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 (Lütken & Mortensen, 1899) (Ophiosphalmidae) and Ophiacantha cosmica (Lyman, 1878) (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 Ophiosphalma glabrum is epibenthic, occurring as single individuals, Ophiacantha cosmica often form 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 Ophiosphalma 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 Ophiacantha cosmica. Both species are presumptively lecithotrophic, with vitellogenic-oocyte feret diameters exceeding 250 µm. Oocyte feret diameters at times exceeded 400 µm in Ophiosphalma 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).
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Language prereferences The authors prefer the article to be formatted in ‘British’ English
During the collation of new sequence data (from the 2019, SO268 cruise) for submission to GenBank, an error in species ID in the 2015 data was discovered. A small number of specimen IDs (all from APEI3) had since been revised and assigned a different species from their initial assignation of O. glabrum; this new information was not known at the time of histological analysis / writing. Consequently, the three "O. glabrum" males and one female collected from APEI3, are now known to be an undescribed Ophioroglypha species that bears a striking resemblance to Ophiosphalma. This means that no samples remain for APEI3 and thus this station was dropped from analyses. All figures and analyses have been repeated in the absence of this data with very little impact on the MS (only one female, with a low number of oocytes, was implicated). This neither changes the interpretation of results, nor the quality of the data presented. Several supplementary figures were also upgraded to main text figures. The Introduction was slightly expanded and certain arguments in the discussion were removed (e.g. infaunal feeding in females), have subsequently been considered too speculative.