THE MOLLUSCAN EPIFAUNA OF THE ALGA HALOPTERIS SCOPARIA IN SOUTHERN SPAIN AS A BIOINDICATOR OF COASTAL ENVIRONMENTAL CONDITIONS

Abstract

The effects of certain environmental factors on spatial variation of the mollusc community associated with the alga Halopteris scoparia in Algeciras Bay (southern Spain), located in the Strait of Gibraltar, are studied. Environmental gradients from the exterior to the interior of the bay have been detected, related above all to water movements, solids in suspension, organic content of sediments, and structural complexity of plants. These gradients are seen as differences in the composition of the mollusc communities of the outer zones (Isla de Las Palomas, San Garcma, and Cucareo) and inner ones (Guadarranque and Crinavis). Although the conditions of the inner zone restrict the growth of some groups such as crustaceans and polychaetes, mollusc populations have developed well there in both number and diversity. Species are found exclusively at the exterior of the Bay (include Cingula amabilis, Pisinna glabrata, and Nodulus contortus) and for the interior (Alvania montagui, Alvania rudis, and Rissoa similis), the latter being the most discriminating between the two major zones of the bay. The environmental parameters having the greatest effect on the molluscs associated with Halopteris scoparia are water movement, the morphological characteristics of the substrate, and most importantly the availability of food.