Rhopalophthalmus egregius Hansen, 1910

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 : 2-8

Rhopalophthalmus egregius Hansen, 1910

( Figs. 1–3 )

Rhopalophthalmus egregius Hansen, 1910: 49 , pl. 6, fig. 3a–k, pl. 7, fig. 1a–d. — W. Tattersall 1915: 151 (at least in part). — O. Tattersall 1957: 87, fig. 1a. — O. Tattersall 1960: 169.

Rhopalophthalmus phyllodus Murano, 1988: 293 , figs. 1, 2.

Not Rhopalophthalmus egregius — Nakazawa 1910: 255, pl. 8, figs. 12, 22 (= R. orientalis O. Tattersall, 1957 ). — W. Tattersall 1915: 151 (in part= R. chilkensis O. Tattersall, 1957 . — W. Tattersall 1921: 408 (= R. orientalis O. Tattersall, 1957 ). — W. Tattersall 1922: 457 (in part= R. kempi O. Tattersall, 1957 ). — W. Tattersall 1936: 147 (= R. armiger sp. nov.). — W. Tattersall 1940: 330 (= R. dakini O. Tattersall, 1957 ). — O. Tattersall 1952: 161, figs. 3, 4 (= R. terranatalis O. Tattesall, 1957 ). — O. Tattersall 1955: 87 (=? R. terranatalis O. Tattersall, 1957 ). — Pillai 1957: 2, fig. 1 (= R. tattersallae Pillai, 1961 )

Material examined. Syntypes. Sangkapoera roads, Bawean Island, Java Sea, Indonesia: 1 male abdomen (without carapace and cephalic appendages) (BL> 10 mm), 1 ovig. female (BL ca. 10 mm); 12 m depth, Siboga Expedition ( ZMUC CRU- 6334).

Thailand. Surat Thani: 7 females (BL ca. 10–10.1 mm), 17 ovig. females (BL ca. 9.4–12 mm), 5 abdomens (all more or less damaged); 16 Oct 1978, coll. T. Rojanasarumpakit (Topotypic specimens of R. phyllodus Murano, 1988 ) ( NSMT Cr 21207). — Bang Plasoi, Chonburi: 5 males (BL ca. 6.5–9 mm), 1 female (damaged), 1 juv. (BL 4 mm), 2 abdomens ( NSMT Cr 21208); — 1 male, 1 ovig. female, 2 abdomens (all damaged) ( NSMT Cr 21209); 22 Aug 1980, coll. Marine Fisheries Laboratory, Department of Fisheries, Thailand. — Klong Don Sak, Surat Thani: 1 female (damaged); 14 Apr 1987, coll. Marine Fisheries Laboratory, Department of Fisheries, Thailand ( NSMT Cr 21210). — Sapum Bay, Phuket Is.: 15 males (BL ca. 6.5–10 mm), 15 females (BL ca. 7–11 mm), 1 ovig. female (BL ca. 11.5 mm), 19 juvs. (BL ca. 4–6.5 mm), 4 abdomens; 19 Feb 1980, coll. Marine Fisheries Laboratory, Department of Fisheries, Thailand ( NSMT Cr 21211).

Singapore. 1 male (BL ca. 9.0 mm), 9 juvs. (BL 3.5–5.0 mm); St. B44 2(c) (NHM 1964.1.21 5446/5558).

Malacca Strait. 2 males (BL ca. 8.2, 8.7 mm), 1 female (BL 7.0 mm), 1 juv. (BL 4.0 mm); St. B51 1(c) (NHM 1964.1.21 5546/6558). — 1 male (BL ca. 8.0 mm), 3 females (BL ca. 6.5– 9 mm), 6 juvs. (BL ca. 3–3.5 mm); St. B51 1(c) (NHM 1964.1.21 5558/6563).

Malaysia. Matang mangrove estuary: 5 males (BL 6.3–8.8 mm), 1 female (BL 6.0 mm), 2 juvs. (BL 4.0, 5.0 mm); St. 2, 4˚50.6' N, 100˚35.3' E, sledge, 4.5 m deep, 8 Apr 2005, coll. Y. Hanamura ( NSMT Cr 21201). — 1 male (BL ca. 8 mm); off shore of St. 4 (4˚51.4' N, 100˚32.9' E), sledge, 2 m depth, 14 June 2006, coll. Y. Hanamura ( NSMT Cr 21202). — Merbok mangrove estuary: 2 males (BL 6.2, 8.8 mm), 1 female (BL 7.7 mm), 4 ovig. females (BL 9.1–10.5 mm); St. A, 5˚40.1' N, 100˚22.2' E, sledge, 1.5 m depth, 14 Oct 2004, coll. Y. Hanamura ( NSMT Cr 21203). — 25 males (BL 5.3–8.8 mm), 17 females (BL 5.5–8.1 mm), 17 ovig. females (BL 8.6–12.0 mm); St. A, 5˚40.1' N, 100˚22.2' E, sledge, 0.8 m depth, 16 Dec 2004, coll. Y. Hanamura ( NSMT Cr 21204). — 4 males (BL 7.0– 9.3 mm), 6 females (BL ca. 6–9.5 mm); St. A, 5˚40.1' N, 100˚22.2' E, sledge, 2.1 m depth, 16 Dec 2004, coll. Y. Hanamura ( FRI Cr 005).

Indonesia. Segara Anakan, Java: 4 males (BL 7.7–8.8 mm), 1 female (BL 8.7 mm), 4 ovig. females (BL ca. 8.0– 9.5 mm); mid-night, surface tow with larva net, 19 Sept 1988, coll. Mulyadi ( NSMT Cr 21198). — Cilacap Bay, southern coast of central Java: 14 males (BL 6.5–9.6 mm), 9 females (BL ca. 5.8–11 mm), 4 ovig. females (BL ca. 8.5–11.7 mm), 17 juvs. (BL ca. 2.4–5.5 mm); 24 Aug 1992, coll. Mulyadi ( NSMT Cr 21199). — Tegal, central Java: 1 female (BL 7.7 mm), 30 juvs. (BL 2.7–4.5 mm), 2 abdomens; 3 June 1994, coll. Mulyadi ( NSMT Cr 21200).

India. Chilka Lake, Orissa: 2 males (BL ca. 10, 10.5 mm), 8 ovig. females (BL ca. 9–12 mm); presented by Indian Museum (NHM 1914.12.15.11–30).

Aberrant form (having 4 stout spines on either side of antennal sympod). Malaysia: Merbok mangrove estuary: 1 ovig, female (BL 10.2 mm); St. A, 5˚40.1' N, 100˚22.2' E, sledge, 3–4 m depth, 11 Jan 2005, coll. Y. Hanamura ( NSMT Cr 21205). — 1 male (BL 6.1 mm); St. A, 5˚40.1' N, 100˚22.2' E, sledge, 1.5 m depth, 17 May 2005, coll. Y. Hanamura ( NSMT Cr 21206). —1 ovig. female (BL 8.2 mm), 1 male (damaged); St. A, 5˚40.1' N, 100˚22.2' E, sledge, 0.9–1.2 m depth, 20 Dec 2005, coll. Y, Hanamura ( FRI Cr 006).

Description. Body moderately robust ( Fig. 1 a).

Anterior dorsal part of carapace between postorbital spines ( Figs. 1 b–d) weakly produced, forming wide, semicircular or sub-triangular rostral plate; postorbital spine, extending as far as anterior end of rostral plate, carina supporting spine feeble and short; cervical sulcus marked dorsally and laterally around anterior one-third of carapace; anterior dorsal median nodule ( Figs. 1 a, b) placed immediately behind cervical sulcus small but well defined, posterior nodule situated close to posterior dorsal margin; posterior dorsal margin of carapace excavate, leaving last 3 thoracic somites uncovered in dorsal aspect; cheeks evenly concave; antero-lateral spine sharp, extending as far as anterior end of rostral plate.

Eyes sub-pyriform ( Figs. 1 c, d), length of cornea slightly shorter than eye stalk. Antennules sexually dimorphic ( Figs. 1 g, h); male first segment of peduncle slightly longer than combined length of anterior 2 segments, bearing several long, inwardly curving setae along lateral margin (slightly deformed in figure due to artifact); second segment shortest, shorter than wide, with a few short to moderately long setae along mesial and lateral margins; third segment as long as wide, possessing a few to several hooked sensory setae on mesial margin and several long setae around disto-mesial part; lateral flagellum swollen at basal part, forming male lobe and its mesial margin hirsutid densely with long hair. Female antennular peduncle more slender than that in males, with first segment distinctly longer than combined length of anterior 2 segments, bearing several long, inwardly curving setae along lateral margin (slightly deformed in figure due to artifact); second segment shortest and shorter than wide; third segment slightly longer than wide, without short sensory setae at disto-mesial part but with several moderate to long setae. Antenna ( Figs. 1 a–d, i, 3b) with scale extending slightly beyond end of antennular peduncle, slightly more than 4 times as long as wide in female and approximately 5 times as long as wide in male; disto-lateral spine sharp, extending slightly beyond anterior margin of lamella; distal suture present; sympod normally possessing 3 long graduated spines increasing in length from mesial to lateral and usually 2 small spines situated near base of longest spine.

Labrum truncated anteriorly, without median spine. Mouth parts typical of genus (see Murano 1988; figs. 1d–h, as R. phyllodus ).

Thoracic appendages: third thoracic endopod ( Figs. 2 a, b) slightly stouter than fourth one, with carpo-propodus of 3–5 articles, fourth to sixth thoracic limbs ( Figs. 2 c, d) similar in shape, carpo-propodi of endopods with 4– 6, most commonly 5, articles, but infrequently 7 articles. Seventh thoracic endopod ( Figs. 2 e, f, 3d) extending well beyond cervical sulcus of carapace, its carpo-propodus composed of 5–7, infrequently 8, articles and longest distoventral setae bearing up to 7 stout setules around middle part, followed by feeble but distinct denticles on ventral margin. Rudimentary eighth thoracic endopod in males ( Figs. 2 g, 3c) 3-segmented, normally curving anteriorly, falling short of end of basal plate of exopod when extended; second segment short, bearing several long setae, third segment elongated, with or without short terminal seta. Rudimentary eighth thoracic endopod in females ( Fig. 2 h) reaching or falling just short of distal end of basal plate, un-articulated but often showing incipient articulation-like structure at mid-length and 1 small seta around mid-length.

Abdominal somites ( Fig. 1 a) rounded dorsally, first 5 somites sub-equal in length, sixth somite 1.2–1.4 times as long as fifth one; first somite of male rounded ventrally to form pleuron, with shallow excavation at anterior part.

Pleopods in males ( Figs. 2 i–k) biramous: first pleopod with endopod un-articulated, possessing several short, marginal setae, exopod articulated; second pleopod with exopod elongated, composed of about 13 articles, basal articles each bearing long seta, distal 5 articles comparatively long and enire except for terminal one bearing pair of apical setae and proportionately long sub-apical seta, endopod multi-articulated; third to fifth pleopods similar in shape with multi-articulated endopod and exopod of sub-equal length. Pleopods in females ( Figs. 2 l–n) un-articulated, increasing in length on posterior somites.

Uropod ( Fig. 1 f) with 2-segmented exopod and endopod; exopod longer than endopod, distal segment slightly shorter than half-length of basal one; endopod with basal segment about 2.5 times as long as distal one, possessing stout seta on ventral surface posterior to statocyst.

Telson ( Figs. 1 e, 3a) 1.2–1.4 times as long as sixth abdominal somite and slightly more than 2.5 times as long as basal width, reaching articulation of uropodal endopod and stout apical spinose setae of telson extending beyond articulation of uropodal exopod by about two-thirds of its length, abruptly narrowing near base to form waist and slightly broadened near mid-length, then gradually narrowing distally; rounded posterior margin of telson with four stout spinose setae, mesial pair being shorter than lateral one, and their tip curving outwards, setules of distal telson setae becoming broader and consequently take on somewhat leaf-like form particularly in distal ones, lateral margin with 13–16, commonly 14 or 15, setae, increasing in length posteriorly and bearing minute 1–4 microscopically small setules on posterior part of larger posterior setae but such setules usually not present in short anterior setae.

Body length. Largest recorded male: BL 10.5 mm, largest ovigerous female: BL ca. 12 mm.

Egg size. Females of BL 9.2–10.8 mm carrying 5–9 embryos, stage I embryos (eggs) oblong in shape, 0.60– 0.75 mm along longer axis (N=11).

Colour. Entire body semi-transparent when alive. In fresh specimens collected from west coasts of Peninsular Malaysia with telson possessing basal semi-circular red chromatophore and slightly larger, elliptical red one at distal one-third to one-fourth.

Remarks. Rhopalophthalmus egregius was established by Hansen (1910) on the basis of materials collected off Bawean Island in the Java Sea, Indonesia. Later, several authors reported this species from various localities worldwide due to somewhat insufficient original descriptions coupled with superficia inter-specific resemblance within the genus Rhopalophthalmus . Tattersall (1957) examined considerable numbers of specimens collected from various localities of the entire geographical range of Rhopalophthalmus and suggested that the following features could be useful in discriminating species within the genus: 1) the presence/absence of the dorsal median nodules on the carapace, 2) the ornamentation of spines on the antennal sympod, 3) the number of the carpo-propodal articles of the thoracic endopods, 4) the shape of the rudimentary eighth thoracic endopod, and 5) the structure and armature of the telson.

Tattersall (1957) also re-examined the type materials of R. egregius housed in the Amsterdam Museum and supplemented the following diagnostic features for the species: 1) the presence of the anterior dorsal median nodule on the carapace, 2) the antennal sympod with three graduated stout spines, 3) the carpo-propodus of the third to seventh thoracic endopod with 4–6 articles, and 4) the lateral posterior pair of spinose setae of the telson slightly longer than the inner pair (see also Ii, 1964). Nevertheless, the exact nature of R. egregius has remained obscure for mysid researchers.

We were able to examine the syntype specimens, which are deposited in the Zoological Museum of Copenhagen University. These syntypes were more or less damaged (one female and one male without anterior body part), but we could observe several key features. In the female syntype with the carapace, the anterior median nodule is less developed, represented only by an obtuse elevation. This nodule, however, is often hardly visible in damaged specimens of Rhopalophthalmus . In contrast, this nodule is usually found in non-type specimens referred to this species. The antennal sympod of the female syntype possesses three stout graduated spines and two additional small spines ( Fig. 3 b), of which the latter character was not reported by Tattersall (1957). The seventh thoracic endopod of the syntype appears to have a six-segmented carpo-propodus, each segment with long seta bearing several rather long setules, followed by several denticles near the apex ( Fig. 3 d). Furthermore, the short setules of the apical telson setae are more or less flat and leaf-shaped rather than jagged, particularly the distal ones, and the lateral setal series of the telson were also furnished with microscopically small setules, particularly the posterior ones ( Fig. 3 a). All of these features are shared with the type specimens of R. egregius and R. phyllodus Murano, 1988 , and a notable number of additional specimens having the combination of these features have been captured from the coastal waters around Java Island, Indonesia, the Gulf of Thailand, the west coast of the Malay Peninsula, and across Chilka Lake in eastern India. Based on these observations, we concluded R. phyllodus to be a junior synonym of R. egregius .

Although we found several specimens with four large spines on one side of the antennal sympods (see Fig. 1 j), the spine arrangement on the antennal sympod was found to be consistent through ontogenetic stages in the observed species of Rhopalophthalmus .

Rhopalophthalmus egregious shows a similarity to R. chilkensis O. Tattersall, 1957 in having three stout spines on the antennal sympod, and these two species may co-occur in Chilka Lake. As reported by Tattersall (1957), R. egregius differs from the Indian species in having the anterior dorsal nodule on the carapace (absent in R. chilkensis ), a more slender antennal scale that is seven times as long as it is wide (about five times in R. chilkensis ), and the mesial pair of the spinose apical telson setae that are shorter (longer in R. chilkensis ) than the lateral pair.

Taylor (2008) reported R. egregius from the east coast of Australia. However, these Australian specimens have subsequently been determined to be R. brisbanensis Hodge, 1963 (Taylor pers. com.). The identity of Colosi’s (1920) specimens from the south-western Pacific remains uncertified.

Distribution. Known with certainty from Indonesia, Gulf of Thailand, Singapore, Malacca Strait, and Chilka Lake in India ( Hansen 1910; Tattersall 1957, 1960; Murano 1988; present study).

Habitat. Rhopalophthalmus egregius is one of the three most abundant species of the genus in the mangrove brackish water system of north-western Peninsular Malaysia, where it had a tendency to occur in abundance in the mouth area rather than the middle to upper reaches of the estuarine river ( Hanamura et al. 2007: as Rhopalophthalmus sp. 3).

Figures

FIGURE 1. Rhopalophthalmus egregius Hansen, 1910. Male (BL 9.5 mm) (a), male (BL 9.7 mm) (b, c, e – g, i), ovigerous female (BL 12.0 mm) (d, h), and ovigerous female (BL 10.2 mm) (j) collected from Merbok, Malaysia: 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) (circles indicate position of red chromatophores); f, right uropod (dorsal view); g, right antennule of male (dorsal view); h, right antennule of female (dorsal view); i, right antenna (ventral view); j, aberrant form in sympod spines of right antenna.

FIGURE 2. Rhopalophthalmus egregius Hansen, 1910. Male (BL 9.5 mm) (a – g, i – k) and ovigerous female (BL ca. 10.0 mm) (l – n) collected from coastal waters of Java, Indonesia, and ovigerous female (BL 12.0 mm) (h) collected from Merbok, Malaysia: a, b, right third thoracic endopod and its carpo-propodus; c, d, right sixth thoracic endopod and its carpo-propodus; e, f, left seventh thoracic endopod and its carpo-propodus; g, right eighth thoracic limb of male; h, rudimentary endopod of right eighth thoracic limb of female; i – k, right first, second, and third pleopods of male; l – n, right first, third, and fifth pleopods of female.

FIGURE 3. Rhopalophthalmus egregius Hansen, 1910. Syntype male (BL unknown) (a, c) and syntype female (BL ca. 10 mm) (b, d) from Java Sea: a, posterior part of telson (dorsal view); b, sympod spines of right antenna; c, rudimentary endopod of left eighth thoracic limb of male; d, carpo-propodus of left seventh thoracic endopod.