Mesocletodes simplex 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 : 79-83

Mesocletodes simplex sp. n.

Material examined.

One dissected female holotype mounted onto five slides (ICML-EMUCOP-130207-01); Talud X cruise; February 13, 2007; coll. S. Gómez.

Type locality.

Southern Trough of Guaymas Basin, Gulf of California, México (27°01'N, 110°53'04"W), 1642 m depth (see Fig. 1); coll. S. Gómez.

Diagnosis (based on the female only).

Body subcylindrical. Cephalothorax with dorsal cuticular process curved posteriorly. Genital somite and third urosomite incompletely fused dorsolaterally. Anal somite quadrate from dorsal view, with simple dorsal cuticular process curved posteriorly. Caudal rami subcylindrical, 2.5 times as long as wide, with seven setae. Antennule octa-segmented, second segment with protrusion bearing a long seta, but not as pronounced as in other species of the genus. Antenna with basis and uni-segmented exopod. Gnathobase of mandible with grinding face, and tri-segmented palp. Maxillary syncoxa with two endites, proximal endite with one, distal endite with three elements; endopod uni-segmented, with two setae. Syncoxa of maxilliped with one seta. P4ENP2 with four setae. Outer setae of the P5EXP issuing close to each other.

Description of female.

Body: total length 725 µm measured from anterior margin of rostrum to posterior margin of caudal rami, subcylindrical, tapering slightly posteriorly, without clear demarcation between prosome and urosome (Fig. 9A, B). Rostrum fused to cephalothorax, with two sensilla. Cephalothorax 0.24 times as long as entire body length; ornamented with sensilla and spinular patches as shown; with dorsal cuticular process curved posteriorly, the latter as in Fig. 9C. P2-P5-bearing somites with sensilla and small spinules along posterior margin, with minute spinules laterally. Genital somite and third urosomite (genital double-somite) incompletely fused dorsolaterally (Fig. 9A, B) (i.e. posterior margin of genital somite indicated by suture with transverse row of spinules and few sensilla dorsolaterally), completely fused ven trally (Fig. 10A); first half of genital double-somite with medial genital field proximally (Fig. 10A), with few minute spinules close to lateral margins, second half with more dense spinular patches ventrally as shown and with minute spinules along posterior margin between pair of long ventral sensilla. Fourth urosomite with short row of small spinules laterally (Fig. 9B), ventrally with transverse row of larger spinules along posterior margin interrupted by longitudinal row of minute spinules along posterior margin flanked by two long sensilla (Fig. 10A). Fifth urosomite as preceding somite except for shorter transverse row of minute spinules ventrally, without sensilla (Fig. 10A).

Anal somite (Figs 9A, B, 10A, 11A, B) quadrate from dorsal view, nearly as long as two preceding somites combined; posterior margin cleft medially; anal operculum rounded and smooth, flanked by two sensilla; with dorsal (Figs 9A, 11A), lateral (Figs 9B, 11B) and ventral (Fig. 10A) spinules as figured; with simple dorsal cuticular process curved posteriorly (Figs 9A, B, D, 11A, B), the latter with a tiny aperture (Figs 9D, 11B).

Caudal rami (Figs 9A, B, 10A, 11A, B) subcylindrical, slightly tapering posteriorly, nearly as long as anal somite and 2.5 times as long as wide; with seven setae as follows: setae I and II located midway lateral margin, the former ventral to the latter and shorter; seta III as long as seta II, arising close to outer distal corner; setae IV and V situated distally; seta VI smallest, arising at inner distal corner; dorsal seta VII bi-articulated, somewhat shorter than seta II.

Antennule (Fig. 12A) octa-segmented; first segment with one proximal and one subdistal row of spinules; second and third segments with longitudinal short row of strong spinules; second segment somewhat globose and with protrusion bearing a long seta not as pronounced as in other species of the genus (indicated by an asterisk in Fig. 12A); third segment two times as long as wide; fourth segment with one, fifth segment without spinules; sixth segment with short transverse row of smaller spinules; seventh segment with one, eighth segment without spinules. Armature formula as follows: 1-[0], 2-[5sp+3se], 3-[5sp+1se], 4-[1sp+(1sp+ae)], 5-[1sp], 6[2sp], 7-[1sp+3se], 8-[5se+acro]. Spinose, spiniform elements (sp) with STE.

Antenna (Fig. 12B). Coxa small, with few strong spinules. Basis with inner spinules. Exopod uni-segmented, with two setae. Endopod bi-segmented; first segment with strong inner spinules; second segment with some outer small spinules, inner margin with strong spinules and two thin lateral spines with STE, and six apical elements (one inner strong spinulose spine, two geniculate spinulose and one bare element, and two outer elements fused basally of which innermost longer and with STE).

Mandible (Fig. 12C, D) with robust coxa. Gnathobase with row of surface spinules, three distal single teeth as shown and several fused tooth-like elements forming a broad grinding face (Fig. 12C). Palp tri-segmented (Fig. 12D); basis with medial and distal small spinules, with one strong seta; exopod uni-segmented, small, with two setae of which innermost smaller; endopod quadrate, with six setae as shown.

Maxillule (Fig. 13A, B, C). Praecoxal arthrite with some very long spinules, two surface setae, and seven apical spines as shown (Fig. 13A). Coxa with five elements, strongest fused to coxa (Fig. 13B). Basis with seven setae (Fig. 13C).

Maxilla (Fig. 13D). Syncoxa with slender outer spinules, and with comparatively stronger spinules close to allobasis; with two endites; proximal endite with one slender seta, distal endite with one strong spinulose element and two slender smooth setae. Allobasis with few outer spinules, with one strong spinulose spine fused to allobasis, one slender seta and one spinulose spine. Endopod uni-segmented, very small; with two setae.

Maxilliped (Fig. 13E) subchelate, strong. Syncoxa with one spinulose strong seta slightly longer than basis; the latter with outer and inner spinules as shown. Endopod uni-segmented, fused to strong spinulose claw.

P1 (Fig. 14A). Coxa with sets of outer, medial, and inner strong spinules, and with outer long slender spinules. Basis with strong spinules at base of outer and inner spine. Exopod tri-segmented; exopodal segments subequal in length, with spinules as depicted; EXP1 and EXP2 without inner armature; EXP3 with four elements, of which two outermost spines with STE, innermost slender and reduced. Endopod bi-segmented, not reaching tip of EXP1; ENP1 with few outer spinules and one inner seta, shorter than ENP2; the latter with few outer spinules and three elements.

P2-P4 (Figs 14B, 15A, B). Praecoxa as in P4 (see Fig. 15B), small, with transverse row of small spinules close to coxa. Coxa with small spinules close to basis, and with stronger outer spinules on anterior face and some medial strong spinules on posterior face. Basis with strong spinules at base of outer element, between rami, and at base of endopod, and with slender long inner spinules; basis of P2 (Fig. 14B) with outer spiniform element, of P3 and P4 (Fig. 15A, B) with outer slender bare seta. Exopod tri-segmented; segments slender and elongate, ornamented as shown; EXP1 without, EXP2 with inner seta; P2EXP3 and P3EXP3 with two outer spines, two apical elements, and two inner setae (Figs 14B, 15A), P4EXP3 (Fig. 15B) with two outer spines, two apical elements and one inner seta.

P5 (Fig. 10B) with some spinules on baseoendopodal setophore; with long outer basal seta. Endopodal lobe poorly developed, with three setae (innermost lost during dissection), of which outermost and medial elements close to each other, innermost separated from the former two elements. Exopod distinct, long, slender, 6.7 times as long as wide (maximum width at distal part), with outer and inner spinules as figured, with three outer and two apical setae, and one outer distal tube pore.

Armature formula as follows:

Description of male.

Unknown.

Etymology.

The specific epithet is derived from the Latin adjective simplex , meaning simple, and refers to the simple (not bifurcated) dorsal process on the anal somite. The name is an adjective in the nominative singular.

Remarks.

Mesocletodes simplex sp. n. is attributed here to Bodin’s (1968) abyssicola group. The dorsal process on the anal somite is simple in most species within this genus, but it is bifurcated in M. brevifurca , M. katharinae Soyer, 1964, M. meteorensis , M. monensis , and M. soyeri Bodin, 1968. The appendages of the species of this group exhibit an amalgam of conditions, most of which are shared with some species of the inermis group sensu Bodin (1968). The antenna of most species of Bodin’s (1968) abyssicola group possesses a basis, but the condition of the antenna is inconclusive for M. bathybia Por, 1964a and M. brevifurca . Also, the antennary exopod of most species of this group is uni-segmented with two setae, but M. abyssicola seems to be unique among these species in that it is represented by one seta only (the condition of the antennary exopod of M. monensis is inconclusive, and the exopod of M. bathybia remains unknown). The mandibular palp of most species of this group is bi-segmented (exopod incorporated to basis, endopod uni-segmented), but uniramous in M. soyeri , M. bathybia and M. katharinae , and tri-segmented (with basis, uni-segmented exopod, and uni-segmented endopod) in M. simplex sp. n. The palp of the maxillule is uni-segmented in M. abyssicola , M. katharinae , M. meteorensis , M. robustus Por, 1965, M. simplex sp. n., and M. soyeri . Also, the proximal and distal endites of the maxilla possess one and three setae, respectively, in M. katharinae , M. meteorensis , M. simplex sp. n., and M. robustus , but two setae only in M. soyeri . The syncoxa of the maxilliped possesses two setae in most species, but one seta only in M. brevifurca , M. simplex sp. n., and M. abyssicola (the maxilliped of M. monensis , M. bathybia , and M. dolichurus Smirnov, 1946 remains unknown). The P1ENP is uni-segmented with three setae in M. abyssicola , M. robustus , and M. soyeri , but uni-segmented with one seta only in M. bathybia . A uni-segmented P1ENP is present also in the species of Bodin’s (1968) inermis group (e.g. M. makarovi Smirnov, 1946, M. guillei Soyer, 1964, M. inermis , M. trisetosa Schriever, 1983, and M. quadrispinosa ). The P1ENP is bi-segmented with an armature complement of 0,3 in the first and second segment, respectively, in M. brevifurca , M. dolichurus and M. katharinae , and 1,3 in M. meteorensis and M. simplex sp. n. The P2-P4ENP is uni-segmented in M. monensis , M. dolichurus , M. robustus and M. soyeri , and probably also in M. abyssicola and M. bathybia , but bi-segmented in the other species of the group of which, only M. katharinae lack the inner seta on P2ENP1. The female P5EXP and endopodal lobe possess five and three setae, respectively, in all the species of this group, except for M. abyssicola and M. soyeri (EXP with four, endopodal lobe with two setae). Similar armature complements and structure of P1-P5 is present in several species of the inermis group sensu Bodin (1968). The caudal rami are more than 10 times as long as wide in most species, but 2.5 to 3 times as long as wide in M. brevifurca , M. meteorensis , and M. simplex sp. n., and 6 times as long as wide in M. katharinae . Mesocletodes simplex sp. n. shares the relatively short caudal rami with M. brevifurca and M. meteorensis . The former can be easily separated from the latter two species by the dorsal process on the anal somite (simple in M. simplex sp. n., but bifurcated in the other two species). Additionally, M. simplex sp. n. seems to be more closely related to M. meteorensis than to M. brevifurca by the relative position of the outer setae of the female P5EXP (both setae separated by a wide gap in M. brevifurca , but both setae issuing close one of each other in M. simplex sp. n. and M. meteorensis ), and number of setae on the P4ENP2 (three setae in M. brevifurca , but four setae in M. simplex sp. n. and M. meteorensis ).