Isannaia, Jeratthitikul & Sutcharit & Prasankok, 2024

Jeratthitikul, Ekgachai, Sutcharit, Chirasak & Prasankok, Pongpun, 2024, Two new genera and three new species of exceptionally rare and endemic freshwater mussels (Bivalvia, Unionidae) from the Mekong Basin, Zoosystematics and Evolution 100 (4), pp. 1333-1345 : 1333-1345

Genus Isannaia gen. nov.

Type species.

Isannaia fortunata gen. et sp. nov., by present designation.

Species included.

The new genus is currently composed of two species, the type species and I. occultata sp. nov.

Diagnosis.

The new genus is distinguished from other genera in the Pseudodontini by its thin and moderately inflated shell, and with rhomboidal to ovate outline (Fig. 4 ). The overall shape is somewhat elongated but relatively shorter when compared to those of Bineurus , Pilsbryoconcha or Namkongnaia . The anterior margin is rounded, shouldered, and the posterior margin is truncated to rounded. Its pseudocardinal teeth are rather small, one on each valve, hill-like or triangulate shape, and less developed on the left valve. Its umbo is slightly elevated and the umbo cavity moderately deep. It is also represented as a distinct clade in multi-locus phylogenetic analyses (Fig. 1 ).

Description.

Shell medium-sized, rather thin, rhomboidal to ovate outline, elongate, very inequilateral, moderately inflated; anterior margin rounded, shouldered; posterior margin truncated to rounded. Umbo slightly elevated and usually eroded. Ligament very narrow. One shallow hill-like or triangulate pseudocardinal tooth on right valve; left valve with one less developed hill-like to tubercule-like pseudocardinal tooth. End of hinge structure with V-shaped fossette. Anterior adductor muscle scar shallow to well-developed, ovate, contiguous with pedal retractor and anterior protractor muscle scars; posterior adductor muscle scar round, very shallow. Pallial line faint to well-marked, continuous. Umbo cavity moderately deep.

Etymology.

The generic name “ Isannaia ” is from the word “ Isan ”, the name of the region of northeastern Thailand, and Greek word “ naiad ” meaning freshwater mussel. The name of this genus thus means “ freshwater mussels from northeastern Thailand ”. The gender is feminine.

Distribution.

Endemic to tributaries of the Mekong River in northeastern Thailand (Fig. 3 ).

Remarks.

Phylogenetic analyses have failed to recover a robustly supported position of Isannaia gen. nov. among other Pseudodontina genera. However, in terms of genetic distance, it is nearest to Namkongnaia , with a separation of 11.77 % p-distance of the COI gene (Table 1 ). Isannaia gen. nov. can be easily distinguished from Namkongnaia by having a much shorter shell ( Jeratthitikul et al. 2021 b).

In general, Isannaia gen. nov. bears conchological resemblance to certain species within the Sundadontina , such as S. brandti Bolotov et al., 2020 or S. plugpomenica Konopleva et al., 2023 ( Bolotov et al. 2020; Konopleva et al. 2023). Nevertheless, its thin shell with less developed pseudocardinal teeth set it apart from the latter genus, which typically features a thicker and sturdier shell with stout and tubercular-like pseudocardinal teeth ( Bolotov et al. 2020).

Figures

Figure 1. Maximum likelihood (ML) tree of freshwater mussels in subfamily Gonideinae based on 1,935 - bp concatenated alignment dataset of COI and 16 S rRNA mitochondrial genes, and 28 S rRNA nuclear gene. The numbers on nodes represent the bootstrap support (BS) from the ML analysis and the bipartition posterior probability (bpp) from the BI analysis, and are shown as ML / BI. Scale bar indicates the branch length.

Figure 3. Map showing type localities of the new species: circle Lannanaia kokensis sp. nov., triangle Isannaia fortunata sp. nov., and square Isannaia occultata sp. nov. Map was generated using QGIS v 3.24. 3 with the outline of Mekong basin from the Freshwater Ecoregions of the World (Abell et al. 2008), river and lake topology from the HydroSHEDS database (https://www.hydrosheds.org), and the map raster data from the NASA EARTHDATA (https://www.earthdata.nasa.gov/).

Figure 4. Shells of Isannaia gen. nov., showing the inner side of the right valve, the outer side of the left valve, and the dorsal view of both valves. A. Holotype (MUMNH - UNI 1950) and B. Paratype (MUMNH - UNI 1952) of Isannaia fortunata sp. nov.; C. Holotype (MUMNH - UNI 2895) and D. Paratype (MUMNH - UNI 2897) of Isannaia occultata sp. nov. Scale bars: 10 mm.