Larval microhabitat associations of the non-native gastropod Crepidula fornicata and effects on recruitment success in the intertidal zone

Habitat-specific distributions of marine benthic invertebrates can be caused by several processes acting prior to, during or after settlement, including differential settlement and varying levels of mortality between habitat types following adaptation of the benthic mode. The non-native gastropod Crepidula fornicata is known for its gregarious settlement patterns, yet associations with other shellfish species are also common. In the present study, a series of no-choice and choice laboratory assays were undertaken in which larvae were offered different settlement substrata, separately and simultaneously, to investigate whether differential settlement of C. fornicata larvae occurs in favour of specific microhabitat types. A field experiment was also conducted to test if recruitment success in the intertidal differed between microhabitat types, by comparing densities of young (< 2 weeks) and older (< 8 weeks) settlers. The laboratory studies indicated that settlement occurs in larger numbers in association with certain habitats. However, settlement in association with specific microhabitat types was not observed in the intertidal. Instead, the distribution of C. fornicata recruits is established after settlement, as the distribution of older recruits, but not younger ones, differed between microhabitat types. Our findings show that the availability of certain complex structures in the intertidal zone is highly important in determining survival success of C. fornicata, due to varying levels of post-settlement mortality.