Composition of The Larval Shell
Studies show that the prodissoconch, the shell of the free-swimming veliger larval stage of C. virginica is composed of aragonite, as opposed to the calcite composition of a post larval adult oyster shell. The epithelium of the oyster’s mantle secretes both the prodissoconch and the post larval shells, but at different times. Tests were ran to try to determine the reason why larval and adult shells have different compositions. At the Biological Laboratory of the U.S. Bureau of Commercial Fisheries in Milford, Connecticut, larvae from the eastern oyster C. virginica were reared in breeding tanks and were then, collected, washed with distilled water, and dried as they died. The sample included a variety of larval stages starting with the straight-hinge veliger larva with its shell, the protostracum, to the last stage of the umbo larva with its shell, the prodissoconch. Both of these larval stages have shells which are very thin, hyaline, and translucent. The study showed the specific gravity of aragonite is 2.95 and calcite is 2.72, so as far as weight is concerned, there really is not an advantage for a larval oyster to have a shell made of one composition over the other. Then, it was decided to compare oyster larval shells not with adult shells, but instead, with other bivalvia larval shells. The study concluded that all, or almost all, bivalvia have aragonitic larval shells because the majority of them have aragonitic adult shells, and it can be assumed that C. virginica oyster larvae have an aragonitic shell simply to conform to the general pattern in the bivalvia. There is no adaptive need for free-swimming larvae to have shells of a composition other than argonite, they have that composition because their ancestors did. The study then posed the question, “Why do larval oysters suddenly begin depositing calcite after they have attached to a substratum and begun metamorphosis?” It was concluded that adaptations for a thicker shell are required for defense against predators because the oysters are permanently immobilized and therefore live in a different environment than that of the free-swimming larvae.
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