Palaemonella okunoi, Komai, Tomoyuki & Yamada, Yusuke, 2015

Komai, Tomoyuki & Yamada, Yusuke, 2015, A new species of the palaemonid shrimp genus Palaemonella Dana, 1852 (Crustacea: Decapoda: Caridea) from Okinawa Island, Ryukyu Islands, Japan, Zootaxa 3964 (3), pp. 352-362 : 353-359

Palaemonella okunoi n. sp.

[New Japanese name: Gareba-mukashi-kakure-ebi] Figs. 1–5

Material examined. Holotype: Cape Maeda, Onna Village, Okinawa Island, Ryukyu Islands, 30 m, coral rubble, 6 July 2012, coll. Y. Yamada, ovigerous female (cl 3.6 mm), CBM-ZC 13081.

Paratype: Awa, Nago, Okinawa Island, 5–10 m, coral rubble, 25 April 2013, coll. N. Shirakawa and S. Komai, 1 ovigerous female (cl 3.7 mm), CBM-ZC 13082.

Diagnosis. Rostrum slightly descending, reaching distal end of antennular peduncle, but far falling short of distal margin of antennal scale, armed with 7 or 8 dorsal teeth, including 2 or 3 postrostral, and 2 or 3 ventral teeth; posteriormost tooth of rostral series located at about anterior 0.2 of carapace, not markedly separated from other teeth. Carapace without supraorbital teeth; hepatic tooth normal in position; postorbital ridge distinct. Eyestalk not narrowing proximally. Second pereopod with carpus unarmed on distomesial margin, slightly longer than palm; merus without conspicuous tooth at ventrolateral distal angle; ischium unarmed. Third to fifth pereopods slender, propodus of third pereopod 19–20 times as long as wide; dactyli slender (6.0–6.5 times as long as wide); propodi each with 4 or 5 spinules on flexor margin (except for terminal spinule).

Description. Body ( Figs. 1 , 2 A, E) subcylindrical; tegmental surface smooth, glabrous.

Rostrum ( Fig. 2 B, C) moderately slender, slightly descending, nearly straight, about 0.6 times as long as carapace, reaching distal end of antennular peduncle; dorsal margin gently sloping anteriorly, armed with 7 (holotype) or 8 (paratype) moderately small teeth, including 2 (holotype) or 3 (paratype) posterior to rostral base (only one tooth posterior to level of hepatic tooth), none of them markedly curved, posteriormost tooth not particularly separated from other teeth, located at about 0.2 of carapace length; ventral margin slightly convex in lateral view, with 2 (paratype) or 3 (holotype) teeth in distal 0.4; lateral surface with weak carina extending to distal 0.2, posteriorly merging into orbital margin. Carapace ( Fig. 2 A–C) with low but distinct postrostral median ridge extending to midlength; postorbital ridge distinct, delimiting deep orbital concavity accommodating basal part of eyestalk; suborbital angle produced into triangular subacute lobe not reaching beyond antennal tooth; antennal tooth moderately small, directed forward; no trace of subraorbital tooth; pterygostomial angle slightly produced, rounded; hepatic tooth normally located, distinctly posterior to anterolateral margin and inferior to level of antennal tooth (not branchiostegal in position).

Epistome ( Fig. 3 C) with pair of low, rounded protuberances on either side of midline. Labrum moderately large ( Fig. 3 D); anterior surface sharply carinate medially. Fourth thoracic sternite ( Fig. 3 A) with spine-like median process directed anteroventrally, flanked by pair of small, medially directed teeth. Fifth sternite (Fig.) 3A with pair of acute submedian teeth slightly curving laterally. Posterior sternites unarmed.

Pleon ( Fig. 2 E) dorsally rounded. First to fourth pleura rounded, fifth and sixth pleura each with small posteroventral tooth. Sixth pleomere 1.7 times as long as fifth pleomere and 1.4 times as long as high; posterolateral process terminating in sharp tooth. Telson ( Fig. 2 G) about 1.3 times as long as sixth pleomere and 2.7 times as long as wide, armed with 2 pairs of long dorsolateral spines (anterior spine located at 0.4 of telson length, posterior pair at 0.7); lateral margin parallel in anterior 0.4, tapering to subtruncate posterior margin in 0.6; posterior margin ( Fig. 2 F) armed with 3 pairs of spines, lateral pair short, intermediate pair elongate, about 0.3 times as long as telson, mesial pair slender, about 0.6 length of intermediate pair.

Eye ( Fig. 2 B, C) relatively small; cornea semispherical, not dilated, darkly pigmented, shorter than eyestalk, its basal width equal to width of eyestalk; ocellar spot evident. Eyestalk subcylindrical, not narrowing proximally.

Antennular peduncle ( Fig. 2 A, C) reaching distal one-fourth of antennal scale. First segment broad, lateral margin slightly straight, distolateral angle produced, terminating in small sharp tooth, distal margin mesial to distolateral tooth gently convex; dorsal surface concave; ventromesial margin with small acute tooth at about midlength; stylocerite short, acute, located slightly proximal to midlength of first peduncular segment. Second segment less than 0.3 length and half width of first segment. Third segment slightly longer than second segment. Lateral flagellum biramous, 9 proximal articles fused; shorter ramus short, consisting of 4 articles; 2 or 3 moderately long aesthetascs present on ventrodistal margin of each article of free shorter ramus and 1 article basal to 2 rami ( Fig. 3 B).

Antenna ( Figs. 2 A, C; 3C) with stout basicerite bearing small, curving acute tooth on distolateral margin. Fifth segment of antennal peduncle (carpocerite) cylindrical, reaching proximal one-third of antennal scale. Antennal scale about 0.6 of carapace length, 2.9 times as long as wide, lateral margin nearly; distolateral tooth moderately strong, distinctly overreaching rounded distal lamella. Flagellum about 3 times as long as carapace.

Mandible ( Fig. 3 D–F) with small, uniarticulate palp bearing few short setae; molar process robust, truncate distally, with 3 principal peripheral teeth; incisor process large, armed with 3 strong, unequal teeth distally. Maxillule ( Fig. 3 G) with palp deeply bilobed, outer lobe tapering distally; inner lobe short, strongly curved inward; proximal endite extending as far as distal endite, slightly tapering to roundly truncate tip, bearing stiff setae distally; distal endite subrectangular, with double row of spines and stiff setae on truncate distal margin. Maxilla ( Fig. 3 H) with distal endite divided into two elongate lobes; proximal endite rudimentary; palp moderately stout, curving mesially, tapering distally to blunt tip. First maxilliped ( Fig. 3 I) with proximal endite somewhat thickened, mesial margin doubled; distal endite suboval, separated from proximal endite by deep notch followed by transverse suture; endopod moderately stout, tapering distally to subacute tip, reaching as far as distal margin of distal endite; exopod well developed, with long flagellum, caridean lobe moderately broad; epipod slightly bilobed. Second maxilliped ( Fig. 2 J) with dactylus and propodus clearly separated; mesial margin of dactylus with numerous stiff setae; propodus with row of moderately long stiff setae on mesial margin; carpus with prominent triangular tooth on outer side; ischium and basis incompletely fused; coxa with mesial margin produced into rounded process; exopod moderately long; epipod moderately large, subrectangular, without podobranch.

Third maxilliped ( Fig. 4 A) with endopod moderately slender, falling slightly short of distal margin of antennal scale. Coxa with large oval lateral plate (= epipod). Basis short, partially fused to ischiomerus. Ischiomerus strongly bowed in lateral and dorsal views, thickened distally, with row of spinules on distal half of ventral margin ( Fig. 4 B). Carpus 0.7 length of ischiomerus, 4.8 times as long as wide. Ultimate segment 0.6 length of carpus, 4.2 times as long as wide, tapering distally to blunt tip, with numerous stiff setae on margins and mesial face. Exopod well developed, reaching distal end of ischiomerus. Single arthrobranch minute, non-lamellate, bud-like.

First pereopod ( Fig. 4 C) slender, reaching distal margin of antennal scale by mero-carpal articulation. Chela ( Fig. 4 D) 4.9 times longer than wide; dactylus 1.3 of palm length; cutting edges of fingers entire, forming thin edge; palm subcylindrical, 2.2 times as long as wide. Carpus slightly widened distally, 1.3 length of chela, 9 times as long a wide. Merus 0.8 length of carpus. Ischium 0.7 length of merus. Grooming apparatus consisting of 4 short transverse rows of setae on proximomesial portion of palm and transverse row of short setae plus tuft of long setae located near ventromesial distal angle of carpus ( Fig. 4 E).

Second pereopods ( Figs. 1 ; 5A) subequal, similar, moderately slender, overreaching distal margin of antennal scale by length of chela and carpus. Chela ( Fig. 5 B) 1.7 of carapace length, 5.9 times longer than wide in dorsal view; surfaces smooth and glabrous. Dactylus slender, 0.9 length of palm, terminating in curved, acute tip; fixed finger similarly slender, slightly deflexed, terminating in curved, acute tip crossing tip of dactylus; cutting edges of both fingers forming thin plate, each with 2 low teeth engaging alternately in proximal one-third. Palm subcylindrical, slightly swollen, 3.3 times longer than wide. Carpus slightly widened distally, 0.7 of chela length, 1.5 length of merus, 6.4 times longer than high, with distinct constriction subdistally; surfaces smooth; mesiodistal margin with 2 blunt lobes divided for each other by conspicuous notch ( Fig. 5 C). Merus 4.9 times longer than high, without conspicuous distal or subdistal tooth on ventrolateral margin. Ischium distinctly shorter than merus.

Ambulatory legs (third to fifth pereopods) relatively slender. Third pereopod ( Fig. 4 F) overreaching distal margin of antennal scale by full length of propodus; dactylus ( Fig. 4 G) simple, compressed laterally, feebly curved, 0.2 of propodal length, 6.0–6.5 times as long as wide, unguis clearly demarcated, bearing tuft of short setae on extensor margin distal to midlength; propodus 19–20 times as long as wide, with 1 pair of spinules on flexor distal margin and 5 or 6 widely spaced spinules on flexor margin; carpus 0.6 of propodal length; merus about 11–12 times as long as wide; ischium 0.7 length of merus. Fourth pereopod ( Fig. 4 H) similar to third pereopod, overreaching distal margin of antennal scale by 0.8 length of propodus. Fifth pereopod ( Fig. 4 I) generally similar to third and fourth pereopods, but slightly longer than them, overreaching distal margin of antennal scale by 0.6 length of propodus; dactylus ( Fig. 4 J) longer than that of third pereopod; propodus about 26 times as long as wide, with tuft of short setae, representing grooming apparatus, on distal part of flexor surface, followed by 5 widely spaced spinules; merus 14 times as long as wide.

Protopods of pleopods slender. First pleopod with endopod about 0.4 length of exopod, moderately narrow. Second to fifth pleopods each with appendix interna arising at about midlength of endopod.

Uropod with protopod bearing small acute posterolateral tooth. Exopod distinctly overreaching tip of telson (except for terminal spines), with submarginal row of setae along lateral margin; lateral margin nearly straight, terminating in small acute tooth, with 1 movable spines just mesial to posterolateral tooth. Endopod subequal in length to exopod.

Coloration in life. Body and appendages entirely translucent, without conspicuous markings; scattered, minute red dots on carapace and pleon. Corneas darkly pigmented. Eggs green. See Fig. 1 .

Distribution. Known only from Okinawa Island, Ryukyu Islands, at depths of 5– 30 m.

Habitat. The two specimens examined in this study were collected from substrata consisting of coral rubble. During sampling, no association with other invertebrates was observed, and therefore, it can be considered that the present new species is free living.

Etymology. It is our pleasure to dedicate this new species to our colleague, Dr. Junji Okuno (Coastal Branch of the Natural History Museum and Institute, Chiba), in recognizing his contributions to the taxonomy and systematics of decapod crustaceans, in particular, Pontoniinae .

Remarks. The present new species is assigned to Palaemonella in reference to the presence of a hepatic tooth on the carapace and the presence of a small, uniarticulate palp on the mandible, although in the holotype, the palp of the left mandible was damaged during dissection. The presence of the mandibular palp was confirmed on the non-dissected right mandible for the holotype ( Fig. 3 D) and also in the paratype with non-dissected mandibles.

Palaemonella okunoi n. sp. appears close to P. hachijo , presently known from the Japanese Archipelago and New Caledonia ( Li & Bruce 2006). These two species are distinguished from all the other congeneric species by the absence of a supraorbital tooth on the carapace, the normally positioned hepatic tooth on the carapace (not branchiostegal in the position), the presence of a distinct postorbital ridge delimiting a deep depression accommodating the basal part of the eyestalk, and the relatively slender ambulatory legs (third to fifth pereopods) with entire, non-subdivided propodi ( Okuno 1999, 2000). Nevertheless, the new species differs from P. hachijo in the following particulars ( Okuno 1999, 2000): (1) the rostrum reaches the distal end of the antennular peduncle in the new species, while overreaching it in P. hachijo ; (2) the posteriormost postrostral tooth is more anteriorly located in P. okunoi n. sp. than in P. hachij o (at the anterior 0.2 versus anterior 0.3); (3) the suborbital angle is more clearly delimited in P. okunoi n. sp. than in P. hachijo ; (4) the fourth thoracic sternite bears a pair of submedian teeth flanking the spiniform median process in P. okunoi n. sp., whereas such a pair of submedian teeth is absent in P. hachijo ; (5) the antennal scale is relatively wider (2.3 times as long as wide versus 4.0 times) with a rounded distal lamella in P. okunoi n. sp. rather than having a subtruncate distal lamella in P. hachijo ; (6) the first pereopod is relatively stouter in P. okunoi n. sp. than in P. hachijo (for example, the merus is 7.5 times as long as wide in the new species versus more than 15 times as long as wide in P. hachij o); (7) the merus of the second pereopod is devoid of a conspicuous tooth at the ventrolateral distal angle in P. okunoi n. sp., rather than having a conspicuous acute tooth in P. hachijo ; (8) the third to fifth pereopods are stouter with less elongate dactyli in P. okunoi n. sp. than in P. h ac h i j o (for example, the dactylus of the third pereopod is about 6.5 times as long as wide in the new species versus more than 8.0 times as long in P. hachijo ).

Palaemonella burnsi Holthuis, 1973 and P. pusilla Bruce, 1975 are also somewhat similar to P. okunoi n. sp. in the general rostral armature, the absence of a supraorbital tooth on the carapace, moderately slender dactyli of the third to fifth pereopods and the unarmed carpus of the second pereopod. Nevertheless, the new species is immediately distinguished from these two species by the presence of a distinct postorbital ridge on the carapace, which is absent in P. burnsi and P. pusilla . Furthermore, P. okunoi n. sp. differs from P. bur nsi in the shape of the eyestalk (proximally not narrowing in P. okunoi n. sp. versus proximally narrowing in P. bur nsi ) and the antennal scale with a distolateral tooth slightly overreaching the distal lamella (versus exceeded by the distal lamella in P. burnsi ) ( Holthuis 1973). The less narrowing antennal scale with a rounded distal lamella and the more slender third to fifth pereopods distinguish P. okunoi n. sp. from P. pusilla . In P. pusilla , the distal lamella of the antennal scale is obliquely truncate with a produced mesial angle ( Bruce 1975). In addition, the carpus of the second pereopod is longer than the palm in P. okunoi n. sp., whereas vice versa in P. pusilla .

Of the 21 known species of Palaemonella , the following seven species are known from Japanese waters ( Hayashi 2001; Bruce 2005; updated by Hayashi 2009): P. burnsi Holthuis, 1973 , P. hachijo , P. longidactylus Hayashi, 2009 , P. po t t s i, P. rotumana (Borradaile, 1898) , P. spinulata Yokoya, 1936 , and P. t e n ui pe s Dana, 1852. The present new species represents the eighth of the genus known from Japanese waters. All of these eight species are shallow water inhabitants. There is little doubt that the future study will eventually reveal the presence of more species in Japanese waters, particularly from sublittoral to bathyal depths or from submarine caves.

Identification keys to the Indo-West Pacific species of Palaemonella are available in previous references (Bruce 2002, 2010), but an updated key is proposed herewith to accommodate P. okunoi n. sp. For preparation of the key, the following literature were referred: Kemp (1922); Holthuis (1973); Bruce (1975, 1978, 1991, 2002a, b, 2003, 2008; 2010); Okuno (1994; 1999); Hayashi (2001; 2009); Li & Bruce (2006); Li et al. (2008); Marin (2008). Two unnamed species are included following to the previous keys to show potential diversity of the genus. The rostral dentition is used case by case, because the number of the rostral teeth is not always reliable as a key character because of intraspecific variation.

Figures

FIGURE 1. Palaemonella okunoi n. sp., holotype, ovigerous female (cl 3.6 mm), CBM-ZC 13081, entire animal in lateral view, showing coloration in fresh condition.

FIGURE 2. Palaemonella okunoi n. sp., holotype, ovigerous female (cl 3.6 mm), CBM-ZC 13081. A, carapace and cephalic appendages, lateral view; B, rostrum and anterior part of carapace, lateral view; C, rostrum, anterior part of carapace and cephalic appendages, dorsal view (marginal setae on left antennal scale omitted); D, orbital region of carapace, showing suborbital angle, dorsolateral view; E, pleon, telson and left pleopods and uropod, lateral view; F, telson, dorsal view; G, same, posterior part, dorsal view. Scale bars: 0.5 mm for B, F, G; 1 mm for A, C, D, E.

FIGURE 3. Palaemonella okunoi n. sp., holotype, ovigerous female (cl 3.6 mm), CBM-ZC 13081. A, fourth and fifth thoracic sternites and coxae of first and second pereopods, ventral view; B, proximal part of left outer antennular flagellum, lateral view; C, left antenna and epistome, ventral view (setae omitted); D, labrum, paragnath and basal part of right mandible, ventral view (allow indicating mandibular palp); E, left mandible, outer view (palp missing); F, same, inner view; G, left maxillule, outer view; H, left maxilla, outer view; I, left first maxilliped, outer view; J, left second maxilliped, outer view. Scale bars: 0.5 mm.

FIGURE 4. Palaemonella okunoi n. sp., holotype, ovigerous female (cl 3.6 mm), CBM-ZC 13081, left thoracic appendages. A, third maxilliped, lateral view; B, same, distal part of ischio-merus, mesial view (setae omitted); C, first pereopod, lateral view; D, same, chela, extensor view; E, same, flexor mesial view, showing grooming apparatus; F, third pereopod, lateral view; G, same, dactylus and distal part of propodus, lateral view; H, fourth pereopod, lateral view; I, left fifth pereopod, lateral view; J, same, dactylus and distal part of propodus, lateral view. Scale bars: 0.5 mm for A, B, D, E, G, J; 1 mm for C, F, H, I.

FIGURE 5. Palaemonella okunoi n. sp., holotype, ovigerous female (cl 3.6 mm), CBM-ZC 13081. A. left second pereopod, lateral view; B, same, chela, extensor view; C, proximal part of palm and distal part of carpus, mesial view. Scale bars: 0.5 mm for C; 1 mm for A, B.