Parabrachidontes leucostictus ( von Martens, 1897 ) Tan & & & & Ambarwati & Reni, 2024

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 : 215-216

Parabrachidontes leucostictus ( von Martens, 1897) , new combination

Modiolus leucostictus von Martens, 1897: 86–88 , plate X, figs. 18–21.

Modiola evansi Smith, 1903: 368 , figure.

Modiola evansi — Lynge, 1909: 132; Annandale, 1916: 93; Suvatti, 1939: 102

Brachidontes arcuatulus — Brandt, 1974: 257 pl. 18 fig. 21 (not Arcuatula arcuatula ( Hanley, 1843))

Limnoperna siamensis — Swennen et al., 2001: 62 and figure 008 [not Limnoperna siamensis ( Morelet, 1866) ]

Arcuatula leucosticta — Huber, 2010: 107 and 110 (not figured)

Brachidontes evansi — Huber, 2010: 116, figure.

Brachidontes setiger — Ngo et al., 2018: 176, fig. 4C (not Brachidontes setiger Dunker, 1857 )

Parabrachidontes leucostictus — Tan et al., 2023: 304–308 and figs. 1, 3, 4, 6B, 7, S1B, S2B, C

Diagnosis. Shell surface faintly corrugated radially, with corresponding dark brown to black bands over a dark green background. Umbones subterminal. Dorsally directed edges of ascending lamellae of both outer and inner demibranchs are attached by tissue fusion to mantle surface.

Material examined. Modiolus leucostictus holotype ( ZMA 135170 ) and paratypes ( ZMB 108.841 ) , Maros River , Makassar, South Sulawesi ( MZBC Pel.2214); Modiola evansi : 10 syntypes from Tale Noi , Songkhla Lake, Thailand ( NHMUK 1901.2.4.140-149; ANSP 98099 ) ; 20 specimens from Thale Noi and Thale Luang, Songkhla Lake, Thailand ( PMBC 30687-88 ; ZRC.MOL.24971-72) ; 8 specimens from Santubong & Buntal, Sarawak, East Malaysia ( NHMUK 94.7.14.27–33) ; 20 specimens from Sarawak River, Kuching, East Malaysia ( ZRC.MOL. 24958) ; 10 specimens from Maros River , Makassar, South Sulawesi, Indonesia ( ZRC. MOL. 24959) .

Description. Shell up to 30 mm in length, thin, elongate, mytiliform, somewhat flattened laterally but with a moderately strong keel. Valve surfaces dominated by fine, closely spaced commarginal lines over a faintly corrugated surface. The raised regions correspond to the dark brown to black radial pigment bands present across the posterior region of the valves, while the furrows between the dark pigment bands are greenish yellow. A few of these dark bands bifurcate towards the shell margin. These bands are narrower and closer together along and ventral to the keel but become progressively wider dorsally where the bands are also farther apart. The region ventral to the keel is brownish yellow, where the radial bands are obsolete or absent altogether. At the anteriormost region, 4–6 faint radial furrows are present. Umbones are subterminal. Byssal hairs absent. Interior of shell is tinged iridescent light purple. Inside edges of dorsal and anterior regions of shell are weakly crenulate. The ligament is narrow, devoid of resilial pits. Shell under ligament has no crenules, while posterior to the ligament, there are 10–12 crenules on the inside edge of the shell. The region anterior to the ligament and ventral to the umbones has about 15 denticles. The posterior shell margin is faintly crenulate, corresponding to the termination of the radial bands on the shell surface. The anterior adductor muscle scar traces a shallow arc just inside the antero-ventral edge of the shell. The posterior adductor muscle is oval and elongate antero-posteriorly (3.5 mm x 2 mm) and merges with the posterior byssal retractor muscle scar, which is unusually long and extends anteriorly beyond a quarter way from the posterior end of the ligament. Shell microstructure comprising a thin (10-15µm) subperiostracal homogeneous layer (likely of calcite) and thicker 250-300 µm nacreous aragonitic layer. A simple prismatic myostracum occurs as the innermost layer. Shell is thus mostly aragonitic (97–98% w/w) with a small amount of calcite that is likely associated with the uppermost shell layer just under the periostracum. Animal. Labial palps small (about 48 folds), short, about ¼ length of ctenidium (3 mm vs 16 mm). Edges of ascending lamellae of outer and inner demibranchs attached along its entire length to inner surface of adjacent mantle along its mid-region. Inner demibranchs about half width of outer demibranchs (outer: inner demibranch width 2.8 mm: 1.4 mm). Plicate organs absent. Foot elongate, muscular. Midgut and style separate. The intestine makes a loop along the left side of the animal. The pericardial complex is located dorsal to the gap between the anterior and posterior sets of the posterior byssal retractor muscle complex (Category 3 of Morton, 2015). Posterior region of mantle edge forming the inhalant aperture is thickened containing yellowishwhite subcutaneous pigment grains. The crenulate margin is thrown into multiple folds, whose surface is lightly shaded with greyish-black pigment. The inside edge of the mantle edge is smooth, and guard papillae are absent from the inhalant siphon.

Geographical distribution. Currently known from southern Thailand (Thale Noi and Thale Sap, Songkhla Lake; Annandale, 1916), East Malaysia ( Sarawak River, Kuching) and Indonesia (Maros River, Makassar, South Sulawesi).

Taxonomic remarks. Parabrachidontes leucostictus ( von Martens, 1897) was described from material collected in the Maros River near Makassar in South Sulawesi, Indonesia. Von Martens (1897: 86) described the shell as ‘…olivaceofusca, punctis albis adspersa…’, the latter referring to the whitish spots present on the valve surfaces of the holotype and some paratypes (see Tan et al., 2023: fig. 3) for which the species name is presumably derived. On careful examination of images of the type material however, these ‘spots’ are actually the bases of byssal hairs adhering to the shell surface. Such hairs are absent in the syntypes of Parabrachidontes evansi ( Smith 1903) described several years later from Thale Noi, the innermost freshwater lake of the Songkhla Basin in southern peninsular Thailand ( Smith, 1903; not Malacca as erroneously referred to in the title of the paper). We also did not observe byssal hairs on the material collected recently from the type locality in Indonesia as well as from Malaysia and Thailand. However, the deposition of these byssal hairs may be a habitat-specific response and it is not uncommon to observe individuals with and without such hairs from different environments, as in Limnoperna fortunei (e.g., Montalto & Molina, 2014) and Xenostrobus securis (Tan KS, pers. obs.). At the same time, the type materials of both P. leucostictus and P. evansi have fine radial ribs that are tinged dark brown. Based on these observations, together with additional morphological details seen in this study, we conclude that the two species are likely to be the same.

Brandt (1974) erroneously referred to Modiola evansi as ‘ Brachidontes arcuatulus ’, while Swennen et al. (2001) mistook it as a species of Limnoperna . More recently, P. leucostictus (as evansi ) was placed in the genus Brachidontes by Huber (2010). It remains to be seen if brackish water specimens from India identified as ‘ Brachidontes striatulus ’ (see Tan et al., 2021; not Byssogerdius striatulus ) are related to Parabrachidontes leucostictus . Apart from their resemblance in shell colour and sculpture, the position of the pericardium in ‘ Modiolus striatulus ’ from Calcutta as observed by Morton (1977) is similar to P. leucostictus observed in this study (i.e., Category 3 of Morton, 2015; see above).