Perils of drifting encapsulated embryos of the periwinkle Littorina scutulata from failures at launch and unscheduled landings

Abstract

Unusually for benthic animals, mothers in many species of littorinid snails release drifting egg capsules that experience hazards of either the plankton or seafloor. At a site sheltered from large waves, numerous egg capsules of Littorina scutulata remained in high intertidal pools near mothers, and the embryos died. Mortality was also high for embryos in capsules caught in high intertidal clumps of algae. Capsules in shallow water were at early cleavage stages, indicating exposure to intertidal predators only for the hours of early development. In the laboratory, small intertidal pagurid hermit crabs and grapsid crabs ate capsules, and barnacles captured capsules, though with ingestion uncertain. An anemone and a mussel were not hazards. Embryos in capsules exposed to microbial and physical hazards on submerged sediments suffered little mortality, in contrast to previously observed high mortality at the same site for embryos of an echinoid. Predation rates could not be estimated for any location, but other hazards were less on low intertidal and subtidal sediments than in the high intertidal zone. The number of embryos per capsule and capsule sizes varied within a mother’s spawn, between spawns of mothers of similar size, and with maternal condition. Packaging several embryos in a capsule did not increase sinking speeds much beyond sinking reported for planktonic single unhatched blastulae in other taxa. Some embryos died through failure to launch, but capsules provided low sinking speeds during drift, brief residence in shallow intertidal waters, and protection from microbial and chemical hazards on sediments when sunk.