Full of eggs and no place to lay them: hidden cost of benthic development

Abstract

The scarcity of sites suitable for attaching eggs may be a hidden cost to depositing embryos in benthic aggregations. We tested this hypothesis with the bubble-shell snail Haminaea vesicula, which lives on mud or sand but requires firm substrata for attaching its eggs. In pools on a sandflat where firm substrata were scarce, the preferred substrata were but a small fraction of the total available firm substrata. Abundant drifting green algal blades comprised 1 to 11% cover but were rarely used for egg deposition. Substrata commonly used for egg deposition totaled less than 1% cover, and included attached and drifting eelgrass, drifting branched red algae, bivalve shells, and attached green algal blades. In choice experiments, the snails preferred to deposit eggs on branched red algae and eelgrass, with bivalve shells next and green algal blades last. To test the hypothesis that substratum availability limits egg deposition, we added artificial eelgrass made of duct tape to tidepools inhabited by the snails. The mean number of egg ribbons laid m -2 in the exper- imental areas was 20 to 360 times that in control areas, and the mean number of adult snails present and egg ribbons per adult snail were greater in the experimental areas. Also, more egg ribbons per adult were deposited on artificial eelgrass when it was positioned away from natural eelgrass patches, indicating that distances of a few meters affect availability of substrata for deposition. The results imply that in this habitat the snails travel to scarce substrata for egg deposition, with whatever costs in time, energy, or risk that may be associated with travel. Scarcity of suitable sites for benthic egg masses can limit an apparently safe mode of development. Habitats that regularly support adult populations can differ greatly in suitability for egg deposition, but this limitation may not be appar- ent without experimental intervention.