Aim To test the hypotheses that seamounts exhibit high rates of endemism and/or species richness compared to surrounding areas of the continental slope and oceanic ridges.
Location The south-west Pacific Ocean from 19–57° S to 143–171° E.
Methods Presence/absence museum data were compiled for seamount and non-seamount areas at depths between 100 and 1500 m for the Ophiuroidea (brittle-stars), an abundant and speciose group of benthic invertebrates. Large-scale biogeographical gradients were examined through multivariate analyses at two spatial scales, at the scale of seamounts (< 1° of latitude/longitude) and regions (5–9°). The robustness of these patterns to spatially inconsistent sampling effort was tested using Monte Carlo-style simulations. Levels of local endemism and species richness over numbers of samples were compared for seamount and non-seamount areas using linear regressions. Non-seamount populations were randomly generated from areas and depth ranges that reflected the typical sampling profile of seamounts.
Results Seamount ophiuroid assemblages did not exhibit elevated levels of species richness or narrow-range endemism compared with non-seamount areas. Seamounts can exhibit high overall species richness for low numbers of samples, particularly on seamounts supporting a dense coral matrix, but this does not increase with additional sampling at the rates found in non-seamount areas. There were relatively few identifiable seamount specialists. In general, seamount faunas reflected those found at similar depths in surrounding areas, including the continental slope. Seamount and non-seamount faunas within the study area exhibited congruent latitudinal and bathymetric species turnover.
Main conclusions Seamount faunas were variable for ophiuroid faunal composition, species richness and narrow-range endemism, reflecting their environmental diversity and complex history. The continental slope was also variable, with some areas being particularly species rich. Broad geomorphological habitat categories such as ‘seamounts’ or ‘continental slope’ may be at the wrong scale to be useful for conservation planning.