New evidence for a cosmopolitan holothurian species at deep-sea reducing environments

The deep-sea holothurian Chiridota heheva Pawson & Vance, 2004 was recently recognised as a cosmopolitan species which exploits the organic enrichment at three types of deep-sea reducing environments: hydrothermal vents, cold seeps, and organic falls. Here, we apply phylogenetic reconstruction and species delimitation approaches using new COI and 12S sequence data to show that C. heheva is genetically congruent with the only other hydrothermal vent holothurian, Chiridota hydrothermica Smirnov & Gebruk, 2000, with strong supporting morphological, ecological, and biogeographical parallels between the two. As such, we propose that C. heheva is a junior synonym of C. hydrothermica, and that subsequent chiridotid holothurians discovered at deep-sea reducing environments likely also belong to this single globally distributed species. As a species endemic to deep-sea reducing environments yet known at sites across the Atlantic, Indian, and Pacific Oceans, this unique holothurian provides an interesting case study for connectivity, biogeography, and speciation in the deep sea, which has important implications for deep-sea conservation planning.