The effect of grain size on the burrowing of two Donax species

Donax serra and Donax sordidus are bivalves inhabiting the intertidal area of sandy beaches in the Eastern Cape, South Africa. This study investigated the effect of grain size on the burrowing performance of these two species in the laboratory to elucidate the influence of grain size on the longshor e distribution of Donax populations and, consequently, the importance of pa rticle diameter in swash exclusion. Burial time, burrowing cycles and a bur rowing rate index were obtained for both species in nine well-sorted sedime nts with grain sizes ranging 90-2000 mum. The burial times of both D. serra and D. sordidus were positively correlated with shell length. Burrowing pe rformance of both species was influenced by grain size. Fastest burrowing t imes were measured in the fine and medium sediments (125-500 mum) but incre ased towards the very fine (90-125 mum) and coarse extremes (500-2000 mum). Burial time experiments were extended to test how D. serra would behave in sediments with more natural properties and were, therefore, repeated in mi xed sands of different sortings. Sediment sorting had a positive influence on the burrowing times of D. serra, Burrowing time was fastest in well-sort ed sediments and slowest in moderately sorted sediments, especially those c ontaining fractions of sands > 500 mum. Burial times approximating the swas h periods on dissipative beaches (similar to 32 s) were measured for grain sizes < 500 mum. It is, therefore, predicted that most D. serra individuals are able to burrow between swashes on dissipative beaches. However, only t he smallest individuals (similar to 15 mm) would be able to burrow between swashes on reflective beaches with swash periods approximating 15 s, The sm aller (maximum) size of D. sordidus reduces burial time and should, therefo re, enable this slower burrowing species to bury successfully under most di ssipative/intermediate conditions. (C) 2001 Elsevier Science B.V. All right s reserved.