ANTI-PREDATORY SPINES ON BRACHIOPODS? EXAMINING LITTLE BRACHIOPODS WITH DENTS

Many organisms possess spines but spine function has been interpreted in various ways. Bivalve spines are generally thought to be anti-predatory, however, brachiopod spines have more often been interpreted to be of use for maintaining position on substrate. Can spines also serve as an anti-predatory mechanism for brachiopods by increasing the effective size of the organism, and thus inhibiting shell-crushing predators? A size refuge is the size at which the predator will not take the prey because the prey is too large to handle. Having spines allows the brachiopod to increase its effective size without increasing the actual shell size – an energetically more efficient strategy than growing a larger shell. This increase in effective size allows the organism to achieve a size refuge at a much smaller size and earlier in life. If spines did permit brachiopods to achieve a size refuge at a smaller size, then spines may serve as an anti-predatory mechanism, as a previous study (Leighton, 2001) suggests. To test this hypothesis, two taxa of concavo-convex brachiopods, Douvillina and Devonoproductus, were collected from three beds of the Devonian-aged Lime Creek Formation of Iowa. These two genera are similar in size and shape but differ in that the latter has spines. We predict that the spinose taxon would achieve a size refuge, indicated by an abrupt drop in the frequency of predation scars, at a smaller size than the smooth taxon. A total of 99 Douvillina and 116 Devonoproductus specimens were obtained. Specimens were examined under a microscope for repair scars. The position of the scar was measured to identify the smallest size the brachiopod could have been when the attack occurred. Data analysis indicates that the spinose Devonoproductus did in fact achieve a size refuge earlier than the smooth Douvillina. This suggests that the spines were used to increase the effective size of the organism, thereby acting as a defense mechanism against potential predators.