Rhopalophthalmus hastatus Hanamura, Murano & Alias

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 : 8-13

publication ID	https://doi.org/ 10.5281/zenodo.207815
DOI	https://doi.org/10.5281/zenodo.6193430
persistent identifier	https://treatment.plazi.org/id/03EB87F0-F17B-FF84-FF2E-E7AEFAF3BFBB
treatment provided by	Plazi
scientific name	Rhopalophthalmus hastatus Hanamura, Murano & Alias
status	sp. nov.

Treatment

Rhopalophthalmus hastatus Hanamura, Murano & Alias sp. nov.

( Figs. 4–7 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 )

Material examined. Holotype. Malaysia. Merbok mangrove estuary: male (BL 9.9 mm), St. C, 5˚38.5' N, 100˚24.9' E, sledge, 5.4 m depth, 14 Oct 2004, coll. Y. Hanamura ( NSMT Cr 21212).

Paratypes. Malaysia. Merbok mangrove, 154 males (BL 5.5–9.9 mm), 32 females (BL 5.6–9.7 mm), 33 ovig. females (BL 8.4–10.3 mm), 9 juvs. (BL 3.5–4.9 mm); other data same as holotype ( NSMT Cr 21213). — 12 males (BL 5.5–8.5 mm), 7 females (BL 5.1–8.0 mm), 1 juv. (BL 4.8 mm); St. B, 5˚39.2' N, 100˚23.9' E, sledge, 1.9 m depth, 10 Aug 2005, coll. Y. Hanamura ( NSMT Cr 21214). — Matang mangrove estuary, 17 males (BL 5.0– 8.6 mm), 14 females (BL 5.3–9.0 mm), 2 ovig. females (BL 7.7–9.4 mm), 72 juvs. (BL 2.6–4.6 mm); St. Ex 1, 4˚53.1' N, 100˚37.8' E, sledge, 2.0 m depth, 23 Feb 2005, coll. Y. Hanamura ( FRI Cr 007).

Additional material examined. Thailand. Surat Thani: 1 female (BL ca. 10.5 mm); 16 Oct 1978, coll. Marine Fisheries Department, Thailand ( NSMT Cr 21222). — Khlong Don Sak, Surat Thani: 2 males (BL 8.8, 10.0 mm), 1 female (BL ca. 10 mm) (more or less damaged); 14 Apr 1987, coll. Marine Fisheries Department, Thailand ( NSMT Cr 21215).

Malaysia. Merbok mangrove estuary: 2 males (BL 8.1, 9.5 mm); St. B, 5˚39.2' N, 100˚23.9' E, sledge, 4 m depth, 14 Oct 2004, coll. Y. Hanamura ( NSMT Cr 21216). — Matang mangrove estuary, 23 males (BL 5.4–9.5 mm), 11 females (BL 6.2–9.7 mm), 5 ovig. females (BL 9.2–10.2 mm), 10 juvs. (BL 2.8–4.5 mm); St. 2, 4˚50.6' N, 100˚35.3' E, sledge, 3.5 m depth, 8 Apr 2005, coll. Y. Hanamura ( NSMT Cr 21217). — 2 females (BL 7.6, 8.7 mm), 1 juv. (BL 4.5 mm); offshore of St. 4 (4˚51.4' N, 100˚32.9' E), sledge, 2 m depth, 14 June 2006, coll. Y. Hanamura ( NSMT Cr 21218).

Description. Body moderately robust ( Fig. 4 View FIGURE 4 a).

Anterior dorsal part of carapace between postorbital spines weakly produced, forming evenly rounded rostral plate ( Figs. 4 View FIGURE 4 b–d); postorbital spine sharp, extending as far as anterior end of rostral plate, carina supporting spine short and feebly developed; cervical sulcus marked dorsally and laterally around anterior one-third; anterior dorsal median nodule absent or hardly traceable or represented infrequently by very low elevation just posterior to cervical sulcus, posterior dorsal median nodule small but well defined near posterior dorsal end of carapace; posterior dorsal margin of carapace excavate, leaving last 3 thoracic somites uncovered in dorsal aspect; cheeks shallow, evenly concave or nearly vertical; anterior ventral corner sharply pointed, its tip reaching as far as anterior end of rostral plate.

Eyes ( Figs. 4 View FIGURE 4 a–d) sub-pyriform, barely reaching anterior end of second segment of antennular peduncle, length of cornea slightly shorter than eye stalk. Antennules sexually dimorphic ( Figs. 5 View FIGURE 5 a, b), that of male somewhat robust and proportionately shorter than that of female; male first segment of peduncle longest, slightly longer than combined length of distal 2 segments, armed with several long inwardly curving setae along lateral margins; second segment shortest, shorter than wide, armed with a few to several setae on lateral and mesial margins; third segment approximately as long as wide, bearing several short, hooked setae on mesial margin and several long setae around anterior mesial part; lateral flagellum basally thickened, forming male lobe and its lateral margin hirsutid densely with long hair. Female with basal segment of antennular peduncule about 1.5 times as long as combined length of distal 2 segments, armed laterally with several long inwardly curving setae; second segment shortest, with a few to several setae on mesial and lateral margins; third segment about 1.5 times as long as wide, with several setae on mesial margin and anterior mesial part. Antennal scale ( Fig. 5 View FIGURE 5 c) about 5 times as long as wide, slightly overreaching end of antennular peduncle; lateral margin naked and ending in sharp spine extending slightly beyond anterior margin of lamella; distal suture present; sympod with single stout spine accompanying 1 short most mesial spine and 1 or 2, sometimes 3, small lateral spines.

Labrum with anterior margin truncated or weakly rounded, without median spine ( Fig. 5 View FIGURE 5 d). Mouth parts as illustrated ( Figs. 5 View FIGURE 5 e–i).

Throracic appendages: third thoracic endopod ( Figs. 6 View FIGURE 6 a, b) extending as far as anterior end of carapace, slightly stouter than fourth one, carpo-propodus with 5-segmented articles. Fourth to sixth thoracic endopods ( Figs. 6 View FIGURE 6 c, d) similar in shape, carpo-propodi of endopods with 6 or 7 articles. Endopod of seventh thoracic limb ( Figs. 6 View FIGURE 6 e, f) fully reaching anterior end of carapace, carpo-propodus with 6 or 7 articles, each with long spinose disto-ventral seta bearing a few to several rather stout setules around mid-length, followed by a row of small subterminal denticules. Rudimentary endopod of eighth thoracic limb in males ( Fig. 6 View FIGURE 6 g) usually curving anteriorly, fully reaching to slightly overreaching distal end of basal plate when extended, 3-segmented, with shortest second article bearing several long setae, third segment elongated, armed distally with 2 short setae. Female rudimentary endopod of eighth thoracic limb ( Fig. 6 View FIGURE 6 h) un-articulated or incompletely 2-segmented at around basal one-third, fully reaching or slightly overreaching distal end of basal plate, with short seta near base while unarmed distally.

Abdominal somites ( Fig. 4 View FIGURE 4 a) rounded dorsally, without any ornamentations; first to fourth somites sub-equal in length, fifth somite 1.0–1.2 times as long as fourth, sixth somite 1.3–1.5 times as long as fifth; first somite of male with rounded, semi-circular lobe forming pleuron with shallow excavation anteriorly.

Pleopods in males ( Figs. 7 View FIGURE 7 a–d) biramous, first pleopod with endopod un-articulated, bearing several short marginal setae, exopod multi-articulated; second pleopod with long exopod constituted of more than 10 articles, each of basal articles with long seta but distal articles unarmed except for terminal one bearing pair of apical setae and one proportionately long sub-apical seta, endopod multi-articulated; third to fifth pleopods similar in shape with both ramous multi-articulated and sub-equal in length. Pleopods in females ( Figs. 7 View FIGURE 7 e–g) un-articulated, increasing in length on posterior somites.

Uropod ( Fig. 4 View FIGURE 4 f) with 2-segmented exopod and endopod, setose all around margins, exopod slightly longer than endopod, distal segment about half-length of basal segment; basal segment of endopod slightly more than twice length of distal segment, with stout seta on ventral surface near mid-length.

Telson ( Fig. 4 View FIGURE 4 e) 1.1–1.3 times as long as sixth abdominal somites, 2.2–2.6 times as long as basal width and again slightly more than 3.5 times as long as width of sub-basal constricted part, abruptly narrowing near basal part and forming marked waist and slightly broadened near mid-length, then gradually narrowing distally; posterior end of telson nearly rounded, barely reaching articulation of uropodal endopod and stout spinose apical setae barely reaching articulation of uropodal exopod, mesial pair of apical setae distinctly shorter than lateral pair, its setules rather sharp throughout entire margins; lateral margin of telson armed with 11–14, commonly 11 or 12, setulated and/or naked setae, slightly increasing its length towards posterior end, posterior setae usually armed with single to several minute setules on posterior basal part.

Body length. Largest recorded male: BL 10.0 mm, largest ovigerous female: BL 10.3 mm.

Egg size. Females of BL 7.9–9.6 mm carrying 5–8 embryos and stage I embryos (eggs) oblong in shape, 0.60– 0.74 mm along longer axis (N=10).

Colour. Semi-transparent when alive, telson with basal red chromatophore and slightly smaller red chromatophore around mid-length.

Etymology. The name of species (“ hastatus ” arming a spear in Latin) was chosen for the spear-like structure in the arrangement of teeth on the antennal sympod.

Remarks. Rhopalophthalumus hastatus sp. nov. shows closest similarity to R. egregius by having: 1) the carpo-propodus of the seventh thoracic endopods bearing long setae possessing stout setules and 2) two red chromatophores on the telson, which also possesses most of lateral setae bearing small setules on the posterior basal part. However, the new species differs from R. egregius in the characteristic structure of the ornamentation of the antennal sympod by the possession of a single stout spine as opposed to three graduated spines in R. egregius . The anterior dorsal nodule of the carapace is usually absent in the new species while it is well marked in R. egregius . The carpo-propodus of the seventh thoracic endopod of the new species is composed of six or seven articles compared to, most commonly, five articles in R. egregius . Furthermore, the posterior red chromatophore on the telson seems to be smaller than that of R. egregius .

Rhopalophthalmus hastatus also shows a closer resemblance to R. indicus Pillai, 1961 View in CoL . However, the spine arrangement of the antennal sympod is notably different between the two species. The rudimentary endopods of the eighth thoracic limbs in both sexes in the new species appear to be more elongated than those in R. indicus View in CoL . Furthermore, the body size of the Indian species is noticeably larger than that of the new species, exceeding 15 mm, in contrast to at most 10 mm or so in the new species, Rhopalophthalmus africana O. Tattersall, 1957 View in CoL shows a similar structure in the spine arrangement of the antennal sympod. However, the new species differs from the African species in having 1) a proportionately longer telson with a marked “waist” and bearing more than 10 lateral setae (vs. a relatively short telson with a feeble “waist” and bearing less than 10 setae in R. africana View in CoL ), 2) the carpo-propodi of the third to seventh thoracic endopods with 5–7 articles (vs. three articles in R. africana View in CoL ), and 3) proportionately long rudimentary endopods of the eighth thoracic limbs, fully reaching to overreaching the basal plate of the exopod in both sexes (vs. noticeably short endopod falling far short of the basal plate in R. africana View in CoL ).

Distribution. Gulf of Thailand and west coast of Malay Peninsula (present study).

Habitat. This species was commonly captured in the middle to upper area of mangrove riverine waters on the north-western coast of Peninsular Malaysia, and it was the innermost resident among the three species of Rhopalophthalmus occurring in that estuarine system ( Hanamura et al. 2007: as Rhopalophthalmus sp. 1).