Bancos de cholga Aulacomya atra atra (Molina) (Bivalvia: Mytilidae) del golfo San José (Chubut, Argentina): Diversidad y relaciones con facies afines

Abstract

The relationships between different assemblages of the grazer-suspensivore biocoenosis and their diversities were studied at the San José Gulf (Chubut Province, Argentina: 42°20'S; 64°20'W), with emphasis on the facies of the ribbed mussel Aulacomya atra atra. The ordination analysis of the assemblages species composition was performed using detrended correspondence analysis and the relationships of the assemblages with the environmental factors were analysed using factorial correspondence analysis. The Hill diversity indices (N0, N1, N2), and the evenness derived index (E=N2/N1) were calculated. It was concluded that: (1) The ribbed mussel beds on hard sustrate have high species richness and high equitability, because the rocks provides firm surfaces for byssal attachment and these beds are usually found in protected localities. (2) The soft bottom assemblages with the mussel Mytilus edulis platensis, under conditions of environmental stability, evolve towards mixed beds with ribbed mussel; this process implies increased sustrate complexity, increased spatial heterogeneity and higher species richness. This successional process can be reverted if the mixed bed are affected by strong physical disturbance which can dislodge the assemblages. (3) Beds of the scallop Aequipecten tehuelchus are found at sites permanently disturbed by waves generated by local winds, they have low equitability and low species richness. When the environmental disturbance is very intense the bed is dislodged and big quantities of individuals are beach stranded. (4) The mixed beds of ribbed mussel and Aequipecten tehuelchus are facies which are alternative to Aequipecten beds and they are found in protected sites.