Rhynchoplax hondai Takeda & Miyake, 1971

Komai, Tomoyuki & Poore, Gary C. B., 2016, Resurrection of Rhynchoplax Stimpson, 1858, with the description of two new species from Japan and Australia (Crustacea: Decapoda: Brachyura: Hymenosomatidae), Zootaxa 4093 (4), pp. 515-524 : 516

Rhynchoplax hondai Takeda & Miyake, 1971

( Fig. 1 )

Rhynchoplax hondai Takeda & Miyake, 1971: 168 –170, fig. 3.—Sakai 1976: 149–150 (English text), 93 (Japanese text). Rhynchoplax ? hondai —Suzuki 1985: 54.

? Halicarcinus hondai —Ng et al. 1999: 81, 92 (key).—Ng & Jeng 1999: 532.—Ng et al. 2001: 15. –– Ng et al. 2008: 108 (list). Halicarcinus hondai —Ng & Chuang 1996: 6 (list; part).

Not Halicarcinus hondai —Lucas 1980: 183–185, figs 3H, 6O, 7H, 9K (= Rhynchoplax magnetica n. sp.).

Material examined. Holotype. Japan. Central Ryukyu Islands, Amami Is, Tokunoshima I., Omonawa, 22 August 1968, coll. K. Honda & M. Takeda, ZLKU 13088, presently deposited in KMNH (immature female, cl 1.5 mm).

Diagnosis. Carapace 1.25 times as wide as long, without prominent protuberance on subhepatic region; anterolateral margin unarmed, but with faint angle hardly recognisable in dorsal view; epibranchial tooth sharp, directed anterolaterally; dorsum demarcated by short gastro-cardiac groove only; outer orbital angle not produced; pterygostomial region with 1 large tubercle lateral to buccal frame. Rostrum 0.5 times as long as carapace, slender, rod-like, tapered, slightly ascending, without pseudorostral angles; with tuft of 4 long setae at apex, pair of subterminal setae; ventral surface rounded, not markedly carinate; supraocular eave ill-defined anteriorly, covering about one quarter of eyestalks. Epistome similar to that of R. minutula n. sp. Eyestalks compact, each with protuberance on anteromesial surface. Maxilliped 3 merus with anterolateral angle slightly produced, rounded. Cheliped merus almost straight in dorsal view; fingers unarmed on occlusal margins. Pereopods 1–5 meri each with prominent distal tubercle on upper margin; dactyli of ambulatory legs all falcate, each with 7 or 8 sharp erect or recurved teeth on flexor margin.

Distribution. Central Ryukyu Islands, Japan (28°N); possibly Taiwan (Suzuki 1985) but this record needs to be verified.

Remarks. The holotype is an immature female with a narrow pleon, which is not convex, and non-excavated, laterally exposed thoracic sternum; sutures separating pleomeres are discernible, but pleomeres 3–5 seem to be at least partially fused ( Fig. 1 ). No gonopores are differentiated. Pleopods 3–5 are differentiated, all short and poorly setose, but biramous; pleopods 1 and 2 seem to be absent. Consequently, it is impossible to assess the characters of the adult female. Takeda & Miyake (1971) stated that the rostrum was horizontal in the holotype, but we confirm here that the rostrum is actually slightly directed upwards. Rhynchoplax hondai is characterised by lacking distinct pseudorostral angles proximolaterally on the rostrum and its demarcation on the dorsum of the carapace being poorly developed with only a short gastro-cardiac groove evident. The presence of a prominent dorsodistal tubercle on the merus of pereopods 1–5 also distinguishes R. hondai from the two new species described in this study. In R. magnetica n. sp., meri of pereopods 2–4 possess slightly developed dorsodistal tubercle ( Fig. 3 f, g), while in R. minutula , there is no dorsodistal tubercle on merus of any pereopod ( Fig. 4 e, g–j).

It is noteworthy that with a carapace length of 1.5 mm, the immature holotype female is still larger than the smallest ovigerous female of R. minutula n. sp., also from the Ryukyu Islands. Suzuki (1985) doubtfully recorded Halicarcinus hondai from Taiwan. His record was subsequently cited by Ng et al. (1999, 2001) and Ng & Jeng (1999) but no new specimens were reported. In spite of our efforts, the whereabouts of the material studied by Suzuki (1985) remain unknown. Prof. H. Suzuki kindly informed us that the material was deposited in the Manazuru Marine Center for Environmental Research and Education, Yokohama National University (MMCER), but he did not know about the present condition (H. Suzuki, personal communication, 26 November 2015). At our request, Dr T. Kikuchi searched the biological collections in MMCER but no material corresponding to the material studied by Suzuki (1985) was located. Consequently, the record of Suzuki (1985) is questionably included in the synonymy. The specimens referred to Halicarcinus hondai by Lucas (1980) from Australia are described below as a new species, R. magnetica .

Figures

FIGURE 1. Rhynchoplax hondai Takeda & Miyake, 1971. Holotype, immature female, cl 1.5 mm, KMNH-ZLKU 13088. a, carapace and eyes, dorsal view; b, rostrum and cephalic appendages, lateral view; c, frontal part of carapace, ventral view; d, left maxilliped 3, outer view; e, thoracic sternum and pleon, ventral view; f, right cheliped (pereopod 1), upper view; g, same, chela, extensor (outer) view; h, right pereopod 2, upper view; i, left pereopod 3, upper view; j, right pereopod 4, upper view; k, left pereopod, 5, upper view. Scale bar = 0.5 mm.

FIGURE 3. Rhynchoplax magnetica n. sp. Paratype female, cl 1.97 mm, QM W. 4067. a, dorsal carapace; b, rostrum and eyestalk, ventral view; c, rostrum, antennnules, antennae and epistome, ventral view; d, posterior thoracic sternum, pleomere 1; e, pleon (half scale); f, pereopod 2; g, pereopod 5. Scale bar = 1 mm.

FIGURE 4. Rhynchoplax minutula n. sp. Holotype, ovigerous female, cl 1.5 mm CBM-ZC 13121. a, carapace and eyes, dorsal view; b, rostrum and cephalic appendages, lateral view; c, frontal part of carapace, ventral view; d, right maxilliped 3, outer view; e, right cheliped (pereopod 1), upper view; f, same, chela, extensor (outer) view; g, right pereopod 2, upper view; h, right pereopod 3, upper view; i, right pereopod 4, upper view; j, left pereopod, 5, upper view. Scale bars = 0.5 mm.