Carapace tapers anteriorly so that maximum height is at about five-sixths of the length (C. borealis Habitus 1, C. borealis Habitus 3). Shoulder vaults well developed and sharp-edged. Angle of postero-dorsal corner about 75°, where right valve carries a series of small spines. The posterior margin curves smoothly into the ventral margin. In ventral aspect the maximum width is posterior to mid-length, and there is quite a sharp taper both anteriorly and posteriorly (C. borealis Habitus 2, C. borealis Habitus 4). The right asymmetrical gland opens at the postero-ventral corner, with just dorsally a large group of supplementary glands, a further group at two thirds height on the posterior margin. The carapace is covered with a rhomboidal pattern of reticulation.
Female, frontal organ well differentiated, the stem just longer than first antenna (C. borealis 1). The slightly down-turned capitulum spinose on its basal dorsal third and along most of its length ventrally; terminates in a slightly curved point. First antenna with a long hirsute dorsal seta extending to the midpoint of the capitulum, podomeres partially hirsute, "e" seta twice the length of the limb, and slightly flattened distally, "a"-"d" setae a third the length of the "e" seta.
Male, frontal organ similar to female's (C. borealis 3). First antenna with S-shaped "a" seta swollen at base; "c" seta about half the length of the first segment; the "b" seta with a ridged pad distal of which are 5-6 spines; the "d" seta is just shorter than the "b" seta, carrying a fringe of fine spines level with the "e" seta armature. The "e" seta about 5/4's the length of the "b" and "d" setae; its armature a double row of 50-55 spines (C. borealis 5). Distal of the armature, the seta bent becoming flattened into a sword-shape. On the endopodite of the second antenna, the longest seta ("f") is slightly flattened, is double the lengths of the sensory setae, righthand hook appendage bent through ca. 120°near its base and distally curves smoothly through a further 60°; its end is ridged and rounded (C. borealis 6).
Species is similar to the single Arctic endemic, C. maxima (Conchoecia maxima)and to its Southern Ocean counterpart, C. antipoda ). It shows a marked latitudinal submergence, occurring as a rare bathypelagic species off Bermuda. Distinguishable from C. maxima (Conchoecia maxima) by its smaller size, its better-developed sharpedged shoulder vaults, and the larger numbers of spines in the armature of the first antennal "e" seta of the male.
North Pacific and North Atlantic 30°N-65°N. Mesopelagic at latitudes >40°N, bathypelagic and rarer at lower latitudes. One of the dominant species at deep mesopelagic in the oceanic waters off the British Isles. 1, 2, 3, 4 (R.R.S. Discovery Map).
None designated; status of original material uncertain.
Lofoten Islands off north coast of Norway.