The role of surface fouling in the development of encapsulated gastropod embryos

Surface fouling is a potential environmental stress for intertidal egg masses and may be detrimental to encapsulated molluscan development. This study examines the influence of egg mass structure (capsular vs gelatinous) and spawning habitat (shaded, partially shaded and full sun) on the amount of surface algal fouling on gastropod egg masses collected along the southeastern Australian coast. In addition, we examine the effects of surface fouling on embryonic mortality and encapsulation period. Egg masses from 18 species were collected and subjected to standardized aquarium conditions under UV-blocking filters. Algal fouling levels were calculated visually (F_v) and biochemically through chlorophyll content (F_c), but only visual quantification was deemed appropriate. The presence or absence of protists was also recorded. The level of fouling on the egg masses was not related to the habitat in which they were spawned. Gelatinous egg masses were fouled by both algae and protists at a significantly higher rate than leathery capsules. Egg masses colonized by protists had a higher level of algal fouling, and overall these egg masses had a significantly higher incidence of embryonic mortality. No effects of protists were observed on the period of encapsulation. When the effect of algal fouling on embryonic mortality was analysed for each species separately, significant positive relationships were found for several species. As F_v increased, the embryonic mortality of Conuber sp., Austraelois ornata, Bembicium nanum and Hoplodoris nodulosa increased. Algal fouling was significantly correlated with encapsulation period in only two species. As fouling increased, encapsulation period decreased for Aplysia sydneyenis and Dendrodoris fumata. Overall, this study reveals that both surface colonization and the effects of fouling on gastropod encapsulated development are species-specific.