Isannaia fortunata, 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

Isannaia fortunata sp. nov.

Figs 1 , 3 , 4 A, B

Type material.

Holotype. Thailand • Nakhon Phanom Province, Tha Uthen District, Tha Uthen Subdistrict, Thuai River ; 17.5621 ° N, 104.6090 ° E; E. Jeratthitikul, P. Prasankok, and K. Wisittikoson leg.; MUMNH -UNI 1950 ; shell length 54.13 mm, shell height 29.96 mm, and shell width 14.39 mm . Paratypes. Thailand • 3 shells; same collection data as for holotype; MUMNH -UNI 1951 to UNI 1953 .

Description.

Shell medium-sized (shell length 43.3–61.4 mm, shell height 24.5–35.1 mm, shell width 13.4–18.4 mm; Table 2 ), rhomboidal, elongate (H / L ratio = 0.55–0.57), rather thin, very inequilateral, moderately inflated. Anterior margin rounded, rather constricted, shouldered; posterior margin truncated, slightly elongate ventrally; ventral margin slightly curved. Dorsal margin slightly curved; anterior low, posterior end slightly higher. Umbo slightly elevated, heavily eroded even in young specimens. Posterior ridge wide and obtuse, not prominent; posterior slope with two faint lines running from umbo to posterior margin. Periostracum thin, brownish-black, eroded part white to coppery-brown. Shell surface with visible growth lines. Ligament long, very narrow, dark brown. One very shallow hill-like or triangulate pseudocardinal tooth on right valve; pseudocardinal tooth on left valve less developed, tubercule-like, or almost absent. Posterior end of hinge structure with small V-shaped fossette. Anterior adductor muscle scar shallow, ovate, contiguous with pedal retractor and anterior protractor muscle scars; posterior adductor muscle scar round, very shallow, almost invisible. Pallial line faint, continuous. Umbo cavity moderately deep, with 1 to 2 tiny muscle scars in the cavity. Nacre blue-whitish with yellowish tint around the umbo cavity.

Exhalant aperture almost smooth, with a row of tiny tubercle-like papillae on edge of aperture, shorter than inhalant. Inhalant aperture with a row of conical papillae, varying in length. Small epithelial fold divides exhalant and inhalant apertures. Gills elongated and slightly ribbed; inner gills slightly wider, and anterior margin of inner gills slightly longer than that of outer gills. Labial palps elongate, slightly curved, pointed at tip. Glochidia unknown.

Etymology.

The species name “ fortunata ” is derived from the Latin adjective “ fortunatus ” meaning lucky. This name refers to the fortuitous discovery of this rare freshwater species.

Distribution.

This new species is known only from the type locality.

Remarks.

This new species is rare and restricted to the Thuai River. Specimens were collected from hard clay along the river bank. It was found sympatrically with several unionid species endemic to Songkhram Basin and nearby tributaries of the Mekong Basin, including Lens maenamensis Pfeiffer et al., 2021 , Physunio pellucidus Pfeiffer et al., 2021 , Ensidens jaculus ( Rochebrune, 1882) , Scabiellus songkramensis ( Kongim et. al., 2015), Pseudodon cambodjensis ( Petit de la Saussaye, 1865) and Scabies scobinatus ( Lea, 1856) .

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.