The epitome of data paucity: Deep-sea habitats of the Southern Indian Ocean

Vulnerable marine ecosystems (VMEs) are protected from bottom-fishing impacts in international waters by UN resolutions through Regional Fishery Management Organizations. VMEs include deep-sea benthic taxa whose life-history traits make them vulnerable to disturbance. Conservation measures for VMEs require regulatory frameworks informed by biodiversity maps. Here we evaluate biogeographic patterns of VME biodiversity of the Southern Indian Ocean to understand conservation avenues for the Southern Indian Ocean Fisheries Agreement (SIOFA) management organization. We synthesised knowledge on the distribution of deep-sea benthic taxa and explored the quality and quantity of available data. Next, we explored how taxa are structured into bioregions using biogeographical networks. We found astounding Wallacean and Linnaean shortfalls within SIOFA's area, which is virtually devoid of distributional data. Across the entire area, results suggest that only 48 % of the expected deep-sea taxa has been sampled at most, and most sampled cells are inadequately sampled. Yet, our bioregionalization analysis identified multiple bioregions, some only observed within SIOFA's area. Whilst the Wallacean and Linnean shortfalls are so important for VMEs that they severely impede to make adequate maps for conservation planning, results suggest that SIOFA hosts a unique faunal composition that must be safeguarded. Predictive approaches to compensate for these shortfalls exist but will likely be insufficient and uncertain. Within SIOFA's area, there is no satisfying solution to cope with the data shortfalls. Yet, biodiversity maps are a global responsibility. This study makes a call to invest in biodiversity inventories in this region to promote informed conservation decisions.