Rhopalophthalmus hastatus Hanamura, Murano and Man 2011

Rhopalophthalmus hastatus Hanamura, Murano and Man 2011

( Fig. 1 View Figure 1 )

Rhopalophthalmus hastatus Hanamura et al., 2011: 8–13 View Cited Treatment , figs. 4–7.

Material examined. Two adult males (BL 9.0 [not dissected], 10.1 mm [dissected]) and two ovigerous females (BL 11.5 mm [not dissected], 11.8 mm [not dissected]), PSUZC 20181118 View Materials - 10.01 View Materials , Thailand, Ban Khok Rai, Tambon Pawong, Muang Songkhla District, Songkhla Province, Thale Sap Songkhla, 07°9’48.71”N 100°34’28.17”E, muddy substrate, shallow zone, 18Nov 2018, N. Tubtimtong, R GoogleMaps . Yolanda and V. Lheknim coll.

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Diagnosis. Body smooth, without hairs, spines or folds, ventral sternites without process; first 5 somites sub-equal in length, sixth somite slightly longer than fifth. Eyes sub-pyriform with well-developed cornea occupying 1/3 of whole eye and width slightly wider than eyestalk.Anterior part of carapace short, produced as sub-triangular process with pair of postorbital spines on dorsal keels. Pair of nodules present at posterior 2/3 of carapace, in lateral aspect. Antennular peduncle with 3 articles, female more slender than male, armed with several long curving setae along lateral to distal margins, male lobe well developed with long dense setae. Antennal scale long and narrow, outer margin naked with strong outer distal spine and distal suture, sympod armed with 2 short spines, 1 mesial spine and 1 strong spine. Labrum sub-globular in shape. First thoracopodal endopod (maxilliped 1) short and basis well developed. Second thoracopodal endopod (maxilliped2) stout. Carpopropodus of third to seventh thoracic endopods composed of five to six articles, eighth endopod reduced in size showing pronounced sexual dimorphism. Pleopods in male biramous, second exopod elongated and armed at distal end with modified setae, while uniramous in female and size gradually increasing to last pleopod. Uropods two segmented in first 1/3, endopod with single stout spine below statocyst. Telson linguiform, entire with lateral margin armed distally with row 13–14 sharp spines increasing in length posteriorly and bearing 3 small spines, posterior margin bearing 4 long setae, armed with several small sharp spines on margin.

Typelocality. Merbok mangrove estuary, Malaysia

( Hanamura et al., 2011).

Habitat and Distribution. This species is known from Merbok and Matang mangrove estuary, Malaysia; Khlong Donsak estuary ( Hanamura et al., 2011) and Songkhla Lagoon, Thailand (Yolanda, 2021; this study) (see Fig. 2 View Figure 2 ).

Remarks. The mysid R. hastatus is very close to its ally congeners, R. egregius and R. orientalis , but,the first species can be distinguished from the other two based on the following characteristics: Rhopalophthalmus hastatus - 1) possess one large spine,one mesial spine, and one to three small spines on the antennal sympod; 2) the antennal scale slightly overreaching the end of the antennular peduncle; 3) carpopropodi of the seventh thoracic endopod composed of six or seven articles; 4) two red chromatophores on the telson but the posterior one smaller than in R. egregius ;5) lateral margin of the apical spine forming sharp spine-like process. Rhopalophthalmus egregius - 1) possess three or four large spines and two small spines on antennal sympod; 2) the antennal scale slightly overreaching the end of the antennular peduncle; 3) carpopropodi of the seventh thoracic endopod composed of five articles; 4) two red chromatophores on the telson; 5) lateral margin of apical spine forming leaf-like process or sharp spine. Rhopalophthalmus orientalis - 1)possess two large spines and two small spines on antennal sympod; 2) the antennal scale barely reaching the end of the antennular peduncle; 3) carpopropodi of the seventh thoracic endopod composed of five articles; 4) one red chromatophore present at the mid-anterior of telson or sometimes without chromatophore; 5) lateral margin of apical spine forming leaf-like process ( Hanamura et al., 2011).

Morphological observation of R. hastatus from the Songkhla Lagoon shows a little variation from the original description by Hanamura et al. (2011). In our specimens, there is a small nodule right after the cervical sulcus (see arrows in Fig. 1A, B View Figure 1 ), while there is no nodule in the original description ( Hanamura et al., 2011). Variation in the occurrence of this small median nodule in the group Rhopalophthalminae is also found in other species, for example, the small nodule in R. egregius from the Zoological Museum of Copenhagen University was absent/less developed, meanwhile, a well-developed anterior nodule was present from Southeast Asia specimens ( Hanamura et al., 2011). Similarly, the specimens of R. longipes from Malaysia also bear a median small nodule ( Tan and Azman, 2017), meanwhile the specimens from Japan are without this nodule ( Ii, 1964). Interestingly, we didn’t find any R. egregius or R. orientalis in this lagoon as previously mentioned by W.M Tattersall (1921). We expect that our findings may contribute to the diversity of Mysidacea in the Songkhla Lagoon.