Longqi is ecologically distinct among known hydrothermal vent fields, hosting species not yet recorded from other locations, and known species in abundances that contrast with populations elsewhere. The species richness of 21 mega- and macrofaunal taxa in our samples is within the range of values for well-studied vent fields on neighbouring seafloor spreading centres (4 to 35 taxa at vent fields on the Central Indian Ridge2,20,22,26; 17 to 43 taxa at Mid-Atlantic Ridge vent fields2,25,27,28,29,30; 12 to 14 taxa at vents on the East Scotia Ridge3,31,32,33; see Supplementary Information for full details), providing confidence of adequate sampling at Longqi for comparative analysis in this study.

The majority of known mega- and macrofaunal species found at Longqi are previously recorded from the Central Indian Ridge, with which this Southwest Indian Ridge vent field therefore has closest affinity in species composition. COI gene sequence data reveal significant differentiation, however, between Southwest Indian Ridge and Central Indian Ridge populations of the scaly-foot gastropod Chrysomallon squamiferum34, consistent with low connectivity across the ~2300 km between those sites via the lecithotrophic larvae inferred for this species19. The extent of contemporary connectivity has yet to be determined between Southwest Indian Ridge and Central Indian Ridge populations of species with planktotrophic larval development such as Rimicaris kairei, whose congener R. exoculata exhibits panmixia in microsatellite markers over a distance of ~7100 km among vent fields in the Atlantic35.

Several species in our samples from Longqi exhibit an affinity at higher taxonomic level with seafloor spreading centres beyond the Indian Ocean. Kiwa n. sp. “SWIR” is morphologically most similar among the Kiwaidae to K. tyleri32 from the East Scotia Ridge, with a molecular phylogeny based on nine gene sequences indicating divergence at 2.6 to 0.6 (median 1.5) Ma18. Similarly, Gigantopelta aegis is closely related to G. chessoia from the East Scotia Ridge, with 4.43% COI divergence and molecular clock calibrations suggesting a common ancestor around 1.85 to 1.54 Ma17. Among eolepadid barnacles, a split between Neolepas sp. 1 and Vulcanolepas scotiaensis of the East Scotia Ridge is also indicated at 3.8 to 0.4 (median 1.7) Ma21. Changes in the latitudinal range of the Antarctic Circumpolar Current, such as those inferred between 1.2 Ma and 650 ka, may have increased hydrographic isolation of the Southwest Indian Ridge from the East Scotia Ridge18, possibly contributing to the allopatric speciation of these taxa. A chiridotid holothurian has been reported at vents on the Central Indian Ridge20, and Chiridota hydrothermica is known at vents in the back-arc basins of the western Pacific and on the southern East Pacific Rise in similar distribution and abundance to the species at Longqi36, but further comparison is required to confirm the affinity of the species on the SW Indian Ridge.

The discovery of a polynoid species at Longqi shared with vent fields ~6000 km away on the East Scotia Ridge, however, and Hesiolyra cf. bergi potentially shared with the East Pacific Rise, is consistent with the most widely-distributed species at hydrothermal vents being polychaetes. The amphinomid species Archinome tethyana and A. jasoni, for example, have been found at vents on the Mid-Atlantic Ridge and the Central Indian Ridge37. These trans-oceanic polychaete species are therefore responsible for the “non-zero” faunal similarity values between some vent fields in different biogeographic provinces (Fig. 6d). The potential trans-oceanic distribution of H. bergi may be extended further if future studies confirm that the hesionid resembling H. bergi on the Mid-Atlantic Ridge25 is conspecific with populations on the Southwest Indian Ridge and East Pacific Rise. Similarly, we identified a spionid specimen from Longqi as Prionospio cf. unilamellata (Table 1) on the basis of morphology, and P. unilamellata is known from Mid-Atlantic vents25, but paucity of material prevented more detailed morphological investigation or molecular analysis.

A negative correlation between faunal similarity and along-ridge-axis distance between vent fields has previously been noted at genus level38, and here we show an overall negative correlation between species-level faunal similarity and Great Circle distances between vent fields across three ocean regions (Fig. 5d). This relationship may be weaker, however, where neighbouring vent fields vary in levels of hydrothermal activity as a result of their ephemeral nature. The “Dodo” vent field on the intermediate-spreading Central Indian Ridge, for example, is waning in activity compared with the nearby “Solitaire” vent field26, and these vent fields consequently differ markedly in faunal composition (Sørensen’s Index 24%) despite being only 145 km apart (Fig. 5a). Such variation may be less likely on slower-spreading ridges, however, where individual vent fields exhibit greater longevity of hydrothermal activity6, and this may contribute to the negative correlation remaining apparent among vents on the Mid-Atlantic Ridge (Fig. 5d).

The extensive inactive sulfide deposits at Longqi are consistent with a prolonged history of hydrothermal activity at the vent field, as expected on a very slow spreading ridge. Our comparison of species on chimneys with contrasting levels of hydrothermal activity suggests that when activity wanes for an individual chimney, its fauna will follow a temporal succession that matches the spatial zonation around the vents. The low abundance of Rimicaris kairei on active vent chimneys at Longqi contrasts with the high-abundance aggregations of this species in the same environment at vents on the Central Indian Ridge20,22,26, and the low abundance of Kiwa n. sp. “SWIR” close to vent fluid sources also contrasts with the aggregations of closely-related K. tyleri in the same location in zonation at vents on the East Scotia Ridge39. We did not observe the large provannid gastropod Alviniconcha hessleri, which occurs in high abundance at several vent fields on the Central Indian Ridge20,22,26. More peripheral taxa in the faunal zonation at Longqi, however, occur in comparable abundances to populations of shared or related species elsewhere, such as the aggregations of Gigantopelta aegis resembling those of closely-related G. chessoia at vents on the East Scotia Ridge39, and Neolepas sp. 1 occurring in high abundance as found at vents on the Central Indian Ridge20,21,22,26.

Despite differences in overall faunal composition compared with vent fields on other ridges, carbon and nitrogen stable isotope composition of species analysed from Longqi are generally similar to those of shared or related species elsewhere, suggesting similar trophic roles. Bathymodiolus gill and foot δ13C values are at the upper range of values expected from carbon fixed by the Calvin Benson Bassham cycle, and may also contain contributions of organic carbon produced by methane-oxidisers40, consistent with dual endosymbiosis known in bathymodiolin mussels elsewhere41,42,43,44,45,46. δ13C values of Gigantopelta aegis are similar to those of G. chessoia on the East Scotia Ridge47 (reported as Peltospiroidea sp.), and δ13C values of Neolepas sp. 1 are similar to Vulcanolepas scotianesis on that ridge47 (reported as Vulcanolepas sp.). The values for Neolepas sp. 1 are lower than conspecific values at the Kairei vent field on the Central Indian Ridge45 (~−16‰), however, indicating possible site-specific differences in composition or δ13C values of microbial food sources. The highest δ13C observed among species analysed from Longqi were in Chiridota sp., similar to the values found in a chiridotid holothurian at the Solwara-1 vent field in the western Pacific48 (δ13C = ~−24‰).

Bathymodiolus was the only taxon analysed from Longqi with negative δ15N values, which are mid-range among those reported for bathymodiolin mussels at hydrothermal vents (~−17‰ to ~6‰)49,50. Positive δ15N values of Gigantopelta aegis, Neolepas sp. 1, and Chiridota sp. are within ~1.3‰ of each other, indicating a comparable inorganic nitrogen source. G. aegis δ15N is similar to that of other large peltospirid gastropods46,47,51, and δ15N of Neolepas sp. 1 at Longqi is within the range for stalked barnacles at other hydrothermal vents (~5‰ to ~11‰)46,47,48.

As this is the first ecological investigation of hydrothermal vents on the Southwest Indian Ridge, further exploration is needed to determine whether the faunal assemblage at Longqi is typical of vent fields on this very slow to ultraslow-spreading ridge. Until such information is available, the Longqi vent field appears to meet several criteria that may define an “Ecologically or Biologically Sensitive Area” under the UN Convention on Biological Diversity (CBD), for example an area that “contains unique, rare, or endemic species, populations or communities”52. Assessing the impacts of mineral exploration activities already licensed at Longqi by the UN International Seabed Authority (ISA)14 should therefore include investigation of other vent fields detected on the Southwest Indian Ridge and the relationships of their fauna with populations at Longqi.