Athanas exilis Komai & Henmi, 2023

Athanas exilis Komai & Henmi, 2023

( Figs. 6–10 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 View FIGURE 10 )

Athanas exilis Komai & Henmi, 2023: 364 View Cited Treatment , figs. 1–4.

Material examined. CBM-ZC 17869 , 1 male (cl 5.4 mm), Japan, Honshu , Wakasa Bay, off Ine, 2018, dredge ; CBM-ZC 17870 , 1 male (cl 6.4 mm,), Japan, Honshu , Wakasa Bay, off Ine, 35°46’03”N 135°20’21”E, 95 m, dredge, coll. Y. Henmi, 13 July 2021, DNA voucher ( LC832869 View Materials ) GoogleMaps ; CBM-ZC 17871 , 2 males (cl 5.7 mm, one damaged), Japan, Honshu , Wakasa Bay, off Ine, 35°44’39”N 135°20’00”E, 95 m, dredge, coll. Y. Henmi, 12 May 2022 GoogleMaps ; CBM-ZC 17872 , 1 ovigerous female (cl 4.2 mm), Japan, Honshu , Wakasa Bay, off Ine, 35°45’10”N 135°19’55”E, 92 m, dredge, coll. Y. Henmi, 21 June 2023 GoogleMaps ; CBM-ZC 17873 , 1 male (cl 5.7 mm), Japan, Honshu , Wakasa Bay, off Ine, 35°45’19”N 135°19’60”E, 92 m, dredge, coll. Y. Henmi, 19 July 2023 GoogleMaps .

Supplemental description. New material generally similar to female holotype except for antennae, male first pereopods (chelipeds) and male second pleopods.

Rostrum ( Fig. 6A–C View FIGURE 6 ) 0.2–0.25 times as long as carapace, falling short of distal margin of basal article of antennular peduncle. Antennular peduncle ( Fig. 6A–C View FIGURE 6 ) proportionally longer in males than in females. First pereopods ( Figs. 7–9 View FIGURE 7 View FIGURE 8 View FIGURE 9 ) carried flexed back at mero-carpal articulation, subequal or slightly unequal, somewhat dissimilar in shape of dactylus and carpus; when flexed, fingertips reaching to base of ischium.

Chela strongly compressed and expanded, leaf-shaped, 1.5–1.7 times as long as wide. Dactylus 0.2–0.3 length of palm, obliquely articulated against strongly deflexed pollex, tapering to acute or subacute tip, crossing with pollex, curved, more strongly curved in major chela; extensor margin with 1–2 small subdistal tubercles and sometimes blunt protuberance proximally; occlusal margin with subrectangular or subtrapezoidal proximal tooth and excavation, better developed in major chela. Pollex short, triangular, terminating in subacute tip, with subrectangular or subtrapezoidal tooth medially on occlusal margin, better developed in major chela, fitting into excavation on occlusal margin of dactylus; flexor margin with row of small tubercles. Palm slightly thickened along midline, deeply constricted proximally; extensor margin convex, proximal margin expanded to reach distal margin of carpus; flexor margin more convex than dorsal margin, bearing row of low irregular, usually obtuse teeth or tubercles; proximal margin produced to reach distal margin of carpus; mesial surface shallowly concave on ventral part. Carpus somewhat dissimilar between sides, about half-length of chela, 2.9–3.2 (major chela) or 3.1–4.6 times (minor chela) as long as wide at its widest point at distal one-third, tapering proximally, with shallow constriction subdistally; extensor and flexor margins each with row of small tubercles, mesial face sometimes with additional row of small tubercles adjacent to dorsal margin; lateral face smooth. Merus 1.6–1.7 times length of carpus, 4.4 times as long as wide; widest point proximal to mid-length, slightly broader than ischium; dorsal margin faintly sinuous, smooth or with row of minute tubercles; lateral surface almost flat; mesial face bisected by blunt median ridge bearing row of minute tubercles, dorsal half smooth or with scattered minute tubercles, ventral half shallowly excavated; lower margin faintly sinuous, with row of minute tubercles at least in distal half. Ischium 0.8 times as long as merus; dorsal margin slightly convex, armed with 8–10 long spiniform setae in proximal half; mesial face with longitudinal row of 3 or 4 long spiniform setae in proximal one-third and longitudinal row of minute tubercles along midline, ventral half slightly concave; lateral face nearly flat or faintly sulcate; ventral margin nearly straight of faintly sinuous, with row of minute tubercles over entire length and sometimes with 1 long spiniform seta proximally. Basis with minute, bud-like exopod. Coxa large, rounded, similar from left to right.

First pleopod endopod ( Fig. 6D View FIGURE 6 ) about half length and width of exopod, gradually tapering to pointed apex, slightly curved mesially. Second pleopod with appendix masculina rod-like, 1.3 times longer than appendix interna ( Fig. 6E View FIGURE 6 ), with about 10 stiff setae terminally or subterminally.

Variation in cheliped. The first pereopods (chelipeds) are subequal to slightly unequal in length with dissimilar fingers and carpi. In the major chela, the dactylus is more strongly curved; the occlusal teeth on the dactylus and pollex are stronger; and the carpus is stouter. In three specimens (CBM-ZC 17869, 17871 and 17873), the right cheliped is major, while the left cheliped is major in one specimen (CBM-ZC 17870); thus, the handedness does not seem to be fixed on one side. In the ovigerous female (CBM-ZC 17872), the right cheliped is normally developed, while the left cheliped is much shorter, slenderer and with a soft integument, indicating a regeneration process.

Colouration in life. Generally similar to holotype ( Komai & Henmi 2023: 68, fig. 1A, B). First pereopod with chela generally translucent, thickened median part whitish; fingers yellowish white; carpus with tinge of pale red; merus and ischium with scattered pale red spots ( Fig. 10 View FIGURE 10 ).

Distribution. Presently known only from Wakasa Bay, Sea of Japan, at depths of 90– 95 m.

Remarks. Athanas exilis was originally described based on a single ovigerous specimen from Wakasa Bay collected at a depth of 90 m. In the original description, Komai & Henmi (2023) compared A. exilis with A. sydneyensis Anker & Ahyong, 2007 from eastern Australia and A. amazone Holthuis, 1951 from the eastern Atlantic. With the present male specimens, it is now clear that A. exilis is closest to A. phyllocheles Banner & Banner, 1983 , presently known only from the western Indian Ocean off Réunion, at depths of 345–450 m, with more distant affinities to A. amazone . All three species have unusually expanded, leaf-shaped chelae on the first pair of pereopods in males ( Fig. 7 View FIGURE 7 ; Banner & Banner 1983: fig. 13d–f; Anker & Ahyong 2007: fig. 5). The morphological differences between A. exilis and A. phyllocheles are as follows: (1) the rostrum falls short of the distal margin of the basal article of the antennular peduncle in A. exilis ( Fig. 6A–C View FIGURE 6 ; Komai & Henmi 2023: fig. 2B, C), rather than reaching it in A. phyllocheles ( Banner & Banner 1983: fig. 13a, b); (2) the rostrum is devoid of a dorsal carina in A. exilis ( Fig. 6C View FIGURE 6 ; Komai & Henmi 2023: fig. 2C), which is present in A. phyllocheles ( Banner & Banner 1983: fig. 13a); (3) the extracorneal tooth falls far short of the distal corneal margin in A. exilis ( Fig. 6B, C View FIGURE 6 ; Komai & Henmi 2023: fig. 2B, C), whilst reaching to the distal corneal margin in A. phyllocheles ( Banner & Banner 1983: fig. 13a, b); (4) the male chelae are much more expanded in A. exilis than in A. phyllocheles (cf. Figs. 7A, C View FIGURE 7 , 8A View FIGURE 8 ; Banner & Banner 1983: fig. 13d–f); (5) the dorsal and ventral proximal margins of the male chelae are expanded into rounded lobes in A. exilis ( Figs. 7A, C View FIGURE 7 , 8A View FIGURE 8 ), but not in A. phyllocheles ( Banner & Banner 1983: fig. 13d–f); (6) the meri and ischia of the male chelipeds are ridged along the midline and bear a row of minute tubercles on their mesial face in A. exilis ( Fig. 7A, C View FIGURE 7 ), whereas in A. phyllocheles , the mesial faces of the meri and ischium are smooth ( Banner & Banner 1983: fig. 13d–f); and finally, (7) the dactylus of the third pereopod is proportionally longer in A. exilis than in A. phyllocheles (0.6 times as long as the propodus versus 0.4 times as long).