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Boeck, 1872

Short description:

Aetideus armatus is an aetideid calanoid copepod known after both sexes (female 1.60-2.00 mm and male 1.30-1.53 mm in total length). Species recorded from epi- and mesopelagic layers of the Atlantic, Pacific and Indian oceans.

Taxonomic description:

Female. Total length 1.60-2.00 mm. Cephalothorax about 3 times longer than abdomen. Crest absent. Rostral rami with wide base. Ratio of width to depth in rostral excavation about 0.9-1.2. Cephalothorax anteriorly more prominent than in A. pacificus. Points of Th5 posterior corners exceeding posterior border of genital segment. A1 about same length as body. Ri Mx1 with 13 setae. Oral appendages and P1, as P3-P4 are typical for Aetideus. Ri P2 incompletely 2-segmented. There are about 15 - 17 teeth at terminal spines of Re3 P2-P4. There are about 4 small teeth at P4 coxopodite near the base of seta.

Remarks. In addition to A. armatus previously recorded from the Pacific Ocean (Farran, 1929; Wilson, 1950; Tanaka, 1957a; Vervoort, 1957; Fleminger, 1967). Bradford (1971a) recorded 3 more species she had established: A. pseudarmatus , A. truncatus and A. divergens. Bradford (1971a) pointed out difficulties with quantitative expression for some of characters used for identification. So some problems in distinction of these species still exist.

Male. (Description after Bradford (1971a) with modifications). Total length 1.30-1.53 mm. Cephalothorax about 2.73 times longer than abdomen. Points of Th5 reaching about posterior border of genital segment. Caudal rami 2.58-2.70 times longer than wide. A1 slightly longer than cephalothorax. Oral parts reduced compared with female. P1-P4 like that in female. The number of teeth at terminal spines of Re P2-P4 more than that in female (18-24). Width length second-fourth P5 segments ratio: 1:(2.3-3.1), 1:(5.7-7.0) and 1:(5.65-6.80).

Vertical distribution:

Occurs mainly in lower epi- upper mesopelagic layers, however sometimes is recorded deeper than mesopelagic waters (Jespersen, 1934, etc.). Results of studies of vertical migrations demonstrated that the largest numbers of specimens recorded at 100 and 250m (Roe, 1984), 183-366m (Farran, 1936b) and in the layer 500-600m in day and night time (Roe, 1972a, 1972b). The species was also found near bottom at 240 m (Matthews, 1964 accord. Bradford, 1971). A. armatus is a food for decapods Systellaspis debilis , Gennades elegans and Sergestes arcticus (Roe, 1984), also to fish Azgyropelecus hemigymnus (Roe and Badcock, 1984).

Geographical distribution:

A. armatus widespread in the Atlantic Ocean from 62°N (Jespersen, 1934) to 54°S (Hardy and Gunther, 1935), more common in the Northern Hemisphere. According to Bradford (1971a) With's (1915) assumption about continuous distribution of the species from the north to the south is proved. Species recorded for the Indian Ocean, the Bay of Bengal, off the Malay Archipelago (Wolfenden, 1911; Sewell, 1929; A. Scott, 1909). However A. armatus might be mixed with 3 species described after 1971 (Bradford, 1971a) so it is difficult to discuss its distribution in the Pacific Ocean. Very possible that the northernmost locality - the Bering Sea (Motoda and Minoda, 1974), the southernmost - 52°11' S 167°25' E (Farran, 1936b).

Type locality: off the Norwegian coast (Boeck, 1872).

Material examined:

1 female from sample 456. See examined samples module.

Aetideus armatus