Mesocyclops longisetus longisetus (Thiébaud, 1912)

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 : 20

Mesocyclops longisetus longisetus (Thiébaud, 1912)

Fig. 22

Diagnosis. Adult female, 1,510 µm in length excluding caudal setae. Four groups of spinule rows on the anterior side of the P4 basis (Fig. 22A). Ducts of seminal receptacle curved posteriorly (Fig. 22B). Caudal ramus is 3.2 times longer than wide and with setules on inner margin (Fig. 22C). One row and two patches of spinules at maxilliped exopod (Fig. 22D). Hyaline membrane of last segment of antennule is strongly serrulate, strongly indentation medially developed (Fig. 22E). Antenna basis with three small rows of spinules in anterior view (Fig. 22F) and two larger rows of spinules in posterior view, without small spinules (Fig. 22G). P5 apical seta 2 times longer than the median spine. P4-endopod terminal segment 2.5 times longer than wide.

Remarks. The specimen examined was found in the Iguaçu River, in the Foz do Areia Reservoir. It is widely distributed in the upper Paraná River Basin, as shown by several reports ( Perbiche-Neves et al. 2014). This species has a wide geographic distribution in South America, and is usually found in low numbers, possibly because it is large and predatory. It is eurythermal and produces egg-sacs with many eggs (>100). In Brazil, the subspecies M. longisetus curvatus has been widely reported, but the validity of this subspecies has been doubted, as the details used to identify it are rarely observed. We decided in this study to be conservative, however a revision of these subspecies especially in the Neotropics is necessary to validate the difference. Considering the results of Wyngaard et al. (2010), these authors do not find significant difference between populations of M. longisetus from USA and M. longisetus curvatus from Brazil. This species can occur in various environments, such as the pelagic zone, littoral of rivers and lakes, among aquatic macrophytes, river channel, puddles, etc.