Prodissoconch morphology, planktonic shell growth, and site at metamorphosis in Dreissena polymorpha

Larval shell morphology and morphometry of the zebra mussel, Dreissena polymorpha, were examined using scanning electron and light microscopy. Larval shell morphology is typical of bivalves having planktotrophic development. The prodissoconch I (PI) has a pitted-punctate surface contrasting markedly with the conspicuous fine concentric or commarginal lines of the prodissoconch II (PII) and dissoconch (D) shells. Length-frequency distributions of the prodissoconch, which were obtained from disarticulated valves of bleached early juveniles (0.5-2 mm) (specimens from west-central Lake Erie), indicated a mean length (umbo to ventral margin) of 76.4 and 220 mu m for PI and PII, respectively; these results corroborated those from direct measurements of early D-shaped veligers from the plankton and from early juvenile settlers from collectors (24- to 36-h deployments), respectively. Measurements of shell lengths at the PII-D boundary thus provided accurate information on size distributions of veligers at the time of settlement and metamorphosis. The estimated minimum and maximum larval shell growth or increment (PII length - PI length, or Delta P) (92 and 195 mu m, respectively), combined with Smylie's in situ larval growth rates (3 -20 mu m d(-1)), suggest a surprisingly wide range in the duration of the planktonic period of D. polymorpha in Lake Erie: 1-9 weeks.