Acanthocyclops robustus subsp. robustus

Acanthocyclops robustus robustus (G.O. Sars, 1863) Figs 5, 6 Diagnosis. Adult female, 1,260 µm in length excluding caudal setae. Genital double- somite 1.1 times longer than wide, ducts divided horizontally into two parts (Fig. 5A). Antenna basis with two rows of spinules (Figs 5B, C, 6E, F). Basis of P4 has well-developed spinous process on distal margin (Fig. 6B, C). Coxopodite with few short rows of small spinules (Figs 5D, 6C); terminal outer spine of P4 endopod 1.2 times longer than median spine (Figs 4B, H, 5E). Inner spine of P5 shorter than whole segment (Fig. 6A, D). Caudal rami approximately 4 times longer than wide (Fig. 6G). Remarks. Illustrated specimen from the Machadinho Reservoir, Uruguay River. Einsle (1996) stated that this “European” species has wide seasonal and spatial variability. In South America, its morphology and variability need to be better investigated, as some taxonomic characters do not fully agree with the description of A. robustus robustus. Molecular techniques should be used to compare the South American populations with populations from other continents. This species is distributed in southeastern and southern Brazil, and also occurs in Uruguay and Argentina. A. robustus robustus is found in the Tietê and Iguaçu rivers, but has not been recorded in the Paranapanema River, which is located between them. It is considered that A. robustus robustus prefers lower and moderate temperatures. It was found in the Barra Bonita Reservoir (Tietê River), a relatively shallow reservoir with few months of low temperatures during the year. It seems to be abundant in eutrophic reservoirs (Perbiche-Neves et al. 2016).