Lannanaia, 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 Lannanaia gen. nov.

Type species.

Lannanaia kokensis gen. et sp. nov., by present designation.

Species included.

The new genus currently contains only one species, L. kokensis sp. nov.

Diagnosis.

This new genus is distinguished from other Pseudodontini genera by its compressed, rather short and high shell, with sub-trigonal outline, and distinct posterior wing. The hinge plate is without dentition, or with very rudimentary broad pseudocardinal tooth in each valve, and posterior end of the hinge structure with rather wide V-shaped fossette (Fig. 2 ). It is also represented as a distinct clade in multi-locus phylogenetic analyses (Fig. 1 ).

Description.

Shell medium-sized, thin, rather short and high, winged, sub-trigonal outline, inequilateral, rather compressed. Anterior margin rounded; posterior margin angulate, somewhat pointed. Umbo tiny, not prominent, eroded. Ligament long, very narrow. Hinge without dentition, or with very rudimentary broad pseudocardinal tooth in each valve; posterior end of the hinge structure with rather wide V-shaped fossette. Anterior adductor muscle scar very shallow, ovate, contiguous with anterior pedal retractor muscle scar, but separated from anterior protractor scar; posterior adductor muscle scar round, very shallow, almost invisible. Pallial line very faint, continuous. Umbo cavity shallow.

Etymology.

The generic name “ Lannanaia ” is from the word “ Lanna ”, a name of the kingdom located in present-day northern Thailand during the 13 th to 18 th centuries, and the Greek word “ naiad ” meaning freshwater mussel. The name of this genus thus means “ freshwater mussels from Lanna ” or “ freshwater mussels from northern Thailand ”. The gender is feminine.

Distribution.

Endemic to Kok River, a tributary of the Mekong River in northern Thailand (Fig. 3 ).

Remarks.

The phylogenetic tree generated in this study places Lannanaia gen. nov. as a sister group to Songkhlanaia , with strong support from both analyses (BS = 100 %, bpp = 0.99; Fig. 1 ), and with a 11.42 % p-distance based on the COI gene (Table 1 ). Both genera share general characteristics of a short shell outline and rather compressed shell. However, Lannanaia gen. nov. can be distinguished from Songkhlanaia by having a sub-trigonal shell outline and a curved ventral margin (vs a rectangular shell outline and an almost straight ventral margin in Songkhlanaia ), distinct posterior wing (vs absent posterior wing), and not prominent umbo (vs slightly elevated and easily visible umbo) ( Konopleva et al. 2023). In addition, the biogeography of the two genera is also distinct. The monotypic genus Songkhlanaia was described from the Songkhla Lake Basin in southern Thailand ( Konopleva et al. 2023), while Lannanaia gen. nov. is restricted to the Kok River, a tributary of the Mekong River in northern Thailand.

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 2. Shells of Lannanaia kokensis sp. 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 0825; B. Paratype MUMNH - UNI 0824. Scale bars: 10 mm.

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/).