POECILOGONY IN AN ESTUARINE OPISTHOBRANCH - PLANKTOTROPHY, LECITHOTROPHY, AND MIXED CLUTCHES IN A POPULATION OF THE ASCOGLOSSAN ALDERIA-MODESTA

A San Diego population of the opisthobranch mollusc Alderia modesta (L oven, 1844) exhibits poecilogony, the presence of two development mode s within a single species. In spring, half of the adults spawned masse s containing similar to 300 eggs with a mean diameter of 68 mu m. Afte r 3 d, these egg masses hatched planktotrophic veligers with a maximum shell dimension of 116 mu m. The remaining adults spawned masses cont aining similar to 30 eggs with a mean diameter of 105 mu m. These egg masses hatched after 5 to 6 d, releasing lecithotrophic larvae with a maximum shell dimension of 186 mu m. About 1% of field-collected adult s produced mixed clutches containing a continuum of larval sizes, span ning the size extremes of planktotrophy and lecithotrophy and hatching larvae with a mean maximum shell dimension of 152 mu m. Adults produc ing planktotrophic and lecithotrophic larvae were interfertile, and no hybrid breakdown was observed through the F3 generation. When starved , adults which previously produced only lecithotrophic larvae switched to producing planktotrophic larvae or mixed clutches with both plankt otrophic and lecithotrophic larvae. Sequence-polymorphisms from a port ion of the mitochondrial cytochrome c oxidase I gene support the concl usion that the two reproductive morphs represent a single species. Mos t of the lecithotrophic larvae and a small percentage of the larvae fr om mixed clutches were metamorphically competent within 3 d of hatchin g. A. modesta is the only molluscan species as yet known to have both planktotrophic and pelagic lecithotrophic development within a single natural population.