Stirring the Deep, Disentangling the Complexity: Report on the Third Species of Thermochiton (Mollusca: Polyplacophora) From Haima Cold Seeps

Abstract

This study documents a new deep-sea chiton from the Haima cold seeps. Thermochiton xui. nov. is the third species of the genus Thermochiton and the first occurrence of this genus in the South China Sea. This species is identified by its morphological characteristics and the molecular sequence of a Thermochiton species is reported for the first time. The placement of the new species is determined in the phylogenetic tree of Ischnochitonidae by Maximum Likelihood (ML) and Bayesian inference (BI) methods, based on the sequences of the mitochondrial cytochrome c oxidase subunit I (COI), 16S ribosomal DNA (16S), and nuclear 28S ribosomal DNA (28S) gene regions. Bayesian evolutionary analysis with an uncorrelated relaxed clock approach indicated that this new species is estimated to have diverged from its most closely related shallow-water ischnochitonid taxa 5.10–10.07 million years ago in the Late Miocene. A regional ocean general circulation model was used to estimate the potential dispersal ability of the three species of Thermochiton. Because it is highly unlikely for one species to have spread between the northwest and southwest Pacific to the localities in which this genus has been found to date, we propose that ‘stepping-stone’ habitats and/or ‘bridge species’ were involved in the dispersal and evolution of these cold-seep endemic chitons.

Same (sea) bed different dreams: Biological community structure of the Haima seep reveals distinct biogeographic affinities

“…22 The patellogastropod limpets mainly living on mussel shells at Haima were exclusively B. lactea; but at Site F the dominant species was Bathyacmaea nipponica which co-occurred with a few B. lactea. 41,42 Clusters of the vestimentiferan tubeworm Paraescarpia echinospica were found from at least three localities at Haima; 29 nevertheless, only one individual of this species was found at Site F. 20 Brittle stars were highly abundant at Haima, but they were scarce at Site F. 37 At least 35 of the species found at Haima (~54%) appear to be endemic to this site, but only seven of them have been named, including the vesicomyid clam Archivesica marissinica, 21 the glass scallop Catillopecten margaritatus, 43 the bathymodioline mussels Gigantidas haimaensis 22 and Nypamodiolus samadiae, 35 the limpet Paralepetopsis polita, 44 the chiton Thermochiton xui, 36 the brittle star Histampica haimaensis, 37 and the tubeworm Sclerolinum annulatum. 34 Many others are also almost certainly undescribed species.…”

“…The bathymodioline mussel Gigantidas haimaensis exhibited the lowest δ 13 C value (-54.3 ‰), since it harbours methaneoxidizing bacteria (MOB) in its gill. 22,58 A number of species were suggested to harbour sulphur-oxidizing bacteria (SOB), 34,35,59,60 including Acharax sp., Archivesica marissinica, "Bathymodiolus" aduloides, Nypamodiolus samadiae, Paraescarpia echinospica, Sclerolinum annulatum, and Vesicomyidae indet. Haima.…”

“…In addition, several species at Haima have recently been described or taxonomically revised. 21,[31][32][33][34][35][36][37] As such, we aimed to (1) revise and update the species list of the Haima seep via a combination of morphological observation and molecular barcoding, (2) revise the genus-level species list at the seeps off India and Pakistan based on published photographs and online video, (3) compare the macrofauna compositions of the SCS seeps (mainly Haima and Site F) with those in the Western Pacific and the Indian Ocean, and thereafter (4) investigate the biogeographic affinity of Haima and Site F within an Indo-West Pacific context.…”