SMALL-SCALE SPATIAL HETEROGENEITY AND SEASONAL-VARIATION IN A POPULATION OF A CAVE-DWELLING MEDITERRANEAN MYSID

Seasonal changes in small-scale spatial distribution patterns of abund ance and composition of Hemimysis speluncola Ledoyer (1963) (Crustacea , Mysidacea) were studied in a population of cave-dwelling mysids off the Medes Islands (NW Mediterranean Sea). The distribution pattern was characterized all year round by marked spatial segregation of juvenil es, which occupied areas closer to the cave entrance, while adults wer e concentrated mainly in the innermost part of the cave. The small-sca le spatial heterogeneity observed appears to be regulated by biologica l factors, particularly social and reproductive behaviour. Nevertheles s, a certain adaptation of the distribution of the swarms to hydrodyna mic factors suggests that physical factors may also play a role. The h eterogeneous aggregation pattern recorded would clearly appear to be a n adaptive strategy by the population to enable it to thrive within it s habitat. That adaptation is designed to reduce predation mortality, enhance mating efficiency and regulate the population generally. Marke d seasonal fluctuations in the density and composition of the populati on were recorded. with high density levels in winter and throughout th e spring. Two methods were employed to quantify the population: hauls with plankton nets towed by divers and collection of faecal pellets. T he patterns observed using both these methods were similar, although t he latter method yielded total density values that were an order of ma gnitude greater.