Parabrachidontes amnicus Tan, Tan, Sanpanich, Duangdee & Ambarwati, 2023

Tan, Koh Siang, , Samuel Hui Ming Tan, , Kitithorn Sanpanich, , Teerapong Duangdee, Ambarwati, & Reni, 2024, Validation of Parabrachidontes, a new genus of fresh- and brackish-water mussels (Bivalvia: Mytilidae) from Southeast Asia with a redescription of the type species and P. amnicus Tan et al., 2023, Raffles Bulletin of Zoology 72, pp. 214-218 : 216-218

Parabrachidontes amnicus Tan, Tan, Sanpanich, Duangdee & Ambarwati, 2023

( Fig. 1)

ZooBank registration. urn:lsid:zoobank.org:act:B92166DD-5FA0-4656-BDD1-DC2C4AA51A4D

‘ Parabrachidontes ’ amnicus Tan et al., 2023: 7 , figs. 1, 2, 6A, 7; Suppl. Info., pp. 2–4, figs. S1A, S2A.

Holotype. THAILAND, Satun Province, Khlong Pak Bala , upper reaches of river, 17 May 2018 ( PMBC 30680 ) Paratypes. Same location data as for holotype, 6 individuals preserved in ethanol ( PMBC 30681 , sequenced) ; PMBC 30682 ; ZRC.MOL.29857 ; ZRC.MOL.29858).

Other material. Same data as for holotype, 6 individuals fixed in formalin ( ZRC.MOL.24957) .

Etymology. The species is named for the riverine (Latin: ‘amnicus’, of a stream) habitat from which it was collected.

Diagnosis. Valve surface with numerous narrow but distinct radial ribs across the dorsal half of the shell that become obsolete towards the anterior end; periostracum greenish at posterior half of shell and along the posterior margin, while the anterior half of shell is brown.

Description. Shell thin, small (SL to 20 mm), elongate, mytiliform, somewhat flattened laterally with a moderate keel. Shell surface with broad radial ribs crossed by irregularly spaced major commarginal lines interspersed with fine, closely spaced minor commarginal lines. Radial ribs number about 26 dorsal to keel, sometimes bifurcating towards the shell margins; finer, closely set ribs are present ventral to the keel, becoming obsolete towards the anterior, but reappearing again at the anterior end as broad but closely spaced radial ribs. Umbones subterminal. Periostracum green at the posterior half of the shell and along the posterior valve margins, while the anterior half of the shell is dark brown. Byssal hairs absent. Interior of shell tinged with purple; inside margins of dorsal and anterior regions crenulate, which correspond to the terminations of the radial ribs on the shell surface. The ligament is robust, relatively wide, resilium pits absent. The inside shell margins immediately posterior to the ligament bears about 10 crenules, whilst those at the anterior margin ventral to the umbones has 17 strongly impressed denticles that become smaller towards the posterior. The anterior adductor muscle scar traces a shallow arc just inside the antero-ventral edge of the shell. The posterior adductor muscle scar is relatively large, ovatecircular in shape (diameter about 2.7 mm) and is conjoined with the broadly elongate posterior byssal and foot retractor muscle scar that extends about ¼ way along the length of the ligament. In addition, a series of small circular attachment scars (about 25 in total, but not always visible) occurs along the inside shell surface of the keel from the posterior mantle attachment towards the anterior, ending at the mid-region of the shell. Shell microstructure comprising a thin (10–15µm) subperiostracal homogeneous layer and thicker 250–300 µm nacreous layer. A simple prismatic myostracum occurs as the innermost layer. Shell is predominantly aragonitic (98.8% w/w) with presence of trace amounts of calcite (0.8%). Animal with very short labial palps (about ¼ length of ctenidium) with about 22 folds. Upper edges of the ascending lamellae of outer demibranchs attached to the inside surface of mantle about ⅔ way dorsally. Upper edges of ascending lamellae of inner demibranchs attached to roof of mantle cavity. Plicate glands absent. Foot elongate, muscular. The posterior pedal/byssal retractor muscle complex is distinctly separated into two regions, comprising an anterior set that is barely split into two equal bundles near the shell attachment region, and a posterior set with two equally elongate bundles that is each subdivided into two smaller bundles at the attachment region. The pericardium is located between the anterior and posterior sets of the posterior byssal retractor muscle complex (Category 3 of Morton, 2015). Midgut and style sac separate. The posterior region of the thickened mantle edge has up to 10 short, simple, unbranched papillae in a single row on either side of the inhalant region, where subcutaneous white pigment grains are present. The mantle margin is entire and slightly crenulate.

Shell (holotype). Shell length 15.2 mm (PMBC 30680) (see Fig. 1A, B)

Geographical distribution. Currently known only from Pak Bara in Satun Province, southwest Thailand.

Taxonomic remarks. The prominent and numerous radial ribs present on the external shell surface of Parabrachidontes amnicus distinguish this species from its congener P. leucostictus (see Tan et al., 2023). In the latter species, radial ribs are present but are weak and generally flattened. The prominent radial sculpture in P. amnicus is reminiscent of many marine Brachidontes species often found intertidally on seashores around the world. However, apart from their convergence in shell sculpture, P. amnicus can be easily distinguished from true Brachidontes species in having the terminal edges of the ascending lamella of its outer demibranch attached to the mantle lobe. As far as we are aware, the ascending lamellae of the outer demibranchs of Brachidontes species are free and not attached to the mantle surface. The disparate positions of the two genera in phylogenetic trees based on multiple genes (see Fig. 1 in Tan et al., 2023) also attest to their evolutionary divergence.