Abundance,diversity and temporal variability in a California intertidal nudibranch assemblage

A 40-month study of a nudibranch assemblage within a 250 m² intertidal area at Asilomar State Beach, California, USA, indicated that dominance was shared among 9 species which comprised 87% of the total number of individuals enumerated. These 9 species also showed high frequency of occurrence in the area. The number of nudibranch species counted per month was shown to be independent of weather and tide conditions, but the numbers of individuals and diversity values were correlated with wave action, lower values and numbers being tabulated during times of storms. Cumulative plots of diversity and species number based upon 15 min enumeration times indicated adequate assessment of diversity and species number after 60 min of sampling. The number of species and individuals and the diversity varied from month to month over the 40 months, but these variations were shown to be statistically insignificant or to be correlated with weather. Hence, it was concluded that the assemblage was a stable one, structured primarily by the 9 dominant species. No statistically significant seasonal or annual changes were observed in the assemblage. Diversity was found to correlate most highly with species number and less with number of individuals. The average number of species found per month was 13, the average number of individuals 104, and the average monthly diversity (HH}) was 1.82. Significant positive and negative correlations between abundances of certain species were found. These were attributed to occurrence of prey items or to synchrony of reproduction. There was no evidence of migration, and resident status for one dominant species was established. Comparison of the present study with earlier work suggested that a stable nudibranch assemblage was not merely a local phenomenon.