Rhynchothorax monoceros, Matsushita & Kakui, 2024

Rhynchothorax monoceros sp. nov.

Figs 1d, e View Fig , 4 View Fig , 5 View Fig , 6 View Fig

[New Japanese name: ‹jbḩ‹fflÙṢḤŦ (Ikkakuibo-umigumo)]

Etymology. The specific name is the Latin noun (nominative case) for unicorn, referring to the long, anteriorly directed apical extension of the ocular tubercle.

Material examined. Holotype: Female ( ICHUM 8640 View Materials ), six slides and one vial; Nishikomi , Setouchi Town, Amami-Oshima Island, Kagoshima Prefecture, Japan (28°13'14.0''N, 129°10'35.0'' E), 15-30 m depth, among dead coral collected by SCUBA diving, 5 November 2015, coll. by Keiichi Kakui. GoogleMaps Paratypes: Two females ( ICHUM 8641 View Materials , one vial; ICHUM 8642 View Materials , seven slides); same collection information as for holotype. GoogleMaps

Diagnosis (female). Trunk three segmented, with third and fourth trunk segments fused; trunk less than four times as long as proboscis; dorsal tubercles lacking on third lateral process; eyes present; ocular tubercle with long, anteriorly directed apical extension overhanging approximately proximal half of proboscis; anterodorsal extensions of cephalic segment lacking, seventh segment of oviger without mid-ventral spines, first coxa of fourth leg with dorsal tubercle; auxiliary claws absent.

Descriptions of holotype female

Trunk ( Figs 1d View Fig , 4a- d View Fig ) three segmented, decreasing in size anterior to posterior. Dorsomedian tubercle present on cephalic, second trunk, and third trunk segments, respectively; all tubercles conical, about half as tall as trunk height, with rounded apex. Total length about three times trunk width at second lateral process. Anterolateral border of cephalic segment not produced into horns. Third and fourth trunk segments fused; fourth trunk segment fan shaped. Lateral processes all separated by less than half their diameter at base. First, second, and fourth lateral processes each with large dorsal tubercle. First lateral process directed anterolaterally; second lateral process directed slightly anteriorly; third lateral process directed slightly posteriorly; fourth lateral process directed posterolaterally. Two dorsal conical tubercles present posterior to ocular tubercle, about half as tall as dorsomedian tubercles on trunk.

Proboscis ( Fig. 4a- d View Fig ) egg shaped, slightly bent down, with vertical mouth, gentle mid-dorsal bulge, and two ventroproximal bulges. Anterior tip of proboscis not reaching distal tip of palp in lateral view.

Ocular tubercle ( Fig. 4a- d View Fig ) present on anterodorsal surface of cephalic segment, as tall as dorsomedian tubercles on trunk, with eyes. Long, anteriorly directed apical extension of ocular tubercle overhanging approximately proximal half of proboscis.

Abdomen ( Fig.4a- d View Fig ) tapering-cylindrical, reaching beyond distal margin of second coxa of fourth leg; ventral surface flat; tip slightly bifurcate.

Cephalic segment lacking chelifore and anterodorsal extensions.

Palp ( Fig. 4e, f View Fig ) with five segments. First segment short, without setae. Second segment cylindrical, with many tiny setae, one dorsodistal, one ventrosubproximal, and one ventrodistal setae, and large dorsodistal tubercle; dorsodistal tubercle cylindrical, length equal to second-segment width, with distal seta. Third segment cylindrical, with mid-dorsal bulge, mediodistal robust seta, four ventral setae (two median, two distal), and one mid-lateral seta; mid-dorsal bulge with three setae. Fourth and fifth segments cylindrical, with many setae.

Oviger ( Fig. 5a- c View Fig ) with 10 segments. First segment (not illustrated) cylindrical, originating from shallow ventral concavity of trunk. Second and third segments cylindrical. Fourth to sixth segments cylindrical; fourth segment with one dorsodistal and two mid-dorsal setae. Seventh segment cylindrical, with thick tip and two ventrodistal bifurcate spines. Eighth and ninth segments oval, with four mid-ventral spines; spine shapes on eighth segment (from medial to lateral) conical, bifurcate, bifurcate, conical; spine shapes on ninth segment (from medial to lateral) conical, bifurcate, conical, conical. Tenth segment oval, with large terminal claw, distal seta, medial conical spine, and one mid-ventral conical and one mid-ventral bifurcate spines.

Legs ( Fig. 6a- d View Fig ) slender; first leg longest. First to third coxae cylindrical, with several tiny setae; first coxa of each leg with dorsal tubercle taller than dorsal tubercle on lateral tubercle ( Figs 4a- c View Fig , 6b, c View Fig ); second coxa with sexual pore on posterior surface of fourth leg ( Figs 1e View Fig , 6b View Fig ). Femur and first and second tibiae cylindrical, each with long dorsodistal seta (detached in holotype) and several tiny setae as illustrated; femur thicker than tibiae, with mid-dorsal seta; second tibia with three ventrodistal setae. Tarsus with two ventral setae.Propodus cylindrical, slightly arched ventrally, with many setae. Claw long and broad. Auxiliary claws absent.

Measurements. Primarily based on holotype; in parentheses are corresponding measurements for paratypes ICHUM 8641 (left) and ICHUM 8642 (right); nd, no data

Proboscis length 0.37(0.37,0.33), trunk length 0.65(0.62,0.58), abdomen length 0.15(0.15,0.14), total length 1.17 (1.14,1.05), proboscis width 0.16 (0.18, 0.16), trunk widths at first and second lateral processes 0.33(0.40,0.34), 0.37(0.40,0.35), length of anteriorly directed apical extension of ocular tubercle 0.19(0.19,0.14).

Lengths for right palp: first to fifth segments 0.02 (nd,0.02), 0.20 (nd,0.17), 0.11(nd,0.10), 0.04 (nd, 0.04), 0.02 (nd, 0.02) [total length 0.40 (nd, 0.36)]; dorsal tubercle on second segment 0.07 (nd,0.08). Lengths for left palp: first to fifth segments nd (nd, 0.02), nd (nd,0.17), nd (nd, 0.11), nd (nd, 0.04), nd (nd, 0.02) [total length nd (nd, 0.36)]; dorsal tubercle on second segment nd(nd, 0.08).

Lengths for right oviger: second to tenth segments nd(nd, 0.05), nd (nd, 0.05), nd (nd, 0.11), nd (nd, 0.07), nd (nd, 0.08), 0.05 (nd, 0.05), 0.05 (nd, 0.05), 0.04 (nd, 0.05), 0.05 (nd, 0.05); terminal claw 0.04 (nd, 0.04) [total length for second to tenth segments nd(nd,0.59)]. Lengths for left oviger: first to tenth segments nd(nd,0.05), nd(nd,0.05), nd(nd,0.04), 0.11 (nd, 0.11), 0.08 (nd, 0.06), 0.09 (nd, 0.09), 0.05 (nd,0.05), 0.04 (nd, 0.05), 0.04 (nd,0.05), 0.05(nd, 0.05), terminal claw 0.03(nd,0.04) [total length nd(nd,0.62)].

Lengths for right first leg: first coxa 0.10(nd, nd), second coxa 0.08 (nd,0.07), third coxa 0.06 (nd, 0.07), femur 0.28 (nd,0.26), first tibia 0.27(nd, 0.27), second tibia 0.22(nd, 0.21), tarsus 0.04 (nd, 0.04), propodus 0.20(nd, 0.20), claw 0.09 (nd, 0.09) [total length 1.33 (nd, nd)]. Lengths for left first leg: first coxa nd(nd, 0.07), second coxa nd (nd, 0.07), third coxa nd (nd, 0.07), femur nd (nd, 0.25), first tibia nd (nd, 0.26), second tibia nd (nd, 0.21), tarsus nd (nd,0.04), propodus nd (nd, 0.19), claw nd (nd, 0.09) [total length nd (nd, 1.25)].

Lengths for right second leg: first coxa nd (nd, 0.07), second coxa nd (nd, 0.06), third coxa nd (nd,0.07), femur nd (nd, 0.22), first tibia nd (nd, 0.18), second tibia nd (nd, 0.14), tarsus nd (nd, 0.03), propodus nd (nd, 0.19), claw nd (nd, 0.09) [total length nd (nd, 1.06)]. Lengths for left second leg: first coxa 0.05 (nd, nd), second coxa 0.06 (nd, 0.06), third coxa 0.06(nd, 0.06), femur 0.21 (nd, 0.21), first tibia 0.19 (nd, 0.18), second tibia 0.15 (nd, 014), tarsus 0.04 (nd, 0.04), propodus 0.20 (nd, 0.18), claw 0.09 (nd,0.10) [total length 1.05 (nd, nd)].

Lengths for right third leg:first coxa 0.06(nd,0.06), second coxa 0.06 (nd, 0.06), third coxa 0.05(nd, 0.06), femur 0.21(nd, 0.20), first tibia 0.18 (nd, 0.17), second tibia 0.14(nd, 0.14), tarsus 0.04 (nd, 0.03), propodus 0.19 (nd, 0.19), claw 0.10 (nd, 0.10) [total length 1.03 (nd, 1.02)]. Lengths for left third leg: second coxa nd (nd, 0.07), third coxa nd (nd, 0.05), femur nd (nd, 0.20), first tibia nd (nd, 0.17), second tibia nd (nd,0.14), tarsus nd(nd,0.04), propodus nd (nd, 0.19), claw nd (nd, 0.10) [total length nd (nd, nd)].

Lengths for right fourth leg: first coxa 0.05, second coxa nd, third coxa 0.06, femur 0.20, first tibia 0.17, second tibia 0.15, tarsus 0.04, propodus 0.19, claw 0.10 [total length nd (nd, nd)]. Lengths for left fourth leg: first coxa (nd,0.05), second coxa (nd, 0.06), third coxa 0.05 (nd, 0.05), femur 0.20(nd, 0.19), first tibia 0.17(nd, 0.17), second tibia 0.14 (nd,0.14), tarsus 0.03 (nd, 0.03), propodus 0.20 (nd, 0.19), claw 0.10 (nd,0.10) [total length nd (nd, 0.99)].

Remarks. Rhynchothorax monoceros sp. nov. is similar to 10 species in the malaccensis - and mediterraneus - groups sensu Krapp (1973) ( R. barnardi Child & Hedgpeth, 1971 ; R. crenatus Child, 1982 ; R. malaccensis Stock, 1968 ; R. mediterraneus Costa, 1861; R. monnioti Arnaud, 1974 ; R. oblongus (Pushkin, 1977) ; R. orientalis Child, 1988 ; R. sidereus Sabroux, Hassanin & Corbari, 2022 ; R. tiahurensis Müller, 1989 ; R. unicornis Fage & Stock, 1966 ) in having eyes, and in lacking auxiliary claws and anterodorsal extensions of the cephalic segment. It differs from all of them except R. mediterraneus in having the ocular tubercle with a long, anteriorly directed apical extension that overhangs approximately the proximal half of the proboscis, and a dorsal tubercle on the first coxa of the third leg. Additional characters distinguishing R. monoceros sp. nov. from the above nine species and R. mediterraneus are listed in electronic Supplementary material: Table S1.

Rhynchothorax monoceros differs from R. mediterraneus as follows (character states of R. mediterraneus in parentheses; Dohrn 1881): no dorsal tubercles on the third lateral process (one); first coxa of the fourth leg with a dorsal tubercle (tubercle lacking); seventh segment of the oviger with two ventrodistal spines (one mid-ventral and two ventrodistal spines); tenth segment of the oviger with a median conical spine (spine lacking).

Miyazaki & Stock (1995) reported sea spiders from Tanabe Bay, Shirahama, Wakayama Prefecture, Japan, as R. mediterraneus , but their species lacked dorsal tubercules on the third and fourth lateral processes, which distinguishes it from R. mediterraneus sensu Dohrn (1881) and from R. monoceros .