Alona cf. ossiani Sinev, 1998 (Figs. 54–57)

Material examined: Three females from Silvituc (Mexico).

Body 0.63 mm, oval, with ventral part slightly convex (Fig. 54). Postero­ventral margin of valves with a submarginal row of setules, sometimes arranged in a row not welldefined, sometimes arranged in 5–6 groups of 4–5 setules each (Fig. 56). Two major head pores connected, post pore distance 1.5 times larger than inter pore distance, lateral pores slightly anterior to the level of first major pore (Fig 55). Postabdomen (Fig. 57) with dorsal part slightly convex and distal part more or less rounded, with 12–14 anal spines and 9–10 lateral fascicles of setae. Each anal spine armed with 3–4 spinules.

Alona ossiani was described recently by Sinev (1998). According to him the main differences between affinis and ossiani are in the morphology of the males, e.g. in the gonopore, the length of the postabdominal claw, and in whether a seta is present on the internal distal lobe of limb I or not. In contrast, females are quite similar, and Sinev (1998) based the separation of females on tiny features such as the setation of the postero­ventral corner of the valves which each is armed with 3–4 groups of 2–3 setules in A. ossiani, and 7–8 groups of 4–8 setules in A. affinis . However, in the present study we found the setulation pattern on the valves to be variable (see Fig. 56) within the same population.

Unfortunately, we did not collect any male so we cannot make a definite conclusion concerning the taxonomical identity of the material examined in this study. Our material agrees with the original description of the female except for the setulation of the posteroventral corner, something also found in the material assigned to A. affinis from Venezuela (Zoppi de Roa and Vasquez 1991). Further studies on more material from South America on the variability of A. ossiani should be made before a definite conclusion concerning the validity of this taxon and its distribution can be made.