Indonaia rectangularis (Tapparone-Canefri, 1889) Tapparone-Canefri, 1889

Bolotov, Ivan N., Vikhrev, Ilya V., Lopes-Lima, Manuel, Mikhail Y. Gofarov,, Konopleva, Ekaterina S., Lunn, Zau, Chan, Nyein & Bogan, Arthur E., 2019, Indonaiarectangularis (Tapparone-Canefri, 1889), comb. nov., a forgotten freshwater mussel species from Myanmar (Bivalvia, Unionidae), ZooKeys 852, pp. 23-30 : 24-26

Indonaia rectangularis (Tapparone-Canefri, 1889) comb. nov. Figure 1 A–F

= Unio rectangularis Tapparone-Canefri 1889: 354.

= Margaritanopsis rectangularis (Tapparone-Canefri, 1889). - Prashad 1922: 93, pl. 2, fig. 5.

Type.

Holotype in MSNG, without ID number (Fig. 1 A–C). Original label: " Unio rectangularis Tapp. Can. Teinzo, Mti E. di Bhamo (L. Fea)" (Fig. 1D). Secondary labels: " Margaritanopsis var.? juv. detto B. Prashad!" (Fig. 1E) and " Margaritanopsis var.? Young specimen" (Fig. 1F). The original label does not have a collecting date, but Fea’s sample of another freshwater mussel species from Teinzo residing in the MSNG is dated "Marzo 1886" that may also be applicable to the holotype of U. rectangularis .

Type locality.

Teinzo (presently Teinthaw village), 24.3978N, 97.2519E, Moolay River (Mole Chaung in Burmese), hills E of Bhamo (L. Fea), alt. 110 m a.s.l., Ayeyarwady Basin, Myanmar ( Tapparone-Canefri 1889).

Diagnosis.

As Tapparone-Canefri (1889) stated, the shell of Indonaia rectangularis is not similar to any other freshwater mussel species known from Myanmar. This species cannot be mistaken with the two congeners, Indonaia andersoniana and I. subclathrata (Fig. 2), that were described from the Ayeyarwady Basin. Both these species differ from Indonaia rectangularis by a more elongated shell and less developed lateral, and pseudocardinal teeth. The pseudocardinal teeth in I. rectangularis are lamellar, thick, and very elongated. Additionally, a unique feature of I. rectangularis is the presence of regular ridges crossing the growth lines in the posterior-dorsal area forming a clear rectangular pattern that was never seen in other species (Fig. 1C).

Redescription.

Shell length 34.2 mm, height 20.4 mm, width 16.2 mm. Shell thick, obovate, inequilateral, with broader posterior side. Dorsal margin slightly convex. Ventral margin nearly straight. Anterior margin rounded. Posterior margin slightly elevated posteriorly, with an inconspicuous wing. Umbo prominent, elevated, rounded, slightly eroded. Shell surface mostly smooth. In the posterior-dorsal area, regular ridges cross the growth lines and form a clear rectangular pattern. In the anterior-dorsal area, curved, lamella-like ridges closely spaced along growth lines. Periostracum light olive-brown, with two parallel, slightly curved green bands along posterior-dorsal area; inner band with a broad greenish extension posteriorly. Nacre silver-white. Umbo cavity deep. Anterior adductor scar round, shallow but well marked. Posterior adductor scar oval, very shallow, unclear. Mantle attachment scars absent. Pseudocardinal teeth are thick, lamella-like, very elongated, two teeth in the right valve and one tooth in the left valve. Lateral teeth well developed, thick, elongated and straight, one tooth in the right valve and two teeth in the left valve. Soft body morphology and anatomy unknown.

Remarks.

Unio rectangularis was originally described based on a single specimen with a shell 34.2 mm long, 20 mm high and 16 mm wide ( Tapparone-Canefri 1889). The single shell labelled as Unio rectangularis and deposited in the MSNG studied here corresponds in dimensions to the specimen described by Tapparone-Canefri. The original label (Fig. 1D) also affirms that this shell is the holotype of Unio rectangularis designated by monotypy.

Distribution.

This species is known only from the type locality, a tributary of the Ayeyarwady River (Fig. 3). It appears to be a rather restricted and rare species, because it has not been recorded since the original description, even during a recent broad-scale survey of freshwater mussels in Myanmar ( Bolotov et al. 2017a, b, 2018; Konopleva et al. 2019). However, there has been no studies on freshwater mussels from the Mole River published since Tapparone-Canefri (1889).