Results
Margaritifera laosensis was detected in two of 10 tributaries of the Nam Ou River. Specimens
were found only in the Nam Long and Nam Pe rivers, two mountainous, oligotrophic headwaters
with circumneutral pH (Table 1). Populations were restricted to reaches at altitudes of 460 to
890 m (elevation of the surrounding mountain ranges is 1,400–1,800 m). Calcium concentration
in both streams was ~30 mg/l (Table 1). Occupied mesohabitats included riffles and runs with
relatively coarse substrates, including gravel and fine gravels associated with boulders and
numerous bedrock outcroppings (Appendix 1). Few macrophytes were observed in streams