Mesocyclops meridianus ( Kiefer, 1926 )

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 : 21-22

Mesocyclops meridianus ( Kiefer, 1926)

Figs 23 , 24

Diagnosis. Adult female, 1,100 µm in length excluding caudal setae. Genital double-somite is 1.3 times longer than wide. Seminal receptacle has narrow, almost filiform and horizontal lateral expansions ( Fig. 23A ). P4 endopod outer terminal spine is approximately in the same length as its segment ( Figs 23B , 24E). Hyaline membrane on the last segment of antennule with small indentations in the proximal 2/3 and heavily indented, more than three large invaginations in the distal 1/3 ( Figs 23C , 24C). Distal margin of P4 intercoxal sclerite smooth ( Fig. 23D ); P4 basis with six groups of spinules in posterior view: three short rows of small spinules proximally, one row in the medial region, one group transversely on the lateral outer margin, and a row on the inner distal margin ( Fig. 23D ). Spinule pattern of antenna basis anterior surface: two converging rows proximally, near the concave margin; two rows of different lengths near the insertion of the outer seta, and a short row, medially, on the inner margin ( Fig. 23E ). Mandible endopod segment with proximal, middle, and distal rows of spinules in anterior view ( Fig. 24A). Caudal ramus 2.6 times longer than broad, smooth, or finely hirsute on the inner margin ( Fig. 24B).

Remarks. Specimen collected in the Middle Paraguay River. This species is widely distributed in the La Plata Basin. It was described by Kiefer (1926) from samples from the Paraguay River. It was, and still is, confused with Mesocyclops brasilianus , described from the Amazon region by Kiefer (1933). Dussart (1984) suggested a possible synonymy of M. meridianus with M. brasilianus . Gutiérrez-Aguirre and Suárez-Morales (2001) confirmed some morphological differences between these two species, and identified some specimens from São Paulo state as M. meridianus . Later, supporting the previous hypothesis, Gutiérrez-Aguirre et al. (2006) reviewed the Neotropical Mesocyclops and observed that M. brasilianus occurs in the Amazon region and M. meridianus more southward on the continent. Possibly, records in northern Brazil are of M. brasilianus , but they need to be verified. The original description of M. meridianus was based on specimens from Paraguay, and several studies indicate that M. meridianus occurs in almost the entire La Plata Basin. However, further studies are needed, as it is possibly a complex of species that cannot be reliably identified based on morphological characters only, since specimens with intermediate characteristics between these two species are often found. In this study, all specimens of this species were checked in detail and confirmed as M. meridianus . However, molecular analyses are needed, comparing organisms from different regions across South America and looking for variations within populations. When comparing this species with others presented by Holyńska et al. (2003), it can be confused with Mesocyclops meridionalis , Mesocyclops pseudomeridianus and Mesocyclops paranaensis .

Figures

Figure 23. Mesocyclops meridianus, female: (A) genital double-somite; (B) terminal P4-endopod; (C) last segment of antennule, arrow indicate hyaline membrane ornamentation; (D) P4 coxa, basis and intercoxal sclerite, arrows indicate the absence of spines; (E) basis of antenna, anterior view. Scale bars: A, B, D, E = 100 µm, C = 50 µm.