The influence of sediment grain size on burrowing, growth and metabolism of Donax trunculus L. (Bivalvia: Donacidae)

Abstract

The bivalve Donax trunculus L. commonly inhabits the intertidal and shallow subtidal zone of exposed sandy beaches along European and North African coasts. We examined the responses of burrowing time, respiration rate and growth of this species to sediment grain size. Burrowing time was examined for individuals from 5 to 45 mm shell length in artificially prepared sorted sediments varying from fine sand (125–250 μm) to medium sand (250–500 μm), coarse sand (500–1000 μm), very coarse sand (1000–2000 μm), gravel (>2000 μm), and native sediment (432 μm mean grain size). The shortest burrowing time for individuals between 5 and 25 mm was observed in medium and coarse sands. For individuals of 25–45 mm shell length, the shortest burrowing time occurred in fine and medium sands. The longest burrowing time for all size classes was observed in gravel. These results were analysed within the framework of the swash exclusion hypothesis. According to this hypothesis, burrowing time of D. trunculus in relation to wave period determines to what extent individuals are drifted from the zone where they preferentially live. The shorter burrowing time of smaller clams, and their ability to burrow faster than larger individuals in coarse sands suggest that juveniles may be able to withstand more reflective morphodynamic conditions than adult individuals. Effects of sediment grain size on metabolic activity and growth rates were compared by maintaining D. trunculus (26–28 mm shell length) in different sediment grades. Metabolic activity and growth rate were highest in medium and coarse grades of sand. In gravel and very coarse sand, growth rates and metabolic rates were lower by as much as 43%. Our data indicated that a small decline in metabolism was accompanied by a greater decrease in growth. Thus, our results agree with the swash exclusion hypothesis: the coarsest granulometries, predominant on the reflective type of beaches, reduce the ecological efficiency of individuals by decreasing either their burrowing rate, and/or growth and metabolism.