Anti-predation strategy, growth rate and extinction amongst Pliocene scallops of the US eastern seaboard

Abstract

Placopecten, Chesapecten and Carolinapecten are scallop genera occurring in the Pliocene of the US eastern seaboard. The first (extant) is a smooth, streamlined form, adept at escaping predators by swimming (‘flight’ strategy). The other two (extinct) are plicate forms. Plication facilitates a ‘resistance’ strategy towards predators which is benefited by large size and high shell thickness - maximally so if these states are achieved early in life. Oxygen isotope profiles show that in early ontogeny, Pliocene Placopecten grew at the same moderate rate as modern Placopecten. By contrast, Chesapecten grew as fast as the fastest-growing modern scallop and developed an unusually thick shell, while Carolinapecten grew substantially faster still, this probably enabled by high primary productivity. Extinction of these genera, and survival of Placopecten, can be attributed to a decline in productivity which prevented a maximally effective ‘resistance’ strategy towards predators but had no deleterious impact on a ‘flight’ strategy.