Comparative population structure of two edible Indo-Pacific coral reef sea cucumbers (Echinodermata: Holothuroidea)

Echinoderms are the targets of considerable global artisanal and commercial fisheries, but efforts to effectively manage them suffer from poor understanding of population demographics and connectivity. Here we report population genetic data (mitochondrial COI sequence) for two congeneric sea cucumbers, Holothuria atra (Jaeger, 1833) and Holothuria whitmaei (Bell, 1887), throughout the Hawaiian Archipelago and Johnston Atoll to inform resource management. These two species share a wide range across the Indo-Pacific region, and are the most ubiquitous species on Hawaiian coral reefs. Both species have roughly similar haplotype diversity (h = 0.88 for H. atra, 0.83 for H. whitmaei), but the nucleotide diversity (π = 0.0087 and 0.0067, respectively) and effective number of alleles (A_E = 8.3 and 5.9, respectively) were both lower for H. whitmaei. Regardless of the metric of population differentiation used, H. atra shows evidence for restricted gene flow relative to the congeneric H. whitmaei; global φ_ST was 0.165 with 12.65% of variation in analysis of molecular variance is distributed among groups for H. atra, whereas global φ_ST was –0.006 and 100% of the variation is within the three groups (Main Hawaiian Islands, Northwestern Hawaiian Islands, Johnston Atoll) for H. whitmaei. These data contribute to the growing body of literature cautioning against the extrapolation of single-species exemplar studies to management and highlight that even for such broadly-distributed species, local-scale management is justified because migration between the Main and Northwestern Hawaiian islands does not occur within ecologically relevant time frames.