Dense beds of the ophiuroid Ophiacantha abyssicola on the continental slope off Nova Scotia, Canada

Megafaunal distributional patterns in the bathyal and abyssal environments remain largely unknown. Ophiuroids are a significant component of the deep-sea megafauna occurring throughout the worldʹs oceans, generally at densities of <10 s individuals m−2. In this study, we describe the abundance, biomass and size frequency distribution of a dense assemblage of Ophiacantha abyssicola in Northeast Channel on the continental slope off Nova Scotia, Canada. This area is characterized by highly irregular topography and strong tidal currents. Three sites, ranging in depth between 430 and 498 m, were sampled (Middle Canyon, Hell Hole West and Hell Hole East) and 1–4 dives were done at each site with the remotely operated vehicle ROPOS between 20 and 22 August 2001. At each dive, all ophiuroids from 1 to 2 quadrats (0.025–0.163 m2) were collected by suction sampling, and their diameter and wet mass measured. Dry (DM) and ash free dry mass (AFDM) were recorded in 10 individuals from each sample, and 25–35% of individuals from each sample were dissected to determine their gender. Relationships between the three measures of mass derived from the subsamples were used to calculate DM and AFDM for all individuals. Density and biomass were estimated for each quadrat by calculating the size of the sampled area from video frames with image analysis. Percentage cover was calculated on selected frozen video frames (0.197–0.369 m2) by the point method. Mean disk diameter ranged from 6.77 to 9.17 mm, and was smallest at Hell Hole West and greatest at Hell Hole East. Mean abundance at each site ranged from 390 to 1200 individuals m−2, and was greater at Hell Hole West than the other two sites. Percentage cover ranged from 35 to 62%, and was lower in Hell Hole East than the other two sites. The patterns in biomass were similar to those in abundance. Such dense aggregations of ophiuroids are rare in the bathyal and abyssal environments, but have been documented in shallow habitats where they have been attributed to (1) reduced predation intensity or (2) increased food supply. We found only two individuals (out of 50 per dive examined) with evidence of arm regeneration, but observed sustained feeding activity coupled with strong currents. Thus, we propose that the formation and maintenance of the ophiuroid beds that we observed at Northeast Channel is associated with maximizing particle capture rather than reduced predation intensity.