Coralline algal rhodoliths enhance larval settlement and early growth of the Pacific calico scallop Argopecten ventricosus

Abstract

The features of heterogeneous rhodolith beds (maerl) that contribute to their role as nursery habitats in coastal ecosystems are poorly understood. Rhodoliths are branched, unattached coralline algae that form complex benthic substrates and support diverse communities worldwide. Pacific calico scallops Argopecten ventricosus = circularis (Sowerby II, 1842) occur in high densities in rhodolith beds in the Gulf of California, Mexico. In this study, we found that Lithophyllum margaritae rhodoliths enhanced larval scallop settlement and early post-settlement growth, and examined the settlement cues responsible. In both field and laboratory experiments, larval settlement was signifi- cantly higher on (1) rhodolith derived vs. non-coralline sedimentary substrates, (2) living vs. non- living coralline surfaces, and (3) substrates with higher (whole, branching rhodoliths) vs. lower (rhodolith fragments or sediment) structural complexity. In the field, larval settlement (mean ± SE) onto rhodoliths was 30 to 35 times higher on live rhodoliths (55.0 ± 13.4 and 84.4 ± 8.8 larvae cm -2 ) than on non-carbonate sediment (1.8 ± 0.8 and 2.4 ± 0.9 larvae cm -2 ) relative to the surrounding rhodolith or sand habitat, respectively. In a laboratory preference experiment, when comparing live vs. dead coralline surfaces respectively, settlement density was 3.3 times greater (55.8 ± 14.6 vs. 17.0 ± 4.9 larvae cm -2 ) on whole rhodoliths and 7 times greater (24.2 ± 4.7 vs. 3.4 ± 1.3 larvae cm -2 ) on frag- mented rhodoliths. The strong cueing to live coralline surfaces may have resulted from live coralline algal surfaces or surface biofilms. Growth, presented as post-settlement size, was significantly greater in scallops that settled onto whole vs. fragmented rhodolith substrates for both live (246.6 ± 1.9 vs. 238.9 ± 4.4 µm) and dead (244.2 ± 2.8 vs. 234.7 ± 5.6 µm) coralline surfaces. The structural and coralline cues provided by live, intact rhodoliths and their large-grained sediments contribute to the importance of rhodolith beds as nursery habitats by increasing both scallop settlement and post- settlement growth. Protection of living rhodolith habitats can enhance scallop and other invertebrate populations as well as the sustainability of scallop fisheries by enhancing early life stages.