Sars, 1900
Short Description:
Gaetanus brevispinus is an aetideid calanoid copepod known after both sexes (female 3.60-4.90 mm and male 2.30-4.10 mm in length). Cosmopolitan species, found mostly in meso- bathypelagic, in high latitudes may occur in epipelagic.
Taxonomic Description:
Female. Total length 3.60-4.90 mm. Cephalothorax 3.0-3.7 times longer than abdomen. Frontal spine absent. Posterior Th5 corners with moderate spine, covering about one third of genital segment length (dorsal view) and directed slightly to the ventral side of specimen (lateral view). A1 length variable, may be longer than cephalothorax: reaching Abd2, or the end of caudal rami. Re1 A2 lacking setae, Re2 A2 with 2 setae. Md palp base with 2, Ri1 Md with 2, Ri2 Md with 9 terminal setae. Second Mx1 internal lobe with 4 setae, Ri with 14-15 setae. Mxp protopodite with lateral plate, its shape variable. Re P1 3-segmented, Re2 and Re3 with external spines, second external spine not reaching the base of next spine. Ri P2 2-segmented, P2-P4 typical of the genus, P4 coxopodite with about 23-27 spines.
Male. Total length 2.30-4.10 mm. Frontal spine absent. Cephalothorax about 3-4 times longer than abdomen. Length of spines on Th5 posterior corners variable. A1 slightly shorter than body. Number of setae on Re1-Re2 A2 reduced. After With's evidence (With, 1915: pl. 3, fig. 1e) setation of the third internal Mx1 lobe reduced, Ri supplied with 12 setae, Re with 11 setae. Mxp protopodite with only 2 distal setae (With, 1915, pl. 3, fig. 1). Re1 P1 lacking external spine, Re2 P1 with small poorly visible spine. P2-P4 as in females, but spines on P4 coxopodite absent. Left Re3 P5 not bilobated, elongated rather triangular then stylet-like, but wide. It is shorter than Re2 of its leg.
Remarks. G. brevispinus described from Arctic Basin (Sars, 1900). The descriptions of close species G. affinis from the North Atlantic (Sars, 1905) and G. intermedius from Antarctic (Wolfenden, 1905) appeared later. The following features were proposed as distinguishing for these species: shape of lateral plate on Mxp protopodite, length of spines on Th5, position of rostrum between A1 (when looking laterally) and body length. These features are highly variable. It seems that in classifying the specimens to particular species the researchers paid more attention to the geographical locality of finds than to the morphological structure. Thus it was considered that G. brevispinus occurs mostly in Arctic Basin, G. affinis in temperate Atlantic and tropical waters of the Atlantic and the Pacific Oceans and G. intermedius in the Southern Hemisphere mostly to the South from 35°S (Sars, 1905; Vervoort, 1952, 1957, 1963; Park, 1978; Bradford and Jillett, 1980; etc.). G. affinis and G. intermedius were recognized as very close species (Vervoort, 1957). Park (1978) proposed their identity, but did not synonymize them. I have studied the specimens of these 3 species from regions adjacent to their type localities and failed to verify the validity of earlier proposed features as distinguishing for the species. It was found that these features are very much variable not only in specimens from distant localities, but in specimens of the same sample. Furthermore the degree of variability on populational level was found more pronounced than between distant populations. No significant differences in body size were found. For groups of specimens from the central part of the Arctic Ocean, the Norwegian Sea and the north-western part of the Pacific Ocean (localities considered to be typical for G. brevispinus ) total body length was 4.40-4.85 mm, for specimens from region of 40°N 66°W (close to typical for G. affinis locality): 3.85-4.65 mm and for specimens from Antarctic and south-eastern Atlantic (localities typical for G. intermedius ): 4.30-4.90 mm. P5 structure of G. affinis and G. intermedius males after Park (1975b) is nearly identical and also in my opinion identical to that of G. brevispinus. Thus Park's assumption concerning G. affinis and G. intermedius identity is absolutely right and in my opinion both species are identical to G. brevispinus.
Vertical distribution:
In high latitudes the species sometimes occurs near surface, but usually deeper than 200 m, in other regions of the World Ocean it was found in hauls from meso- and bathypelagic (Jespersen, 1934; Vervoort, 1957; Markhaseva, 1996, etc.) and in a big number of total hauls from depths from 3000 m.
Geographical distribution:
Cosmopolitan species (except the Indian Ocean), but in the equatorial and subtropical zones rather infrequent; recorded from the Gulf of Guinea, the region of the Malay Archipelago, the south-eastern and tropical parts of the Pacific Ocean (Vervoort, 1949, 1963; Wilson, 1950; Bjornberg, 1973; etc.). However the species is often recorded in boreal zones of the Atlantic and Pacific oceans and also to the North to the central part of the Arctic Ocean to 86°N (Markhaseva, 1996), species is distributed circumpolar in subantarctic and antarctic zones where it is recorded mostly to the south between 35°S and 76°S (Farran, 1929).
Type locality: Arctic Basin (Sars, 1900).
Material examined:
72 females and 9 males from samples: 18, 61-64, 72, 73, 79, 90-94, 118-121, 132, 134-136, 145, 146, 156-160, 171-174, 188-191, 195, 218, 251, 294, 359, 376, 384, 393, 429, 431, 435, 436, 447, 449, 453, 456, 476, 525, 527, 530, 532, 533, 535, 539, 542, 543, 545-547, 551-555, 570, 574. See examined samples module.