Thermocyclops decipiens Kiefer, 1929

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 : 35

Thermocyclops decipiens Kiefer, 1929

Figs 36 , 37

Diagnosis. Adult female, 850 µm in length excluding caudal setae ( Fig. 36A ). Caudal ramus 2.5 times longer than broad; lateral seta inserted in the distal third of the ramus; inner terminal seta is 3 times longer than the outer terminal seta ( Fig. 36A ). Seminal receptacle with posteriorly curved aliform projections ( Fig. 36B ). P5 terminal spine is 1.1 times longer than the terminal seta ( Figs 36C , 37D ). Inner spine of terminal P4-endopod segment is 2.4 times longer than the out- er one ( Figs 36D , 37A, B ).P4 intercoxal sclerite has ornamented spatula-shaped bilaterally symmetrical structures on the distal margin ( Figs 36E , 37C ). Mandible endopod has two patches of spinules in anterior view, one close to the distal outer margin, and another semicircular patch proximally ( Fig. 37E, F ).

Remarks. The specimen illustrated was found in the Uruguay River, in the Machadinho Reservoir. It is the most widely distributed species of Cyclopoida in reservoirs of the La Plata Basin, occurring in Brazil, Argentina, Paraguay, Uruguay, and other countries of the continent. Several studies report its wide occurrence in Brazil ( Silva and Matsumura-Tundisi 2005, Nogueira et al. 2008).It is also present in Asia, Africa, and North America ( Reid 1989) and Australia ( Mirabdullayev et al. 2003). It is well adapted and often dominant in reservoirs and lakes, associated with increased trophy in some reservoirs ( Sendacz and Kubo 1982, Landa et al. 2007, Silva 2011, Perbiche-Neves et al. 2016, 2021). However, it is also present in more oligotrophic locations, in both storage and run-of-river reservoirs ( Nogueira et al. 2008). In this study, this species was found in almost all sampled environments, occurring both in reservoirs with oligotrophic characteristics, such as Furnas Reservoir in the Rio Grande, and in eutrophic reservoirs, such as Barra Bonita in the Tietê River Basin and Foz do Areia in the Iguaçu River Basin, where it was the dominant species. These results suggest that the use of this species as a bioindicator must be viewed with caution. In addition, the widespread use of the ratio Thermocyclops decipiens / Thermocyclops minutus should be critically addressed ( Perbiche-Neves et al. 2021). Long-term or laboratory studies could shed some light on this question, with experiments designed to test hypotheses about the ratio of Thermocyclops decipiens / Thermocyclops minutus in different degrees of trophy. Studies of this nature could also include the other co-occurring species of the genus, T. inversus . This species can be distinguished from T. minutus and T. inversus by the shape of the cephalothorax, which is more fusiform in Thermocyclops decipiens , and by the curved caudal setae.

Figures

Figure 36. Thermocyclops decipiens, female: (A) adult female with detail of caudal rami, arrow indicates the position of caudal setae; (B) ventral view of genital double-somite; (C) P5, arrow indicates the inner terminal spine; (D) terminal segment of P4-endopod; (E) intercoxal sclerite and basis of P4, arrow indicates spinules in the intercoxal sclerite. Scale bars: A = 100 µm, B–D = 50 µm.

Figure 37. Thermocyclops decipiens, female: (A) last segment of P4-endopod; (B) detail of terminal spines of last segment of P4-endopod; (C) intercoxal sclerite, with detail of spinules; (D) P5; (E) right maxilla and the maxilliped; (F) maxilliped basis, showing spinule clusters. Scale bars: A = 100 µm; B, C, D, F = 20 µm; E = 50 µm.