Abstract
The Longqi hydrothermal vent field is the first deep-sea active vent field to be explored on the ultra-slow spreading Southwest Indian Ridge. Although a number of larger taxa has been described or characterised, many smaller and less conspicuous animals remain undescribed. Here, two small (< 7 mm) coiled gastropods belonging to the vent-endemic family Peltospiridae are characterised and formally named from Longqi. Lirapex politus n. sp. is characterised by its entirely smooth shell lacking in axial sculpture, which distinguishes it from the three described congeners from East Pacific Rise and Mid-Atlantic Ridge. Dracogyra subfuscus n. gen., n. sp. is conchologically most similar to Depressigyra globulus from the northeastern Pacific, differing in having an almost closed umbilicus and lacking a basal notch in the outer lip. Radula characteristics clearly distinguish the two, however, with Dracogyra n. gen. having a much wider, shorter, sturdier central tooth and stronger laterals. Molecular phylogeny reconstruction using the cytochrome c oxidase I (COI) barcoding fragment indicate that Dracogyra n. gen. is in fact most closely related to Gigantopelta and Lirapex is sister to Pachydermia. The pairwise distance in COI between Dracogyra n. gen. and other peltospirid genera (14.4%~26.6%, mean 21.3%) are sufficient to justify separate genera. Both new species were found around diffuse flow venting areas in association with giant holobiont peltospirid snails Chrysomallon and Gigantopelta. The addition of these two new species increases the total macrofauna species known from Longqi field to 23.
Highlights
The Southwest Indian Ridge is an ultra-slow spreading ridge running from the Rodrigues Triple Junction in southwest Indian Ocean to the Bouvet Triple Junction in southeast Atlantic Ocean (Dick et al, 2003)
The entire clade appears to be restricted to chemosynthetic ecosystems and Peltospiridae is so far only known from hot vents (Chen et al, 2015b)
Lirapex politus n. sp. is the first species of its genus to be discovered from Indian Ocean, meaning the distribution of Lirapex spans Pacific, Indian, and Atlantic oceans; a first for the clade Neomphalina as a whole
Summary
The Southwest Indian Ridge is an ultra-slow spreading ridge running from the Rodrigues Triple Junction in southwest Indian Ocean to the Bouvet Triple Junction in southeast Atlantic Ocean (Dick et al, 2003). The Chinese manned submersible HOV Jiaolong visited the Longqi field on-board R/V Xiangyanghong 9 on several occasions (Zhou et al, 2017). Up to this point, 21 megafauna species have been reported from the Longqi field (Copley et al, 2016). Among the dominant fauna of the Longqi field are two large snails in Peltospiridae, the scaly-foot gastropod Chrysomallon squamiferum Chen et al, 2015 and Gigantopelta aegis Chen et al, 2015 which often co-occur and form large aggregations side-by-side on diffuse flow venting areas (Chen et al, 2015a; Copley et al, 2016). Two other gastropods are reported from the Longqi field, including a Lepetodrilus species (Vetigastropoda: Lepetodrilidae) which is likely a grazer or filter-feeder and a species of Phymorhynchus (Neogastropoda: Raphitomidae) which is a carnivore/scavenger (Copley et al, 2016)
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