Cyclocaris Guilelmi Chevreux, 1899: 148–152, figs 1–5;— Sars, 1900: 20–25, pls 2–3;— Damas and Koefoed, 1907: p. 438;—Stephensen, 1917 (1913): 107–108;— Chevreux, 1935: 28–30, pl. 16, figs 3, 8, 24, 26.
Cyclocaris guilelmi Stebbing, 1906: 31;— Brüggen, 1907: 215–216;— Stephensen, 1923: 52–54, chart 10;— Schellenberg, 1927: 664–665, fig. 59;— Stephensen, 1933: 8–9, map, fig. 3;— Stephensen, 1935: 43, figs 2–3;— Gurjanova, 1951: 181–182, fig. 53.— Barnard, 1958: 91.— Barnard, 1959: 116–117, figs 4–5.— Gurjanova, 1962: 87–88, fig. 18.— Barnard & Karaman, 1991: 479.— Bucklin et al., 2010: 42, 46.
Cyclocaris faroensis Norman, 1900: 197–198, pl. 6, figs 5–15.— Thurston & Allen, 1969: 358.
Remarks. This species differs from all others in the genus by the permanent yellow-brown ocular pigment covering the whole surface of the head. The absence of eyelobes separates it from C. lowryi and C. franki. The comparison is based on Barnard (1959), the best available account of the species.
C.guilelmi is a well known Arctic species. It occurs widely at mesopelagic and bathypelagic depths, mostly at 500-2000+m (Stephensen, 1923, Østvedt, 1955, Birstein & Vinogradov, 1958, 1970), but has been recorded in the epipelagic zone (Sars, 1900, Stephensen, 1933). The species is attracted to sediment traps in large numbers (Seiler & Brandt, 1997, Kraft et al. 2013) and has been taken occasionally in baited traps, both benthic, at 1095 m (Chevreux, 1899, 1935), and midwater, at 1800 m (Barnard, 1959). The species appears to breed throughout the year (Kraft et al. 2013).
Distribution. Disjunct: Arctic Ocean, north-west Pacific Ocean. Norwegian Sea, Greenland Sea, south to the Faroe-Shetland Channel (Norman, 1900, Stephensen, 1923, Østvedt, 1955, Seiler & Brandt, 1997, Vinogradov, 1997, Kraft et al. 2013); Baffin Bay (Stephensen, 1933, Buchanan & Sekerak, 1982); Arctic Ocean, Nansen, Amundsen, Makarov and Canadian Basins (Sars, 1900, Barnard, 1959, Kosobokova et al., 2011); Pacific Ocean north of 39˚N and west of 170˚E (Birstein & Vinogradov, 1955, 1958, 1970).