Chydorinae, Dybowski & Grochowski, 1894

Chydorinae , Amapá, Araguari river , UHE Cachoeira Caldeirão

In recent years, research on Cladocera in Brazil has been increased with description of new species, report of new occurrence records, regional checklists, and lists of species from previously unsurveyed areas (e.g., Santos-Wisniewski et al., 2001; 2011; Lopes et al., 2006; Serafim Jr et al., 2006; Kotov and Elmoor-Loureiro, 2008; Sousa et al., 2009; 2016; 2017; Güntzel et al., 2010; Sinev and Elmoor-Loureiro, 2010; Gazulha et al 2011; Rocha et al., 2011; Soares and Elmoor-Loureiro, 2011; Van Damme et al., 2011; Sousa and Elmoor-Loureiro, 2013; 2018a; 2018b; Elmoor-Loureiro, 2014; Farias et al., 2017; Zanata et al., 2017). These works have been filling gaps in the knowledge about Brazilian Cladocera biodiversity and distribution, although this task is still incomplete.

Among the new taxa described, some Chydoridae species are notably interesting because of the specificity of their habitat or habitus. Alona elisae Sousa, Elmoor-Loureiro and Santos, 2016 , for example, was found in a single rock pool in the state of Goiás, although numerous samples of Alona intermedia -like were investigated ( Sousa et al., 2016). Ephemeroporus quasimodo Elmoor-Loureiro, 2014 is exclusive to ultraoligotrophic ponds from Central Brazil. Monospilus macroerosus Sousa, Elmoor-Loureiro and Panarelli, 2017 was found exclusively in the wet leaf litter on the hydromorphic soil in a gallery forest.Nonetheless, the most impressive of these new taxa is Kisakiellus aweti Sousa and Elmoor-Loureiro, 2018 whose unique morphology seems to be related to resistance against water flow. So far, K. aweti was known only from the type locality, that is, two first order streams located in Upper Xingu river basin, state of Mato Grosso, Brazil. The present study aims to report a new record of K. aweti from the Amazon Hydrographic Region.

A single specimen of K.aweti was found in a plankton sample taken from Araguari river , state of Amapá, Brazil ( Fig. 1 View Figure 1 ), in November 2014, during monitoring of the area of influence of the hydroelectric dam of Cachoeira Caldeirão, at that time still in construction. For sampling, a plankton net 60 µm mesh size was used. The specimen is deposited in Elmoor-Loureiro collection under access number EL02974 .

The site from where the sample was taken (0°43’0.57”N 51°25’18.37”W) was about 350 m wide and located near to Porto Grande and upper to the dam, having forested margins and with presence of Eichornia azurea (Sw.) Kunth and Montrichardia arborescens (L.) Schott. According to Cunha et al. (2011), from the spring to the district of Porto Grande, Araguari river flows over a high declivity (0.50%), presenting high velocities. At the sampling site, the river was very shallow (ca. 2 m deep) and characterized by rapid currents, few aquatic macrophytes on the banks, nutrient-poor and neutral water (pH = 7.06), high water temperature (29.7°C), low dissolved oxygen (3.93 mg /L) and electrical conductivity (22.55 µs/ cm). The average flow for the river in November was 200 m 3 /s. The low depth observed is typical for the Araguari river basin in November, the period when the minimum water flow occurs ( Cunha et al., 2014).

Kisakiellus aweti was initially found in first-order streams, and it is considered a benthic species, with morphological specializations that seem to be related to resistance against water flow ( Sousa and Elmoor-Loureiro, 2018b). The present specimen was found in a plankton sample, but it is believed to be an accidental occurrence, due to low water depth and rapid current observed during the sampling. These river conditions should have displaced and suspended the specimen to the water column. Chydoridae specimens, as part of the bottom-dwelling meiofauna, always undergoes being dislodged from its habitat and were accidentally resuspended in the water column due to water turbulence ( Viroux, 2002; Perbiche-Neves and Nogueira, 2010).

The specimen ( Fig. 2 View Figure 2 ) presented the general characteristics described for K. aweti by Sousa and Elmoor-Loureiro (2018b): body elongated and relatively low in lateral view, with a shallow depression between the articulation of the valves and the headshield ( Fig. 2A View Figure 2 ). The ventral margin has wide flanges in its anterior portion ( Fig. 2B View Figure 2 ). The posterior margin is strongly concave, naked, with posterodorsal angle as a short and obtuse projection ( Fig. 2E View Figure 2 ). In dorsal and ventral views, the body is broadly oval and exhibits a flap that surrounds the entire valves ( Fig.2B View Figure 2 ). Headshield with a broad, rounded anterior margin; fornices projected backwards ( Fig. 2D View Figure 2 ), with two isolated main head pores ( Fig.2F View Figure 2 ); lateral pores absent. Postabdomen ( Fig. 2C View Figure 2 ) relatively short, not laterally compressed; ventral margin strongly arched; preanal margin relatively long, anal and postanal margins armed with setulae. Postabdominal claw curved, longer than the postanal and anal margins; ventral margin covered by spines. Basal spines absent.

Besides the characteristics indicated above, the specimen also possess other features presented in the original description, such as: the labral keel elongated, not developed, its apex armed with long setulae; sensory seta on the antennule thick and curved; antennal formula (exo/endo) of spines 001/101 and setae 013/003; first limb ODL (Outer Distal lobe) bearing a short and thick seta armed with spine, first seta on the exopodite of the fifth limb long plumose and posteriorly inserted.

This new record extends the known geographical distribution of K. aweti in more than 1,500 km, but, so far, its occurrence is restricted to the oriental part of the Amazon Hydrographic Region ( Fig.1 View Figure 1 ). Nevertheless, this restriction could be considered circumstantial, once the knowledge about the cladoceran distribution in the Amazon region is still scarce and this taxon was established very recently being, in consequence, even little known. So, K. aweti is expected to possess a wider distribution.