Abstract
Deep-sea mining in the Pacific Clarion-Clipperton Fracture Zone (CCZ), a low-energy sedimentary habitat with polymetallic nodules, is expected to have considerable and long-lasting environmental impact. The CCZ hosts extraordinarily high species diversity across representatives from all Domains of Life. Data on species biology and ecology remain scarce, however. The current study describes the reproductive biology of Ophiosphalma glabrum (Ophiosphalmidae) and Ophiacantha cosmica (Ophiacanthidae), two ophiuroids frequently found in the CCZ. Specimens collected in Spring 2015 and 2019 in four contract areas were examined morphologically and histologically. Size-class frequencies (disc diameter and oocytes feret diameters), sex ratios, gametogenic status, putative reproductive mode, and a simple proxy for fecundity are presented. Habitat use differs in each. While O. glabrum is epibenthic, occurring as single individuals, O. cosmica often forms size-stratified groups living on stalked sponges, suggesting gregarious settlement or retention of offspring (though no brooding individuals were found). Further molecular analyses are needed to establish whether O. cosmica groups are familial. In O. glabrum, for which sample sizes were larger, sex ratios approximated a 1:1 ratio with no size-structuring. In both species, individuals were at various stages of gametogenic maturity, but no ripe females were identified. Based on this, O. glabrum is most probably gonochoric. Reproductive mode remains inconclusive for O. cosmica. Both species are presumptively lecithotrophic, with vitellogenic-oocyte feret diameters exceeding 250 μm. Oocyte feret diameters at times exceeded 400 μm in O. glabrum, indicating substantial yolk reserves. Estimates of instantaneous fecundity (vitellogenic specimens of O. glabrum only) were confounded by interindividual variability in gonad characteristics. The well-furnished lecithotrophic larvae of O. glabrum would be capable of dispersing even under food-impoverished conditions. The current study examines ophiuroid reproductive biology over multiple localities in the CCZ concurrently for the first time, at sites characterised by differing productivity regimes. The reproductive biology of each species is thus discussed with reference to past evolutionary (habitat stability), contemporary (food supply), and future environmental drivers (potential impacts of deep-sea mining).
Highlights
The challenges of exploring remote deep-sea abyssal environments have, far, insulated the deep-sea benthos from the impacts of mineral resource extraction
As part of a wider concerted effort to address significant knowledge gaps in our ecological understanding of nodule-rich seabeds while this habitat remains relatively pristine, the current study describes the reproductive biology of the brittle stars Ophiosphalma glabrum (Lütken and Mortensen, 1899; Ophiosphalmidae) and Ophiacantha cosmica (Lyman, 1878; Ophiacanthidae) in a nodule-rich environment, based on the histological analyses of specimens collected from several mining-contract areas within the eastern Clipperton Fracture Zone (CCZ)
Molecular analyses of the arms of specimens in the current study used for species’ confirmation in a subset of individuals have been compiled into dedicated DOI-indexed dataset containing accession codes (Genbank), BOLD IDs and photos, trace files and collection data, and specimen metadata
Summary
The challenges of exploring remote deep-sea abyssal environments have, far, insulated the deep-sea benthos from the impacts of mineral resource extraction. Recent studies that have sought to describe the benthic fauna that typify the CCZ have revealed extraordinarily high taxonomic diversity across representatives from all Domains of Life (e.g., Amon et al, 2016; De Smet et al, 2017; Shulse et al, 2017; Wilson, 2017; Goineau and Gooday, 2019; Hauquier et al, 2019; Brix et al, 2020; Christodoulou et al, 2020), making the CCZ of critical importance for biodiversity conservation In many of these studies, data (e.g., species-abundance curves) strongly suggest that many – arguably most – species remain unaccounted for, with high species turnover over relatively short spatial scales even in groups that are brooders like Isopoda (e.g., Wilson, 2017; Brix et al, 2020). Remoteness of habitat and spatial variability in species composition of this sort present challenges for performing robust ecological studies, as evidenced by the scarcity of ecologically meaningful data available for even the most conspicuous “common” epifaunal species in the CCZ (Danovaro et al, 2017), including many echinoderm species (Amon et al, 2016)
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