Chiridius pacificus is an aetideid calanoid copepod known after both sexes (female 2.70-3.50 mm and male 2.36-2.65 mm in length) from the Pacific Ocean, mainly meso- and bathypelagic.
Female. Total length 2.70-3.50 mm. Cephalothorax 2.7-3.5 times longer than abdomen. Posterior points of Th5 corners always reaching the middle length, or often exceeding the midlength of genital segment. In the majority of specimens Th5 corners not divergent, even slightly converged. Genital segment is the widest between first third of its length and the second third, or in its second third. A1 reaching the midlength of genital segment, or the midlength of Abd2. Ri2 Md with 7 long setae and 1 small thin posterior seta. Re Mx1 with 10 setae. External spines of Re1-Re3 P1 comparatively long, usually exceeding the base of next spine; third external spine on Re P1 is 1.30-1.47 times longer than second one. P2 usually with 3-segmented Re and 2-segmented Ri, but not all specimens have complete subdivision, sometimes separation between Re1 and Re2 indistinct. First and second external spines on Re3 P2 relatively long: reaching, or even exceeding the base of following spine, terminal spine with 32-35 denticles. P3 and P4 with 3-segmented rami, terminal spines with 25-34 denticles. First, second and third external spines of Re3 P4 significantly thinner and smaller than those in P3.
Male. Total length 2.36-2.65 mm. Cephalothorax 3.2-3.5 times longer than abdomen. Cephalon pressed anteriorly at the level of A1 base. Posterior points of Th5 corners longer than Abd1. A1 slightly shorter than cephalothorax. Ri2 Md with 8 long, strong and 1 short, thin posterior seta, Mx1 rudimentary, its third (?) internal lobe with 3 setae, protopodite near the base of Ri with 3 setae, Ri with 9 setae, Re with 9 setae. P2 with 2-segmented Ri. First and second external spines on Re3 are of the same length as in female. P3 and P4 with 3-segmented rami. P5 with rudimentary Ri in both rami, this is distinguishing feature of the species from all other species in the genus.
Remarks. Females are very close to Ch. polaris. Difficulties in distinguishing between these species arise of great variability of Ch. pacificus. The main differences between Ch. pacificus and Ch. polaris are: Ch. pacificus is of smaller size, usually has longer points of Th5 posterior corners and higher ratio of second and third external spines on P1. The assumption (Bradford and Jillett, 1980) that Ch. pacificus and Ch. carnosus are identical has been confirmed. Ch. pacificus is also similar to Ch. mexicanus , but Ch. pacificus has larger size and longer Th5 points, A1 and also external spines on Re1 and Re2 P1 and Re P2.
Most probably meso- upperbathypelagic species; found in hauls between 500 and 2000 in the Kurile-Kamchatka region, more often in range 500-1000 m (Markhaseva, 1996). Also was found in series of total hauls from 1000 to 1500 m.
The Pacific Ocean: in the Northern Hemisphere - southern part of the Sea of Okhotsk, region of the Kurile-Kamchatka Trench (Markhaseva, 1996) and the Sagami region (Tanaka, 1957a); in the Southern Hemisphere between 41-45°S and 168-175°E off New Zealand (Bradford and Jillett, 1980).
Type locality: the southern part of the Sea of Okhotsk (Brodsky, 1950).
22 females and 1 male from 15 samples: N 64, 80, 86, 91-93, 119, 121, 140, 157, 158, 171-173, 246. See examined samples module.
Lectotype: 1 female, N 39802; Kurile Sakhalin expedition, 26 August 1948; R/V "Toporok"; sta 14, sample 76, depth of site 1450; stratum of sampling 1450-0 m.