Morphological comparisons of collected specimens with type descriptions [35] suggest that M. laosensis may represent a distinct (and not multiple) taxon. Nacre color of all specimens was white, often with small oily spots (Fig. 3). The mean length of M. laosensis sampled was 65.5 mm (SD = 21.4 mm, range 26.1-110 mm, Figs. 4A, B). In the Nam Long River, the numerically dominant size class (66.7 %) of living mussels were 31-70 mm long, and the numerically dominant size class of individuals (40.9 %) were 51–70 mm long (Fig. 4C). In the Nam Pe River, the numerically dominant size class (41.7 %) of mussels were 71–80 mm long (Fig. 4D). In the Nam Long River, collected mussels had died from natural causes; most of them were between