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.