Conjoined Twins, Triplets, and Quadruplets in the Gastropod Crepidula fornicata

Although molluscan conjoined larvae are rarely observed, over 600 conjoined twin, 11 triplet, and 2 quadruplet veligers were found among 125,000 normal veligers in 118 untreated egg masses produced in the laboratory over 5 months by slipper-shell snails, Crepidula fornicata. Seven twin morphologies were recorded, but two patterns (top-to-top, side-by-side) accounted for ~80% of all twins. Twinned larvae at hatching were similar in size to normal larvae but developed more slowly. Attached co-twins developed at the same rate and metamorphosed "spontaneously" on the same day. In the presence of an artificial metamorphic cue (elevated K+level), either the attached co-twins both metamorphosed or neither did. Despite these observations, it is unknown whether the twinned larvae arose by fusion of two embryos, by incomplete fission of one embryo, or by blastomere equalization during early cleavage. By the end of the study period, some twins had survived as juveniles for six weeks; those that had died appeared unable to feed following either head loss (from unequal autotomy) or shell loss. Twins occurred in both static and nonstatic cultures. Some parents produced more twins per hatch and were more likely to produce twins in successive hatches than were other parents. Although the cause of twin production in untreated capsules is unknown, many active twins were produced by briefly exposing young egg capsules to acidified seawater. Twinned larvae and twinned juveniles of C. fornicata may be useful systems for addressing questions about environmentally-mediated sex determination, early development, and metamorphic competence.