Mesocletodes unisetosus sp. n.

Gomez, Samuel, 2018, New species of the genus Mesocletodes Sars, 1909 from the deep Gulf of California (Copepoda, Harpacticoida), ZooKeys 751, pp. 75-112 : 85-89

Mesocletodes unisetosus sp. n.

Material examined.

One male holotype preserved in alcohol (ICML-EMUCOP-270800-04), one male paratype preserved in alcohol (ICML-EMUCOP-270800-03), and one male paratype dissected and mounted onto seven slides (ICML-EMUCOP-270800-05); Talud IV cruise; August 27, 2000; coll. S. Gómez.

Type locality.

Southern Carmen Basin, Gulf of California, México (25°54.7'N, 110°11'W), 2018 m depth (see Fig. 1); coll. S. Gómez.

Dignosis (based on the male only).

Body subcylindrical. Cephalothorax, free prosomites and urosomites, except for anal somite, with posterior margin serrated. Cephalothorax dorsoventrally flattened, without dorsal cuticular process. Anal somite quadrate, with dorsal cuticular process. Caudal rami 14 times as long as wide, with seven elements. Antennule octa-segmented, haplocer, second segment with strong protrusion bearing one strong seta and two strong spinules. Antenna with basis, without exopod. Mandibles, maxillules, maxillae and maxillipeds strongly atrophied, nontraceable. P2 and P3ENP1 with one inner seta. P5 endopodal lobe with one seta.

Description of male.

Body: total length ranging from 655 µm to 695 µm (mean= 670 µm; n= 3) measured from anterior margin of rostrum to posterior margin of caudal rami, subcylindrical (Fig. 16A, B), tapering slightly posteriorly, without clear demarcation between prosome and urosome. Cephalothorax, free prosomites and urosomites, except for anal somite, with posterior margin serrated (Fig. 16A, B), of cephalothorax and free prosomites less pronounced, of urosomites comparatively coarser; lateral margin of cephalothorax plain (Fig. 16B). Rostrum fused to cephalothorax. The latter dorsoventrally flattened, without dorsal cuticular process, with few sensilla on surface and along posterior margin. P2-P4-bearing somites without spinular ornamentation, with sensilla along posterior margins. P5-bearing somite with four medial spinules dorsally (Fig. 16A) and some spinules laterally (Fig. 16B). Genital somite as preceding somite dorsally (Fig. 16A) and laterally (Fig. 16B); ventrally (Fig. 17A) without spinular ornamentation and with two sensilla, sixth leg represented by asymmetrical plate. Third and fourth urosomites as preceding somites dorsally and laterally (Fig. 16A, B), ventrally (Fig. 17A) with serrated posterior margin and long posterior spinules. Fifth urosomite with two medial spinules dorsally (Fig. 16A), laterally (Fig. 16B) and ventrally (Fig. 17A) as two preceding somites, without sensilla.

Anal somite (Figs 16A, B, 17A) quadrate; dorsally without (Fig. 16A), laterally (Fig. 16B) and ventrally (Fig. 17A) with spinules as shown; with dorsal cuticular process flanked by pair of sensilla (Fig. 16A, B).

Caudal rami (Figs 16A, B, 17 B–F) slender, exceedingly elongated, 14 times as long as wide (maximum width measured at the base of ramus), as long as urosome, almost straight in lateral view (Figs 16B, 17D), covered with small spinules; with seven elements as follows: seta I and II issuing laterally in distal part of first third of ramus, the former ventral to and smaller than the latter (Fig. 17D, F); seta III situated subdistally on dorsal surface (Fig. 17 B–E); seta IV reduced, fused basally to seta V (Fig. 17B, C, E), the latter longest; seta VI reduced, somewhat smaller than seta IV, arising at distal inner corner (Fig. 17B, C); dorsal seta VII bi-articulated, situated subdistally on posterior part of first half of ramus (Fig. 17B, D, F); with large outer pore distally (Fig. 17 B–E).

Antennule (Fig. 18A) octa-segmented, haplocer; first segment without armature, with some medial and some distal spinules; second segment with strong protrusion bearing one strong element (arrowed in Fig. 18A), with two strong spinules; third and fourth segments smallest; fifth segment somewhat swollen, with aesthetasc fused basally to slender seta, with four spiniform elements, two of which modified; sixth segment with one normal and one modified element; sixth to eight segments elongated, subequal in length. Armature formula as follows: 1-[0], 2-[6sp+2se], 3-[2sp+1se], 4-(1sp), 5-[4sp+ (1se+ae)], 6[1sp+1se], 7-[3se], 8-[2sp+7se+acro]. Spinose, spiniform elements (sp) with STE.

Antenna (Fig. 18B). Basis elongate, with few inner spinules on distal corner. Exopod absent. Endopod bi-segmented; first segment with strong inner spinules, as long as basis; second segment with inner strong spinules proximally and medially, with outer spinules on distal half of segment, laterally with one well-developed lateral spine with STE, and one very reduced element (the latter indicated in Fig. 18B, C), and with four apical elements (two spinulose spines, of which innermost smaller, and two geniculate elements).

Mandibles, maxillules, maxillae and maxillipeds strongly atrophied, non-traceable.

P 1 (Fig. 19A). Praecoxa small, with spinular row as shown. Coxa with small median and longer outer spinules. Basis with inner spinules, with outer and inner spines, the former somewhat longer. Exopod tri-segmented; EXP1 as long as EXP3, EXP2 shortest; with spinules as depicted; EXP1 and EXP2 without inner armature; EXP3 with four elements, of which outer and apical element with STE. Endopod bi-segmented, reaching proximal fourth of EXP3; ENP1 seemingly without spinular ornamentation, with one inner seta; ENP2 with few inner spinules, with one outer, one apical and one inner element.

P2-P4 (Figs 19B, 20A, B). Praecoxa small, with few spinules as depicted. Coxa with outer spinules. Basis with inner spinules; outer element of P2 spiniform (Fig. 19B), of P3 and P4 setiform (Fig. 20A, B). Exopod tri-segmented; EXP1 and EXP3 elongated, subequal in length, EXP2 shortest; EXP1 without, EXP2 with inner seta; P2EXP3 and P3 EXP 3 with two outer spines, two apical elements, and two inner setae (Figs 19B, 20A), of P4EXP3 with two outer spines, two apical elements and one inner seta (Fig. 20B). Endopod of P2 reaching tip of EXP2 (Fig. 19B), of P3 and P4 reaching slightly beyond EXP2 (Fig. 20A, B); ENP1 with one inner small seta (Figs 19B, 20A, B); ENP2 with three (P2; Fig. 19B) and four (P3 and P4; Fig. 20A, B) setae.

P5 (Fig. 21A, B) with few strong spinules on baseoendopodal setophore. Endopodal lobe poorly developed, with one seta (Fig. 21A). Exopod distinct, long, slender, 3.5 times as long as wide (maximum width measured at its base), with few inner spinules subapically, with two outer, one apical and one inner seta, and one subdistal tube pore.

Armature formula as follows:

Description of female.

Unknown.

Etymology.

The specific epithet is derived from the Latin prefix ūni, meaning one, and the Latin noun seta, meaning hair, and refers to the presence of one seta only on the endopodal lobe of the male P5. The name is a noun in the nominative singular.

Remarks.

The only species for which the male is known are M. angolaensis , M. elmari Menzel, 2011, M. fladensis , M. nudus and M. unisetosus sp. n. Of these, the female is known only for M. elmari . These species are attributed to Bodin’s (1968) inermis group. The males of M. fladensis , M. angolaensis , and M. unisetosus sp. n. possess a dorsal process on the anal somite only and lack mouth parts. Mesocletodes nudus and M. elmari , lack the dorsal process on the cephalothorax and anal somite, and of these, only the male of M. nudus lacks mouth parts. Menzel and George (2009) suggested that the lack of mouth parts in the males of M. fladensis and M. angolaensis , and consequently in M. unisetosus sp. n. and M. nudus , might support a monophylum of derived Argestidae (but see below). The same trend has been observed, for example, in the families Aegisthidae (e.g. Nudivorax Lee & Huys, 2000, Scabrantenna Lee & Huys, 2000, Andromastax Conroy-Dalton & Huys, 1999) and Pseudotachidiidae (e.g. Paranannopus Huys, 2009; see Willen 2005). Menzel (2011) suggested that, regardless the developmental stage of the mouth parts (well-developed, strongly reduced or absent), the males of Mesocletodes become non-feeding during the last moult as an adaptation to the sparsely populated and oligotrophic deep-sea environments, and could represent a derived character. Also, Menzel (2011) noted that the sexual dimorphism in M. elmari (attributed to the inermis group), the only species for which both sexes have been described, is expressed, among other characters, in the antennule, P5, P6 and most interestingly, in the armature formula of the P1-P4ENP2 (but the bi-segmented condition of the rami of the swimming legs is the same in both sexes), and that the mouth parts are present in both sexes regardless of whether the male is non-feeding or not. The males of M. fladensis , M. angolaensis , and M. unisetosus sp. n. are non-feeding and lack mouth parts. Therefore, the option for comparing the armature formula of P1-P4 of the males of these three species only was chosen, viz. with a dorsal process on the anal somite only. Among these species, M. angolaensis and M. unisetosus sp. n. possess a serrated posterior margin of the cephalothorax, and on the posterior margin of P2-bearing somite to penultimate urosomite, and only M. fladensis exhibits posterior spinules instead. An antennary basis is present in M. angolaensis and M. unisetosus sp. n., and only M. fladensis possesses an allobasis. Mesocletodes angolaensis , M. unisetosus sp. n., and probably M. fladensis share the lack of the exopod of the antenna. The Mexican species, M. unisetosus sp. n., seems to be more closely related to M. angolaensis than to M. fladensis . In addition to the characters above shared between these two species, they also share the presence of an inner seta on the P2 and P3ENP1 (this seta is missing in M. fladensis ) and the exceedingly elongated caudal rami more than 13 times as long as wide. Briefly, the male of the Mexican species, M. unisetosus sp. n., is unique and can be separated from the male of M. angolaensis , and M. fladensis , by the number of setae on the P5 endopodal lobe (two setae in M. angolaensis , and M. fladensis , but one seta only in the Mexican M. unisetosus sp. n.).