Gaetanus simplex is an aetideid calanoid copepod known after both sexes (female 3.20-3.50 mm and male 2.65-3.10 mm in length) from the Sargasso Sea and the Pacific Ocean. Species found in epi-, meso- and bathypelagic.
Female. Total length 3.20-3.50 mm. Cephalothorax about 3.5-4.0 times longer than abdomen. Frontal spine present, not large, curved to rostrum. Spines on Th5 posterior corners situated in dorsal part of posterior Th5 border (lateral view), usually exceed the midlength of genital segment, but may vary in length. A1 shorter than body, usually reaching genital segment. Re1 A2 lacking seta, Re2 A2 with 2 setae. Md palp base and Ri1 Md with 2 setae, Ri2 Md with 9 terminal setae. Second internal Mx1 lobe with 4 setae, Ri Mx1 with 14 setae. Lateral plate on Mxp protopodite absent. P1 with 2-segmented Re supplied with external spine on each segment, sometimes indistinct traces of subdivision between Re1 and Re2 are visible. P2 with 2-segmented Ri. P4 coxopodite with about 12-17 spines.
Male. Total length 2.65-3.10 mm. Cephalothorax about 3 times longer than abdomen. Frontal spine nearly the same as in female. It is not removed posteriorly as in close species G. armiger. Spines on Th5 slightly not reaching the posterior border of genital segment. Oral parts in comparison with that in female rudimentary. Setation of A2 is the same as in female. Ri Mx1 with 8-9 setae, Re with 11. Md palp base lacking setae. Ri1 Md with 1, Ri2 Md with 9 setae. Mxp protopodite with 2 setae distally. Re P1 indistinctly 3-segmented (after Park (1973) - 2-segmented). P2-P4 as in female, but P4 coxopodite lacking spines. P5 very close to that of G. armiger.
Remarks. When genera Gaetanus and Gaidius were united (Park, 1975b) the new name Gaetanus campbellae Park, 1975 was proposed for this species. However, this species already had earlier name: G. simplex.
The species was recorded from epipelagic, more often in meso-, upper bathypelagic, single findings known from lower bathy- and abyssopelagic (Markhaseva, 1996).
The species is known from the Sargasso Sea: region of 32°N (Deevey and Brooks, 1977); in the Pacific Ocean once recorded in the south-eastern part (Bjornberg, 1973) and widespread in the northern part of the Pacific Ocean; in the Sea of Okhotsk and the Sea of Japan, the region of the Kurile-Kamchatka Trench (Markhaseva, 1996), the Bering Sea (Motoda and Minoda, 1974) and the north-eastern part of the Pacific Ocean (Park, 1973).
Type locality: the north-western part of the Pacific Ocean (Brodsky, 1950).
More than 200 females and 18 males from samples 11, 19, 61, 62, 71, 72, 77-79, 89-91, 118, 132, 145, 156, 157, 165, 169-172, 182-186, 189, 190, 195, 216-218, 222, 227, 228, 235, 241-243, 245, 247, 248, 250, 251, 258, 259, 474. See examined samples module.
Lectotype: female, N 39804, high latitude expedition of Arctic Scientific Institution, 25 July 1946; ice-breaker "Severny Polus"; depth of site 4600 m, stratum of sampling: 4000-0 m. Kept in the Zoological Institute, Russian Academy of Sciences, St. Petersburg.