Activity: Reading Information from Empty Shells

The features of bivalve shells can reflect much about the lifestyle of the living bivalve that made the shell. This is especially important to paleontologists (who never see the living animals) but also for biologists, who often have only empty shells available. Ask your students to identify any of the following from the shells available in your classroom, and at the same time, ask them to think about adaptation and the relationship between function and structure.

Deep burrower — Bivalves that burrow deep in the sediment must have siphons to maintain contact with the water. Those siphons are controlled by muscles that leave scars on the shells in the form of an embayment called the pallial sinusPallial Sinus: An embayment in the posterior part of the pallial line that indicates the attachment of siphonal retractor muscles and demarcates that part of the mantle cavity into which the siphons can retract in bivalves.. The deeper the sinus, the longer the siphons (see Hard-shelled Clam, Soft-shelled Clam). What must we conclude about a bivalve shell without a pallial sinus?

Jet propulsion — The “ears” or auriclesAuricles:
Ear-shaped structures on the shells of scallops, one of their defining characteristics. of scallops (below) have tiny gaps through which jets of water are expelled when the valves close suddenly. This results in a kind of jet propulsion that creates the scallop's swimming movements as it claps its valves together.

Byssal attachment — Some bivalves that produce byssal threadsByssus:
A tuft of long, tough filaments which are formed in a groove of the foot, and issue from between the valves of certain bivalve mollusks, by which they attach themselves to rocks, etc. for attachment on a hard surface have a gap in the edge of the shell even when the valves are closed (see Arks). This indicates a relatively large byssus that stays attached even when the bivalve must close tightly to avoid a predator.

Cementation — Cementing species (see Oysters) show clearly the place on one valve that has been cemented to a rock. This sometimes assumes the shape of the surface to which it is attached. For example, an oyster on the outside surface of a scallop might show the wavy surface of the scallop shell.

Predation scars — Bivalves and other mollusksMollusk: A member of the phylum Mollusca; also spelled mollusc (most especially in the United Kingdom). have the ability to repair breaks in their shells, if the damage is not sufficient to cause death of the bivalve. Crabs might chip at the edges of the shell, but fail to break into the shell to consume it. Such a minor break can be repaired, but leaves a scar on the shell. Moon Snails (family Naticidae), Murex Snails (family Muricidae), and Octopuses (Phylum Mollusca, Class Cephalopoda) all bore holes in other mollusk shells using their radular teeth, sometimes with the help of chemicals to soften the shells. These holes have beveled edges in the case of the snail predators, and straight-sided in the case of the Octopus. Such a hole, if complete (at right, in a snail shell), is a clear indication of the cause of death of that particular mollusk.

EpifaunaEpifaunal:
Living on top of the sediment, i.e., unburied; also called epibenthic. — Not all features of an empty shell reveal something about the living bivalve. For example, you might think that barnacles growing on the outside of a bivalve shell means that the bivalve lived epifaunally, that is, not buried below the sediment. This is not the case, because the barnacles might have attached to the shell after its death, when the empty shells were scattered on the sea floor.