Testing the hypothesis of tolerance strategies in Hiatella arctica L. (Mollusca: Bivalvia)

Abstract

The physiological and biocenotic optima of Hiatella arctica L. inhabiting shallow water fouling communities of the White Sea were compared. The biomass and proportion of H. arctica in communities were used for the estimation of biocenotic optima or community success. The physiological state of populations was assessed by means of the fluctuating asymmetry. The fluctuating asymmetry of H. arctica was calculated using the valve weights. It was determined that the shell of H. arctica possesses a slight directional asymmetry, the right valve usually being larger (and heavier) than the left one. The relationship between fluctuating and directional asymmetries is discussed. High biomass and proportion of H. arctica in the community generally correspond with high levels of fluctuating asymmetry. Thus, a discrepancy between physiological and ecological optima is observed, which is recognised as being characteristic of a tolerance strategy. However, in the absence of pressure from major filter-feeding competitors such as the mussel Mytilus edulis L. and the solitary ascidian Styela rustica L., populations of H. arctica possess a high biomass and low levels of fluctuating asymmetry. It appears that H. arctica prefers to inhabit shelters or depressions, or to be covered by other organisms. Populations of H. arctica existing in the absence of shelter had extremely high levels of fluctuating asymmetry despite the absence of filter-feeding competitors. Thus, the strategy adopted by H arctica L. in the investigated upper 5 m layer of water in the White Sea can be described as a stress- and competitor-tolerant strategy.