Ra. Wright et al., SELECTIVE PREDATION BY BLUE CRABS ON THE GASTROPOD, BITTIUM-VARIUM - CONFIRMATION FROM OPERCULA FOUND IN THE SEDIMENTS, Estuaries, 19(1), 1996, pp. 75-81
Small blue crabs (Callinectes sapidus Rathbun, 43-70 mm carapace width
) can influence the size-distribution of the gastropod, Bittium varium
, strongly reducing the contribution of snails >3 mm shell length in f
ield enclosures, We test the hypothesis that these size-dependent effe
cts are due to size-selective predation rather than size-dependent emi
gration from the field enclosures. In laboratory feeding trials, blue
crabs showed negative selectivity for snails <2.5 mm and positive sele
ctivity for snails >3.3 mm. When feeding, blue crabs crush Bittium she
lls, but the opercula are deposited undamaged in the sediment, Sedimen
t from a field enclosure experiment contained 6.5x more Bittium opercu
la from enclosures with blue crabs than from enclosures without blue c
rabs. We reconstructed the size distribution of Bittium killed by blue
crabs from the opercula recovered from sediment. This distribution qu
alitatively matched those ''missing'' from the size frequency distribu
tion of surviving snails. Estimates of selectivity from laboratory fee
ding trials predicted the pattern of size-selection from Bittium kille
d in the enclosures. We also estimated Strauss's linear index of selec
tivity and Chesson's alpha based on the size distribution of snails av
ailable during the field enclosure experiment. These indices predicted
both the pattern of selectivity and the size distribution of Bittium
killed in enclosures with blue crabs. We conclude that size-selective
predation by blue crabs can explain the observed shifts in Bittium siz
e distributions.