Greetings, Clamrades..

By Ronald L. Shimek

The phylum Mollusca contains several large and ecologically successful groups. Probably the most homogeneous of these is the Class Bivalvia which contains the clams.

Three Tridacna , in a depression of the reef flat at Palau.

There are a lot of superlatives that can be attached to this group. Excluding the great cephalopods, the largest mollusks are clams. Furthermore, clams are adapted to exploit life at the substrate-water interface, and so dominate this habitat that in some cases they can exclude of all other organisms (Paine 1966, 1974; Paine and Levin, 1981; Virnstein, 1977). Clams are amongst the best filtering machines in the animal kingdom. Finally, clams are exceptionally economically important, both as food items, and as the source of decorative materials such as pearls. With all that, like Rodney Dangerfield, "clams don't get no respect."

Even though many of them are exceptionally beautiful animals, with the exception of the tridacnid clams of the Indo-Pacific, relatively few clams are kept by hobbyists. The probable reason is that many of these animals are relatively difficult to maintain. Yet, clams are common members of both fresh water and marine ecosystems and deserve to take their place of honor in our captive ecosystems.

What makes a clam?

The name bivalve means "two shell" and this implies clams have two shells. This is indeed the case. Alone amongst mollusks, clams have two shells throughout their lives. Some snails have two shells, but they start their larval existence with only one. All clams have two, always, no exceptions. These shells are connected together at the top of the animal by rubber-like proteinaceous ligament (Boggild, 1930; Kozloff, 1990; Ruppert and Barnes, 1994). Depending on the type of clam, this ligament is situated to be either under compression when the animal has the shells closed or it is stretched when the animal has its shells closed. In either case, relaxation of the powerful adductor muscles which close the shell causes the shells to open or gape due the relaxation of the stress on the hinge ligament. Consequently, clams close due to active muscle power, and open when the muscles relax or die. So, if you are keeping clams in your system, start to worry if they are wide open, because it means that the animal can't pull against the resistance of the ligament to close the shell. Generally, with the exception of tridacnids, a happy clam is a closed clam, or at best slightly open clam.

Inside the shells, the body structures of most clams are relatively consistent. It is not true that if you have seen one, you have seen them all, but certainly it is true that if you have seen one you have seen the majority of them. At the front end, the clam body consists of a mouth with a pair of lips. It truly can be said that clams are the mollusks that have lost their heads. Typical mollusks are represented by animals like snails. Snails have a good head, with well-developed sense organs and a good brain. Clams have dispensed with all of these unessential structures; after all who needs a brain if you basically sit in one place through your life and filter water?

What clams DO have is a set of exceptionally large gills. The typical molluscan gill consists of a basal rod with thin filaments extending down off either side. Blood is pumped though the gills internally by a muscular heart and water is moved over them externally by beating hair-like cellular projections called cilia. Gas exchange occurs across the thin tissues of the gills. Most mollusks bathe their gills in mucus. This mucous layer collects particles from the water that get trapped on the gills. The mucus moves due to the action of the cilia covering the gill and helps flush these particles off the gill.

Most bivalves have modified the basic gill to make it a suspension-feeding organ. The gill filaments have become highly elaborated and enlarged; far larger than is necessary for respiration. Adjacent filaments become attached to one another, with small holes in the attachments, and in effect the gill turns into a large net. These clams keep the surface of the gill bathed in mucus to collect particles which impinge upon the gill, but now instead of moving the particles off the gill to clean it, the particles are moved to food grooves and conveyed to the mouth in flowing stream of mucus propelled by underlying cilia. The ultimate use of these gills in feeding is seen by a few members of the genus Solemya , which have lost their gut completely and gain nutrition by the engulfment of bacteria by the gill filament epidermal cells (Bernard, 1980; Morse, and Zardus, 1997).

Oddly enough, one small group of bivalves has bucked the trend and has not elaborated the gills for feeding. Instead their lips have grown very large and have many folds; to a student dissecting

Lopha cristagalli , the zig-zag oyster. These animals are common in the Indo-Pacific. They feed on phytoplankton.

one of these clams for the first time the lips look like the gills of other clams and are often mistaken for them. These lips get covered in cilia and bathed in mucus, and function to collect food much in the way that gills do in the other bivalves. Thus a similar function has lead to two different structures to have a similar appearance. These clams are mostly deposit-feeding animals sucking up mud and other fine sediments, and are using their lips for sorting of the mud particles (Salvini-Plawen, 1988; Kozloff, 1990; Ruppert, and Barnes, 1994; Morse and Zardus, 1997).

The clam shells are secreted by tissue, called the mantle, that extends down from the dorsal surface cover the entire animal on either side. In the area where the shells meet, the bottom edges of the mantle come together to form a curtain that can enclose the cavity between the shells. This cavity contains the clam's body and the expanded filtering gills. In many clams, this tissue curtain becomes fused together to form a tube or siphon.

Many clams have a siphon or "neck." The siphon is simply a tube for conveying water and food to the animal, and water and feces away from the animal. Most clams have two siphons, which are sometimes fused and sometimes not. Some years ago a researcher interested in determining why clams are so successful ecologically, determined that siphons were invented at least seven different times, so there are seven basic types of clam siphons (Stanley, 1968). Interestingly, in all cases, the siphonal position on a clam marks the posterior surface or the animal. The mouth is at the opposite end of the shell.

Most clams use the siphons to extend up into the water column to suck water into the shell while the clam remains safely buried out of sight. A few other clams use the siphons to vacuum the sediment surfaces around the burrow collecting organic debris. Organic debris is a fancy way of saying "food" to a clam. This organic debris is generally rich in bacteria which in turn are rich in nitrogen and are a good protein source. Those animals that eat deposits or mud, don't actually digest the mineral components of mud but rather eat the bacteria and other micro-organisms living in the mud.

After the food is conveyed to the mouth it is swallowed in a strand of mucus and transported to the stomach where it is mixed with some enzymes. Although some digestion occurs in the stomach, most digestion occurs in a pair of large digestive glands located on either side of the stomach. The food enters these glands through small pores in the sides of the stomach wall. After digestion, undigestible residue is expelled from the digestive glands and sent back to the stomach. From here it goes to the intestine where it is compacted into feces and expelled from the gut. There is no musculature around the gut except in the region of the intestine, which typically passes through the heart. The only large and evident muscles in the body of the clam are generally located on either side of the foot. These muscles, called adductors are used to pull the shell shut. There is much diversity in the position of these muscles, but most bivalves have two, one near the mouth and the other near the anus (Salvini-Plawen, 1988; Morse and Zardus, 1997).

Bivalves have a central foot located between the gills. In most clams this organ is also muscular. However, here, the muscles are in sheets or bands, not coalesced into a compact mass. The foot can extend out of the body and is used either in locomotion or in fastening to the substrate. Although most bivalves can move exceptionally rapidly (Trueman, 1967), most of them generally move very little, and most of them are relatively sluggish.

Many clams that live on rocks or on coral substrates rather than in mud or sand. These animals can fasten themselves to the substrate by the means of secretions from a secretory area, called the byssal gland in the bottom center of the foot. This gland secretes liquid adhesive chemicals that harden on contact with water. In most cases, the animal places the byssal gland opening on the substrate and secretes some of the glue. The foot is then rapidly pulled away from the substrate and the glue hardens into a thread. The animal holds on to its end of the thread with a strong set of muscles, and they secretion sequence can occur again and again, so that finally the animal is held in place by mass of byssal threads.

Byssal attachment is remarkably durable and very strong. Consider that mussels in the intertidal can with stand the pounding surf with being detached. On shallow coral reef flats, the attachment of Tridacna to the substrate under them is by a byssal attachment and is really quite durable; and is also capable of withstanding the pounding of surf. One of the very neat things about byssal threads is that the animal can voluntarily release them when it wants to move. If you keep mussels in an aquarium you will be able to track their movements around the tank by the trail of byssal threads they leave behind. As they hang on to some, they reach with their foot and secrete some new ones and then let go of the older ones. The old ones will remain attached to the aquarium walls for months or until they are scraped off.

Swimming scallops can also fasten to the bottom using byssal threads. In the northern Pacific pink scallop, Chlamys hastata , the scallop can release the byssal thread and start to swim within a tenth of second.

Chlamys hastata , the Northeastern Pacific pink scallop. These scallops can release their byssus and swim away if threatened by a predatory sea star.

Scallops are one of the few groups of clams that have rapid locomotion. They can swim by clapping their valves together forcing a jet of water out near the hinge in the back which moves the animal rapidly forward.

Both sexes of clams generally reproduce by spawning freely into the water, there is no sexual dimorphism in the group, and very few clams have any parental care. Sexes are generally separate, but some hermaphroditic species are known. A few clam species, characterized mostly by being small animals, brood their juveniles.

In these animals, the female retains the developing embryos in the cavity around the gills, and small juveniles are released some time after spawning. In most marine clams, the eggs are quite small and develop rapidly to small feeding larvae called veligers. These larvae typically feed on unicellular algae. Growth is relatively rapid, and generally the larvae settle out of the plankton in a few weeks as very small versions of the adults. If conditions are right, growth can be and generally is rapid until the animal reaches sexual maturity, after which they grow very slowly (Strathmann, 1987).

The large geoduck (pronounce "gooey duck") clam of the Pacific coast, Panope abrupta , can generally reach its adult size of 3 to 5 kg within about 10 years, after which it can live a very long time. Some geoducks with ages over 150 years have been collected, and ages of 120 years or more are fairly common in some populations. In the Indo-Pacific tropics, individuals of the various Tridacna species can reach lengths of 2 to 4 cm in a year or two, but large adult individuals of the huge Tridacna gigas may be many decades old (Haderlie and Abbott, 1980).

For more information and pictures about bivalves follow these links:

http://www.york.biosis.org/zrdocs/zoolinfo/grp_moll.htm#bivalvia

http://www.photovault.com/Link...../AAMVolume01-02.html

Fresh-water clams generally have a different type of life history, and in many cases this involves a stage parasitic on fishes. Consider for a moment the problem of a clam living in a good habitat in a rapidly-flowing stream. If it spawned out normal swimming larvae they would be swept far downstream and into a very different environment in very short order. Many of these clams have a larvae called a glochidia which looks like a small clam with teeth around the edge of the shell. This larvae fastens onto the gills or fins of a fish and gains nourishment from the fish blood. Additionally, as the fish is unlikely to leave its habitat, so the clam has a mechanism of staying in the right locality. When the larvae have grown large enough to survive, they release from the fish, and take up the more normal clam life style of suspension-feeding.

For some pictures and information about fresh-water clams follow this link:

http://rivers.oscs.montana.edu/dlg/aim/mollusca/bivalvia0.html

This particular parasitism does not generally kill the fish, but neither is it particularly benign, opening wounds that can lead to infections, and stealing nourishment. Aquarists that maintain freshwater clams and mussels should be aware of the potential problems that can occur should they keep fishes in the same tanks. In nature the spawn of a single clam is spread over a large area by the flow of the stream, and the number of larvae that can infect any one fish is relatively small. In the enclosed system of a fresh-water aquarium of only a few gallons, the infestation rate can be exceptionally high. In some situations, the fish can be killed by the parasite load. Of course, the baby clams die too, but most aquarists, unfortunately, consider that to be of little consequence.

The Problem with Clams...

Several types of clams are occasionally offered for sale to aquarists, and a few, primarily the Tridacnids, are offered consistently. The latter group consisting of the species Hippopus hippopus and several species in the

This Tridacna gigas was found at a depth of 10 m (33 feet). It was almost 2 m long.

genus Tridacna are maintained with some moderate success by many aquarists. All of these species contain zooxanthellae in their tissues and for good health need a lot of light. In the real world they are seldom found deeper than 10 m (33 ft) and the majority of individuals are much shallower, even living on the reef flats in very shallow waters. As you might imagine, in the equatorial tropics they receive a VERY significant amount of sunlight.

Additionally, all of the tridacnids are suspension-feeding animals and there have been a couple of recent references indicating that feeding of the clam is necessary for good growth and health of the zooxanthellae, which in turn benefits the clam (Klumpp and Griffiths 1994; Klumpp, and Lucas, 1994; Hoegh-Guldberg, 1996; Knop, 1996). In culture situations, the sea water containing the clams is enriched with both nitrate and phosphate based fertilizer. As a result of this, good water, and lots of sunlight the clams grow very rapidly. In the average reef aquarium, phosphates and nitrates are minimized and the light really isn't that intense. To gain the necessary materials for good health, the clams must feed. In nature these clams seem to feed primarily on unicellular green algae and bacteria, so it will benefit the clams for green water (a culture of unicellular green algae) to be added frequently to the system. Most tanks with a good sand bed, as are found in tanks utilizing Berliner/Jaubert methodology have a lot of bacteria and natural microplankton in the water as well. These will assist in clam growth.

For information about, and pictures of, Tridacna, follow this link:

http://www2/hawaii.edu/~delbeek/delb7.html

The other clams that are offered to aquarists include flame scallops ( Lima species), thorny oysters, ( Spondylus species), true scallops ( Pecten species) and zig-zag oysters ( Lopha cristagalli ). Occasionally, other species are found.

Lima lima, the flame scallop, photographed in Palau. These animals are difficult to maintain because of their dietary requirements

Numerous species of freshwater clams are also found on the market. None of these animals has zooxanthellae, and all are suspension-feeders. Clams also have a property that might seem odd for their sedentary life-style, that is they have a relatively high metabolic rate. So... They need a lot of food. In general, none of these clams can be kept for any length of time without supplemental feeding of the aquarium with green water. They typically are put into an aquarist's system and they appear to live well for three to six months after which they rather suddenly die. It appears that they utilize all of their fat and other energy reserves and in effect, live on borrowed time. When the reserves run out, they don't get enough nutrient from the system that they are in and perish.

Clams are often beautiful animals, particularly those that live exposed on coral reefs. They often have pleasing patterns and colors to their shells and soft tissues, and given the appropriate foods they can be quite hardy. However, clamrades, unless they are well nourished, either with supplemental feeding in the case of Tridacnids or with a defined feeding regime, in the case of all others, they are unlikely to thrive in a captive system.

References Cited:

Bernard, F. R. 1980. A new Solemya s. str. from the northeastern Pacific (Bivalvia: Cryptodonota). Venus. 39:17-23.

Boggild, O. B. 1930. The shell structure of mollusks. Det Kongelige Danske Videnskabernes Selskabs Skrifter Naturvidenskabelig og Mathematisk Afdeling. 9:231-326.

Hoegh-Guldberg, O. 1996. Nutrient enrichment and the ultrastructure of zooxanthellae from the giant clam Tridacna maxima. Marine Biology. 125: 359-363.

Klumpp, D. W. and Griffiths, C. L. 1994. Contributions of phototrophic and heterotrophic nutrition to the metabolic and growth requirements of four species of giant clan (Tridacnidae). Marine Ecology Progress Series. 115: 103-115.

Klumpp, D. W. and J. S. Lucas. 1994. Nutritional ecology of the giant clams Tridacna tevoroa and T. derasa from Tonga: influence of light on filter-feeding and photosynthesis. Marine Ecology Progress Series. 107: 147-156.

Knop, D. 1996. Giant Clams, A comprehensive Guide to the Identification and Care of Tridacnid Clams. Daehne Verlag, Ettlingen, Germany. 255 pp. ISBN 3 921684 23 4

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

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. 1966. Food web complexity and species diversity. American Naturalist. 100:65-75.

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. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

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.

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.

Strathmann, M. F. 1987. Reproduction and development of marine invertebrates of the Northern Pacific coast. University of Washington Press. Seattle. 670 pp.

Trueman, E. R. 1967. The dynamics of burrowing in Ensis (Bivalvia). Proceedings of the Royal Society, London, Series B, Biology. 166: 459.

Virnstein, R. W. 1977. The importance of predation by crabs and fishes on benthic infauna of Chesapeake Bay. Ecology 58: 1199-1217.