Keratella lenzi ( Hauer , 1953 ) : a new occurrence for Acre State , Brazil , specifically in Igarapé Jesumira located in Parque Nacional da Serra do Divisor Erlei

Rotifers are very important freshwater plankton, and they occupy an ecological niche of small filter feeders. The extent of their distribution includes floodplains, because they tolerate environmental changes caused by an increase in water levels that leads to a diversification of niches. However, the biodiversity of floodplains is still poorly known, especially in Northeast Brazil in the state of Acre. The organisms were collected using pond nets with a 50μm mesh size, and they were preserved in 4% formaldehyde. This paper registers the first occurrence of Keratella lenzi in Acre State. The species was identified in plankton samples from Igarapé Jesumira (7o28’10.2”S and 73o33’54.6”).


Introduction
In aquatic ecosystems, rotifer fauna is generally the most diverse group among the freshwater invertebrates, and it is still relatively poorly known (Segers and Shiel, 2003).This Phylum shows basic characteristics: bilaterally symmetry; body has more than two cell layers, tissues and organs; body cavity is a pseudocoelom; body possesses a complete gut with an anus; body is covered by an external layer of chitin called a lorica; and there is a nervous system with a brain and paired nerves, but no circulatory or respiratory organs.Reproduction is mostly parthenogenetic, otherwise sexual and gonochoristic, and they feed on bacteria and protists, or are parasitic (Segers, 2004).
In general, the organisms of these aquatic microfauna inhabit floodplains.The river-floodplain systems undergo a marked temporal variation due to physical, chemical and biological factors associated with alternating flooding and dry phases, where these modifications are influenced by the hydrological regime of the main river (Junk et al., 1989), causing the organisms to migrate from the principal river to lakes or bayous, environments that are common in Amazonian waters.
The Amazonian waters have about seven species of Keratella with two varieties according to studies conducted by Robertson and Hardy (1984) in Amazonas and Pará, and Sendacz and Melo-Costa (1991) in Amazonas and Acre, and also Keppeler (2003) and Keppeler and Hardy (2004) in Acre and Amazonas.Thus, the present work contributes to the citation of another new rotifer occurrence in the state of Acre.

Material and Methods
Rotifers were collected in Igarapé Jesumira (7º28'10.2"Sand 73º33'54.6")using pond nets (Wetzel and Likens, 1991) with a 50μm mesh size (Segers, 1993) and were preserved in 4% formaldehyde.In the laboratory, the samples were examined, and specimens were photographed using a Bioval binocular microscope, Model L2000A, equipped with a digital camera designed specially for microscopes, with Image Output USB and Max Resolution 640×480, Color Max 1.64 million 24-bit color, and automatic range of wave adjustment.The windows 2000-XP (Professional-SP4) operating system were employed on an Intel Celeron Acer Aspire 3690-2023 notebook.
The limnological characterization of the aquatic environment was carried out according to the temperature, pH, dissolved oxygen and turbidity, utilizing a Troll 9500 multiparametric limnological probe.Transparency was determined with a Sechii disk, and depth was measured with a previously marked ruler.