Plesionika unicolor Komai & Tsuchida, 2014, n. sp.

Plesionika unicolor n. sp.

(Fig. 2–5)

Material examined. Holotype: RV “Natsushima”, NT10-13 cruise, ROV “Hyper-Dolphin”, dive #1165, Northeast Nikko Seamount, 23°06.787’N, 142°21.936’E, 564 m, 31 July 2010, slurp gun, ovigerous female (pcl 13.0 mm), NSMT-Cr 22719.

Non-type: same dive, collecting point not specified, 1 juvenile (pcl 5.8 mm), JAMSTEC 1100023729.

Description. Holotype. Body (Fig. 2) moderately stout for genus; integument firm.

Rostrum (Fig. 3 A, B) gently upturned, distinctly overreaching antennal scale, slightly shorter than carapace (approximately 0.9 of carapace length); dorsal margin armed with moderately strong 11 teeth (including 8 postrostral), 3 on proximal half of rostrum widely spaced, fixed, but 7 of 8 postrostral series basally articulated; none of dorsal rostral series barbed at tip; 7 basally articulated teeth closely set, decreasing in size posteriorly, posteriormost one located at about 0.4 of carapace length; no subterminal teeth on dorsal rostral margin; ventral margin with widely spaced 6 small teeth decreasing in length distally; lateral surface with sharp carina extending to distal 0.3, merging into orbital margin. Carapace (Figs. 2, 3 A, B) with low but distinct postrostral crest extending to midlength, with peak at posteriormost dorsal spine; dorsal surface with scattered microscopic setae and tegmental scales; orbital margin nearly vertical in lateral view, with row of stiff setae; suborbital lobe not clearly delineated; antennal tooth broadly triangular, sharply pointed; branchiostegal tooth also small, not exceeding antennal tooth.

Pleon (Fig. 2) rounded, unarmed dorsally. First and second pleomeres each with distinct transverse groove concealed by row of setae on dorsal surface (Fig. 3 C). Third pleomere with slightly produced posterodorsal margin. First to third pleura rounded, fourth and fifth each with small posteroventral tooth. Six pleomere 1.7 times as long as fifth, 1.8 times longer than high, with small posteroventral tooth and acutely pointed posterolateral process. Telson damaged.

Eye (Fig. 3 A, B) large, subspherical; corneal width about 0.2 of carapace length; ocellus distinct, not constricted at base. Eyestalk short, cup-shaped.

Antennular peduncle (Fig. 3 A, B) slightly overreaching midlength of antennal scale. First segment longer than distal 2 segments combined; dorsodistal margin elevated, with several stiff setae; ventromesial ridge with tiny tooth; stylocerite acuminate, reaching distal margin of first segment, bearing distinct protuberance proximolaterally. Second segment with 4 or 5 minute spinules on dorsodistal margin. Outer flagellum exceeding 3 times as long as carapace, aesthetasc-bearing portion about 0.8 of carapace length.

Antennal peduncle (Fig. 3 B, D) with stout basicerite bearing moderately strong ventrolateral tooth. Carpocerite reaching midlength of antennal scale. Antennal scale about 0.6 times as long as carapace, 3.8 times as long as wide; lateral margin slightly convex; distolateral tooth just reaching roundly truncate distal lamella.

Third maxilliped (Fig. 4 A) slender, overreaching antennal scale by half length of penultimate segment (carpus). Ultimate segment subequal in length to penultimate segment, both with scattered long setae on dorsal, lateral and ventral surfaces, and with slender terminal spine. Antepenultimate segment subequal in length to distal two segments combined. Coxa with rounded lateral pate bearing strap-like epipod. Exopod reaching beyond midlength of antepenultimate segment.

Strap-like epipods on first to fourth pereopod and corresponding setobranchs on second and third pereopods; epipod on fourth pereopod smaller than others, no corresponding setobranch on fourth pereopod.

First pereopod (Fig. 4 B) slender, elongate, microscopically chelate, overreaching antennal scale by half length of carpus. Propodus tapering distally, 0.6 times as long as carpus, bearing grooming setae on proximal 0.3 of mesial surface ventrally. Carpus with scattered long setae. Merus-ischium combined slightly shorter than propodus-carpus combined; ischium with row of 7 minute spinules on mesial surface adjacent to ventral margin (Fig. 3 F).

Second pereopods (Fig. 4 C, D) subequal in length, overreaching antennal scale by length of chela. Chela slightly less than 0.2 length of carpus; dactylus subequal in length to palm; carpus divided into 16 (right) or 19 (left) articles, distalmost article longest and bearing 2 prominent tufts of setae. Merus slightly shorter than ischium, without annulation; ischium with row of curved stiff setae in proximal 0.6.

Third to fifth pereopods elongate, slender, similar in shape, bearing sparse long setae on each segment, fifth slightly shorter than third and fourth. Third pereopod (Fig. 4 E) overreaching antennal scale by 0.1 length of merus; dactylus (Fig. 3 G, H) less than 0.1 length of propodus, terminating in clearly demarcated unguis, bearing 2 accessory spinules on distal half of flexor margin, ultimate accessory spinule less than half length of unguis; propodus (Fig. 3 H) 0.8 times as long as carpus, with spinules arranged in 2 irregular rows on flexor margin; carpus with 7 small spines on lateral surface adjacent to flexor margin; merus subequal in length to propodus-carpus combined, about 1.4 times as long as carapace, armed with 12 lateral spines and 6 ventromesial spines; ischium with 2 ventral spines. Fourth pereopod (Fig. 4 F) just reaching antennal scale by tip of merus; carpus with 2 small spines; merus about 1.4 times as long as carapace, shorter than propodus-carpus combined, armed with 12 lateral spines and 9 ventromesial spines; ischium with 1 ventral spine. Fifth pereopod (Fig. 4 G) reaching midlength of antennal scale by tip of merus; merus 1.1 times as long as carapace, with 10 lateral spines; ischium unarmed ventrally.

Pleopods and uropod without distinctive features.

Eggs small, about 0.4 x 0.6 mm.

Non-type (juvenile). Generally similar to holotype. Rostrum just reaching distal margin of antennal scale, 0.7 of carapace length; dorsal margin armed with 12 teeth, including 4 on rostrum proper and 8 postrostral (of them posterior 7 teeth basally articulated); ventral margin with 5 teeth. Telson (Fig. 3 I) slightly falling short of posterior margin of uropodal rami, terminating in acute posteromedian projection, with 4 pairs of dorsolateral spines; posteromedian projection armed with 3 pairs of spines (second pair longest). Antennular peduncle with stylocerite reaching only midlength of first segment. Epipods on first and second pereopods normally developed, that on third pereopod rudimentary, no epipod on fourth pereopod. Carpi of second pereopods each divided in 17 articles. Meri of third to fifth pereopod with 9, 8, 8 lateral spines and 8, 7, 1 ventral or ventromesial spines, respectively.

Coloration. Body and appendages generally orange. Cornea brown, with reflective pigment. Propodi and carpi of posterior 3 pairs of pereopods whitish. See Fig. 5.

Distribution. Known only from the type locality, Northeast Nikko Seamount, 564 m.

Remarks. Plesionika Spence Bate, 1888 is the most species-rich genus in the caridean family Pandalidae, currently represented by about 90 species worldwide (De Grave & Fransen 2011; Cardoso 2011; Li & Chan, 2013). Identities of several species are still confusing in spite of recent revisionary studies (e.g., Chan & Crosnier 1991; 1997; Chan 2004; Fransen 2006). The present new species appears closest to P. picta Chan & Crosnier, 1997, presently known only from French Polynesia. Shared diagnostic characters include: rostrum relatively short, with proportional ratio against carapace length included within range of 0.8–1.0; most of postrostral teeth basally articulated; orbital margin nearly vertical posteriorly in lateral view, with a row of stiff setae; antennal tooth of carapace broadly triangular, not spiniform; telson with 4 pairs of dorsolateral spines; second pereopods subequal in length; dactyli of the third to fifth pereopods about 0.1 times as long as propodi. Morphologically, P. unicolor n. sp. can be distinguished from P. picta by some minor or subtle differences. The postrostral crest appears to be more distinct in P. unicolor n. sp. than in P. picta (cf. Fig. 3 B and Chan & Crosnier 2004: Fig. 20 a, e). Basally articulated teeth on the postrostral crest are distinctly longer in P. unicolor n. sp. (Fig. 3 B) than in P. picta (cf. Chan & Crosnier 2004: Fig. 20 a, e). The antennal tooth of the carapace is acuminate in P. unicolor n. sp., rather than blunt or at most subacute in P. picta (cf. Chan & Crosnier 2004: Fig. 20 a, e). The ultimate accessory spinules on the third pereopod dactylus is distinctly less than half length of the unguis in P. unicolor n. sp. (Fig. 3 G), whereas it is only shorter than the unguis in P. p i c t a, making the dactylus distally biunguiculate (Chan & Crosnier 2004: Fig. 20 d). In addition, the epipod on the fourth pereopod is prominent on either side in the adult holotype of P. unicolor n. sp., whereas in P. picta, the fourth pereopod is normally devoid of epipod (Chan & Crosnier 1997). As noted below, however, variation will discount the diagnostic significance of this character, but still may be useful. The color in life is quite different between the two species. As shown by previous studies, coloration is a very useful character in distinguishing species in Plesionika (e.g., Chan & Crosnier 1991; 1997; Chan 2004). In P. unicolor n. sp., the body is entirely orange without conspicuous markings. In contrast, in P. picta, the carapace is generally red with a whitish posterior margin; the abdomen is alternated with red and white bands.

It is remarkable that the development of pereopodal epipods is different between the adult and juvenile in P. unicolor n. sp. As described above, the adult holotype bears epipods on the anterior four pereopods. On the other hand, the juvenile has normally developed epipods only on the first and second pereopods; epipod on the third pereopod is rudimentary; and no epipod is present on the fourth pereopod. It is likely that epipods on the third and fourth pereopods develop with growth. Intraspecific variation in the development of the epipod on the fourth pereopod is also known in the closely related P. picta (cf. Chan & Crosnier 1997).

Etymology. Named after the uniformly orange color of the body. Used as a noun in apposition.