Abstract
The structure of decapod crustacean assemblages living in shallow waters (5 to 25 m depth) in an area of the Alboran Sea (Mediterranean Sea) neighbouring the Strait of Gibraltar was studied. The relatively high richness found in this area is probably due to the diversity of substrata and the confluence of Atlantic and Mediterranean waters. The most abundant species was the hermit crab Diogenes pugilator due to the prevalence of sandy substrate, the shallow depth-range, and the species’ life history. The coralligenous bottom showed the highest species richness and diversity values due to the complexity of microhabitats in this type of bottom and probably due to the food flux associated with currents. Conversely, shallow, well calibrated, fine to medium sandy bottoms of 5 m had the lowest values and the maximum densities due to the high abundance of a few well adapted species. The analysis of the different stations showed significant spatial differences according to depth and sedimentary characteristics. Depth is the environmental variable that correlates best with the decapod assemblages, with a particularly significant boundary between 5 m and 15 m. Nevertheless, there was a continuous transition between the assemblages. These results evidence the importance of quantitative studies in differentiating decapod assemblages. Relationships between these assemblages should also be taken into account in coastal management, since altering a substrate could have repercussions for the structure of the communities of neighbouring substrates. No seasonal significant differences were found in the overall analysis of the area, but there were differences between spring and autumn and spring and summer in relation to depth and substrate (crossed analysis). Finally, we present the species which allow us to discriminate the different assemblages according to sediment and depth, as well as the species’ contributions.
Highlights
The Alborán Sea is a biogeographically remarkable area due to its location within the Atlantic-Mediterranean Province (Ekman, 1953) and Lusitanian Province (Briggs, 1974)
The major hydrological features of this sea are the mixture of Atlantic and Mediterranean water masses with different physicochemical characteristics, its peculiar hydrodynamic movement (Lacombe and Tchernia, 1972; Lanoix, 1974; Parrilla and Kinder, 1987; Tintoré et al, 1991), and the Almeria-Oran semipermanent hydrographic front (Millot, 1987, 1999)
In spite of its biogeographical interest, there have been few studies on the structure of decapod assemblages of the different shallow bottoms in the Alborán Sea area (García Raso, 1987, 1990; García Raso et al, 1996, 2006a), and none of them are devoted to the western sector. This is why this study aims to describe the structure of decapod communities in the vicinity of the Strait of Gibraltar, and establish the interactions between the different assemblages
Summary
The Alborán Sea (southeast Spain) is a biogeographically remarkable area due to its location within the Atlantic-Mediterranean Province (Ekman, 1953) and Lusitanian Province (Briggs, 1974). Upwelling (Cano, 1977, 1978) supplying nutrient-rich water to the area causes higher plankton richness when compared to other Mediterranean areas (Vives et al, 1975; Greze et al, 1985; Delgado, 1990) All these factors and their temporal variations determine the structure and composition of animal communities and differences in biomass distributions (Rodríguez, 1983; Camiñas, 1983). Four sectors have been proposed within the Alborán Sea (according to the composition and biomass of fish larvae (Rubín, 1997; Rubín et al, 1997)), and are delimited by the 36oN latitude and 4oW longitude This longitude coincides with our study area, where it is possible to recognise an Atlantic influence (e.g., in macroalgal composition, Bellon, 1921; Conde and Seoane, 1982; Flores, 1989). Some of these species may be present as a consequence of temporary or accidental introductions resulting from the surface current through the Strait of Gibraltar, as happens with some deep species (Cartes et al, 2000)
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