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 microscop y. Larval shell morphology is typical of bivalves having planktotrophi c development. The prodissoconch I (PI) has a pitted-punctate surface contrasting markedly with the conspicuous fine concentric or commargin al lines of the prodissoconch II (PII) and dissoconch (D) shells. Leng th-frequency distributions of the prodissoconch, which were obtained f rom disarticulated valves of bleached early juveniles (0.5-2 mm) (spec imens from west-central Lake Erie), indicated a mean length (umbo to v entral margin) of 76.4 and 220 mu m for PI and PII, respectively; thes e results corroborated those from direct measurements of early D-shape d veligers from the plankton and from early juvenile settlers from col lectors (24- to 36-h deployments), respectively. Measurements of shell lengths at the PII-D boundary thus provided accurate information on s ize distributions of veligers at the time of settlement and metamorpho sis. The estimated minimum and maximum larval shell growth or incremen t (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.