Abstract

Four species of archeogastropods, presumed members of three subfamilies of the trochidae, exhibit significant differences in developmental modes and shell coiling. All four species have lecithotrophic development which is reflected in their inflated pausispiral protoconchis; however, Margarites marginatus and Lirularia succincta have benthic development in gelatinous masses, while Margarites pupillus and Calliostoma ligatum have pelagic embryos and swimming larvae with a potential for dispersal over a period of a week or longer. These modes cannot be deduced from the size of the egg, the size or shape of the protoconch, or the size or relative prominence of female pallial reproductive structures. The protoconch of C. ligatum is orthostrophically coiled, but the protoconchs of the other three species are hyperstrophically coiled although their teleconchs are orthostrophic. These three trochoidean species thus share with architectonicoideans, pyramidelloideans, opisthobranchs, and pulmonates the distinctive shell character of heterostrophy, previously unreported for archaeogastropods.

These observations, considered together with others reported in the literature, show: (1) that developmental mode (pelagic or benthic) cannot be inferred from protoconch appearance or taxonomy in major trochoidean ganera; (2) that significant dispersal potential is present in the histories of some trochoidean archeogastropoids; and (3) that character sets (pallial reproductive structures, pairing during spawning, heterostrophic shell coiling) thought not to occur below the mesogastropod level are found in the Trochoidea. These conclusions bring into question the usefulness of these characters in defining higher gastropod taxa and raise additional questions concerning the ancestry of the higher gastropods.

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