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

Abstract

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. Additional key words: metamorphosis, veligers, pH effects, Mollusca Reports of conjoined twins among molluscs are rare. Newman (1923) reviewed all categories of one-egg twinning known in the animal kingdom, and wrote have never seen a reference to a case of twins or double monstrosity in Mollusca .... Apparently the first records of molluscan single-egg twins were in the bivalve Cumingia tellinoides (Titlebaum 1928; Tyler 1930). Since then, duplicate embryonic or larval structures have been induced artificially from other molluscan eggs including those of the bivalves Pholas dactylus and Spisula subtruncata (Guerrier 1970b), the prosobranch gastropod Ilyanassa obsoleta (Ghareeb 1971 in Render 1989; Render 1989) and the scaphopod Dentalium vulgare (Guerrier et al. 1978). All of the above molluscan embryos normally have a first cleavage that produces two blastomeres of unequal size. If first cleavage was equalized by pressure, chemicals, or other techniques, some embryos developed into conjoined twin embryos or larvae. In addition to single-egg twinning, conjoined molluscan embryos may arise by fusion of two embryos. According to Morgan (1927) the first report of molluscan double monsters produced by fusion was by de Lacaze-Duthiers (1875) for the opisthobranch Philine aperta. Heymons (1893 in Conklin 1897) also reported fusion in opisthobranch embryos (Umbrella sp.) and said that fusion could occur late in development, even after formation of the foot anlage and shell-field invagination. Conklin (1897) observed double or multiple embryos in Crepidula but they did not develop. Later, Conklin (1917) stated that fusion could be induced in the gastropod Crepidula plana, but he did not provide protocols and did not diagram or discuss fused embryos beyond the eight-cell stage. While researching the heritability of growth rate and the onset of metamorphic competence in the marine prosobranch Crepidula fornicata (Pechenik et al., in press), I found substantial numbers of larvae with more than the typical number of structures. This is apparently the first study of conjoined twin, triplet, and quadruplet veligers and twin juveniles arising in a molluscan species without experimental manipulation. I report the basic morphology of each major twin type. Because twinned C. fornicata may be useful models This content downloaded from 157.55.39.104 on Wed, 28 Dec 2016 18:06:19 UTC All use subject to http://about.jstor.org/terms