Thor leptochelus ( Xu & Li, 2015 ) Xu & Li, 2015

Komai, Tomoyuki, Okuno, Junji & Minemizu, Ryo, 2015, New records of two species of the coral reef shrimp genus Thor Kingsley, 1878 (Crustacea: Decapoda: Thoridae) from the Ryukyu Islands, Japan, Zootaxa 4013 (3), pp. 399-412 : 400-406

publication ID	https://doi.org/ 10.11646/zootaxa.4013.3.4
publication LSID	lsid:zoobank.org:pub:1F9B736F-25DF-47A9-9D16-CC9ADBDFD8E6
DOI	https://doi.org/10.5281/zenodo.6112410
persistent identifier	https://treatment.plazi.org/id/03959A02-8A64-6B0B-FF2E-FC2CFD88F98A
treatment provided by	Plazi
scientific name	Thor leptochelus ( Xu & Li, 2015 )
status	comb. nov.

Treatment

Thor leptochelus ( Xu & Li, 2015) n. comb.

[New Japanese name: Zunguri-hime-sango-moebi] Figs. 1–4 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 , 5A View FIGURE 5. A

Material examined. Cape Maeda, Onna, Okinawa Island, 7 m, 23 June 2012, SCUBA diving, coll. N. Shirakawa, 1 female (cl 1.5 mm), 1 ovigerous female (cl 1.6 mm), CBM-ZC 13090; Sunabe, Chatan, Okinawa Island, 8 m, 15 November 2013, SCUBA diving, coll. N. Shirakawa, ovigerous female (cl 2.3 mm), CBM-ZC 13091; Shichugama, Kume Island, 8 m, 8 June 2002, coll. T. Kawamoto, 1 ovigerous female (cl 1.8 mm), CMNH-ZC 997.

Diagnosis. Body very robust in adult female. Rostrum short, about half-length of carapace; distal part compressed laterally, proximal part abruptly widened in dorsal view; dorsal margin armed with 1–3 teeth, none postrostral; tip acute; ventral margin unarmed or armed with 1 tooth subterminally. Carapace with pair of supraorbital teeth; suborbital angle well defined, subacute; pterygostomial angle rounded, unarmed. Pleura of fourth and fifth pleomeres each with posteroventral tooth. Telson with 3 pairs of dorsolateral spines; posterior margin convex, with 2 pairs of spines and 1 mesial pair of plumose spines. Antennular stylocerite unarmed on dorsolateral margin proximally. No strap-like epipods on third maxilliped and pereopods. First pereopod with slender chela tapered distally and longer than carpus; merus unarmed in distal half of ventral margin. Meri of third and fourth pereopods each with 1 spine subdistally, that of fifth unarmed.

Description. Body ( Fig. 1 View FIGURE 1 ) very robust particularly in ovigerous females, carapace height greater than length.

Rostrum ( Fig. 2 View FIGURE 2 A, B) short, about half-length of carapace, directed forward, not reaching distal end of antennular peduncle, strongly widened both transversely and vertically at base; dorsal margin sloping anteriorly, armed with 1–3 teeth, rostral carina following last tooth extending slightly beyond level of posterior margin of orbit; tip acute; ventral margin with 1 conspicuous subterminal tooth or unarmed ( Fig. 2 View FIGURE 2 C); ventrolateral carina distinct in proximal two-thirds, merging into orbital margin. Carapace ( Figs.1 View FIGURE 1 , 2 View FIGURE 2 A) with height greater than length; dorsal midline without postrostral tooth, but with minute tubercle; pair of acute supraorbital teeth present; dorsum slightly inflated posteriorly; postorbital region somewhat depressed; antennal tooth small, slightly ascending; suborbital angle produced into subacute triangular process somewhat ascending; pterygostomial angle broadly rounded.

Pleon ( Fig. 1 View FIGURE 1 ) dorsally rounded. First to third pleura rounded, fourth and fifth pleura each with acute posteroventral tooth. Sixth pleomere 1.9 times as long as fifth pleomere and 1.4 times as long as proximal height. Telson ( Fig. 2 View FIGURE 2 D) 1.5 times as long as sixth pleomere, tapering to gently convex posterior margin, armed with 3 pairs of dorsolateral spines; posterior margin ( Fig. 2 View FIGURE 2 E) without median tooth, but with 3 pairs of spines, lateral pair at posterolateral angle of telson short, intermediate pair longest, mesial pair plumose, shorter than intermediate pair.

Eye ( Figs 1 View FIGURE 1 , 2 View FIGURE 2 A) large, stout, nearly subcylindrical; cornea slightly wider and distinctly longer than eyestalk, semispherical, with ocellar spot; inner surface of eyestalk strongly inflated.

Antennular peduncle ( Fig. 2 View FIGURE 2 E, G) somewhat flattened dorsoventrally, perpendicularly erect in habitus. First segment with lateral margin expanded in rounded lobe; stylocerite acuminate, overreaching distolateral margin of third segment, slightly sinuous in lateral view, unarmed on dorsolateral margin proximally; ventral surface with low swelling proximally, mesial margin terminating in small tooth. Second segment very short, with sharp distolateral tooth. Third segment bearing triangular plate articulated at dorsodistal margin. Upper flagellum very stout and fairly long, thickened aesthetasc-bearing portion slightly longer than carapace; aesthetascs extremely thick and fairly long; slender terminal portion less than half-length of thickened portion. Lower flagellum slightly shorter than total length of upper flagellum, each article with very short seta on lower side of distal margin.

Antennal peduncle ( Fig. 2 View FIGURE 2 F, H) with stout basicerite bearing moderate ventrolateral distal tooth. Carpocerite (fifth segment) short and stout, reaching midlength of antennal scale. Antennal scale ( Fig. 2 View FIGURE 2 H) 0.9 times as long as carapace, 1.6 times as long as wide, obliquely ascending (angle against horizontal plane of carapace about 45°); lateral margin straight, distolateral tooth reaching rounded distal margin of lamella.

Mandible ( Fig. 2 View FIGURE 2 I, J) without palp; molar process moderately stout, with complex masticatory surface; incisor process slender, distally with fine, closely spaced acute teeth. Maxillule ( Fig. 2 View FIGURE 2 K) with bilobed endopod, outer lobe much smaller than inner lobe, inner lobe with apparently simple, apical seta; proximal endite damaged during dissection; distal endite broad, truncate mesial margin with numerous spiniform setae. Maxilla ( Fig. 2 View FIGURE 2 L) with short endopod (= palp) far falling short of distal margin of basial endite; distal endite deeply bilobed, finely setose on mesial margin; proximal endite reduced, setose on mesial margin; scaphognathite with anterior lobe large and broadly rounded, posterior lobe small. First maxilliped ( Fig. 2 View FIGURE 2 M) with endopod (= palp) very broad, uni-articulate, bearing numerous short setae along mesial margin; distal endite broad, mesially setose; proximal endite also broad, also setose on mesial margin; exopod with flagellum well developed with numerous plumose distal setae; caridean lobe small with several marginal plumose setae. epipod large, bilobed. Second maxilliped ( Fig. 2 View FIGURE 2 N) normal; dactylus small and narrow, with dense spiniform setae on mesial margin; propodus broad, with slender spiniform setae on mesial margin; carpus with weak tooth on outer distal margin; merus stout; ischium and basis fused, mesial face forming shallow excavation; exopodal flagellum well developed; epipod roundly triangular, with small, non-lamellate podobranch.

Third maxilliped ( Fig. 3 View FIGURE 3 A) stout, overreaching distal margin of antennal scale by 0.3 length of ultimate segment. Ultimate segment about 4.0 times as long as penultimate segment (= carpus), tapering distally, tip encircled by 6 corneous spinules ( Fig. 4 View FIGURE 4 A); dorsal and ventral margins with row of tufts of stiff setae becoming shorter distally; mesial face with numerous transverse tracts of stiff setae, forming grooming apparatus. Penultimate segment (= carpus) with 2 transverse tracts of long, stiff setae on dorsomesial surface ( Fig. 4 View FIGURE 4 B). Antepenultimate segment depressed proximally, with row of long stiff setae on dorsodistal margin; distolateral margin with 1 small tooth obscured by single long spiniform seta ( Fig. 4 View FIGURE 4 C); ventrolateral margin with 1 spinule distally, ventromesial margin also with 1 spiniform seta subdistally ( Fig. 4 View FIGURE 4 B). Coxa somewhat inflated laterally, but without strap-like epipod. Exopod not reaching distal margin of antepenultimate segment.

No strap-like epipod on any pereopod.

First pereopod ( Fig. 3 View FIGURE 3 B) shortest among pereopods, not reaching midlength of antennal scale. Chela ( Fig. 4 View FIGURE 4 D) slender, tapering distally, 1.2 times as long as carpus; dactylus short, about half length of palm, terminating in 2 curved ungues; fixed finger terminating in slender unguis about twice length of those on dactylus; palm with short oblique row of stiff setae proximally on flexor surface. Carpus about twice as long as distal width, slightly shorter than palm, with grooming apparatus on mesial face. Merus slightly inflated ventrally; ventromesial margin with cluster of minute spinules medially ( Fig. 4 View FIGURE 4 E). Ischium shorter than merus, with set of 3 spiniform setae subdistally on ventromesial face ( Fig. 4 View FIGURE 4 E). Ventral margin of merus-ischium distinctly notched at articulation.

Second pereopod ( Fig. 3 View FIGURE 3 C) overreaching antennal scale by length of chela. Dactylus about 0.7 times as long as palm. Carpus 3.2 times as long as chela, divided into 6 articles of unequal length, third article longest. Merus 1.3 times as long as ischium. Ischium wider than merus, with set of few spiniform setae proximally on ventral margin.

Third to fifth pereopods generally similar in structure, but decreasing in length and stoutness toward posterior. Third pereopod ( Fig. 3 View FIGURE 3 D) overreaching antennal scale by length of dactylus. Dactylus ( Fig. 4 View FIGURE 4 F) about 0.3 times as long as propodus, 3.0 times longer than high (including unguis), distally biunguiculate (penultimate unguis curved, shorter but almost as wide as ultimate unguis); flexor margin with 4 or 5 slender accessory spinules over entire length. Propodus with field of minute spinules in distal one-third, followed by 2 rows of spinules, on flexor surface in largest specimen ( Fig. 4 View FIGURE 4 G), and row of several tufts of long setae on extensor surface; flexor surface of right propodus only with 2 rows of spinules in other three specimens ( Fig. 3 View FIGURE 3 E). Carpus about half length of propodus. Merus about 4.0 times as long as wide, armed with 1 spine located near ventrolateral distal angle.

Fourth pereopod ( Fig. 3 View FIGURE 3 F) overreaching midlength of antennal scale by tip of propodus, but not reaching distal margin of antennal scale; flexor margin of dactylus with 4 or 5 accessory spinules in addition to penultimate unguis; flexor surface of propodus with short field of minute spinules in distal one-fourth in either side; merus with 1 spine. Fifth pereopod ( Fig. 3 View FIGURE 3 G) reaching or overreaching midlength of antennal scale by tip of dactylus; flexor margin of dactylus armed with 4 or 5 accessory spinules in addition to penultimate unguis; flexor surface of propodus only with 2 rows of spinules; merus unarmed.

Protopods of first to fourth pleopods ( Fig. 1 View FIGURE 1 ) each with strongly broadened ventrolateral lamella in spawning molt, while moderately broad in non-ovigerous female. Uropod ( Fig. 2 View FIGURE 2 D) with endopod reaching posterior margin of telson; exopod slightly longer than endopod, armed with small spine just mesial to minute posterolateral tooth.

Size. Ovigerous females: cl 1.6–2.3 mm.

Colouration in life. Carapace and anterior three pleomeres mottled milky white, with scattered brownish blotches or spots; fourth to sixth pleomeres and tail fan translucent. Cornea grayish. Antennular peduncle milky white; outer flagellum reddish brown, mottled with white. Antennal peduncle and scale translucent in general, carpocerite with reddish ring distally. Third maxilliped to second pereopods generally brownish. Third to fifth pereopods with dactyli to carpi translucent with brown tint, meri darker brown. Protopods of anterior three pleopods milky white, those of fourth and fifth pleopods translucent.

Distribution. Presently known only from Xisha Islands in the South China Sea (type locality) and Ryukyu Archipelago (Okinawa and Kume islands), at depths of 7– 8 m.

Habitat. No association with other invertebrates was observed during sampling. Individuals were seen in crevices on reefs, but only during the night. They tried to escape immediately when light was used.

Variation. The body is less robust in the non-ovigerous female paratype than in the three ovigerous specimens (holotype and paratypes), and this variation could be size-related.

Notable intraspecific variation is seen in the dentition of the rostrum: there are three dorsal and one ventral teeth in the holotype, one dorsal and one ventral teeth in the ovigerous female paratypes, and one dorsal and no ventral tooth in the non-ovigerous female paratype. Bruce (1976) reported on the occasional absence of ventral tooth in Thor spinosus .

The armature of the flexor surface of the propodi of the third and fourth pereopods is also variable among the specimens examined. In the largest specimen, there is a field of numerous minute spinules on the distal part of the flexor surface of the left third pereopod propodus and the both fourth pereopod propodi, whereas such a spinulose field is absent in the right third pereopod propodus. In the other three specimens, such spinulose field is not seen on every pereopod propodi.

Remarks. The present specimens agree well with the original description of Thinora leptochelus by Xu & Li (2015), particularly in the very robust body, the ascending suborbital angle of the carapace, similar armature of the telson, absence of a conspicuous tooth on the antennular stylocerite and the tapered chela of the first pereopod. Minor discrepancies are as follows. Xu & Li (2015) stated that the antennular stylocerite is “armed with little indistinct tooth near proximal end of lateral margin”, but in our specimens, there is no trace of any tooth-like structure. According to the description by Xu & Li (2015), there are three accessory spinules on the dactylus of the third pereopod, whereas in our specimens the dactylus of the third pereopod is armed with four or five accessory spinules. The rostral armature shows variation in our specimens as mentioned above, but two of the four available specimens match the holotype of Thinora leptochelus in having one dorsal and one ventral teeth.

This species was originally assigned to Thinora in reference only to the rostral armature and the unarmed pterygostomial angle of the carapace ( Xu & Li 2015). Thinora was diagnosed as having a single dorsal and no ventral teeth on the rostrum ( Bruce 1997), but substantial variation in the rostral armature has been reported in species of Thor (e.g., Bruce 1976, 1978). The present specimens show that the rostral armature is also variable in Thinora leptochelus , as described above, and the observation led us to reevaluate the generic assignment of the species. Thinora leptochelus further differs from Thinora maldivensis and is rather similar to species of Thor in the non-dorsoventrally flattened rostrum and the dactyli of the third to fifth pereopods each bearing three to five accessory spinules in addition to penultimate unguis, rather than having only two accessory spinules in Thinora maldivensis . The shape of the pterygostomial angle of the carapace is not reliable in differentiating Thinora and Thor , and none of previous authors did mention this character to be of possible generic significance (e.g., Chace 1997; Bruce 1997). At present, male characteristics need to be verified for Thor leptochelus , but the diagnostic characters noted above clearly shows that the species should be referred to Thor .

Thor leptochelus n. comb. is the fourth species of Thor characterized by the possession of a pair of conspicuous supraorbital teeth on the carapace. The other three species are T. cordelli , T. spinipes and T. spinosus . Thor leptochelus differs from the latter three species in the lack of a small tooth near the proximal end of the antennular stylocerite and the slender, tapered chela of the first pereopod. In T. cordelli , T. spinipes and T. spinosus , the antennular stylocerite bears a small tooth on the proximal end of the stylocerite; the chela of the first pereopod is robust and not markedly tapered ( Bruce 1976, 1983; Wicksten 1996). The extremely robust body in ovigerous females could be also characteristic to the present new species, although this character might be size- or sexrelated. In the structure of the chela of the first pereopod, T. leptochelus is rather similar to T. intermedius (cf. Holthuis 1947) and T. marguitae (cf. Bruce 1973; this study). From T. spinipes , T. leptochelus is further distinguished by the comparatively wider antennal scale (1.6 times as long as wide versus about 2.5 times as long) and the much more robust ultimate segment of the third maxilliped. From T. spinosus , T. leptochelus is easily separated by the different armature of the posterior margin of the telson and the less stout dactyli of the third to fifth pereopods (about 3.0 times as long as wide versus about 2.0 times as long). In T. leptochelus , the posterior margin of the telson bears one mesial pair of plumose setae on either side of the unarmed midpoint, whereas in T. spinosus , there are two mesial pairs of plumose setae on either side of the tiny median tooth. Finally, the new species differs from T. cordelli in the lack of a median tooth on the posterior margin of the telson.

The living coloration of the new species seems to be also characteristic, in particular in the milky white cephalothorax and anterior three pleomeres and the translucent posterior three pleomeres and tail fan (telson + uropods), giving a curious impression that the body of the shrimp can only half be seen. According to Bruce (1983), in T. spinipes , the body is semitransparent but heavily mottled with superficial and deep patches of dull yellow-brown. Thor spinosus also shows a quite characteristic colour pattern on the body: the ground color of the body is dark olive brown with a superimposed pattern of fine striae; a pair of small dorsolateral, dark bluish-red eyespots is present on the third pleomere and a pair of similar spots is present on the endopods of the uropod; a single large median ventral spot is also present on the sixth pleomere ( Bruce 1976). No information on the living colouration is available for T. cordelli .