Rhopalophthalmus orientalis O. Tattersall, 1957

Hanamura, Yukio, Murano, Masaaki & Man, Alias, 2011, Review of eastern Asian species of the mysid genus Rhopalophthalmus Illig, 1906 (Crustacea, Mysida) with descriptions of three new species, Zootaxa 2788, pp. 1-37 : 23-29

Rhopalophthalmus orientalis O. Tattersall, 1957

( Figs. 14–17 )

Rhopalophthlmus orientalis O. Tattersall, 1957: 91 , fig. 2a–f. — Ii 1964: 173, figs. 44, 45. — Tattersall 1965: 79, fig. 1. — Wang & Liu 1997: 205, fig. 4. — Liu & Wang 2000: 116, fig. 28.

Rhopalophthalmus egregious — Nakazawa 1910: 255, pl. 8, figs. 12, 22. — Tattersall, 1921: 408.

Material examined. Japan. Todoro, Miyazaki Prefecture: 18 males (all damaged), 13 females (BL ca. 7.5–11.5 mm), 1 ovig. female (BL ca. 11 mm); 31 Oct 1937, coll. N. Ii ( NSMT Cr 21232). — Susaki, Kochi Prefecture: 3 females (all damaged), 1 juv. (BL 5.0 mm), 20 abdomens of males and females; 20 Nov 1935, coll. N. Ii (No. 241) ( NSMT Cr 21233). — Susaki-Inlet, Kochi Prefecture: 23 males (BL ca. 8.5–12.5 mm), 28 females (BL ca. 7.7–12 mm), 7 ovig. females (BL ca. 10.5–11 mm); 24 Aug 1933, coll. N. Ii (No. 243) ( NSMT Cr 21234). — Coast of Natsushima, Tsuruga, Fukui Prefecture: 43 males (BL ca. 7–12 mm), 33 females (BL ca. 7–11 mm), 1 ovig. female (BL 11.8 mm), 1 juv.; 2 Nov 1938, coll. N. Ii (No. 261) ( NSMT Cr 21235). — Ariake Bay, Nagasaki Prefecture: 3 males (BL ca. 8–12.2 mm), 3 females (BL ca. 9.5–11.5 mm) (slightly damaged due to dried); sledge, 6 m depth, mud, 25 June 1975, coll. T. Takita ( NSMT Cr 21236). — Tateyama Bay, entrance of Tokyo Bay, Chiba Prefecture: 1 male (BL ca. 7.5 mm); 19 June 1975, other details unknown ( NSMT Cr 21237). — Off Kawanoe, Hiuchi-nada, Seto Inland Sea: 1 ovig. female (BL ca. 11 mm); 16 Nov 1994, from stomach of five spot flounder Pseudorhombus pentophthalmus Günther, 1862 , coll. R. Shibata ( NSMT Cr 21238).

Thailand. Sapum Bay, Phuket Is.: 2 males (BL ca. 6.0, ca. 7.0 mm), 5 females (BL ca. 7.5–9.5 mm), 1 abdomen; 19 Feb 1980, coll. Marine Fisheries Laboratory, Department of Fisheries, Thailand ( NSMT Cr 21342).

Malaysia. Matang mangrove estuary: 1 male (BL 9.7 mm); St. 2, 4˚50.6' N, 100˚35.3' E, sledge, 4.5 m depth, 8 Apr 2005, coll. Y. Hanamura ( NSMT Cr 21239). — 15 males (BL 6.5–9.5 mm), 16 females (BL ca. 5.3–12 mm), 11 ovig. females (BL 8.7–11.3 mm); St. 2, 5˚39.2' N, 100˚23.9' E, sledge, 4.5 m depth, 8 Apr 2005, coll. Y. Hanamura ( NSMT Cr 21240). — 44 males (BL 5.0– 9.5 mm), 50 females (BL 5.1–10.0 mm), 11 ovig. females (BL 8.7– 9.5 mm), 45 juvs. (BL 2.6–5.1 mm); offshore of St. 4 (4˚51.4' N, 100˚32.9' E), sledge, 2.4 m depth, 14 June 2006, coll. Y. Hanamura ( NSMT Cr 21241). —Merbok mangrove: 32 males (BL 5.9–9.2 mm), 17 females (BL 5.7–9.7 mm), 7 ovig. females (BL 8.3–9.8 mm), 2 juvs. (BL 2.5, 3.2 mm); St. B, 5˚39.2' N, 100˚23.9' E, sledge, 2 m depth, 10 Aug 2005, coll. Y. Hanamura ( NSMT Cr 21242). — 5 males (BL 5.5–9.7 mm), 2 females (BL 6.8, 8.8 mm), 1 ovig. female (BL 9.3 mm), 2 juvs. (BL 4.2, 4.5 mm); St. C, 5˚38.5' N, 100˚24.9' E, sledge, 5.4 m depth, 14 Dec 2004, coll. Y. Hanamura ( FRI 008).

Indonesia. Tegal, central Java: 1 juv. (BL 3.0 mm); 3 June 1994, coll. Mulyadi (identification tentative) ( NSMT Cr 21243).

Description. Body moderately robust ( Fig. 14 a).

Anterior dorsal part of carapace between postorbital spines weakly produced, forming wide, sub-triangular rostrum with rounded apex ( Figs. 14 a–d, 17a, b); cervical sulcus marked dorsally and laterally around anterior onethird; anterior medial nodule small but well defined, situated just posterior to cervical sulcus, and slightly larger posterior nodule situated close to posterior carapace margin; postorbital spine small and carina supporting spine not developed or feebly ridged; cheeks slightly sinuous; antero-lateral spine moderately long, reaching as far as end of rostral plate.

Eyes ( Figs. 14 a–d, 17a, b) sub-pyriform, fully reaching mid-length of second segment of antennular peduncle, cornea well pigmented, broader than eye stalk. Antennules sexually dimorphic ( Figs. 14 g, h); male peduncle with basal segment as long as combined length of distal 2 segments, with several long inwardly curving setae along lateral margin (slightly deformed in figure due to artifact); second segment shortest, slightly shorter than wide; third segment slightly longer than wide, with several short hooked setae on mesial margin and several long setae around disto-mesial part, lateral flagellum basally swollen, forming male lobe, hirsutid densely on mesial margin with long hair. Female antennular peduncle more slender than that in males, first segment of peduncle longer than combined length of anterior 2 segments, with several long inwardly curving setae (slightly deformed in figure due to artifact); second segment shortest, slightly shorter than wide; third segment slightly longer than wide, with several long ordinary setae around disto-mesial part. Antennal scale ( Figs. 15 a, 17e) barely reaching end of antennular peduncle, about 5 times as long as wide, disto-lateral spine slightly overreaching end of blade, distal suture present; sympod with 2 stout spines of sub-equal length and commonly with 2, rarely 1, small spines at base of stout spines.

Labrum with anterior margin nearly truncated, without median spine. Mouthparts as illustrated in Ii (1964: figs. 44j–l).

Thoracic appendages: endopods of thoracic limbs moderately long, reaching anterior end of carapace ( Figs. 15 b–g, 17f, g); third thoracic endopod slightly stouter than fourth, its carpo-propodus commonly consisting of 4 or 5 articles; fourth to sixth thoracic endopods similar in shape, carpo-propodi with 4–6 articles; seventh thoracic endopod with carpo-propodus composed of 4–6 articles, longest setae rather smooth, without stout setules. Rudimentary endopod of eighth thoracic limb in males ( Figs. 15 h, 17h) 3-segmented, usually curving anteriorly, reaching around distal one-third when extended; second segment shortest, with several long setae; distal segment elongated, rod-shaped. Rudimentary endopod of eighth thoracic limb in females ( Figs. 15 i, 17i) un-articulated, slightly variable in length, fully extending to falling slightly short of distal end of basal plate of exopod.

Abdominal somites ( Fig. 14 a) rounded dorsally, without any ornamentation; first to fifth somites sub-equal in length, sixth somite 1.1–1.4 times as long as fifth one; male first somite rounded ventrally to form pleuron with shallow excavation near anterior part.

Pleopods in males biramous ( Figs. 15 j–l); first pleopod with endopod un-articulated, with several short marginal setae, first pleopod with exopod multi-articulated; second pleopodal exopod greatly elongated, consisting of about 13 articles, terminal article armed with 2 apical setae and one proportionately long sub-apical seta, several basal articles each with long seta, and unarmed in several articles between these armed articles; third to fifth pleopods similar in shape, with multi-articulated endopods and exopods of sub-equal length. Pleopods in females ( Figs. 16 a–c) un-articulated, length increasing on posterior somites.

Uropod ( Figs. 14 f, 17d) with 2-segmented exopod and endopod; distal segment of exopod about half-length of basal segment; endopod with distal segment about two-fifths as long as basal segment, bearing stout seta on ventral margin near statocyst.

Telson ( Figs. 14 e, 17c) 1.1–1.4 times as long as sixth abdominal somite, extending (without spinose apical setae) to articulation of uropodal endopod, and spinose apical setae extending beyond articulation of uropodal exopod by about half of setal length, length of telson about 2.5 times basal width and again about 4 times, or slightly fewer times, as long as wide at sub-basal constricted part, narrowed abruptly at base, forming discernible waist and slightly wider near mid-length, then gradually narrowing toward posterior end; rounded posterior margin with 2 pairs of stout spinose setae, mesial pair extending as far as or slightly beyond lateral pair, setules becoming broadened distally; lateral margin of telson armed with 11–16 (commonly with 14 or 15 in Japanese and 12 or 13 in Malaysian specimens) smooth setae, increasing its length posteriorly.

Body length. Largest recorded male: BL 12.5 mm, largest female: BL ca. 12 mm.

Colour. In Malaysian specimens, entire body almost semi-transparent when alive, while telson with basal red chromatophore but no chromatophore at mid-length, unlike sympatric R. egregius and R. hastatus , both with two red chromatophores.

Egg size. Female of BL 9.0 mm carried 8 embryos, eggs (stage I embryos) spherical, measuring 0.60–0.80 mm along longest axis (N=7).

Remarks. No substantial differences could be found in morphology between our materials and published data for R. orientalis ( Ii 1964; Tattersall 1957, 1965; Liu & Wang 2000). Among the specimens so far examined, however, the Malaysian specimens showed a tendency to have smaller body size than the Japanese ones, and the Malaysian population also possessed slightly fewer numbers of lateral setae on the telson (commonly 12 or 13 setae vs. 14 or 15 in Japanese one) and fewer number of articles in the carpo-propodi of the fourth to seventh thoracic endopods, each commonly composed of four or five articles compared to five or six in Japanese specimens. Furthermore, the rudimentary endopods of the eighth thoracic limbs are proportionately longer in Malaysian specimens than in Japanese ones in both sexes. These differences, however, appeared to be bridged by the Chinese population (cf. Liu & Wang 2000), although the identity of R. orientalis may be a subject of future taxonomic studies over its geographical scale.

Rhoplaophthalmus orientalis can be distinguished from its congeners, which have a combination of following features: 1) the antennal sympod with two stout spines and one or two much smaller lateral spines, 2) the antennal scale barely reaching the end of the antennular peduncle, and 3) the telson possessing a marked “waist” near base.

Rhopalophthalmus anishi Panampunnail & Biju, 2006 , R. mumbayensis Panampunnail & Biju, 2006 , and R. tropicalis Wooldridge & Mees, 2003 have the telson with the lateral pair of the spinose apical setae longer than the mesial pair as opposed to shorter setae in R. orientalis .

Rhopalophthalmus kempi O. Tattersall, 1957 has the third to seventh thoracic endopods with three-segmented carpo-propodus against the four or five-segmented one, and the rudimentary endopods of the eighth thoracopods are proportionately longer in both sexes than those in R. orientalis , extending well beyond the basal plate of the exopod as opposed to barely reaching or slightly shorter in R. orientalis .

Rhopalophthalmus murudana Panampunnail & Biju, 2006 has, as reported so far, a proportionately short telson with the distal margin excluding the spinose apical setae falling far short of the articulation of the uropodal endopod while fully reaching that articulation in M. orientalis . The eyes of R. murudana are also comparatively short with the cornea barely reaching the end of the first peduncular segment of the antennules as opposed to extending well beyond it in R. orientalis .

Rhopalophthalmus tattersallae Pillai, 1961 has a prominent sub-triangular rostrum, and the rudimentary endopod of the eighth thoracic limb in the female is two-segmented and noticeably shorter when compared with an unarticulated endopod in R. orientalis .

On the other hand, Pillai (1964) suggested a similarity between R. macropsis and R. orientalis , but R. macropsis has an elongated endopod in the male eighth thoracopods as well as a proportionately more slender telson with the lateral pair of the spinose apical setae longer than the inner pair.

Distribution. This species has been recorded with certainty from shallow coastal waters of Japan ( Nakazawa 1910; Tattersall 1957; Ii 1964), East China Sea ( Liu & Wang 2000), and Malacca Strait ( Tattersall 1965; present study).

Habitat. In the estuarine systems of Matang and Merbok mangroves, Peninsular Malaysia, R. orientalis were usually captured in the mouth area to middle reaches ( Hanamura et al. 2007; as Rhopalophthalmus sp. 2).

Figures

FIGURE 14. Rhopalophthalmus orientalis O. Tattersall, 1957. Male (BL 11.3 mm) (a – c, e – g) and ovigerous female (BL 10.5 mm) (d, h) collected from Kochi Prefecture, Japan: a, entire body of male (lateral view); b, carapace and cephalic appendages of male (lateral view); c, anterior part of carapace and cephalic appendages of male (dorsal view); d, anterior part of carapace and cephalic appendages of female (dorsal view); e, telson (ventral view); f, right uropod (ventral view); g, right antennule of male (dorsal view); h, right antennule of female (dorsal view).

FIGURE 15. Rhopalophthalmus orientalis O. Tattersall, 1957. Male (BL 11.3 mm) (b – h, j – l) and ovigerous female (BL 10.5 mm) (a, i) collected from Kochi Prefecture, Japan: a, right antenna, ventral view; b, c. right third thoracic limb and carpopropodus of endopod; d, e, right sixth thoracic limb and carpo-propodus of endopod; f, g, right seventh thoracic limb and carpopropodus of endopod; h, right eighth thoracic limb of male; i, rudimentary endopod of right eighth thoracic limb of female; j – l, right first, second and fifth pleopods of male.

FIGURE 16. Rhopalophthalmus orientalis O. Tattersall, 1957. Female (BL ca. 11 mm) from Seto Inland Sea, Japan: a – c, right first, third, and fifth pleopods of female.

FIGURE 17. Rhopalophthalmus orientalis O. Tattersall, 1957. Male (BL 10.2 mm) (a – h) and female (BL 11.3 mm) (i) collected from Matang, Malaysia: a, carapace and cephalic appendages of male (lateral view); b, anterior part of carapace and cephalic appendages of male (dorsal view); c, telson (ventral view) (circle indicates the position of red chromatophore); d, right uropod (ventral view); e, right antenna (ventral view); f, g, right seventh thoracic limb and carpo-propodus; h, right eighth thoracic limb of male; i, rudimentary endopod of right eighth thoracic limb of female.