Assemblages of cryptic animals in coral rubble along an estuarine gradient spanning mangrove, seagrass, and coral reef habitats

Small cryptic animals utilize the interstices of coral rubble as a refuge from predators and form a significant part of the diversity and productivity of coral reef ecosystems. In the present study, we examined the composition of cryptic mobile animals along an estuarine gradient within a mangrove-seagrass-coral reef seascape off Ishigaki Island, southwest Japan. We used mesh traps with defaunated coral rubble to sample cryptic mobile animals quantitatively at four sites: upper-estuary mangrove, lower-estuary mangrove, seagrass bed, and coral lagoon. Sixty-five species of decapod crustaceans, gastropods, fishes, and other mobile animals were identified. The assemblage composition of these identified species was significantly different among the four sites. Low-resolution taxa including copepods, gammarids, and other taxa also showed significant differences in composition among the four sites. Two types of rubble (branch and lump) were used to examine the possible effects of microhabitat structure, but no significant effect on assemblage composition was detected. Results indicate that, despite the close proximity and high degree of water mass mixing among the sites, cryptic animals colonized the trays from the surrounding habitats, and that the assemblage of cryptic animals varied considerably along the estuarine gradient.