POSTER (abstract)
" SPATIAL AND TEMPORAL VARIABILITY OF THE OPISTHOBRANCH MOLLUSCS OF PORT LLIGAT BAY (NE SPAIN) " Studies on population dynamics of opisthobranch molluscs are very scarce. This is probably related to the fact that the number of specimens is usually low and field work is difficult. In order to study the spatial and temporal variability of opisthobranchs of a Mediterranean Bay, we chose Port Lligat (a shallow rocky beach near Cadaqués, Costa Brava), since in the past we had found relatively large populations of some species there. Samples were taken once a month during two years (from February 1997 to January 1999). The sampling consisted of 5 stations along two transects (one perpendicular to the shore, the other one in parallel). Stations 1 and 4 (0.5m depth) had stones with sponges, colonial ascidians and polyplacophora. Stations 2 and 3 (1 m depth) were a mixture of stones, sand and Posidonia oceanica with sponges at the base. In station 5 (3 m depth) they were mainly stones and some P. oceanica. At each one of the 5 stations, 3 samples were taken (3 independent square meters). In each one of the squares, all the stones were lifted, both sides examined, and all the opisthobranchs present were measured and left undisturbed. Only unidentified animals were collected for classification. Egg masses were measured when present.A total of 137 specimens, belonging to 25 species were found. The nudibranch Aldisa banyulensis was the most abundant species, with higher number of specimens at the end of winter and spring, and the largest specimens found during spring time. Egg masses of was the most abundant species, with higher number of specimens at the end of winter and spring, and the largest specimens found during spring time. Egg masses of A. banyulensis were found at late spring-early summer during the first year. The nudibranch Dendrodoris limbata was also abundant, present through the year, and the largest specimens found in winter and spring. A single egg mass of D. limbata was found in May of the first year. Diversity (Shannon-Weaver index) was similar during the two years of the study, but the species distribution was slightly different.
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