A calanid copepod of the genus Calanus. In the North Atlantic, C. glacialis can be confused with congeners C. finmarchicus in the N. Atlantic and C. marshallae in the N. Pacific which both overlap in size and range with C. glacialis. Other species of Calanus overlapping in range with C. glacialis are C. hyperboreus, a significantly larger species and C. helgolandicus, a significantly smaller species.

1. the more elongated outer ramus of the left 5th leg in the male.

2. the female head shape

3. the female 5th leg basipod of C. glacialis has a curved interior margin; C. finmarchicus' is straight.

4. larger size than C. finmarchicus (female prosome 1.86 mm (Murphy & Cohen, 1978) - 3.28 mm (Frost, 1974); male prosome, 2.08 mm - 3.16 mm (Frost, 1974))

5. shape of posterolateral margin of the fifth thoracic segment Fleminger & Hulsemann, 1977)

6. shape of ventral surface of genital segment Fleminger & Hulsemann, 1977)

7. the pore signature pattern of integumental organs as described by Fleminger and Hulsemann (1977).

8. the geographical range of C. glacialis is more northerly than C.finmarchicus in the western N. Atlantic and also occurs in the polar waters of the N. Pacific where C.finmarchicus is absent.

1. the photoreceptor of C. marshallae is very large (Fleminger & Hulsemann, 1977; Frost, 1974)

2. on average, C. glacialis is larger than C. marshallae (female prosome 2.7-4.2, Frost, 1974) although their sizes overlap.

3. the female 5th leg basipod of C. marshallae has a slightly more curved interior margin and fewer denticles, on average (Frost, 1974).

4. shape of ventral surface of genital segment Fleminger & Hulsemann, 1977)

5. the caudal ramus of C. marshallae is about twice as long as wide but less than the width of the anal segment whereas in C. glacialis and C. finmarchicus, the length is usually more than twice the width and about equal to the width of the anal segment. (Frost, 1974)

6. geographic distribution: C. marshallae has been identified only in the N. Pacific.

1. the more evenly rounded frontal part of the female head; C. helgolandicus is more pointed.

2. the longer caudal rami.

3. larger size than C. helgolandicus (female prosome 1.98-2.80mm, Fleminger & Hulsemann, 1977)

4. the geographical range of C. helgolandicus is centered in the eastern N. Atlantic and warmer waters of the western N. Atlantic while C. glacialis is primarily an Arctic species found in both the N. Atlantic and N. Pacific.

C. hyperboreus by:

1. the fifth posterolateral margin of the fifth thoracic segment has pointed tips on C. hyperboreus male, female, C5 and C4.

2. smaller length than C. hyperboreus. Female C. hyperboreus prosome: 5.9-7.4 (Hirche, 1997), total length: 7-10mm (Wilson, 1932), male: total length: 6-7mm (Wilson, 1932)