Search

The structure of gastropod communities was examined from January to June 1999 in four sites of the streams of Mont Saint-Michel Bay along a gradient of salinity, and the occurrence of larval trematodes infecting snails was studied. Abundance and species richness of gastropods increased from polyhaline (95 snails, 1 species) to oligohaline waters (6672 snails, 6 species). Whatever the salinity, the most abundant species was Potamopyrgus antipodarum, an invasive non-indigenous species that represented 80% of the gastropods. Only one male was found in P. antipodarum populations suggesting a predominantly parthenogenetic mode of reproduction. Among 7218 gastropods collected, 1.2% were infected by larval trematodes: 5 species in Lymnaea peregra (4.4%), 4 species in Planorbis planorbis (12.0%), one echinostome in Physa acuta (0.2%), and a new species of Sanguinicola in P. antipodarum (0.5%). This is the first record of infected P. antipodarum in Europe. No parasites were found in polyhaline waters. The prevalence per host population varied from 0 to 100% depending on time of collection, salinity and host species. In the lowest-salinity site, abundance of gastropods and prevalence of trematodes were negatively correlated. The dominance of P. antipodarum in the gastropod communities is discussed in relation with euryhalinity, parthenogenesis and weak rate of parasitism.

The diet of the cuttlefish Sepia officinalis L. was studied from stomach contents. This analysis was carried out on monthly samples throughout the whole biological cycle in the Northern Bay of Biscay focusing on 3 areas: a breeding and hatching area (Morbihan Bay), a winter area (off Belle-Ile island) and a transition area (Morbraz). The list of prey species showed that Sepia officinalis an opportunistic predator with a flexible diet that allows it to survive when its usual prey is unavailable. The diet is discussed in terms of the repletion ratio (R.R.: number of stomachs with remains/number of stomachs examined as %) and the monthly frequency of a prey species. Results show that R.R. was minimal during reproduction (around 25%) whereas during the rest of the life cycle it was higher than 50%. Amphipods (Phtisica marina) are the main prey for the first 3 months. During the growth period, fish (Ammodytiae, Gobiidae, Gadidae) occupied the first place in the diet. In animals returning to coastal waters for reproduction, the diet changed again and crustacea especially Brachyura (Carcinus maenas, Liocarcinus arcuatus) became the most important prey.