Small-scale distribution of a cnidarian population in the western Mediterranean

Abstract

The neritic area of the coast of Catalonia (north-west Mediterranean) has a continental shelf narrower in the northern half and broader in the southern half. This area is crossed from north to south by a coastal current whose intensity decreases to the south. Both conditions, together with other local geographical and hydrographical features, are thought to be the key factors in determining the distribution of planktonic cnidarian assemblages, along two main variational axes: a north-south axis and a coast-open sea axis. Aiming to find support to this hypothesis, the planktonic cnidarians collected in two seasons of the year 1983 have been studied. The first season (April-May) corresponds to the highest abundance and heterogeneity, and the second (September-October), to the epoch when the cnidarian population is lower, but constant, along the coastal area. In a global principal component analysis the factor that best explains the group distribution in the area is the coastal character, as given by the bottom depth of the sampling station. The number of individuals is greater near the coast (due to the dominance of a euryhaline species, Muggiaea atlantica), but the species number increases towards the open sea (due to the addition of more oceanic species, such as Narcomedusae and Trachymedusae and the effect of a permanent hydrographic front at the shelf-slope boundary). The second factor is the latitudinal axis. The northern half of the coast is under the influence of waters favouring a greater number of species and individuals. On the contrary, the southern half shows a less marked influence of the north-south current, their cnidarian assemblages being more constant over time. Although the global cnidarian population follows this general pattern, the most abundant and frequent species (M.atlantica and Aglaura hemistoma) show a broad distribution over all the studied area because they are rather insensitive to hydrographica l variations.