. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 230 A. L. MORAN hance performance during development in the egg capsule (Fretter and Graham, 1962; Lyons and Spight, 1973; Had- fieldand laea. 1989). Egg capsules protect offspring (Shuto, 1974; Spight, 1977; Pechenik, 1984; Hawkins and Hutchinson, 1988; Rawlings, 1990, 1996) and retain progeny within suitable adult habitat (Wells and Wells, 1962; Chapman. 1965; Pechenik, 1979); in many species, capsules also provide a nutrient-rich environment for developing embryos (Fretter and Graham, 1962; Fioroni, 1977, 1988). Larvae of
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 230 A. L. MORAN hance performance during development in the egg capsule (Fretter and Graham, 1962; Lyons and Spight, 1973; Had- fieldand laea. 1989). Egg capsules protect offspring (Shuto, 1974; Spight, 1977; Pechenik, 1984; Hawkins and Hutchinson, 1988; Rawlings, 1990, 1996) and retain progeny within suitable adult habitat (Wells and Wells, 1962; Chapman. 1965; Pechenik, 1979); in many species, capsules also provide a nutrient-rich environment for developing embryos (Fretter and Graham, 1962; Fioroni, 1977, 1988). Larvae of many gastropods with mixed or entirely encapsulated develop- ment have transitory structures that may represent special- izations for consumption of nutritive materials such as al- bumen or nurse eggs (Portmann. 1955; Portmann and Sandmeier, 1965; Gather and Tompa, 1972; Lyons and Spight, 1973; Rivest. 1983, 1992; Rivest and Strathmann, 1995). However, in many species the mechanisms of con- sumption of intracapsular nutrition are poorly known. Like- wise, the extent to which planktotrophic larval characters have been modified in species with nonplunktonic, encap- sulated larvae is not fully understood, and has not previ- ously been addressed by comparing closely related taxa with contrasting developmental modes. The gastropod genus Littorina, the periwinkle snails, contains ~ 19 species found in the high-shore zone through- out the northern Atlantic and Pacific oceans (Reid, 1989; Reid et al., 1996). The genus contains both planktotrophic and nonplanktotrophic species (see Reid, 1989, for review); molecular phylogenetic evidence supports planktotrophy as the ancestral state within the genus (see Rumbak et al., 1994). All species of Littorina undergo early development in egg capsules. Planktotrophic species have "mixed" de- velopment: early developmental stages are contained in complex pelagic capsules (Fig. 1A), within which each larva is encased in an individual egg
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