Unexpected discovery of Diadema clarki in the Coral Triangle

Sea urchins of the genus Diadema, key herbivores in coral reef ecosystems, also provide habitat for other organisms. Our research extended Diadema biogeography in seaways east and west of Sulawesi and identified Diadema species associated with the endemic Banggai cardinalfish (Pterapogon kauderni) using field surveys and molecular DNA barcoding methods. Field observations (20 sites, n = 11,223) found urchins with morphological phenotypes typical of Diadema setosum (≈ 74%, all sites), D. savignyi (≈ 24%, 19 sites) and atypical or mixed traits (≈ 2%, 19 sites). Distribution of these phenotype groups across the three main habitat types (i.e. coral reef, reef flat and seagrass beds) differed significantly (χ2 = 533.03, p < 2.2e−16), indicating overlapping but non-equivalent ecological niches. Pterapogon kauderni associated with all urchin morphological phenotypes present. Diadema mtDNA CO1 sequences were obtained from tissue samples collected (4 sites, n = 62) from specimens with typical D. savignyi and D. setosum phenotypes. Phylogenetic tree analysis resolved the sequences into four clades. Three clades from our analysis were identified as D. savignyi, D. setosum and D. clarki based on additional sequences obtained from GenBank. This unexpected first record of D. clarki mtDNA in the Coral Triangle implies a substantial extension of the known range of this recently resurrected species. Our findings indicate the occurrence and/or introgression of D. clarki may be widespread, and misidentification of Diadema urchins based on external morphology may be relatively common. Further research is required to determine the distribution and functional roles of Indo-Pacific Diadema species, contributing to our understanding of processes underpinning biodiversity.