Predation on hardest molluscan eggs by confamilial snails (Neritidae) and its potential significance in egg-laying site selection

Neritid snails (Gastropoda: Neritimorpha) protect their eggs in a hard capsule, of tough conchiolin, reinforced by mineral particles derived from the faeces and stored in a special sac near the anus and oviduct opening. Predation on this arguably hardest of molluscan egg capsule is described and illustrated here; neritids of the freshwater to brackish-water genera Clithon and Vittina, generally classified as herbivores, feed facultatively on the eggs of various confamilial species after breaking the reinforced capsule lid by means of prolonged radular rasping. Intensive predation pressure by these common inhabitants in Indo-West Pacific coastal streams may have given rise to the remarkable egg-laying behaviour of Neritina on the shells of other living snails. Our laboratory examination showed that Neritina species deposited clusters of egg capsules more frequently on the living shell than on other substrates, and that the predation rate was significantly lower on this moving ‘nursery’. Predation rate was even lower on the small egg capsules of Clithon and Vittina themselves, which were deposited one by one in the depressions on the rough surfaces of stones.