Direct observations of a dense occurrence of Bolinopsis infundibulum (Ctenophora) near the seafloor under the Oyashio and notes on their feeding behavior

A dense occurrence of the ctenophore Bolinopsis infundibulum was observed at depths of approx. 1250 m from the manned submersible SHINKAI 2000 from 1998 to 2000 near the seafloor off Kushiro, Hokkaido, Japan. Body length ranged from 6.5 to 16.5 cm for seven specimens sampled from the submersible. Horizontal transects by the submersible showed that B. infundibulum were scarce shallower than 1000 m, but were abundant within 30 m of the bottom, ca. 0.8 ctenophore m−3, then decreasing gradually to 80 m above the seafloor. The ctenophores forage with extended oral lobes and hanging quiescently in the water column. Ninety ctenophores among 112 observed at 1200–1270-m depth were in the foraging posture. One individual captured at depth had a female C5 stage of Neocalanus cristatus and fragments of calanoids (? Aetideidae) in its gut. Carbon and nitrogen stable isotope ratio data indicated that the B. infundibulum relied on surface photosynthetic products for food and their position in the trophic pyramid is relatively low. B. infundibulum feeds mainly on copepods, at least some part of which consist of carbon-rich copepods such as Neocalanus spp. and Eucalanus bungii, which grow in surface layers but sink to depth when diapausing as C5 or adult stages. Carbon-based estimations showed that one Neocalanus cristatus C5 or 4–6 N. plumchrus+flemingeri C5+female are sufficient for daily maintenance of a medium-sized (∼10 cm) B. infundibulum. Considering the abundance of B. infundibulum, we conclude that this deep ctenophore population may contribute substantially to the mortality of these epipelagic copepods during diapause.