Substrate exploitation and resistance to biotic disturbance in the brachiopod Terebratalia transversa and the bivalve Pododesmus macrochisma

To test the hypothesis that epifaunal bivalves are more resistant to biotic disturbance than brachiopods, I evaluated abundances and body size of the brachiopod Terebratalia transversa and the anomiid bivalve Pododesmus macrochisma, which co-occur in the San Juan Islands (Washington State, USA, eastern Pacific). The proportion of bare space correlated with intensity of biotic disturbance had negative effects and surface rugosity had positive effects on abundance of T transversa. Both rugophilic settlement and post-settlement mortality of juveniles due to biotic disturbance restrict T transversa to crevices and complex substrates formed by solitary ascidians and giant barnacles. Its juveniles are under-represented on flat substrates, implying lower survivorship than in crevices, where juveniles are common. Abundance of the byssally-cemented bivalve F macrochisma is negatively affected by rugosity and does not decrease with increasing bare space. P. macrochisma size patterns do not differ between crevices and flat substrates. Flat valves and high post-settlement mobility probably increase its juvenile survivorship under grazing pressure. T transversa is not able to re-attach after settlement and is thus less resistant to grazing effects than P. macrochisma. This study supports the hypothesis that brachiopods are more negatively affected by biotic disturbance than epibyssate bivalves. Although brachiopods can be less preyed upon by carnivorous predators than epibyssate bivalves, T transversa is affected more negatively by grazing than P. macrochisma. The evolutionary decline of brachiopod abundance in shallow habitats may represent responses to a heavy pressure on juveniles by grazers that evolved during the Mesozoic.