Oxyurella longicaudis Birge 1910

Oxyurella longicaudis (Birge, 1910) (Figs. 65–76)

Material examined: + 10 females from three localities at Tabasco (Mexico).

Body 0.72–0.8 mm, ovoid, with posterior margin convex, ventral margin of valves with ca. 90 setulated setae (Fig. 65). Postero­ventral margin of valves with small spinules (Fig. 67). Rostrum with a rounded tip, less projected than in O. ciliata. Labrum more or less triangular, with a rounded tip, without setules (Fig. 66). Second antenna with spine formula: 1–0–1/0–0–1 and setal formula: 0–0–3/1–1–3, with groups of small spinules on segments of the exopod and the endopod (Fig. 68). Postabdomen with 12–14 anal spines two of which are longer than the rest (Fig. 69). Groups of setules on lateral sides. Postabdominal claw with a basal spine, followed by a row of smaller spinulae (Fig. 70).

Concave side also with a row of spinules. Trunk limb I (Fig. 71, 72), ODL with one seta (Fig. 71); IDL with two setae, endites 1–3 with two, three and four setae respectively. A small accessory seta present near the base of IDL. Trunk limb II (Fig. 73) with eight scrapers increasing in size distally, the three more proximal with a more developed pecten of sclerotized teeth; GT with five filtering elements, two densely setulated setae, and a lobe. Trunk limb IV (Fig. 74, 75) with six filtering setae of different length on the exopodite; gnathobase with five filtering setae (Fig. 75). Endopod with a distalmost seta sclerotized, hook­like and followed by three pectinate setae. Setae of external armature bisegmented. Trunk limb V (Fig. 76) with five filtering setae on EX and no setae on GT.

Oxyurella longicaudis is less rare than O. ciliata. A significant number of specimens were found at three localities in the present study. According to Michael and Frey (1983) O. brevicaudis Michael & Frey also occur in the studied region, but we found only the two previously mentioned taxa. The same authors stressed the difficulties in finding material of Oxyurella in samples from North America. We found these two species to be co­existing in the same pond in Tabasco State (see Table 2), but it is easy to distinguish them on basis of the characters already given (labrum, distal anal spines, their armature, and size of specimens).