Macrothrix undefined-2

13.** Macrothrix sp. 2 (Fig. 3A–D) Localities and specimens found: 25 – 4♀, 2♀ ov, 1j; 26 – 2♀, 1j. Distribution and ecology. This uncertain Macrothrix member was discovered in the open and plant-filled littoral of one of the reservoirs (see Appendix 1). Remarks. At the moment, the exact species identity of these individuals is not clear. More material and further research are needed to resolve this issue. It is possible that this is a new, not-yet-described representative of the genus. Based on the key diagnostic characteristics (dilating distally antennules, short distal segments of setae natatoriae, specific armament of the setae of the outer lobe of the limb I), the Vietnamese Macrothrix sp. 2 is most similar to M. spinosa King, 1853, which is widely known in the Oriental region, and to the Australian M. breviseta Smirnov, 1976 (Smirnov 1992). As in the first species, our individuals have the entire dorsal margin of valves clearly serrated (Fig. 3A, C). At the same time, the structure of their antennules, bearing distally several transverse groups of relatively long setules on the anterior side (Fig. 3A, B), is more similar to those of M. breviseta. However, the dorsal margin of valves of M. breviseta has serrulations mainly in the area of the head pore. In addition, the Vietnamese Macrothrix sp. 2 also differs from both named species by an even shorter distal segment of the natatoriae setae (Fig. 3A, D).