An annotated checklist of the Arcella ( Arcellidae ) from littoral zone of Paranoá lake-Brazil , with a pictorial key

The pictorial key contributes to taxonomic analysis, as it lists the species found in a given environment, and points out the morphological characteristics that differs one specific taxon from the others. Arcellidae Ehrenberg, 1830 is one of the testate amoebae families with highest representativity in terms of richness and abundance, including three genera, that Arcella shows greater dispersion in different types of aquatic biotopes. The zooplankton community in the Paranoá Lake has been extensively studied over the decades. However, there are no studies on testate amoebae in this environment. The study aimed to develop a pictorial key to help in the identification of Arcella in the Paranoá Lake. Samples were taken with plankton net at 13 sites in the littoral zone of the lake. The elaboration of the key was based on morphological characteristics of the shell and pseudostoma of the individuals. There were identified 23 taxa, 16 species and seven subspecies. Two species with low incidence in Brazil were recorded (Arcella catinus and Arcella rota). The species richness was higher than the records published to date in Brazil, thus demonstrating the relevance of the data and the applicability of this pictorial key in other studies both in the Paranoá Lake, as in other regions.

In general, Arcella has a high variability of morphological types, which combined with the paucity of descriptive studies, makes complex interspecific identification (Bobrov & Mazei, 2004).According to Lahr and Lopes (2009), this problem is more critical due to the lack of preserved specimens, and the poor standardization of the descriptive format of available works, hindering the new records.
The development of pictorial keys can be a tool to assist the description of species and facilitate the understanding of the characters described textually (Pimpão & Mansur, 2009).Furthermore, their use facilitates the identification of species, allowing reliable identifications by students and researchers unfamiliar with a particular group (Oliveira, Mianzan, Migotto, & Marques, 2007).Thus, this study aimed to provide the first list of testate amoebae of the Paranoá Lake and develop a pictorial key to the taxa of Arcella recorded in this environment.

Study area
The study was conducted in the Paranoá Lake (15°48'S; 47°50'W, Figure 1), an urban reservoir, occupying approximately 18% of the territory of the Federal District, Brazil (Mattos, Altafin, Freitas, Cavalcanti, & Alvez, 1992), with depht maximum 40m and about 111km perimeter.The Paranoá was created in 1959 from the damming of streams in the region, aiming to improve climate conditions in the region, promote recreation and for landscaping purposes (Ferrer & Del Negro, 2012;Ferrante, Nancan, & Netto, 2001).

Sampling design
Samples were collected at 13 sites flooded in the littoral region of Paranoá lake, all with the presence of marginal vegetation.The collection periods covered the dry season (August and September 2014) and rainy (April and May 2015).The collect took place in shallow areas, ranging 40-60 cm deep.Altogether, 100L of water was filtered with the aid of buckets and horizontal hauls in plankton net of 68μm and fixed in 70% ethanol.

Laboratory analysis
Sample sorting was performed on Sedgewick-Rafter chambers (2.5mL) integrally counted under an optical microscope.For each identified taxon, we took one or more specimens with the aid of pipette, and stored on slides, immersed in glycerin (90%), for better handling and morphological analysis.
Measurements were performed using the AxioVision software through images obtained by light microscopy (Olympus BX41) connected to the image capturing.They measured the height and length of the teak, and the diameter pseudostoma in one or more specimens.
Testate amoebae were photographed with a digital camera (Samsung Galaxy Camera) attached to the eyepiece of the optical microscope (Nikon Eclipse E 100), at lateral and ventral views.The ilustrations were held in image editing programs for higher representativity of the organisms.
The pictorial key was based on morphological characteristics (Figure 2) used to identify testate amoebae species: shape and ornamentation of the shell, presence or absence of undulations on the surface, types of ornamentation of the pseudostoma and presence or absence of borders and invaginations.
Samples of testate amoebae were deposited in the Laboratory of Limnology, Centro Universitário do Distrito Federal, as well as individuals used for taxonomic identification described herein.

Results
There were recorded 16 species and seven taxa infraspecific of Arcella distributed in the littoral region of the Paranoá Lake (Table 1).
Remarks: Lateral face, hemispherical shell, domed with slight undulations on the surface, with a flap on the shell base, and a small, invaginated buccal tube.Ventral face, circular shell, circular and central pseudostoma, pores present around pseudostoma.
Remarks: Lateral face, pyramidal shell with parallel sides, forming only one polygon.Ventral face, rounded shell, pseudostoma circular, centered and with little edge.
Remarks: Lateral face, hemispherical shell.Ventral face, circular shell, edge with projections, similar to a short border.Lobated pseudostoma, invaginated, small and centered.
Remarks: Lateral face, discoid shell, domed, with projections in the form of lateral spines.Spines curved upward, nearly perpendicular to the base, forming a trapezoidal shell.Ventral face with thornshaped projections throughout the circumference of the shell, pseudostoma circular and centered.This is the first record of this subspecies in Brazil.
Remarks: Lateral face, hemispherical shell, rounded with depressions, presence of a small border on the extremity of the shell.Dorsal face, shell forming five vertices and depressions.Ventral face, circular shell with pseudostoma circular and centered.
Remarks: Lateral face, hemispherical shell that may have invaginated buccal tube.Ventral face, circular shell.Pseusostoma circular, small, with collarette around the aperture.
Size: (n=5 Remarks: Lateral face, hemispherical shell, with ripples throughout the shell, presence of invaginated buccal tube and string.Ventral face, circular and irregular shell, with pseudostoma circular and centered.Although the study of Tsyganov and Mazei (2006) has described this subspecies as a new one, entitled Arcella intermedia (Deflandre, 1928), the initial classification prevails.
Remarks: Lateral face, hemispherical shell with small and regular undulations, forming pits.Ventral face, circular shell, pseudostoma circular and centered.
Remarks: Lateral face, oval shell, with high height and forming depressions, with invaginated buccal tube.Ventral face, circular shell with circular pseudostoma, centered, and collarette at the border.

Arcella rota
Remarks: Lateral face, discoid shell.Ventral face, circular shell with a number of small spines on the border of the shell.
Remarks: Lateral face, hemispherical shell, invaginated buccal tube starting at the lateral margin of the shell.Ventral face, circular shell with pseudostoma circular, centered.

Arcella vulgaris
Remarks: Lateral face, hemispherical shell, domed, with a border in the basal portion of the shell.Ventral face, circular shell with pseudostoma circular, centered.
Remarks: Lateral face, hemispherical shell, edge with projections similar to a short border, surface with pits.Ventral face, circular shell.Pseudostoma circular, small, collarette around the aperture.
The first characteristics used for the main division and classification of 23 taxa of Arcella in pictorial key were pseudostoma form (lobes or circular) and the position of the buccal tube (internal or external).To highlight the difference between some taxa with similar characteristics, we used proportion of shell measurements (Figure 4 and 5).
The high species richness found in this study may be associated the structuring of the sampled environments caused by marginal vegetation.The areas along the margin of the lake encompasses a variety of microhabitats (Kuczynska-Kippen, 2005;Meerhoff et al., 2007;Esteves, 2011).This condition could favor the biological richness of testate amoebae (Schwind et al., 2016); Fulone et al., 2005), justifying the rise of taxa recorded in studies on the littoral area (Souza, 2008;Maia-Barbosa et al., 2014).Moreover, the identification of infraspecific taxa can be considered another factor that influenced the record of the expressive richness of testate amoebae in this environment.It is known that some authors do not recognize variations in species, as they may be reflection of taxonomic problems caused by morphological variability (Bobrov & Mazei, 2004;Smith, Bobrov & Lara, 2008).Nevertheless, this study opted for identification of infraspecific taxa (Splitting method), considering that morphological variations may help future studies of taxonomic revision, as seen for example in Tsyganov and Mazei (2006), which suggest changing A. hemisphaerica var.intermedia undulata to Arcella intermedia.The description of testate amoebae species provides more details of their morphological characteristics, which may ultimately confound the identification of specimens by unfamiliar professional and/or students (Walter & Winterton, 2007).In this case, the pictorial key represents an effective tool, as it shows visually and in a comparative way the used characteristics.However, it is noteworthy that the selection of these critical characteristics was based on the differentiation of species found in the studied environment, since there is no standardization in the literature in the elaboration of these tools.

Conclusion
This study showed detailed taxonomic data of Arcella, which, through the use of the pictorial key, sought facilitate the identification of testate amoebae species and provide a basis for comparison of taxa in the Paranoá lake with other environments.Thus, the identification at the infraspecific level, adopted herein, provides a more accurate description of morphological variations of the taxa, which may assist works of taxonomic revision in other aquatic ecosystems.
Furthermore, the species richness recorded in this environment suggests that the Midwest region of Brazil can contain a great diversity of testate amoebae in aquatic environments.Therefore, this study highlights the need to increase researches with protozoa in this region.

Figure 2 .
Figure 2. Representation of different visual planes of Arcella, and morphological characteristics used for the pictorial key.Source: the author.

Figure 4 .
Figure 4. Pictorial key of species of Arcella identified in the littoral region of the Paranoá Lake.Source: the author.

Figure 5 .
Figure 5. Key pictorial of species of Arcella identified in the littoral region of the Paranoá Lake continuation.Source: the author

Table 1 .
List of species of Arcella recorded in the Paranoá lake.