Rhopalophthalmus armiger Hanamura & Murano

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 : 18-23

Rhopalophthalmus armiger Hanamura & Murano sp. nov.

( Figs. 11–13 )

Rhopalophthalmus longipes — Ii, 1964, fig. 47n. — Wang & Liu 1994: 91 (?in part). — Liu & Wang 2000: 114 (?in part). Rhopalophthalmus egregious — W. Tattersall 1936: 147.

Rhopalophthalmus macropsis — Pillai 1973: 56, fig. 24.

Material examined. Holotype. South China Sea. South-western part of South China Sea: male (BL ca. 14 mm); RV Hakuho-maru cruise KH-72-1, St. 50, 06°51.6' N, 108°47.2' E to 06°51.6' N, 108°48.9' E, plankton net installed at mouth of 3 m beam trawl, 132–137 m depth, 11 July 1972, coll. M. Murano ( NSMT 21225).

Paratypes. South-western part of South China Sea: 64 males (BL ca. 7.0– 15 mm), 78 females (BL ca. 8.5–16 mm), 20 ovig. females (BL ca. 11–16 mm), 23 juvs. (BL ca. 5.5–7.0 mm) (most of them more or less damaged); data same as for holotype ( NSMT 21226).

Additional material examined. South China Sea. Off Vietnam: 1 juv. (BL ca. 6 mm); RV Hijun-maru, 16°18’ N, 108°27’ E, 24 Jan 1933, coll. N. Ii (No. 138 in Ii 1964). — Southern part of South China Sea: 1 juv. (BL ca. 4 mm); RV Hijun-maru, St. 50, 3°53' N, 110°13' E, 27 Feb 1933, coll. N. Ii ( NSMT Cr 21227). — 1 female (BL ca. 8.0 mm), 5 juvs. (BL ca. 5.0–6.0 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 21229). — Northern part of South China Sea: 1 female (BL 8.5 mm); RV Hakuho-maru cruise KH-73-2, St. 41, 22°15.3' N, 115°28.2' E to 22°15.7' E, 115°28.9' E, 00:50–01:50, plankton net installed at mouth of 3 m beam trawl, 50–55 m depth, 18 Mar 1973, coll. M. Murano ( NSMT Cr 21230).

Timor Sea. North-western part of Timor Sea: 2 females (BL ca. 5, 5.5 mm), 1 abdomen; RV Hakuho-maru cruise KH-72-1, St. 29, 12°17.3' S, 129°40.9' E to 12°17.2' S, 124° 36.4' E, plankton net installed at mouth of 3 m beam trawl, 49–52 m depth, 24 June 1972, coll. M. Murano ( NSMT Cr 21231).

Australia. Outside Trinity Opening, NSW: 1 female (BL ca. 5.5 mm; damaged); Great Barrier Reef Expedition, St. 29, Bottom Strain Net, 24 June 1928 (NHM 1936.10.8.55).

Description. Body ( Fig. 11 a) moderately stout, somewhat large among eastern Asian members of Rhopalophthalmus .

Anterior dorsal part of carapace between postorbital spines slightly produced, forming evenly rounded rostral plate ( Figs. 11 a–d); postorbital spine sharp, supported by short carina; cervical groove marked dorsally and laterally around anterior one-third; antero-median nodule small, sometimes indistinct due to fragile nature of carapace but often prominent in young specimens, posterior nodule marked well, posterior dorsal margin excavate dorsally, leaving last 3 thoracic somites uncovered in dorsal view; cheeks sinuous; antero-lateral spine comparatively short, falling slightly short of anterior end of rostral plate.

Eyes ( Figs. 11 a–d) somewhat globular, falling short of end of second segment of antennular peduncle, cornea well pigmented, as large as eye stalk. Antennules sexually dimorphic ( Figs. 11 g, h); male peduncle with first segment as long as or slightly longer than combined length of distal 2 segments, armed laterally with several long inwardly curving setae (somewhat deformed in figure due to artifact); second segment shortest, wider than long; third segment stout, as long as wide, no short hooked setae observed on mesial margin but with several long ordinary setae around disto-mesial part, lateral flagellum basally swollen, forming male lobe hirsutid with dense long hair. Antennule in females more slender than that of males, first segment of peduncle as long as or slightly longer than combined length of distal 2 segments, with several long inwardly curving setae on lateral margin; second segment shortest, slightly wider than long; third segment longer than wide, with several long setae on disto-mesial part. Antenna ( Fig. 11 i) with scale extending well beyond end of antennular peduncle, about 7 times as long as wide, disto-lateral spine extending well beyond end of blade, distal suture present; sympod with two long spines at disto-mesial part, mesial second spine slightly longer than most mesial one, accompanying 0–2 short spines at base of longest spine.

Labrum ( Fig. 11 j) truncated anteriorly, without median spine. Mouth parts as illustrated ( Figs. 12 a–e).

Thoracic appendages ( Figs. 12 f–h) remarkable in having proportionately long endopods particularly in seventh one; third thoracic endopod slightly stouter than fourth one, carpo-propodus 3-segmented, basal article nearly twice as long as combined length of distal 2 segments; fourth to sixth endopods similar in shape but length increasing posteriorly, its carpo-propodi divided into 3–5 segments; seventh thoracic endopod longest, and male endopod normally reaching end of antennular peduncle, slightly shorter in female extending to end of cornea, carpo-propodus with 5 or 6 articles, basal article noticeably long, as long as combined length of remaining distal articles or slightly longer, long disto-ventral setae nearly smooth, without stout setules. Rudimentary endopod of eighth thoracic limb in males ( Fig. 12 i) normally 3-segmented, falling short of end of basal plate of exopod when extended; second segment with several long setae (not shown due to damage); third segment elongated, rod-shaped. Rudimentary endopod of eighth thoracic limb in females ( Fig. 12 j) un-articulated, falling slightly short of end of basal plate of exopod, with short seta near mid-length and slightly longer sub-terminal seta.

Abdomen ( Fig. 11 a) smooth, first to fifth somites sub-equal in length, sixth somite 1.3–1.55 times as long as fifth one; first somite in male ventrally rounded to form pleuron with shallow excavation on anterior part.

Pleopods in males biramous ( Figs. 13 a–d); first pleopod with endopod un-articulated, bearing several short marginal setae, exopod multi-articulated; exopod of second pleopod elongated, nearly reaching mid-length of sixth abdominal somite, composed of approximately 15 articles, basal 6 articles each with long seta and distal article with pair of apical setae and proportionately long sub-apical seta, but several unarmed articles present between these armed articles; endopod multi-articulated; third to fifth pleopods similar in shape, with multi-articulated endopods and exopods of sub-equal length. Pleopods in females ( Figs. 13 e–g) un-articulated, generally increasing in length on posterior somites while third one relatively short, as long as first one.

Uropod ( Fig. 11 f) 2-segmented in both exopod and endopod, exopod narrow and slender, distal segment about half-length of basal segment; endopod barely reaching or falling slightly short of mid-length of distal segment of exopod, with stout ventral seta around basal one-third.

Telson ( Figs. 11 e, 13h, i) 1.3–1.4 times as long as sixth abdominal somite, distal margin (excluding posterior spinose setae) reaching articulation of uropodal endopod, comparatively narrow in general appearance, slightly more than 3 times as long as basal width and again about 4 times as long as broad at sub-basal constriction part, forming no discernible waist but having nearly parallel lateral margins; rounded posterior margin with 2 pairs of large spinose setae of sub-equal length, each with setules becoming broad towards distal; lateral margin of telson armed with 9–12, commonly 10 or 11, very stout, smooth setae on posteriorl two-thirds, spaciously set posteriorly and posterior setae distinctly longer than posterior width of telson proper.

Body length. Largest male: BL ca. 15 mm, largest ovigerous female: BL ca. 16 mm.

Etymology. The species name “ armiger ” (armed in Latin) reflects the characteristic ornamentation of the telson, alluding to an armature-like structure.

Remarks. Rhopalophthalmus armiger sp. nov. was regarded to be different from its closest relative, R. longipes , in the structure of the telson, a proportionately long seventh thoracic endopod, as well as a comparatively large eyes (further details can be found in “Remarks” under R. longipes ).

Rhopalophthalmus macropsis collected from the Indian Ocean ( Pillai 1964, 1965, 1973) has occasionally been regarded as a synonym of R. longipes ( Wang & Liu 1994; Liu & Wang 2000). We could not examine the type specimen of R. macropsis despite our attempts to secure permission from the Indian institutions. However, the type of R. macropsis collected from the Arabian Sea appeared to be completely different from R. longipes as well as from its variant in having 1) the lateral pair of the spinose apical telson setae distinctly longer than the mesial pair (subequal length in R. longipes and R. armiger ), 2) the vestigial male endopod of the eighth thoracic limb greatly elongated, overreaching basal plate of the exopod (falling distinctively short of the end of basal plate in R. longipes and R. armiger ), and 3) the carpo-propodus of the seventh thoracic endopod with the basal segment of sub-equal length to the combined length of the remaining distal articles (distinctly longer in R. longipes and R. armiger ).

Meanwhile, a young male reported from the Malacca Strait under the name R. macropsis by Pillai (1973) does not conform to the type of R. macropsis , but agrees with a variant of R. longipes reported by Ii (1964). As Pillai (1964) remarked in his original description, R. macropsis seemed to show a similarity to R. kempi O.Tattersall, 1957 or R. orientalis O. Tattersall, 1957 rather than to R. longipes or its variant, which further justifies reassigning the Pillai (1973) male from R. macropsis to the new species R. armiger .

Unfortunately, we could not access the Chinese material; however, it is probable that some specimens recorded under R. longipes from the South China Sea are R. armiger , as some of them are remarked to have fewer telson setae than the typical R. longipes . Also, a juvenile specimen recorded from off the north-east coast of Australia under the name R. egregius by Tattersall (1936) would belong to the new species for its characteristic feature of the telson (see Fig. 13 i).

Rhopalophthalmus armiger shows a 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 opposed to four sub-segments with an unusually elongated carpus in the new species.

Rhopalophthalmus armiger , together with its closest relative, R. longipes , is remarkable among the Asian members of Rhopalophthalmus in having no hooked setae on the male antennular peduncle as well as a proportionately short third pleopod in females, which suggests that these features may have phylogenetic significance.

Distribution. This species may be an oceanic form of tropical waters and was captured from the South China Sea, Timor Sea, north-eastern Australia, and the Malacca Strait ( Ii 1964; Pillai 1973; Wang & Liu 1994; Liu & Wang 2000; present study).

Figures

FIGURE 11. Rhopalophthalmus armiger Hanamura & Murano sp. nov. Holotype male (BL 14.0 mm) (a), paratype male (BL ca. 15 mm) (b, c, e – g, i, j), and paratype female (BL 16.5 mm) (d, h) collected from South China Sea: 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); i, right antenna (ventral view); j, labrum.

FIGURE 12. Rhopalophthalmus armiger Hanamura & Murano sp. nov. Paratype male (BL ca. 15 mm) (a – i) and paratype female (BL 16.5 mm) (j) collected from South China Sea: a, right mandible; b, right maxillule; c, right maxilla; d, right first thoracic limb; e, right second thoracic limb; f, right third thoracic limb; g, right sixth thoracic limb; h, right seventh thoracic limb; i, right eighth thoracic limb of male; j, right eighth thoracic limb of female.

FIGURE 13. Rhopalophthalmus armiger Hanamura & Murano sp. nov. Paratype male (BL ca. 15 mm) (a – d), paratype female (BL 16.5 mm) (e – g), and juvenile male (BL 6.5 mm) collected from South China Sea (h), and juvenile female (BL ca. 5.5 mm) collected from off north-eastern coast of Australia (i): a – d, right first, second, third, and fifth pleopods of male; e – g, left first, third, and fifth pleopods of female; h, i, telson, ventral and dorsal view, respectively.