Flapjack devilfish in the northern North Atlantic: morphology, biology and ecology of Opisthoteuthis borealis (Cephalopoda, Octopoda, Cirrata)

Deep-sea cephalopods are important in the bathyal ecosystems in terms of both abundance and diversity, but are seriously understudied. One of the most intriguing groups among the deep-sea cephalopods are Cirrata, relatively primitive octopods. Opisthoteuthis is the largest genus among the Cirrata. The least studied species of Opisthoteuthis in the Atlantic, Opisthoteuthis borealis Collins, 2005 was known from nine specimens only prior to our study, and nothing was described about its biology. Four males, all larger than the previously known maximum size (mantle length 78–96 mm cf. 75 mm), are described and COI sequence of the species provided to ease the identification of the Atlantic Opisthoteuthis. Our findings expand the known geographical (North Atlantic from 60° N northward and up to the Davis and Denmark Straits and the Iceland–Faroe Ridge), depth (878–1321 m) and temperature (3.0–3.6 °C) ranges of O. borealis. Arm bifurcation is reported in Cirrata for the first time, suggesting well-developed regeneration is present even in this ancient taxon of cephalopods. Ontogenetic increase of spermatophore length, i.e., when the spermatophores produced later during ontogenesis are larger than those produced earlier, is reported in Cirrata for the first time. The stomachs in all the studied specimens were at least one-third full, suggesting that O. borealis continues to feed and grow after reaching maturity. Polychaetes dominated over crustaceans in the stomach contents. Contrary to the assumption that Cirrata feed on relatively small prey only, large mature males of O. borealis consume polychaetes reaching 41.5–45.9% ML of the specimens.