SEASONAL DISTRIBUTION OF NUDIBRANCHS IN THE SOUTHERN DELTA AREA, SW NETHERLANDS

Between January 1990 and December 1991 quantitative data on the life cycles, distribution and substrata of the nudibranchs in the southern Delta area (Netherlands) were collected using SCUBA. Six locations were sampled each month. A total of 17 species of nudibranchs was found, of which 10 species were considered common. Species with more than one generation each year, which fed on seasonally variable resources, were strongly associated with their dietary species and their distribution was dependent on the distribution of their dietary species. Nudibranch species with annual life cycles, which fed on organisms abundant throughout the year, did not show such dependency on their dietary species. The seasonal occurrence of species with annual life cycles was influenced by (temporal) changes in temperature, in contrast to the species with more generations each year. The total number of the species which fed on seasonally variable resources was not affected by changes in temperature, but by changes in abundance of the dietary species. Because both the distribution and population size of most nudibranch species are dependent on their dietary species, nudibranch predation did not seem to affect community structure.