Fishes associating with the echinoids Diadema setosum and Echinothrix calamaris (Diadematoida: Diadematidae) in the Phi Phi Islands of southwestern Thailand

Many fish species are known to associate with echinoids. We studied the frequency of fishes associating with the diadematid Black Longspine Urchin Diadema setosum and Banded Sea Urchin Echinothrix calamaris in the Phi Phi Islands of southwestern Thailand during July 17–20, 2022. A total of 1373 fish individuals of 20 species associated with 1157 D. setosum, and 11 fish individuals of seven species associated with 76 E. calamaris. The proportion of fish individuals per echinoid host was 8.2 times higher for longer-spined D. setosum (1.19) than for E. calamaris (0.14), and the proportion of echinoids hosting one or more fish was 2.5 times higher for D. setosum (22.9%) than for E. calamaris (9.2%), supporting the hypothesis that fishes prefer to associate with echinoid species with the longest spines, which potentially provide more protection against predators. All fishes associating with echinoids were small (<10 cm); most were adults, but for some species, all were juveniles. All fishes were facultative associates of echinoids. Only three fish species had been previously reported associating with D. setosum and none with E. calamaris; the remaining fish species represent new records of association, including 17 for D. setosum and seven for E. calamaris.