Spatial and temporal variability of the opisthobranch molluscs of Port Lligat Bay, Catalonia, NE Spain

Abstract

Studies on the community structure and population dynamics of opisthobranchs are very scarce. This is probably related to the fact that the number of specimens is usually low and field work is difficult in the habitat where they occur. The spatial and temporal variability of the opisthobranch molluscs inhabiting Sant Antoni beach in Port Lligat Bay (a shallow rocky shore near Cadaqués, Costa Brava, NE Spain) were studied quantitatively from February 1997 to January 1999. This shore was chosen because in previous qualitative studies relatively large populations of some species were found there. Samples were taken monthly during the two years and the sampling consisted of 5 stations along two transects (one perpendicular to the shore, the other one in parallel). At each station, 3 samples were taken (3 independent square meters). In every square, 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 also measured when present. In this quantitative study, a total of 138 specimens, belonging to 26 species were found. The nudibranch Aldisa banyulensis was the most abundant species, with higher numbers of specimens at the end of winter and spring, and the largest specimens found during spring time. Egg masses of A. banyulensis were found in late spring-early summer during the first year. The nudibranch Dendrodoris limbata also was abundant and present through the year. The largest specimens of D. limbata were found during winter and spring. Abundance and species richness of opisthobranchs was significantly different at the five stations studied, probably due to contrasting physical conditions and prey abundance. Qualitative and quantitative studies provide similar values of species richness but different abundances. In total, 45 opisthobranch species are now reported for Sant Antoni beach.