Genetic structure of a population of “peanutfish” (“dragonfish”) sea cucumbers using mitochondrial COI sequences in Marau, Guadalcanal, Solomon Islands

“Peanutfish” (“dragonfish”) is a common name for several species of commercially valuable sea cucumbers in the tropical Indo-Pacific. A “peanutfish” population composed of morphologically diverse individuals was discovered in Hatare, Marau, Guadalcanal, Solomon Islands (9.87°S, 160.82°E). To investigate the species composition and genetic structure of the “peanutfish” population, sequences of 464 bp including mitochondrial cytochrome c oxidase subunit I (COI) were analyzed during 2011–2012. Sequences were from 90 individuals, including three morphotypes having different shapes of dorsal papillae, namely (i) sharp, (ii) blunt, and (iii) intermediate and unclassifiable between (i) and (ii). Based on a phylogenetic tree, all individuals of the three morphotypes matched with Stichopus horrens, except for one individual that matched with S. cf. monotuberculatus. There were 32 mutation sites and 29 haplotypes. Haplotype diversity was not different among the three morphotypes. The degree of genetic differentiation (FST) between the sharp and blunt types was 0.0174 and not significant (P > 0.05), suggesting that these morphotypes were not genetically differentiated and thus were concluded as the same single species (S. horrens) based on the COI partial sequence, provided that no hybridization occurred with other species. These results indicate that S. horrens has high intraspecies variability in its external morphology. Comparison of data in the present study with sequences deposited in GenBank showed that frequent COI haplotypes of S. horrens were common within the Pacific Ocean, and most occurred sympatrically in Marau. This panmictic occurrence implies that these haplotypes are not geographically isolated from each other.