonstrate this process by taking a few fresh shells and burying them in mud, preferably salt-marsh or mangrove mud. Within a few weeks, many of the shells will be blackened. Some shells blacken within days of burial, and others, such as the scallop shell, don’t blacken after months of burial.
Mud is not the only environment in which shells will blacken. On many beaches around the world, seaweed is buried in the sand, and as the plant material rots it forms a microenvironment devoid of oxygen. Shells readily blacken when buried in such layers under these circumstances. We have seen this type of blackening on the beaches of Portugal, South Australia, Brazil, Tunisia, northern France, and western Florida, among other places.
In the absence of buried seaweed on a barrier island beach, the presence of black shells is good evidence of island migration. For example, along the U.S. East Coast from Long Island, New York, south, there is essentially no mud on the continental shelf. The only marine mud along this entire stretch is found in the bays behind the islands, so black shells on the beach must have once resided in the bays, where there is lots of mud to blacken shells. This being the case, the only logical way that large amounts of black shells can come to a barrier island beach is for the island to migrate over a marsh deposit. What was once on the back side of the island is now on the front side. Of course, blackened shells on artificial beaches are a reflection of the sand’s source area, which may include buried marsh sediment from a lagoon or sediment out on the continental shelf left behind as the sea level rose.
Upper Shell hash on an Ocracoke Island, North Carolina, beach shows shells that are predominantly brown stained, a common color for beach shells of the southeastern United States. Most of these shells have been broken up, probably by a combination of vehicle traffic, stingray activity (crushing), and wave action.
Center Microscopic view of brown-stained calcareous sand from the beach at Luquillo, Puerto Rico. This fine sand is nearly 100 percent broken skeletal material from nearshore organisms. Note the many small, stick-shaped grains derived from echinoid spines, as well as clamshell fragments and a foraminifera shell in the center of the photo (coiled). The grains show polish as well as the brown stain.
Lower Shells showing varying degrees of black staining, including marine mollusk shells as well as oysters that once lived in a lagoon or estuary. The black color is secondary, forming after the organism died, and is caused by very fine iron sulfide in the interstices of the shell. This compound is formed when the shells are buried where there is no oxygen, as in mud, in rotting seaweed, or in the sediment at depths of 3 to 6 ft (1 to 2 m), commonly on the adjacent continental shelf.
Buyer beware. This boatful of Caribbean conch shells is not on a Caribbean island but in Ocracoke, North Carolina. These shells, probably from the Bahamas, were for sale for nine dollars each, and this same beautiful species is found at shell shops all over the world. Killing of shelled mollusks for the tourist trade is threatening the existence of some species and has impacted subsistence-food fishing for some islanders.
The continental shelf off the eastern United States has abundant black shells in its sediment cover, a reflection of a history during the last sea-level rise, from eighteen thousand years ago to the present, of barrier islands repeatedly migrating over bays. Thus, old bay deposits with their black shells are smeared over much of the surface of the continental shelf.
It appears anecdotally that the staining in black shells is much more durable than brown staining. Black shells eventually bleach when stranded away from a beach, but at a slower rate (decades rather than the few years required to remove brown staining).
Picking up shells on the beach is a relatively harmless hobby. Who among us on a beach outing can resist collecting a souvenir shell or two for displaying on the bookshelf or desk back home? They are objects of wonder and beauty and remind us of good times and a great environment.
Is this a beach of popcorn? The answer lies on the beach at Fuerteventura in the Canary Islands, where this white, popcornlike material occurs. It is made up of fragments of calcareous algae (red algae), washed up onto the beach and bleached white. The white algal nodules are mixed with black gravel, which consists of volcanic rock fragments (basalt and some black volcanic glass) derived from the underlying wave-cut platform. The camera lens cap is shown for scale. Photo courtesy of Ignacio Alonso.
On the other hand, commercial shells in shell stores are most often collected in bulk while the animal is still alive; the animal is then killed and the shell cleaned before being offered for sale. One clue to the fact that such shells were not picked up on beaches is that, as a rule, such store-bought shells never show evidence of the abrasion or wear caused by rolling around on a beach.
The foot-long Bahamian conch, or queen conch, is an example of a widely sold shell, recognizable by its large size and bright pink color in the shell opening. Every shell store in Europe and America seems to have these beautiful shells in abundance. They are used to make trumpets or souvenir home decorations, and the meat, which is a staple for Bahamians, is even considered to be an aphrodisiac. Now the Bahamian government says the shells are in danger of disappearing in the vicinity of settlements. This story is repeated throughout the Caribbean. A Roatán (Honduras) islander told us he once could obtain the family’s evening meal by thirty minutes of shellfish fishing; then a commercial shell-collector supplier overfished the grounds in less than two years, and today the man may have to fish all day to secure the family meal.
Rare species, such as the Nautilus shell, are sold individually. Less rare forms may be sold by the bucketful, ten to twenty U.S. dollars per bucket. Such exploitation of marine mollusks, echinoderms, corals, and other marine organisms has a negative impact on the ecology of collecting locales and, in the case of rare species, may impact the species’ very existence. Sea turtle shells are another case in point but are rarely seen in shell markets today.
It’s more fun (and more difficult and challenging) to find your own shells and treasures.
