Observation of a high abundance aggregation of the deep-sea urchin Chaetodiadema pallidum A. Agassiz and H.L. Clark, 1907 on the Northwestern Hawaiian Island Mokumanamana

AUV images of deep-sea megafauna on the seamount Mokumanamana in the Northwestern Hawaiian Islands revealed a large aggregation of the diadematoid urchin Chaetodiadema pallidum. This species had not been previously recorded on this seamount, but was known from other locations in the Hawaiian Archipelago. A total of 11,360 individuals were counted by eye from 1,369 seafloor images along the 200, 250, and 300 m depth contours on the west side of Mokumanamana. To estimate the full population size, Inverse Distance Weighting and Empirical Bayesian Kriging interpolation calculated a total population size of between 139,552 and 144,063 individuals. These estimates are 1-2 orders of magnitude greater than any previous estimates of urchin aggregation abundances. Little biological data exists for this species; thus, it is difficult to know if this is a spawning aggregation, a feeding front, a type of "schooling" behavior, or some combination thereof, but a food source did seem to be present in the form of mesophotic green algal detritus.