Evidence, cause and consequence of exceptionally rapid growth in Pliocene scallops of the US eastern seaboard

Scallops are amongst the fastest growing of bivalves, with many species growing in the order of 40 mm per annum in early ontogeny, and reliable evidence of early ontogenetic growth-rates up to 70 mm per annum in wild populations of certain species. From the evidence of oxygen isotope (δ18O) sclerochronology, modern examples of the western Atlantic genus Placopecten grow about 40 mm per annum in early ontogeny. The same approach reveals similar growth rates in Pliocene examples from Virginia and North Carolina. By contrast Pliocene examples of the extinct genera Chesapecten and Carolinapecten from Virginia grew at least 75 mm per annum, faster than has been recorded in any wild modern scallop, and examples of Carolinapecten from Florida grew up to 140 mm per annum, twice the maximum rate in wild modern scallops. The rapid overall growth of Carolinapecten is matched by exceptionally large microgrowth increments. In specimens with the fastest overall growth the number of increments approximately equals the number of days indicated by the oxygen isotopic data (e.g. c. 180 over half an oxygen isotope cycle), implying that deposition was daily. Specimens with slower overall growth do not have smaller increments but have substantially fewer than the number of days indicated by δ18O evidence, showing that reduced overall growth was a consequence of periodic interruptions rather than permanently less favourable conditions. Since few individuals lived more than a year, rapid somatic growth must have been accompanied by gamete production, implying abundant food resources. Intervals of particularly rapid growth (largest microgrowth increments) are fairly closely correlated with increases in δ13C, as might have been caused by phytoplankton blooms. However, there is little evidence that these were stimulated by upwelling since there are few indications of a matching increase in δ18O (i.e. colder water). Instead, primary productivity may have been enhanced by nutrient supply from the land. Whatever the cause of high primary productivity in the Pliocene of the US eastern seaboard, the subsequent demise of two scallop genera with exceptionally rapid growth seems as likely to relate to a decline in productivity as to a fall in temperature.