A REVIEW OF LARVAL DEVELOPMENT IN THE INTERTIDAL LIMPET GENUS SIPHONARIA (GASTROPODA, PULMONATA)

A compilation of distributional and life-history data relating to mode of larval development is presented for 26 species of Siphonaria, a genus of intertidal pulmonates. Most species deposit gelatinous benthic egg masses with two species releasing pelagic egg masses. Fifteen species hatch as planktonic-developing larvae, nine hatch as direct-developing juveniles, and in a further two larvae hatch with both the swimming velar apparatus (associated with planktonic development) and a crawling foot (associated with direct development). Data on mode of larval development are interpreted with respect to same adaptive models. Despite important exceptions, there is support for adaptive models based upon egg capsule size (direct developers hatch from larger egg capsules) and intertidal distribution (direct developers generally occur higher on the shore than planktonic developers). Worldwide, planktonic developers are more widespread than direct-developing species, and individual planktonic species have a greater mean latitudinal range. The evidence for adaptive models relating latitudinal distribution to developmental mode is equivocal. There appears to be no clear relationship between body size and developmental mode in the genus, although the smallest species has direct development and the largest has planktonic development. Tn most siphonariid subgenera, developmental mode appears to be constant, but two subgenera contain a mixture of developmental types.