Rhopalophthalmus longipes Ii 1964

Rhopalophthalmus longipes Ii, 1964

(Figs. 8–10)

Rhopalophthalmus longipes Ii, 1964: 180, figs. 46, 47 (in part, not Fig. 47n = R. armiger sp. nov.). — Wang & Liu 1994: 91, fig. 14 (?in part). — Wang & Liu 1997: 204. — Liu & Wang 2000: 114, fig. 27 (?in part).

Material examined. Syntypes. Ajiro, Shizuoka Prefecture, Japan: 63 males (BL 6.3–ca. 9.5 mm), 75 females (BL ca. 6.5–9.5 mm), 51 juvs. (BL ca. 4.5–6.2 mm) (more or less damaged); 11 Sept 1935, coll. H. Aikawa (No. 217 in Ii, 1964) (NSMT Cr 21219).

Japan. Off Amami Is., south-western Japan: 1 female (BL 6.2 mm); RV Tansei-maru cruise KT-70-2, St. 451, 28°10.3' N, 129°13.8' E, 13:53–14:13, bottom net, 67–68 m depth, 24 Apr 1970, coll. M. Murano (NSMT Cr 21220). — Near mouth of Ohmura Bay, Nagasaki Prefecture: 3 males (BL ca. 7.5 –10 mm) (more or less damaged); sledge, 30 Sept 1977, coll. T. Takita (NSMT Cr 21221).

South China Sea. South-western part of South China Sea: 1 male (BL ca. 7 mm); RV Hakuho-maru cruise KH-72-1, St. 45, 05°13.5' N, 107°00.8' E to 05°13.7' E, 107°01.1' E, plankton net installed at mouth of 3 m beam trawl, 60 m depth, 10 July 1972, coll. M. Murano (NSMT Cr 21223).

Timor Sea. Western part of Timor Sea: 27 males (BL ca. 5.5–8.0 mm), 15 females (BL ca. 5.5–8.0 mm), 18 juvs. (BL ca. 4.5–6.5 mm) + abdomens; RV Hakuho-maru cruise KH-72-1, St. 32, 12°37.3' S, 124°33.9' E to 12°36.0'S, 124°36.4' E, plankton net installed at mouth of 3 m beam trawl, 74–78 m depth, 25–26 June 1972, coll. M. Murano (NSMT Cr 21224).

Description. Body moderately stout (although majority of material examined were more or less damaged). Anterior dorsal part of carapace between postorbital spines slightly produced, forming evenly rounded rostral plate (Figs. 8 a–d); postorbital spine sharp, supported by very short, feebly defined carina; cervical sulcus well marked dorsally and laterally around anterior one-third, small but distinct nodule present just posterior to cervical groove in addition to posterior one; posterior dorsal margin excavate, leaving last 3 thoracic somites uncovered in dorsal view; cheeks concave or slightly sinuous; antero-lateral spine moderately large.

Eyes (Figs. 8 b–d, 10a, b) somewhat globular, falling short of end of second segment of antennular peduncle, cornea well pigmented and developed among Asian members of genus and slightly shorter than eye stalk. Antennules sexually dimorphic (Figs. 8 g, h); male antennular peduncle with first segment distinctly longer than 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, shorter than wide; third segment stout, as long as wide, so far no short hooked setae observed but with several long ordinary setae around mesial and disto-mesial parts, lateral flagellum basally swollen, forming male lobe, hirsutid with dense long hair. Female antennular peduncule slightly more slender than that of males, first segment distinctly longer than combined length of distal 2 segments, with several long inwardly curving setae along lateral margins (slightly deformed in figure due to artifact); second segment shortest, slightly shorter than wide; third segment longer than wide, with several long setae around distomesial part. Antenna (Figs. 8 i, j, 10e) with scale extending well beyond end of antennular peduncle, about 7 times as long as wide, disto-lateral spine extending well beyond end of lamella, distal suture present; sympod spines composed of 2 stout spines, lateral spine longer than most mesial one, and also 1 or 2 short lateral spines near base of longest spine.

Mandibular palp (Fig. 9 a) with distal segment sub-oval, with rather long setae along margins (other mouth parts, see Ii, 1964: figs. 46f, g, 47c–f).

Thoracic appendages (Figs. 9 b–f, 10d) remarkable in having proportionately long endopods particularly in seventh one; third thoracic endopod slightly stouter than fourth one, carpo-propodus 2- or 3-segmented, basal article sub-equal in length to combined length of distal 2 segments; fourth to sixth endopods similar in shape but length increasing posteriorly, its carpo-propodi divided into 3 or 4 articles; seventh endopod longest, fully reaching mid-length of cornea, carpo-propodus with 4 articles, basal one noticeably long, as long as or longer than combined length of second and third articles, longest disto-ventral setae rather smooth, without stout setules. Rudimentary endopod of eighth thoracic limb in males (Figs. 9 g, 10f) normally 3-segmented, reaching mid-length of basal plate of exopod, with several long setae on second articles and a few short setae on lateral part of distal article. Rudimentary endopod of eighth thoracic limb in females (Fig. 9 h) normally un-articulated, barely reaching mid-length of basal plate of exopod.

Abdominal somites smooth, first to fifth somites sub-equal in length, sixth somite about 1.2 times as long as fifth one; first somite of male rounded ventrally to form pleuron with shallow excavation at anterior part.

Pleopods in males (Fig. 9 i) biramous; exopod of second pleopod elongated, composed of about 15 articles, several basal articles accompanying long seta, distal articles without seta except for most distal one, latter with pair of apical setae and proportionately long sub-apical seta. Pleopods in females (Figs. 9 j–l) un-articulated, length generally increasing on posterior somites but that on third pleopod somewhat short, comparable to first one.

Uropod (Fig. 8 f) 2-segmented in both exopod and endopod, exopod narrow and slender, distal segment about half-length of basal segment; endopod barely reaching mid-length of distal segment of exopod, with stout seta on ventral surface around basal one-third, distal segment about one-third length of basal segment.

Telson (Figs. 8 e, 9m, 10c) about 1.3 times as long as sixth abdominal somite, distal margin (excluding spinose apical setae) reaching articulation of uropodal endopod, comparatively narrow, about 3.5 times as long as basal width and again slightly more than 4.5 times as long as wide at sub-basal constriction part, abruptly narrowing near base but not forming discernible waist owing to sub-parallel lateral margins above mid-length, and then gently narrowing towards distal; posterior margin of telson with 2 pairs of spinose setae of sub-equal length, each with setules becoming broader towards distal; lateral margin of telson armed with 14–16 somewhat closely set, moderately long smooth setae on distal two-thirds, increasing in length posteriorly, posterior setae of lateral series as long as posterior width of telson.

Body length. Largest recorded male: BL ca. 10 mm, ovigerous female not collected so far.

Remarks. Rhopalophthalmus longipes was established by Ii (1964) on the basis of specimens collected mainly from Japan. Due to the somewhat poor condition of the specimens, Ii left the presence/absence of dorsal nodules on the carapace undetermined. The specimens available to us including the syntypes were more or less damaged as already mentioned by Ii, although a few specimens were found to possess a small nodule just posterior to the cervical sulcus (cf. Figs. 8 a, 10a); hence we believe that this species possesses this nodule.

Ii (1964) noted that some specimens from the South China Sea have fewer but remarkably greater setae on the lateral margins of the telson as compared with the typical form of R. longipes, and he provisionally dealt with them as a variation of this species. This variety has subsequently been a subject of taxonomic arguments among mysid researchers (Pillai 1973; Liu & Wang 2000). We were able to examine a considerable number of specimens, covering various size ranges, with this characteristic feature in their telson. In those examined so far, the number of lateral telson setae of the variety is noticeably fewer when compared with that of the typical form throughout the entire life stage, and, even in larger specimens exceeding BL 10 mm, the setal counts are within the range of 9–12 as opposed to 14–16 in the typical form of BL 8–12 mm. Moreover, these telson setae of the former specimens are much greater than those in the typical R. longipes as posterior setae in adult specimens are noticeably longer than the posterior telson width relative to the sub-equal length in the typical specimens. The endopod of the seventh thoracopod is proportionately long in the variant, fully reaching the end of the antennular peduncle even in sub-adult males, while in typical R. longipes, it barely reaches the end of the cornea. Furthermore, the eyes of the variety appear to be larger than those of the typical R. longipes, being as large as the second segment of the eye stalk relative to those of slightly smaller eyes. Thus, we concluded that both populations are different at the species level. Meanwhile, Pillai (1964) recorded R. macropsis on the basis of a male specimen collected from the Arabian Sea and later an immature male of this species was reported from the Malacca Strait (Pillai 1973). The identity of R. macropsis has remained disputed as several researchers have regarded it to be a valid species (Mauchline & Murano 1977; Mauchline 1980; Vilas-Fernández et al. 2008) while Chinese taxonomists considered it to be the morphologically variable R. longipes (Wang & Liu 1994; Liu & Wang 2000). In this study, we reached the conclusion that R. macropsis is a valid species, although the eastern Indian Ocean specimen examined by Pillai (1973) might be different from his Arabian Sea species. The identity and taxonomic position of Pillai’s specimens is discussed in “Remarks” under R. armiger sp. nov.

Rhopalophthalmus longipes shows resemblance to R. terranatalis O. Tattersall, 1957 collected from Africa, but the latter species has the carpo-propodus of the seventh thoracic endopod composed of seven sub-segments instead of four sub-segments with an unusually elongated carpus in R. longipes.

Distribution. Recorded with certainty from Japanese coastal waters, South China Sea, and Timor Sea (Ii 1964; present study).