by Amanda Kahn, Invertebrate Zoology and Molecular Ecology Lab

In our Ask a Grad Student page, Leeanna asked a bunch of really good questions, and all revolve around bivalves.  Now, maybe you think you don’t know bivalves well enough to have them over for dinner, but I expect that many of you actually have had them FOR dinner!  Bivalves include clams, mussels, oysters, scallops, and other generally clam-shaped animals with two shells.  Class Bivalvia is within Phylum Mollusca, and its closest neighbors on the evolutionary tree are Classes Monoplacophora (extinct, snail-like animals), Polyplacophora (chitons), Gastropoda (snails and slugs), Scaphopoda (tusk shells), and Cephalopoda (octopuses and squids).  Too much information?  Too much information.  Sorry.  On to the questions!

Q: How do bivalves pump out water?

A: On each side of the foot inside of the bivalve (let’s say, for example, a clam), there are two big hollows, called mantle cavities.  On one end of the bivalve’s shell, there is an inhalant and exhalant siphon, which the clam uses to pump water in and out of the mantle cavities.

Q: How do bivalves eat their food?

A: What do bivalves eat for food?  Oh, sorry, I guess you’re asking the questions…well, to indulge myself, bivalves filter suspended edible material, such as plankton, out of the water that they pass through the mantle cavities with their inhalant and exhalant siphons.  According to the Animal Diversity Web, there are four different ways that bivalves feed.  Those four feeding types are called protobranchs, filibranchs, lamellibranchs, and septibranchs.

“In protobranchs, the ctenida [pronounced ten-ID-ee-uh, basically the gills] are used only for respiration and food is caught by the labial palps [arms around the mouth]. In filibranchs and lamellibranchs, the ctenida trap the food particles in their mucous coating and transfer the food to the labial palps via ciliary action [translation: movement of a bunch of little hairs that carry food along, similar to how people crowd-surf in concerts].”  Finally, a septibranch bivalve pumps food using a membrane stretched across a cavity inside the body.

Pretty crazy, huh?  To read about feeding in greater detail, check out this article by Dr. Ronald L. Shimek.

Q: Do bivalves walk on a foot?

A: Well, some do, kind of.  Mostly, bivalves don’t walk – they just use the foot to hold on to a rock below them.  The one bivalve I can think of that uses its foot to move is the scallop, and that’s only half-true.  Scallops hold on to rock with a foot, but when they’re small, if they really need to move they can let go with the foot and clap the two shells together to swim away.  That’s not really the foot causing the motion, but it’s the foot allowing motion to happen.  So I guess the answer is no, most bivalves do not walk on a foot (or walk at all).  Bivalves do have muscles though, and they use them to open and close their shells.  Also, you might notice a bunch of bivalves’ siphons sticking out of a mudflat, but if you move toward them, they’ll pull back into the mud.

[youtube=http://www.youtube.com/watch?v=HGyf5A3QOdU]

Q: How long does it normally take to grow full size? What is their general life-span?

A: For many bivalves, we don’t know!  Different bivalves can live for different amounts of time.  Some only live a few years, while the oldest bivalve, the geoduck (which, in spite of the way it is spelled, is pronounced “gooey duck”), can be over 140 years old!

Thanks for the questions, Leeanna!

Read more about bivalves at the Animal Diversity Web