Larval settlement preference of a native bivalve: the influence of an invasive alga versus native substrata

Recruitment patterns of marine invertebrates are strongly influenced by the habitat preference of larvae as larvae can choose to settle (or not) in response to positive or negative cues. High abundances of recruits of the native infaunal bivalve Anadara trapezia occur in the invasive alga Caulerpa taxifolia compared to native habitats. Using controlled laboratory experiments, A. trapezia larval habitat preference was investigated through exposure to available native (adult A. trapezia, Zostera capricorni and sediment from unvegetated areas) and invasive (C. taxifolia and sediments from C. taxifolia) substrata that co-occur in estuaries invaded by C. taxifolia in New South Wales, Australia. When exposed to all substrata, larval settlement was significantly higher on adults compared to all other substrata except Z. capricorni. Although settlement to C. taxifolia was low, larvae did not reject it as settlement surface. In pairwise comparisons, larval settlement was always higher on adults compared to all other substrata, although differences were only significant compared to C. taxifolia and unvegetated sediments. There was no difference in settlement when larvae were exposed to Z. capricorni and C. taxifolia. When offered a single substratum, larval settlement was significantly higher on adults and Z. capricorni compared to all remaining substrata. Manipulations of shells of adults indicated that larvae may be responding positively to biofilms on the surface of shells. The data indicate that A. trapezia larvae prefer to settle on adults and, while they do not prefer C. taxifolia, they do not reject it as a settlement surface. Therefore, C. taxifolia may serve as a sink habitat for A. trapezia.