Growth and mortality in two Pleistocene bathyal micromorphic bivalves

Size-frequency and growth-ring data have been used to calculate mortality and growth patterns in two micromorphic bivalve species from pelagic mud of Lower-Middle Pleistocene age on Rhodes (Greece). Phaseolus ovatus Sequenza, a lametilid deposit-feeder, showed increasing mortality and a slow-fast-slow growth pattern accompanied by an abrupt change of shape in the fast-growing interval. This in turn is coincident with a minimum in the size-frequency distribution. The bimodality occurring in all distributions is interpreted as a recurrent population character, probably a non-annual reproduction cycle, while height and spacing of the modes, which differ among the distributions, are environmentally controlled. Kelliella miliaris (Philippi), a vesicomyid suspension-feeder, showed a near-constant mortality and a logarithmic growth rate. The difference in mortality between the two species is related to grain-size of the sediment and to stability of the food source. Growth pattern, mortality, food sources, reproduction.