Rhynchothorax nopperabo, Matsushita & Kakui, 2024

Matsushita, Hiroki & Kakui, Keiichi, 2024, Two new Rhynchothorax sea spiders from Japan, Spixiana 47 (1), pp. 15-28 : 17-20

Rhynchothorax nopperabo sp. nov.

Figs 1a- c , 2 , 3

[New Japanese name: Įjďĵ‹fflÙṢḤŦ (Noppera-ibo-umigumo)]

Etymology. The specific name is for the mythical Japanese faceless ghost “Noppera-bo”, referring to the lack of eyes.

Material examined. Holotype: Female ( ICHUM 8636 ), three slides and one vial; Ikari-kai, Shiriuchi Town, Hokkaido Prefecture, Japan (41°32'08.8''N, 140°25'46.9''E), intertidal zone, among gravel, 27 June 2021, coll. by Aoi Tsuyuki and Yuki Oya . Paratypes: Three females ( ICHUM 8637 , three slides and one vial; ICHUM 8638 , one vial; ICHUM 8639 , one vial); same collection information as for holotype .

Diagnosis (female). Anterolateral border of cephalic segment not produced into horns; no dorsal setae on lateral processes; one posterior tubercle on first lateral process; two posterior tubercles on second lateral process not reaching anterior margin of third lateral process; proboscis with nearly smooth dorsal surface; eyes lacking; cephalic segment with anterodorsal extensions; terminal palp segment unilobed; seventh to ninth segments of oviger with four ventral (one middle and three distal), three ventro-subdistal, three ventro-subdistal terminally-digitiform spines, respectively; auxiliary claws present, longer than half claw length.

Descriptions of holotype female

Trunk ( Figs 1a , 2a, b ) four segmented, decreasing in size anterior to posterior, without dorsal tubercles; total length about three times trunk width at second lateral process. Anterolateral border of cephalic segment lacking horns. Fourth trunk segment fan shaped. Lateral processes all separated by less than half their diameter at base, without dorsal setae.First lateral process directed anterolaterally, with large posterior tubercle; second lateral process directed slightly anteriorly, with two large posterior (not reaching anterior margin of third lateral process) and one small anterior tubercles; third lateral process directed slightly posteriorly, with one small anterior and one small posterior tubercles; fourth lateral process directed posterolaterally, with small anterior tubercle.

Proboscis ( Fig. 2a,b ) egg shaped with two pairs of slight dorsolateral swellings; mouth triradiate. Anterior tip of proboscis not reaching distal tip of palp in lateral view.

Ocular tubercle and eyes ( Fig. 2a,b ) lacking.

Abdomen ( Fig. 2a,b ) conical, not reaching distal margin of second coxa of fourth leg; ventral surface flat; tip slightly trifurcate.

Chelifores ( Fig. 2a,b ) lacking; cephalic segment with flat triangular anterodorsal extensions.

Palp ( Figs 1b , 2c ) with five segments. First segment tiny, without setae.Second segment cylindrical, with dorsodistal seta. Third segment cylindrical, without setae. Fourth segment cylindrical, with large dorsal tubercle at two-thirds of length from proximal end, several ventrodistal, one mid-dorsal, and one lateroproximal tiny setae, and laterodistal seta; dorsal tubercle cylindrical, length about half that of fourth segment, with two latero-subdistal and one distal setae. Fifth segment arched dorsally, unilobed, semicircular in shape, with many setae.

Oviger ( Fig.2d,e ) with 10 segments.First to third segments cylindrical, without setae; first segment originating from flat ventral surface of trunk. Fourth and sixth segments cylindrical. Fifth segment arched ventrally. Fourth to sixth segments without setae. Seventh segment cylindrical, with one mid-ventral and three ventrodistal spines. Eighth and ninth segments oval, each with three ventro-subdistal spines. All spines on seventh to ninth segments terminally digitiform. Tenth segment oval, with large terminal claw, distal seta, medial terminally-digitiform spine, two ventroproximal conical spines, and serrated lamellar projection between terminal claw and proximal-most conical spine.

Legs ( Fig. 3a- d ) slender. First coxa cylindrical, with anterodistal tubercle on first and second legs ( Fig. 3a, b ), and posterior tubercle on third leg ( Fig. 2b ). Second coxa cylindrical, with sexual pore on posterior surface of fourth leg ( Fig. 1c ). Third coxa cylindrical. Femur and first and second tibiae cylindrical, each with long dorsodistal seta and several tiny setae as illustrated; three ventrodistal setae on second tibia. Tarsus with ventral seta. Propodus cylindrical, slightly arched ventrally, with many setae. Claw short, broad. Two auxiliary claws, narrow, longer than half claw length.

Measurements. Primarily based on holotype; in parentheses are corresponding measurements for paratype ICHUM 8637; nd, no data.

Proboscis length 0.51 (0.51), trunk length 0.90(0.90), abdomen length 0.22 (0.21), total length 1.62(1.62), proboscis width 0.27(0.29), trunk widths at first and second lateral processes 0.53 (0.48), 0.50(0.51).

Lengths for palp: second to fifth segments 0.26(0.25), 0.06 (0.06), 0.13 (0.15), 0.08 (0.07) [total length 0.53 (0.53)]; dorsal tubercle on fourth segment 0.07(0.07).

Lengths for oviger: second to tenth segments 0.06(nd), 0.08 (0.08), 0.13 (0.12), 0.09 (0.09), 0.13 (0.13), 0.08(0.07), 0.06(0.06), 0.05(0.04), 0.06(0.06), terminal claw 0.04 (0.04) [total length from second segment to terminal claw 0.78 (nd)].

Lengths for first leg: first coxa 0.14 (nd), second coxa 0.17(0.18), third coxa 0.12(0.08), femur0.28(0.27), first tibia 0.26 (0.26), second tibia 0.26(0.26), tarsus 0.03(0.03), propodus 0.20(0.20), claw 0.07(0.06) [total length 1.52 (nd)]; auxiliary claw 0.05 (0.05).

Lengths for second leg: first coxa 0.10 (nd), second coxa 0.12 (0.14), third coxa 0.10 (0.10), femur 0.21 (0.20), first tibia 0.22(0.18), second tibia 0.20 (0.22), tarsus 0.03 (0.03), propodus 0.18 (0.18) claw 0.06 (0.06) [total length 1.22 (nd)]; auxiliary claw 0.05(0.05).

Lengths for third leg: first coxa 0.07(nd), second coxa 0.11(0.12), third coxa 0.09(0.09), femur0.22(0.22), first tibia 0.20 (0.21), second tibia 0.21(0.21), tarsus 0.03(0.03), propodus 0.20(0.19) claw 0.07(0.06) [total length 1.19 (nd)]; auxiliary claw 0.04 (0.05).

Lengths for fourth leg: first coxa 0.08 (nd), second coxa 0.13 (0.13), third coxa 0.09 (0.09), femur 0.25 (0.23), first tibia 0.24(0.24), second tibia 0.24 (0.22), tarsus 0.03 (0.05), propodus 0.24 (nd, broken) claw 0.07(nd, broken) [total length 1.37(nd)]; auxiliary claw 0.04 (nd, broken).

Genetic information. A partial 18S sequence was determined from one paratype (ICHUM 8639, 1695 bp, LC795945 ). The sequence in the INSD most similar to our 18S sequence, as determined by BLAST searches ( Altschul et al. 1990), was from the pycnogonid Pycnogonum tuberculatum Clark, 1963 ( KX536444 ; identity score 96.36 %, query cover 100%; Sabroux et al. 2017). A single congeneric 18S sequence had previously been deposited in INSD, from nominal R. mediterraneus collected from Japan ( AB292215 ; Nakamura et al. 2007); this sequence appeared in ninth place in our BLAST results (identity score 96.21 %, query cover 99 %). One Hedgpethia ( Colossendeidae ) and six Pycnogonum sequences appeared in the second to eighth places, with 96.02-96.53% identity scores and 100 % query cover.

Remarks. Rhynchothorax nopperabo sp. nov. is similar to the six congeners in the philopsammum -group sensu Krapp (1973) ( R. alcicornis Krapp, 1973 ; R. anophthalmus, Arnaud, 1972 ; R. architectus Child, 1979 ; R. arenicolus Stock, 1989 ; R. philopsammum Hedgpeth, 1951 ; and R. vallatus Child, 1990 ) in lacking eyes, having auxiliary claws, and having anterodorsal extensions on the cephalic segment. It differs from all of them except R. vallatus in having a unilobed terminal segment of the palp, the anterolateral border of the cephalic segment not produced into horns, and the two small posterior tubercles on the second lateral process not reaching the anterior margin of the third lateral process, and in lacking mid-dorsal tubercles on the proboscis.

Rhynchothorax nopperabo differs from R. vallatus as follows (character states of R. vallatus in parentheses; Child 1990): the oviger has a mid-ventral and three ventrodistal spines on the seventh segment, three ventro-subdistal spines on each of the eighth and ninth segments, two ventroproximal conical spines and a medial spine on the tenth segment (one mid-ventral and one ventrodistal spine on the seventh; one mid-ventral spine on each of the eighth and ninth; one ventroproximal spine on the tenth); the spines on the seventh to ninth segments of the oviger are terminally digitiform (conical); each lateral process lacks dorsal setae (one); the first lateral process has one posterior tubercle (two); the auxiliary claws are 0.63-0.81 times as long as the claw (less than half the claw length).

The 18S sequence in the INSD most similar to that from R. nopperabo , as determined by BLAST searches, was not from a confamilial species but from the pycnogonid Pycnogonum tuberculatum . All of the top nine sequences determined by BLAST searches (one from Rhynchothoracidae , seven from Pycnogonidae , and one from Colossendeidae ) showed approximately 96% similarity to our sequence, i. e., our sequence differed from one confamilial and eight non-confamilial sequences to the same extent. This suggests that the 18S data alone may be insufficient to distinguish these three families.

Figures

Fig. 1. New Rhynchothorax species described herein, fixed specimens; Rhynchothorax nopperabo sp. nov., female, holotype, ICHUM 8636 (a, c), and paratype ICHUM 8637 (b, after soft tissues dissolved for DNA extraction); Rhynchothorax monoceros sp. nov., female, holotype (d, e), ICHUM 8640. a, d. Habitus, dorsal view; b. palp first segment (arrow), ventral view; c, e. sexual pore (arrows) on second coxa of fourth leg (c, intentionally stained with black oil-based paint). Scale bars: 1 mm (a,d); 100 µm (b); 50 µm (c,e).

Fig. 2. Rhynchothorax nopperabo sp. nov., female, holotype, ICHUM 8636. a, b. Body, lateral (a) and dorsal (b) views; c. right palp; d. left oviger; e. distal part of right oviger. Scale bars: 1 mm (a, b); 100 µm (c-e).

Fig. 3. Rhynchothorax nopperabo sp. nov., female, holotype, ICHUM 8636. a. Right first leg; b. right second leg; c. right third leg; d. right fourth leg. Scale bar: 100 µm (a-d).