Deltamysis songkhlaensis (Yolanda, Sawamoto & Lheknim, 2019)

Daneliya, Mikhail E., 2023, On the Mysid Crustacean Genus Deltamysis (Mysidae: Heteromysinae), with a New Species and a New Record from Australia, Records of the Australian Museum (Rec. Aust. Mus.) 75 (4), pp. 413-430 : 428

Deltamysis songkhlaensis (Yolanda, Sawamoto & Lheknim, 2019)

Heteromysoides songkhlaensis Yolanda et al., 2019: 536–542 , figs 2–4.

Deltamysis songkhlaensis .— Daneliya, 2021: 4.

Type material. Holotype, allotype, and numerous paratypes, Songkhla Lagoon , Thailand, Songkhla Province, Singha- Nakhon District , Tambon Bang Khiat , Ban Bang Khiat, Thale Sap, 7°20'58.68"N 100°25'31.56"E (see Yolanda et al., 2019) .

Diagnosis. Anterior margin of carapace angular, apically pointed or blunt. Cornea narrower than eyestalk (0.57–0.63 times as wide). Telson rather short, with tapering lateral margins, posteriorly rounded, 1.1–1.2 times as long as wide anteriorly; apically without cleft, rather convex, and spinules nearly undistinguishable in length from neighbouring spiniform setae; its lateral margins with four to seven spiniform setae on each side, gradually increasing in length; longest terminal spiniform setae 0.27–0.32 of telson length. Maxilla 2, exopod rather large, reaching endopod segment 2, with numerous lateral setae; endopod with lateral setae. Maxilliped 2 without lateral processes on carpus and propodus. Pereopod dactylus rather thick. Pereopod 3–6 carpopropodus 4-segmented; segment 1, longer, subequal or only slightly shorter than other segments combined.

Body length. Males 3.2–4.0 mm, females 3.1–3.7 mm (Yolanda et al., 2019).

Comparison. Deltamysis songkhlaensis is the most differentiated species in the genus, with unique shape of the carapace anterior margin (see diagnosis), the deepest reduction of the eye cornea, the telson armature and the segment ratio in pereopod 3–6 carpopropodus ( Table 1).

Distribution. Currently known only from the Songkhla Lagoon system in Thailand (Yolanda et al., 2019).

Habitat. Found in brackish water conditions (0.47–24.8 psu) on the muddy bottom at depths of 0.6–1.5 m (Yolanda et al., 2019).

Remarks. Yolanda et al. (2019) in their detailed description of D. songkhlaensis attributed the species to the genus Heteromysoides ( Daneliya, 2021) . They noticed certain similarities of D. songkhlaensis to a species formerly known as H. nana , now also a member of Deltamysis , as well as other former Heteromysoides species, currently belonging to the genus Platyops (tribe Heteromysini ). Deltamysis songkhlaensis is indeed similar to D. nana , but not more than to the other three species of Deltamysis , equally sharing with them all the generic diagnostic features. In the prior work I transferred H. songkhlaensis to Deltamysis , and here I update the species diagnosis, comparing it with other members of the genus.

Concerning the structure of the telson, it is not possible to say, whether the apical spinules are entirely missing in D. songkhlaensis or they are strongly developed into the large spiniform setae, which are undistinguishable from the lateral spiniform setae. I have been calling these spine-like structures “spinules”, because they are normally attached to the telson without articulation. In D. songkhlaensis the apical spine-like structures are clearly articulated, as it was figured by Yolanda et al. (2019), but to maintain their presumed homology I prefer to call them “spinules”.

ACKNOWLEDGEMENTS. The material was collected by J. K. Lowry and S. J. Keable of the Australian Museum and Research Institute (AMRI). I would like also to acknowledge the assistance of S. J. Keable and H. E. Stoddart in providing the access to and final deposition of the specimens, as well as other staff of AMRI for creating an excellent and memorable working environment during my stay at the museum. I sincerely appreciate the suggested corrections and changes to the manuscript made by the two reviewers, the journal chief editor, and the special issue editors. This study was made possible for me by the AMRI Visiting Collection Fellowship in 2015.