Thermocheres pacificus, Lee & Chang & Kim, 2022

Lee, Jimin, Chang, Cheon Young & Kim, Il-Hoi, 2022, Symbiotic copepods (Cyclopoida and Siphonostomatoida) collected by light trap from Korea, ZooKeys 1115, pp. 1-71 : 1

Thermocheres pacificus sp. nov.

Figs 28 , 29

Material examined.

Holotype ♂ (MABIK CR00250117) preserved in 90% alcohol, Site 22 (Yesong, Bogil Island, south coast, 34°08'11"N, 126°33'49"E), 26 Apr. 2021, leg. J. Lee and C. Y. Chang ; Paratype ♂ dissected and mounted on a slide, Site 15 (Namhae Island, south coast, 34°45'00.5"N, 127°54'33.9"E), 04 Jul. 2020, leg. J. G. Kim. Dissected paratype is retained in the collection of I.-H. Kim .

Description.

Male. Body (Fig. 28A ) cyclopiform, moderately broad. Body length 1.14 mm in dissected and figured paratype (1.25 mm in holotype). Prosome 695 μm long, four-segmented, consisting of cephalothorax and three free pedigerous somites. All prosomal somites with acutely pointed posterolateral corners. Cephalothorax slightly wider than long (477 × 486 μm), consisting of completely fused cephalosome and first pedigerous somite, fringed with membrane along posterodorsal margin. Urosome (Fig. 28B ) six-segmented. Fifth pedigerous somite narrower than genital somite, with tapered lateral apex. Genital somite quadrangular, wider than long (125 × 184 μm), with parallel lateral margins and pointed, tooth-like posterolateral corners; genital operculum well-developed, with one large cusp on distal margin and pair of unequal setae on tip of posterolateral apex. Four abdominal somites 57 × 140 μm, 45 × 125 μm, 36 × 116 μm, and 52 × 114 μm, respectively; first and second abdominal somites with acutely pointed, posteriorly extended posterolateral corners. Caudal ramus (Fig. 28B ) 1.57 × longer than wide (83 × 53 μm), with six pinnate setae, ornamented with setules along inner margin; all setae positioned distally or subdistally.

Rostrum (Fig. 28C ) slightly longer than wide, tapered, with angular apex. Antennule (Fig. 28D ) 368 μm long, 17-segmented, geniculate between antepenultimate and penultimate segments; armature formula 1+aesthetasc, 2+aesthetasc, 2+aesthetasc, 2+aesthetasc, 2+aesthetasc, 2, 2+aesthetasc, 2, 7+3 aesthetascs, 2, 2+aesthetasc, 3+aesthetasc, 1, 2+aesthetasc, 1, 1+aesthetasc, and 11; setae naked, mostly short; aesthetascs thin but that of penultimate segment thicker. Antenna (Fig. 28E ) consisting of coxa, basis, one-segmented exopod, and two-segmented endopod; coxa 35 × 17 μm, unarmed; basis 65 × 17 μm, with few spinules on outer margin; exopod 3 × longer than wide (18 × 6 μm), armed with two unequal setae distally and one seta near middle; first endopodal segment 38 × 15 μm, unarmed but with row of spinules on inner and outer margins; second endopodal segment 35 × 12 μm, terminated in long spiniform seta (107 μm long), armed with one seta on proximal inner margin, three (one minute) setae distally and subdistally, and ornamented with setules on inner and outer margin.

Oral siphon (Fig. 28F ) 454 μm long, evenly tapering from proximal to distal, extending to insertions of leg 2. Mandible (Fig. 28G ) consisting of short basal segment and elongate, slender stylet bearing 11 teeth distally. Maxillule (Fig. 28G ) bilobed; small outer lobe 23 × 9 μm, distally with four setae, one naked, other three pinnate; elongate inner lobe based on segment-like extension, 127 × 15 μm, tipped with three thin, equally long, distally feebly pinnate setae. Maxilla (Fig. 28H ) slender, consisting of syncoxa (189 μm long), basis (205 μm long) and terminal claw; basis with small seta at 70% region and small, tapering membrane distally; terminal claw (Fig. 28I ) 64 μm long, curved, with row of spinules along proximal half of concave margin. Maxilliped (Fig. 29A, B ) consisting of syncoxa, basis, four-segmented endopod, and terminal claw; syncoxa with one seta on inner distal corner; basis longest, with one blunt tubercle on inner margin bearing minute setule on distal margin of tubercle; endopodal segments with two, one, one, and one setae respectively; terminal claw slender, 103 μm long, weakly curved, ~ twice longer than terminal endopodal segment.

Legs 1-4 (Fig. 29C-F ) biramous, with three-segmented rami. Outer seta on basis naked in leg 1 but pinnate in legs 2-4. Outer spine on first exopodal segment of leg 1 large, extending beyond base of first outer spine of third exopodal segment. Second endopodal segment of legs 1-4 with bicuspid outer distal corner. Inner distal seta on third exopodal segment of leg 4 distinctly smaller than proximal setae. Armature formula for legs 1-4 as follows:

Leg 5 (Fig. 29G ) consisting of pinnate lateral seta on fifth pedigerous somite and small exopod; exopodal segment 18 × 14 μm, articulated from somite, with three naked setae (two on distal margin and one on posterior margin). Leg 6 (Fig. 29H ) represented by two naked setae on genital operculum.

Female. Unknown.

Etymology.

The specific name of the new species refers to its discovery in the Pacific Ocean, in contrast with the Indian Ocean in which the type locality, Madagascar, of the type species is located.

Remarks.

The discovery of this new species reinforces the taxonomic status of the genus Thermocheres . The type species of the genus, T. validus Kim, 2010, was described as an associate of a sponge in Madagascar ( Kim 2010b). Although the new species is represented by only a single male, it exhibits diagnostic characters of the genus. In particular, the form of the maxillule in which the inner lobe is elongated and armed with three long, slender setae and the armature condition (formula III, I, 5) of the third exopodal segment of legs 2-4 are shared by the two species. Within the Asterocheridae , the latter character is shared only by Australomyzon Nicholls, 1944 and Bythocheres Humes, 1988. But T. pacificus sp. nov. and T. validus reveal two important differences, i.e., (1) one of four setae on the outer lobe of the maxillule is positioned in middle in T. validus , whereas all of the four setae are distally positioned in T. pacificus sp. nov.; and (2) the third endopodal segment of leg 4 is armed with one spine plus four setae (formula 1, 1+I, 2), but with one spine plus three setae (formula 1, I, 2) in T. pacificus sp. nov. These two differences are so significant that the Korean material should be separated from the type species as a new species. The proportional length of the caudal ramus and the developmental condition of the protopod of leg 5 also appear different between the two species, although these characters are subject to sexual dimorphism. Thermocheres validus was described on the basis of the female only.

Figures

Figure 28. Thermocheres pacificus sp. nov., male A habitus, dorsal B urosome, ventral C rostrum D antennule (open circles indicate insertions of aesthetascs on opposite surface) E antenna F oral siphon G mandible and maxillule H maxilla I distal part of maxilla. Scale bars: 0.1 mm (A, B, F); 0.05 mm (C, D, G, H); 0.02 mm (E, I).

Figure 29. Thermocheres pacificus sp. nov., male A maxilliped B endopodal region of maxilliped C leg 1 D leg 2 E endopod of leg 3 F leg 4 G leg 5 H right genital operculum, ventral. Scale bars: 0.05 mm (A, C-F, H); 0.02 mm (B, G).