Acanthocyclops robustus robustus (G.O. Sars, 1863)

Perbiche-Neves, Gilmar, Corgosinho, Paulo H. C., Previattelli, Daniel, Suárez-Morales, Eduardo, Nogueira, Marcos G. & da Rocha, and Carlos E. F., 2025, Catalogue for identification of the most common lacustrine and riverine cyclopoid copepod (Crustacea) species in plankton of La Plata Basin, South America, Zoologia (e 24023) 42, pp. 1-47 : 9

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).

Figures

Figure 4. P5 of cyclopoid genera into the families found in our study separated in Cyclopinae (A–F) and Eucyclopinae (G–L): (A) Acanthocyclops; (B) Microcyclops; (C) Metacyclops; (D) Thermocyclops; (E). Mesocyclops; (F) Megacyclops; (G) Eucyclops; (H) Macrocyclops; (I) Ectocyclops; (J) Paracyclops; (K) Homocyclops; (L) Tropocyclops.

Figure 5. Acanthocyclops robustus robustus, female: (A) genital double-somite,ventral view,arrow indicates genital receptacle; (B) antenna basis, posterior view, arrow indicates one row of spinules; (C) antenna basis, anterior view, arrows indicate two rows of spinules; (D) intercoxal sclerite, coxa and basis of P4, arrows indicate row of spinules; (E) last segment of P4-endopod. Sacale bars: 50 µm.

Figure 6. Acanthocyclops robustus robustus, female: (A) genital double-somite and P5, ventral view; (B) P4; (C) right P4, coxa, basis, and first segment of endopod and exopod; (D) right P5; (E) basis of antenna, posterior view; (F) basis of antenna, anterior view; (G) caudal rami, ventral view; (H) last segments of P4–exopod and endopod. Scale bars: A, B, G = 100 µm; C, E, F = 50 µm; D = 20 µm.