Chiridius molestus is an aetideid calanoid copepod known after both sexes (female 2.30-2.70 mm and male 1.80 mm) from mostly mesopelagic waters.
Female. Total length 2.30-2.70 mm. Cephalothorax 2.8-3.75 times longer than abdomen. Points of Th5 posterior corners slightly exceeding the midlength of the genital segment or reaching the end of its last third; Abd1 is widest in its second third. A1 equal in length or shorter than cephalothorax. Ri2 Md with 8 terminal setae. Re Mx1 with 8 setae. Park (1975a) noted 9 setae. Few specimens were found with P5 rudiments in females; they have Ri2 Md with 9 setae and Re Mx1 with 10 setae. External spines on Re1 and Re2 P1 variable in length: sometimes reaching or exceeding the base of following spine (or not reaching the base of following spine). P2 with 3-segmented Re and incompletely 2-segmented Ri the line of separation often badly visible. Edge of segment between second and third external spines of the characteristic shape: with bulge near the base of third external spine and with deep excavation close to the base of second external spine. Ri2 P2 reaching or not reaching the border between Re2-Re3 P2, terminal spine with 23-26 denticles. Re3 P3 edge between second and third external spines close to that in Re3 P2. P3-P4 rami 3-segmented, with 22-28 denticles on terminal spines of Re3.
Male. Total length 1.80 mm. (In the first description 2.04 mm, ratio of cephalothorax length to abdomen length 2.3 (Tanaka, 1957a); Park (1975) recorded total length 1.80-1.98 mm, and cephalothorax 2.50-2.68 times longer than abdomen). Cephalothorax 3.2 times longer than abdomen. Anterior border of cephalon strongly pressed at the insertion of A1. Posterior points of Th5 corners reaching the middle of Abd1. A1 as long as cephalothorax, reaching the middle of Abd3. Ri2 Md with 9 setae (one of which is thin and small), Tanaka mentioned 8 setae. Mx1 with 1 seta near Ri base, with 9 setae on Ri, and 9 on Re. Ri1 Mxp with 1 medial seta. External spines of Re1, Re2 P1 shorter than on the same segment of female, Ri not reaching the border between Re2 and Re3 P2. Ri P2 2-, Ri and Re P3-P4 3-segmented. Left P5 is a little bit longer than two thirds of right P5 length.
Remarks. After Bradford and Jillett (1980) and on the form of examined material, I consider Chiridius subgracilis Park, 1975 and Chiridius molestus Tanaka, 1957 identical species (Markhaseva, 1996). The New Zealand authors left the name subgracilis (Bradford and Jillett, 1980), but according to the priority rule, it should be considered as the junior synonym and the name Chiridius molestus should be assumed valid.
Most possible mainly mesopelagic species, found in the hauls 200-500, 500-1000 m (Bradford and Jillett, 1980), recorded in epipelagic (A. Scott, 1909; Bradford and Jillett, 1980); found in total hauls from 4000 m.
Due to possible mixing of the present species with Ch. gracilis , it is difficult to discribe the species distribution area. In the Atlantic Ocean the most northern finding is 48°N (Markhaseva, 1996), known also from the Gulf of Mexico (Park, 1975a) and the region of the Canary Islands (Vives, 1982); it was found in the Indian Ocean (Markhaseva, 1996); from the Izu region (Tanaka, 1957a; Tanaka and Omori, 1970a), from the Malay Archipelago ? (A. Scott, 1909), region of the New Zealand (Bradford and Jillett, 1980; Markhaseva, 1996), the south-eastern part of the Pacific Ocean near the coast of South America (Markhaseva, 1996), in the Pacific Ocean species was recorded to the south not further than 46°S.
Type locality: the Izu region.
45 females and 2 males from samples: 266, 267, 313, 387-389, 422-428, 445, 447-451, 457. See examined samples module.