The Bivalves by Sean Chamberlin
It’s hard to imagine anyone who has not seen or at least heard of some kind of bivalve, the trademark, hinged, two-valve shell of clams, oysters, mussels, scallops, cockles, paddocks or quahogs. And while Tom Cruise didn’t repeat “show me the clams” in Jerry Maguire, he could have. Clams (representing all shelled animals, apparently) have become synonymous with money, owing to the widespread use of shells (mostly, not clams) by native Americans and other first peoples as currency for trade. Today, the products of bivalves, namely pearls, bring the big money. Pearl necklaces, some valued in the millions, have graced the couture of many a celebrity.
Of the more than 6700 species of marine bivalves, nearly all of them live attached to or burrowed within the bottom, hard or soft. A few, like the scallops, can swim for short periods of time. Others, like the rock-boring clams (aka paddocks), drill into hard substrates (hardened clays, shales and limestones) where they find protection. Most filter phytoplankton and other particles from seawater although some use their siphons to feed on surface deposits. Marine bivalves range in size from the tiny (~5mm) gem clam, Gemma, found along the Atlantic and Gulf coasts, to the giant (> 1 meter) clam, Tridanca, found in the South Pacific.
The body plan of bivalves is highly compressed but its mantle is well-developed, exhibiting a pair of lobes, called mantle skirts, that create a spacious mantle cavity. The mantle cavity encloses a pair of large gills which are used both for respiration and feeding. A pair of strong adductor muscles attach the body to the shell, the scars of which are readily visible on the interior of most bivalve shells. In most bivalves, the shells are similar but in some, like oysters, one valve is smaller than the other. Often found protruding between the valves are a pair of siphons—one incurrent and one excurrent—that draw in and expel water, respectively. The incurrent siphon may be modified for feeding, much like a vacuum cleaner, and may be prehensile, allowing it to crawl along the surface like an elephant trunk using sensory cells to find food. On some species, like the suggestive Pacific Northwest geoduck (pronounced “gooey-duck” from the Nisqually Indian “gwe-duk” which means “dig deep”), has a body and siphon so large that it cannot retract into its shell. The geoduck, reaching weights of more than seven pounds and living more than a hundred years, is the largest burrowing clam in the world.
[MARGIN] In the Pike Place Market in Seattle, Chamberlin overheard an elderly woman point to a geoduck and whisper to her husband, “It looks just like you.” [END MARGIN]
A prominent feature of most bivalves is their muscular foot, which they use to burrow into sediments. In a highly coordinated and sometimes very rapid sequence of maneuvers, the bivalve opens its shell and extends its foot into the mud. By contracting its foot above the base, an anchor is formed which allows the animal to pull itself down. At the same time, it expels water by partially closing its shell, which acts to reduce its profile and loosen sediments. By rocking its shell, the animal may also gain wiggle room and “dig” deeper into the substrate. Some bivalves, like the razor clam (found on both coasts), is especially quick at penetrating the sand on beaches where it occurs. Hungry clam diggers have invented clam guns—basically a metal pipe with a handle—to grab a plug of sand containing a clam and spill it onto the surface to grab the animal before it can escape. Other clams, like the geoduck, are noted for their deep digging ability, often burrowing a meter or more into the mud (which is why it needs such a large siphon). Burrowing enables bivalves to exploit habitats that are otherwise unavailable to aerobic animals. By pumping oxygenated seawater into their burrows, they create a kind of “snorkel” by which they may breathe at sediment depths where oxygen is not available.
Burrowing bivalves also alter the properties of the sediments. Burrowing and pumping allows for deeper penetration and diffusion of oxygen, a process called bioirrigation. Bioirrigation facilitates the breakdown of organic matter and influences sediment geochemistry. In addition, by digging out sediments and reworking them during feeding or other processes, they alter the size and distribution of sediments, a process called bioturbation. Burrowing animals contribute to variations in the depth to which oxygen penetrates in sediments, a zone known as the redox discontinuity layer (RDL). Their activities and burrows enhance the diffusion of oxygen into the sediments and strongly influence the depth of the RDL. The presence or absence of oxygen determines the way in which organic matter is processed (i.e. aerobically or anaerobically) and has an effect on whether that carbon is rapidly broken down, slowly decayed and/or buried (e.g., Kristensen, 2000). In addition, the RDL impacts a number of chemical reactions, especially iron and sulfur.
Another adaptation of bivalves is the evolution of glands in their foot that secrete one of the strongest glues on Earth (natural or manmade), byssal threads. A number of bivalves secrete strong, fibrous “guy wires” to attach to hard substrates or anchor their bodies in soft sediments. Representative examples include the ornate and iridescent pen shell, found commonly in muddy bottoms in Florida and elsewhere, and the mussel, found in rocky intertidal zones worldwide. In addition to the anchor they provide for the organism, the byssal threads of mussels provide a microhabitat (a small and specialized place to live) for a number of small invertebrates. In mid-intertidal zones dominated by populations of mussels—the popularly known mussel bed—the seemingly monospecific stand belies a richly diverse community of organisms. As many as 300 species have been found associated with mussel beds (e.g. Schmidt, 1999; Suchnaek, 1992). Similar associations have also been observed in hydrothermal mussel beds. Bysall threads have also caught the attention of researchers interested in biopolymers, natural materials that may provide clues for the manufacture of adhesives.
[MARGIN] Byssal threads were apparently woven into a fine fabric called “linen mist” by the Romans and may have been the fabric of Jason’s legendary golden fleece. Jules Verne adorned Captain Nemo and his captives in Twenty Thousand Leagues Under the Sea with clothing constructed from byssal threads. [END MARGIN]
The evolution of gills and a circulatory system with a heart similarly provided adaptations well-suited to exploit benthic habitats. The development of a water circulation system that allowed water to enter and exit in the same direction meant that burrowing animals could direct their water processes towards the surface. This adaptation provided the means to exploit suspension feeding through modification of the gills to sort and capture food particles from other suspended matter. Some bivalves, however, like the giant clam, obtain most of their nutrition (up to 90% by some accounts) from the zooxanthellae that live in their mantle tissues. Like corals, these clams maintain an obligatory symbiosis with algae, without which, they die.
Reproduction in bivalves involves the external release of gametes from separate males and female. Spawning appears to be synchronized by changes in water temperature—seasonal or induced by processes like upwelling—although other causes are possible. Fertilized eggs produce a planktotrophic trochophore larva that transforms into a veliger. The veligers of oysters may travel more than 1000 kilometers and, like other mollusk larvae, have been shown to be capable of using dissolved organic matter as a food source to supplement their nutrition. |
