Heteranthessius unisetatus sp. nov.

Figs 13, 14

Material examined.

Holotype ♂ (MABIK CR00250127) dissected and mounted on a slide, Site 6 (Jukbyeon Port, Uljin, 36°49'26.4"N, 129°26'52.2"E), 21 Sep. 2020, leg. J. Lee and J. G. Kim.

Description.

Male. Body (Fig. 13A) moderately narrow. Body length 1.92 mm. Prosome 1.08 mm long, comprising cephalothorax and second to fourth pedigerous somites. Cephalothorax 690 × 596 μm, distinctly longer than wide. All prosomal somites with rounded posterolateral corners. Urosome (Fig. 13B) six-segmented. Fifth pedigerous somite 200 μm wide. Genital somite subquadrate, longer than wide (309 × 265 μm), with rounded corners. Four abdominal somites 116 × 153 μm, 91 × 131 μm, 58 × 136 μm, and 91 × 149 μm, respectively. All abdominal somites smooth, without ornamentation. Caudal ramus broad, 1.64 × longer than wide (120 × 73 μm), with six setae; outer seta (seta II) short, naked, positioned at 45% region of ramus length; dorsal seta (seta VII) small and naked; other four setae pinnate.

Rostrum (Fig. 13C) broad, with round posterior margin. Antennule (Fig. 13D) 335 μm long, seven-segmented; armature formula 3, 12+2 aesthetascs, 2, 2+aesthetasc, 4+aesthetasc, 2+aesthetasc, and 7+aesthetasc; all setae naked; aesthetascs on second, fourth, and fifth segments large, broad, longer than antennular segments; aesthetasc on sixth segment small; aesthetasc on terminal segment as long as those of proximal segments but slender. Antenna (Fig. 13E) four-segmented, with armature formula 1, 1, 3, and 4+2 claws; terminal segment (third endopodal segment) gradually narrowed distally, 2.0 × longer than wide (76 × 38 μm); two terminal claws unequal, outer longer and thicker than inner, ~ 0.9 × as long as terminal segment.

Labrum (Fig. 13F) wider than long, with shallow posteromedian incision, fringed with broad membrane along posterior margin, pair of weak, tapering lobes at posteromedial region. Mandible (Fig. 13G) simple, with curved, elongate gnathobase bearing serrate margins. Maxillule (Fig. 13H) as small, digitiform lobe tipped with one naked seta. Maxilla (Fig. 13I) as large lobe tipped with one naked seta. Maxilliped (Fig. 14A) large, consisting of three segments and terminal claw; first segment as long as wide, unarmed; large second segment with one rudiment of seta and one large tubercle ventromedially; small third segment unarmed; terminal claw large, with one spine proximally and denticles on distal half of inner margin.

Legs 1-4 biramous; outer seta on basis small; spines on rami with densely serrate margins. Legs 1-3 (Fig. 14B-D) with three-segmented rami. Leg 4 (Fig. 14E) with three-segmented exopod and one-segmented endopod. Second endopodal segment of leg 1 characteristically with two inner setae. Endopod of leg 4 small, globular, with or without inner seta. Armature formula for legs 1-4 as follows:

Leg 5 (Fig. 14G) represented by small papilla tipped with one pinnate seta 47 μm long. Leg 6 (Fig. 13B) represented by two small setae on genital operculum.

Female. Unknown.

Etymology.

The specific name of the new species is derived from Latin words, referring to the presence of a single seta on the maxillule and maxilla.

Remarks.

The genus Heteranthessius consists of four known species: H. dubius (T. Scott, 1903) from an unknown host in Scotland (T. Scott 1903), H. scotti Bocquet, Stock & Bernard, 1959 from calcareous algae at Roscoff, France (Bocquet et al. 1959), H. furcatus Stock, 1971 from a tunicate in the Mediterranean Sea (Stock 1971), and H. hoi López-González & Conradi, 1995 from an actiniarian at Gibraltar ( López-González and Conradi 1995). Heteranthessius unisetatus sp. nov. is easily distinguishable from the congeners by its unique morphological features: the maxillule bears only a single seta apically, against two setae in the four congeners, the maxilla is unsegmented, with a single seta apically, against two-segmented, with one spine or spiniform process and one seta on the distal segment in the congeners, and leg 5 is represented by a single seta, against two setae in the congeners. The most striking feature of the new species is the possession of two inner setae on the second endopodal segment of leg 1. Because the latter feature is very extraordinary and we have failed to find the same armature condition in other poecilostome copepods, it may be interpreted as an abnormality. However, it is remarkable that both left and right leg 1 display the same setation.