Clams

All reef aquarists are familiar with the beautiful Tridacnid clams found in the shallow waters of coral reefs throughout the Pacific. Tridacnids, however, are very unusual clams; they have symbiotic zooxanthellae and they also have an internal anatomy that is oddly oriented relative to their shells. In this column I will discuss some aspects of basic clam biology and some of the other species that are occasionally available in the aquarium trade. Clams or bivalves are mollusks and members of the Class Bivalvia of the Phylum Mollusca. These are animals with a consistent morphology and physiology. Except for a few structures, the basic internal anatomy and structures are rather similar throughout the class.

Two Shells

Although the name "clam" does not at all clearly indicate what obvious and completely diagnostic features these animals possess, the term "bivalve" suggests these animals' most evident primary structures, namely, the two shells, one on each side of the body. Although it may not be obvious to many people, the term "bivalve" is very descriptive. The term "valve" is an older, somewhat archaic English term for "shell," so "bivalve" is a noun meaning "two shells." The full name of the group in question is then, "bivalved mollusks," but this is generally just shortened to "bivalves," and it is the name used to describe the group of animals we more commonly call "clams." Using the same root terminology, a name occasionally used for snails is "univalved mollusks," or mollusks with only one shell.

The property of having two shells is, interestingly enough, not unique to clams within the mollusks. All of the individuals within one small group of tiny snails also possess two shells. These peculiar snails in the genera Berthelinia and Julia occasionally show up in aquaria (Shimek, 2004). They are odd little animals that are related to the lettuce slugs that prey on Caulerpa. They have a pair of brilliant green shells that are structurally quite similar to those found on clams. As a matter of fact, these animals' shells were collected as fossils in the late nineteenth century, well before the living animals were discovered, and they were scientifically described and named as a type of fossil clam. After the discovery of the living bivalved snails in the late 1940s, that error was corrected and the animals were scientifically rediscribed as the only bivalved gastropods. Except for these few, rather odd snails, no living mollusks other than bivalves have two shells.

One other group of animals is "bivalved" and may be confused with the clams. In fact, these animals, the lamp shells or brachiopods, may look quite "clammy." There are two quite distinct types of brachiopods, both of which are bivalved. Specimens of one type, the so-called "articulate brachiopods," are occasionally found as hitchhikers in aquaria attached to live rock. The shells of these animals differ from clams in that they are never symmetrical and one of them always has a hole for a fleshy stalk that extends through the shell that is used for fastening the animal securely to the substrate. This stalk is stubby, just long enough to lift the animal off the substrate and allow it to pivot. Many types of clams live fastened to the rock, but never by a stalk that allows them to pivot. Examples of a second brachiopod type, called "inarticulate brachiopods," may occasionally be purchased by aquarists. These animals live in sand and have symmetrical, somewhat clam-like shells; however, a long fleshy stalk protrudes from the back of the shells where they are connected together. The stalk extends down into the sediment, fastening the animal in place. A clam may extend its foot down into the sediment, but if it does, that foot extends from the gaping region of the clam's shells, normally opposite the side where the shells are connected. The clam's foot is withdrawn completely within the shells when the animal closes its shells, while the stalk of the inarticulate brachiopod remains outside its shells at all times.

What's on the Mantle?

A clam is an animal with a small body enclosed within a space created by an external shell on each side of its body. The shells generally connect or are hinged together at the top (or "anatomically" dorsal apex) of the body. Differences in the position and structure of the connection between the shells are used to delineate some of the clam subgroupings. A ligament made of a resilient rubbery protein holds the shells together, and acts like a spring pushing the shells open. This means that a clam must actively contract muscles to close its shells. When these muscles relax, the shell opens passively. If you are wondering about the condition of a clam in your tank, an easy first determination of its health may be made by gently touching the shell, or the tissue layer around the shells. If the animal contracts rapidly, it is probably healthy. If it contracts slowly or feebly, it may be in trouble. If it doesn't contract at all, it is dead.

The shells are secreted by a layer of tissue on the outside of each side of the body called the mantle. This mantle lines the shell extending down each side of the body from the dorsal region near the hinge between the shells to the shells' outer edges. Deposition of shell material on the inner shell surface by the underlying mantle thickens the shell. This deposition continues throughout the clam's life. If the shell is broken, but the animal is not killed, subsequent deposition of shell material on the shell's inner surface will heal the fracture. The shells grow by deposition of shell material on their outer edges. Here, the deposition is done by a region near the mantle's outer edge. Fibers extend into the shell from just inside the mantle's outer edges and fasten the mantle to the shell. The mantle's outermost region may extend beyond the shell and be elaborated into sensory tentacles or other sensory structures, such as the eyes found in scallops. These structures generally are thought to function in detecting potential predators. Most bivalves can't move either rapidly or far, and therefore avoid predation by withdrawing into their shells and closing them tightly, so it is important for them to be able to detect the approach of predators.

In many clams, the mantle's posterior edges may fuse together where they meet and form tubes or siphons. The siphons guide incurrent (entering) and excurrent (exiting) streams of water. Clams are basically filtering pumps, and the direction and separation of water currents is vitally important to them. These siphons may be quite long. In the geoduc (pronounced "gooey duck"), Panopea abrupta, of the Pacific coast of North America, the siphons may extend a meter or more.

Inside the shells, the body is suspended directly below the hinge and hangs in the cavity created and enclosed by the shells. Because this cavity is lined by the mantle, it is called the "mantle cavity" and it surrounds the body on all sides. Relative to the size of the body, this cavity is often quite spacious, and is a "hydrodynamic" space through which water is drawn, directed, filtered and used to flush and clean the animal.

Inside the Envelope

Internally, most clams have a consistent, relatively simple and straightforward morphology. Much of this simplicity is due to reductions from the standard molluscan body plan. Normally, mollusks have good heads and sensory organs; however in bivalves, except for the mouth and lips, all normal head structures, including the brain, are absent. Presumably if all you do is sit in one spot and filter water, you don't need much in the way of a brain - and bivalves don't have one. The rest of their nervous system is not terribly sophisticated, either. The major components of their nervous system are a few pairs of small and simple ganglia, or nerve cell aggregations, one on each side of the animal. There are typically three major pairs of ganglia. One pair, located near the mouth, seems to control feeding. Another pair in the visceral mass probably controls digestive and reproductive activities, and a pair in the foot likely controls any locomotion. As one might expect, the array of sensory organs is decidedly limited. Most bivalves have a few sensory tentacles around the mantle or siphon edges. Many have a statocyst, or balance organ, that allows them to determine their orientation in the sediments. A few have enervated pigmented spots, termed "eye spots," that seem to be photosensory. These are generally located on the mantle but in a few clams they may be located on the siphons or viscera. The most sophisticated and complex sensory structures found in any of the clams are the eyes of scallops. These eyes present a paradox; although they are quite capable of forming an image, the animal simply does not have sufficient nerve cells to process or "see" such an image. Instead, these eyes appear to function as sophisticated shadow receptors, signaling when light changes rapidly. This would happen, for example, when a predatory fish swimming over the scallop casts a shadow upon it. This can trigger the scallop to swim by clapping its valves rapidly together; this causes a jet of water to exit from around the hinge, thereby propelling the animal away from harm.

Getting From Here to There… at a Clam's Pace

In contrast to the swimming ability seen in scallops, most clams cannot move much. Many of them have a foot which can extend out of the shell. While a few forms, such as razor clams, can burrow very rapidly, most sediment dwelling clams can't burrow rapidly at all. Other species, such as the large geoducs, are immobile as adults. As juveniles they grow and burrow deep into the sediments, but once they reach adulthood, they live in their hole in the ground, as much as two feet or more below the sediment's surface. When disturbed, they may pull their siphon rapidly down into the sediment, but the rest of the animal's body doesn't move.

In some clams the foot may also fasten the animal to the substrate. The foot of these animals contains a gland which secretes a material called "byssus;" thus the gland is called the byssal gland. Byssus generally consists of proteinaceous strands. The animal secretes these as liquid strands that harden to the substrate and then are extended up into the water as threads or strings. The animal then physically holds on to them. These strands are quite strong, capable of holding a mussel in place in pounding surf, for example. However, the animal may let go of them at will, and this property allows animals such as mussels, which are usually thought of as stationary, to be quite mobile. They move by sequentially depositing and then releasing their byssus. If you have a mussel in your aquarium, it is easy to follow where it has moved by following the trails of byssus that remain attached to the substrate. Incidentally, the rapidly swimming scallops also may secrete byssal attachments. When the scallop wants to swim, it simply lets go of the byssus and swims away.

Visceral Reactions

Being basically designed to sit in one place and convert particulate organic material into clam tissue, the internal structures of clams are not particularly complex. The gut is a good example of this simplicity. The mouth leads to an esophagus that may be, relative to the size of the animal, quite long. This, in turn, empties into a relatively large stomach. A digestive gland is found on either side of the stomach. These glands, sometimes inappropriately called "livers," are the site where most digestion occurs. A fairly short intestine leads out of the posterior end of the stomach and terminates in an anus near the opening of the excurrent siphon.

This basic simplicity aside, however, the processing and digestion of foods is quite unlike that seen in most animals. The typical processes of digestion that the reader may be familiar with in vertebrates are wholly absent, and present a good example of how different animals attack the same fundamental problem of digestion of food. Clams are generally adapted to eat very fine particulate food, and are hence called "microphagous" feeders. Probably the most structurally complicated part of any bivalve is its stomach. This organ is superbly adapted to assist in the digestion of very fine particulate material. In the case of most bivalves, this material is phytoplankton, microalgal cells suspended in the water. The filtered particulate food is collected from the gills onto streams of mucus that enter the mouth, where they are compacted into a single strand that is swallowed. This mucus strand or string extends through the esophagus into the stomach, which is a bag-like structure with several peculiarities. Near its anterior end is located a hardened cuticular plate called the "gastric shield." This plate is made of protein and chitin and forms a roughened, rasp-like surface. At the stomach's opposite end is a pouch. This pouch is secretory and produces a rod, called a "style," made of a mass of hardened mucus and digestive enzymes. This rod may be quite long, often two to three times the length of the shell, and the pouch that holds and produces it may be folded around the stomach. The rod is flexible and bends to follow the shape of the pouch. The rod is spun by beating cilia in the pouch and may rotate at speeds up to about 600 revolutions per minute.

The mucus string coming from the mouth is wrapped around the spinning rod and pulled into the stomach much as rope around a windlass. Secretion adds material to the rod rapidly as it spins, but it doesn't increase in length. Instead, the free end is pressed against the cuticular rasp and ground to pieces, thoroughly mixing the particulate food with digestive enzymes from the rod. The resulting slurry drops to the bottom of the stomach where the stomach's pH changes, causing the mucus to dissolve. The floor of the stomach is covered with corrugations, grooves and ridges, all covered with microscopic beating cilia. These cilia set up currents in this "sorting region" and particles of the appropriate size and density are conveyed by cilia to the digestive gland openings on either side of the stomach. Inedible material, such as mineral grains, or particles that are too large, is moved to the far end of the stomach. In this area, the pH becomes more alkaline, and the mucus again solidifies. Here, the indigestible food and mucus are compacted together into another mucus strand that will pass out of the intestine in a groove called the typhlosole.

Food particles that enter the digestive gland are ingested by individual cells and digested internally. Unlike the condition seen in humans or even some other mollusks, food is not digested in a slurry of digestive juices within the gut cavity, but rather internally within the individuals cells of the digestive gland. Indigestible food residue is conveyed out of the gland and added to the mucus strand which will become feces. Nutrients are released from the digestive gland cells into the clam's blood.

Circulation

Clams follow the typical molluscan circulatory pattern and lack capillaries. Blood is pumped out of the heart and goes through a few large vessels to the tissues where it exits the arteries and enters large spaces where it passes over the cells of the various organs. It then collects into large internal spaces called lacunae from which it passes through the kidney and gills, ultimately returning to the heart to complete the circuit. The heart has two auricles, one on either side, receiving oxygenated blood from the gills. These auricles pump blood into the single ventricle which then pumps blood to the body. An odd peculiarity of the bivalve circulatory system is that the ventricle surrounds the intestine. Consequently, it may be said, correctly, that the gut goes through the heart.

The Next Generation

All clams reproduce in a relatively similar manner. Their reproductive system is simple, consisting of gonads and simple pores to release the gametes into the mantle cavity. The gonads are highly branched and their branches intertwine with those of the digestive gland. Both of these, in turn, extend down into the foot, as do loops of the intestine. The testes are typically bright white, while the ovaries are often intensely colored, orange, green or red, by the yolk that is found in the eggs. The gonads are basically a collection of tubules joining to form the gonoducts. These empty out of the gonopores located near the anus and release eggs or sperm into the excurrent water current. The strong water flow carries them out of the mantle cavity. Fertilization generally occurs in the sea. The embryos typically develop rapidly into small swimming and feeding larvae that develop two tiny little shells. These settle out of the plankton in the appropriate habitat and take up life as juveniles. In a few bivalve species, the females retain the eggs in the mantle cavity where fertilization occurs. The larvae develop in the mantle cavity and are finally expelled as small juveniles, which have bypassed the planktonic stage altogether. Most bivalves have separate sexes, but hermaphroditism is common.

Most shallow-water bivalves are short-lived, having life expectancies of probably less than ten years. Some, however, live much longer. Geoducs from North America's Pacific Northwest coast have been documented with ages of 150 years, and the large tridacnids may live a similar span. Deep-sea bivalves may live much longer still, perhaps many centuries. Some of these latter animals are predatory and have been documented to take over a year to eat a single prey item (see, for example, Allen, 1983; Allen and Morgan, 1981).

Bivalve Diversity

Clams may be subdivided into different groups based upon their gill morphology. There are three fundamentally and structurally different types of gills, hence there are three basic types of clams. These are generally called the protobranchs, lamellibranchs and septibranchs. The root word for gill in Latin is "branch," and these names mean, respectively, "first gill," "flap or layer gill" and "shelf gill." We can't see the gills from outside the shells, but fortunately the basic shell shapes often correlate with gill structure, thus allowing us to determine which type of clam we have in our systems. In all bivalves, the gills are the source of water currents both going into, and out of, the shell.

The name protobranch means first gills, and these animals are presumed to be similar to the ancestral clams. The first bivalved mollusk fossils are found in rocks over 400 million years old, and these tiny shells do bear a passing resemblance to some living protobranchs, so maybe this is not too far off the mark. Protobranchs have small gills and large extended lips referred to as "labial palps." Their gills have no feeding function and are only respiratory. These typically small animals often live in muddy areas and use their palps to suck up mud, which they ingest. They feed on small particulate matter in the mud, such as detritus, bacterial aggregations, and fecal pellets. They also may eat the mineral sediment by digesting the bacteria it contains. Protobranchs are often capable of moving rapidly through sediments. I know of no protobranchs that are kept in the aquarium trade, but they possibly could be kept in a reef aquarium with a well-developed sand bed.

The majority of bivalves are lamellibranchs, and have very small labial palps and very large gills. Although there are several different types of lamellibranch gills, they all function as both respiratory and filter-feeding organs. The gills are bathed in mucus and water is pumped through them, collecting small plankton in the mucus. The mucus is conveyed to the mouth by a series of food grooves where it is eaten and forms the mucous strand described above. Most of the larger lamellibranch clams have limited mobility, although the smaller ones often do move quite well. All of the clams normally seen in reef tanks are lamellibranchs including ark shells, flame scallops, oysters, and tridacnids. Except for the tridacnids, these species are difficult to keep in reef aquaria as very little phytoplankton is available for them to eat. Typically, they live for a few months, while they exhaust their energy reserves and then die. These animals should not be purchased unless you know that you have enough phytoplankton in your system to ensure their survival.

Figure 7. Although commonly kept, for a while, by many aquarists, flame scallops, Lima spp., such as this one photographed in the Caribbean, have a dismal survival record in aquaria. They generally starve after a few months. Only in aquaria where phytoplankton is highly dosed, or whose water contains a lot of particulate material, will these beautiful animals survive. Their long sensory tentacles are extensions from the mantle's edge.

The septibranchs are among the most bizarre clams and, probably because of that, are my personal favorites. These animals' gills are reduced to a muscular shelf or septum that extends across the body. This siphon has "flapper" valves so that when it contracts, water is pulled into the body. They are found in sediments with the tip of their siphon just at the sediment's surface, where they wait for a small crustacean to encounter the siphon. When this occurs, the septum contracts, and the bug is pulled into the mantle cavity and eaten. It is mashed up and digested in the stomach. All septibranchs are predatory. These animals are not seen in the aquarium hobby, but should do well in most systems containing a well-developed crustacean fauna. They are relatively uncommon in shallow water, but are abundant in the very deep waters of the abyss.

Bivalves in the form of tridacnids make beautiful and interesting attractions in our reefs. Other clams may be equally attractive but presently are difficult to keep. When we learn how to consistently provide a phytoplankton food source most of these clams will become attractive additions to our artificial ecosystems.

Selected References:

Here are a group of references selected to give both basic and specific information about bivalves, their morphology, and their importance in some marine communities.

Allen, J. A. 1983. The ecology of deep-sea molluscs. 29-75. In: Russell-Hunter, W. D. (ed.) The Mollusca, Vol.6. Ecology. Academic Press.

Allen, J. A. and Morgan, R. E. 1981. The functional morphology of Atlantic deep water species of the families Cuspidariidae and Poromyidae (Bivalvia): an analysis of the evolution of the septibranch condition. Phil. Tram. Roy. Soc. London, [B] 294: 413-546.

Morse, M. P. and J. D. Zardus. 1997. Bivalvia. In: Harrison, F. W. and A. J. Kohn. (Eds.) Microscopic Anatomy of Invertebrates. Volume 6A, Mollusca II. pp. 7-118. Wiley-Liss, Inc. New. York.

Paine, R. T. 1974. Intertidal community structure; Experimental studies on the relationship between a dominant competitor and its principal predator. Oecologia. 15: 93-120.

Paine, R. T. and S. A. Levin. 1981. Intertidal landscapes: disturbance and the dynamics of pattern. Ecological Monographs. 51: 145-198.

Ruppert, E. E, R. S. Fox, and R. D. Barnes. 2003. Invertebrate Zoology, A Functional Evolutionary Approach. 7^th Ed. Brooks/Cole-Thomson Learning. Belmont, CA. xvii +963 pp.+ I1-I26pp.

Salvini-Plawen, L. 1988. The structure and function of molluscan digestive systems. In: Trueman, E. R. and M. R. Clarke (Eds): The Mollusca, Volume 11, Form and Function. Academic Press, New York, 301-379.

Shimek, R. L. 2004. Marine Invertebrates. 500+ Essential-To-Know Aquarium Species. T. F. H. Publications. Neptune City, NJ. 448 pp.

Stanley, S. M. 1968. Post-paleozoic adaptive radiation of infaunal bivalve molluscs - a consequence of mantle fusion and siphon formation. Journal of Paleontology. 42: 214-229.