THE ROLE OF AUTECOLOGY, NICHE TRADEOFFS AND SPATIAL HETEROGENEITY IN COEXISTENCE OF BRACHIOPODS WITH MODERN BIOTA IN SHALLOW HABITATS

The large-sized brachiopod Terebratalia occurs in highly productive habitats with relatively high consumer pressure in shallow environments of the San Juan Islands. This occurrence contrasts with hypotheses predicting that present-day brachiopods are limited to low-productivity refuges. To resolve this paradoxical occurrence, ontogeny of Terebratalia and its abundances are analyzed on two spatial scales. Terebratalia rapidly loses its rotation ability after it achieves its preferred orientation with respect to ambient water currents. Contraction of dorsal pedicle muscles is severely inhibited because dorsal insertions shift away from the valve midline, rise above the dorsal valve floor, and become limited by a wide cardinal process in the early ontogeny. This reduced mobility might enhance survival of small-sized Terebratalia by maximizing protection against dislodgement. Both rugophilic settlement and post-settlement mortality of juveniles owing to consumer pressure restrict Terebratalia to crevices because its percent covers are separately negatively correlated with the amount of cleared space, and positively correlated with substrate complexity. Grazing effects on juveniles can be a more important factor affecting brachiopods than carnivorous predation on adults because Terebratalia juveniles are rarely found alive on exposed surfaces, size of adults is not lower on exposed surfaces than in crevices, predators in this area do not prefer Terebratalia, and Terebratalia is rare on coralline barrens. The preference of brachiopod larvae for cryptic habitats might evolve as a response to a pressure on juveniles owing to grazers that evolved during the Mesozoic. In contrast to epibyssate bivalves, brachiopods cannot re-attach after settlement and are thus less resistant to grazing effects. On meter-decimeter scales, substrate complexity and disturbance have significant effects on species abundances, and local coexistence is probably enhanced by stabilizing tradeoffs in ability to exploit complex substrates and ability to resist disturbance on smooth surfaces. On decameter scales, small-scale variations in substrate complexity are averaged out, equalizing fitness of benthic groups and enabling regional coexistence of Terebratalia with grazers.