Increased reproductive opportunity: a potential benefit of seasonal aggregation for a little-gregarious and highly sedentary spiny lobster

Abstract

The spiny lobster Panulirus guttatus is a highly sedentary, obligate reef-dweller that exhibits a low degree of gregariousness yet reproduces year-round. Previous laboratory studies revealed that these lobsters were significantly attracted to scents released by conspecifics during a ‘high reproductive activity’ period (HRA,~60% of ovigerous females on average) but not during a ‘low reproductive activity’ period (LRA,~20%), suggesting that becoming more closely distributed at the right time may increase reproductive opportunities for these lobsters. We tested this hypothesis in the field. We marked all crevices harbouring P. guttatus lobsters (‘dens’) on two isolated coral reef patches over six consecutive sampling periods (three during an LRA and three during the ensuing HRA) and recorded the number, sex and size of lobsters in each den. Dens marked at a given time were considered as harbouring zero lobsters at previous times. For each site, a model selection based on parameters of the negative binomial distribution showed that m (mean lobsters/den) varied little over time but that k (dispersion parameter) decreased across the LRA and remained low (indicating a more clumped distribution) across the HRA. These trends in aggregation were further confirmed by values of Lloyd's ‘patchiness’, supporting the hypothesis that seasonal aggregation increases reproductive opportunities for P. guttatus. However, the tendency to aggregate appeared to be modulated by density and mean size of lobsters, which differed with site. These findings have potential implications for exploited P. guttatus populations and invite further study of the complex behaviours that characterize sedentary lobsters.