PATTERNS OF REEF SPONGE DISTRIBUTION IN THE NORTH-WEST URABÁ GULF, SOUTHERN CARIBBEAN, COLOMBIA

Abstract

The North-West of Urabá Gulf in the Southern Caribbean supports a variety of coral reef zones in conditions of high turbulence and fluctuating turbidity and salinity. Sponge composition and density were surveyed and their relationship with physical and structural features of the reef zones were analyzed. In total, 65 demosponges and 1 calcareous sponge species were encountered along belt transects (20 m2) on 11 stations between 1 and 17 m in depth. Sponge density in Urabá tended to be higher than in other areas with water conditions optimal for coral reef development (i.e. Roncador, Serrana and Quitasueño remote oceanic atolls in the SW Caribbean, Colombia) due possibly to higher concentrations of organic suspended materials discharged from rivers. Nevertheless, this effect was not reflected in the Urabá‘s species richness as just a limited number of species (Niphates erecta, Iotrochota birotulata and Mycale laevis) had high densities. Multivariate analyses showed that sponge community composition varies along a depth gradient and across habitats (coral and geomorphological zones), similar to the patterns observed in other Caribbean areas. However, marked taxonomic differences between shallow (<4 m) and deeper (>6 m) zones and the great increase of the sponge density below just 6 m, seem to be a unique feature of Urabá. The absence of many species and the low abundance of sponges in shallow zones (< 6 m) may be due, not only to the variables associated to low depth, like turbulence, which is periodically very strong, but also to great fluctuations in surface salinity due to discharges of the Atrato and other local rivers. The shallow and deep stations that had low sponge richness and density, showed the greatest urchin densities (especially Echinometra viridis); therefore, it is possible that these urchins may be playing an additional controlling role on sponge populations.