Epibiotic associations with the deep-sea pycnogonid Nymphon hirtipes Bell, 1855 as a source of biodiversity

Among arthropods, pycnogonids are generally considered of minor ecological significance in marine ecosystems. However, the fact that they can host a wide diversity and high abundance of epibiotic organisms suggests that their role in benthic communities may be underestimated. The present study characterized the diversity, dynamics and role of the epibiotic community associated with deep-sea pycnogonids (Nymphon hirtipes). Despite their small size, they hosted a mean of 62 macro-organisms belonging to at least 26 species from 11 different phyla. Foraminifera was the dominant phylum while other phyla, such as Bryozoa, Cnidaria (Hydrozoa), Porifera and Brachiopoda were common. The macro-epibiont burden varied from 1 to 235 organisms per pycnogonid; it was consistent across depths and locations but fluctuated as a function of sex and of the reproductive state in males. Specifically, most colonized individuals were females, which are larger and thus offer more surface for epibionts. Among males, burdens peaked in those carrying larvae. Maximum epibiont burden and coverage was correlated with the cessation of grooming at the beginning of the breeding period. Micro-epibionts consisted of bacterial mats covering most body parts. Several epibionts were estimated to have negative effects on their host (e.g. increasing drag, preventing mating, decreasing limb mobility and impeding feeding). However, because this species dies after reproducing, epibiosis may be considered as a positive trade-off since it may provide food to the mobile juveniles living on the male for several months. Furthermore, epibionts could be used as camouflage during this period. N. hirtipes has been identified as a common component of the benthofauna in the deep sea of the North Atlantic and Arctic Oceans, and the present study indicates that it displays the greatest diversity and burden of associates ever reported among pycnogonids.

Automated summary

Deep-sea pycnogonids host a diverse and abundant epibiotic community, with the burden of epibionts varying with sex and reproductive state, playing a potentially important role in benthic communities.