First record of Dero ( Aulophorus ) bimagnasetus Harman ( Oligochaeta ) from Brazil and habitat characteristics

Annelid worms represent a significant part of freshwater benthic communities worldwide and Oligochaeta is a particularly species-rich group. Dero (A) bimagnasetus (Naididae) previously found and described from a small marsh in Surinam in 1974, has now been found for the first time in Barra Lake, MG, Brazil. Due to the scarce biological data and absence of ecological information in the literature regarding this species we are presenting morphological information on the specimens obtained and the physical and chemical characteristics of the habitat they were found. This species occurred only in the littoral zone of Barra Lake, in muddy, low oxygen, low conductivity and low organic matter sediment. The four individuals collected ranged 3.17-4.15 mm total length; 0.25 0.26 mm body width and 0.16-0.21 mm3 total volume. Considering the present anthropic pressures on freshwater biota and fast biodiversity losses worldwide it is now recognized that attention must be paid to low abundance species and the urgency for preservation of their habitats.


Introduction
Aquatic Oligochaeta form an important and particularly rich-species component of benthic and pleustonic invertebrate aquatic environments around the world.There has been a considerable advance in taxonomical research for this group, but this has not so far been accompanied by ecological studies.The understanding of biological diversity in relation to its origin and maintenance requires a great deal of ecological information and there is nowadays a consensus that, in order to conserve biodiversity, both taxonomical and ecological studies must be pursued at a much faster pace.
In this paper we report the first occurrence of Dero (A.) bimagnasetus in Brazil, describing some of its morphological characteristics and the environmental conditions of the habitat in which this species was found.Before the present record, this species was only recorded within the Neotropical region in a small marsh located in the Surinam (Harman, 1974), its type locality.

Material and Methods
Barra Lake (see in Figure 1) is a relatively large lake in the Middle Rio Doce Valley, Minas Gerais state, Brazil (19° 48' 00.2'' S; 42° 37' 17.3'' W).The lake is surrounded by Eucalyptus forest from reforestation, mixed with native Atlantic forest regrowth.
Sampling was carried out at 5 sites on the lake, 1 in the limnetic and 4 in the littoral zone, on one sampling date, March 03, 2004, in the morning (08:00 h -11:00 h).Geographical coordinates were recorded with a GPS Garmin 130.
Water pH, electrical conductivity and dissolved oxygen parameters were measured at 3 depths in the water column and on the bottom with the multi-probe sensor Horiba U-10.Water samples for the analysis of total phosphorus and nitrogen concentrations were collected at the surface layer with a Van Dorn bottle, stored in polyethylene bottles and frozen shortly thereafter.The total nitrogen and total phosphorus concentrations were determined in the laboratory, according to the procedures described in Mackereth et al. (1978).
Sediment samples were taken with a Van Veen grab (337 cm 2 ) for both benthic macroinvertebrates and sediment particle size analysis.The organic matter content of the sediment was determined as in Buchman and Brady (1979) and sand, silt and clay fractions were determined as described by Suguio (1973), and this information was used to classify sediment texture (Camargo et al., 1987).
Two sediment samples were taken in each sampling site.Samples were sieved through a net of 0.21 mm mesh.The retained material was preserved in 8% formaldehyde solution.In the laboratory, organisms were sorted out and identified under a Zeiss Stemi SV 6 stereo-microscope and Zeiss Axioskop 2 Plus microscope, using specialised literature, mainly the taxonomical keys of Righi (1984) and Brinkhurst and Marchese (1989).
Specimens were photographed and measured in a Zeiss Axiocam HCr microscope with an image acquisition system.Individual biovolumes were calculated from linear dimensions using the cylinder equation (Sun and Liu, 2003).

Results
Dero (A.) bimagnasetus (Figure 2) specimens were found among the macroinvertebrates collected at site 4, one of the four littoral sites sampled in Barra Lake.Since this is the first record of this species in Brazilian freshwater environments, a general characterisation of the physical and chemical conditions in site 4 is presented (as shown in Table 1).The water pH was nearly neutral, and the conductivity was low.The dissolved oxygen concentration at point 4 was significantly lower than at the other sites.Also, there was a dense stand of macrophytes (Eleocharis sp., Nymphaea sp. and Utricularia sp.) at this site, not seen at any other sampling site in the littoral zone of the lake (Campitelli-Ramos, 2008).In the previous record of this species in Surinam, D. (A.) bimagnasetus was first recorded among rotting plant debris (Harman, 1974).However, in Barra Lake, D. (A.) bimagnasetus was collected from muddy sediment with little organic matter, as shown in Table 2.
Only 4 individuals of this species were found in the littoral zone of Barra lake, and linear dimensions and biovolumes of the specimens are presented (as shown in Table 3).The total length varied from 3.17 to 4.15 mm; width from 0.25 to 0.32 mm and biovolume from 0.16 to 0.35 mm 3 .

Discussion
D. (A.) bimagnasetus was among the less abundant species sampled in the Barra Lake, which could be the result of a small number of samples taken, instead of an actual species rarity.They were also among the smallest sized organisms in the benthic assemblages when compared with other macroinvertebrates collected in the sediment at site 4.
The distinctive morphological feature of D. (A.) bimagnasetus is the occurrence in the ventral bundles of III of a single giant chaetae and 1 or 2 giant chaetae in the ventral bundle of IV (see in Figure 3 and Figure 4).Among the Naidinae, this character also appears in other species, e.g., Pristina aequiseta Bourne.However, in the genus Dero no other species possess such giant chaetae (Brinkhurst and Wetzel, 1984).Modified from Harman (1974).
bifid or pectinate needle chaetae, beginning at segment V.However, according to Harman (1974), who described this species, the presence of giant chaetae in the ventral bundles indicates a possible sexual reproductive mode, not previously found in this genus.The name of this species is therefore related to the very large ventral chaetae on segments III and IV.
The fact that up to now D. (A.) bimagnasetus had been found only in Surinam and now in Southeastern Brazil, suggests that this might be a low abundance species.Considering the increasing anthropic pressure on freshwater habitats and consequent biodiversity losses by habitat destruction, a great attention should be paid to this species and its habitat, in order to preserve it, as well the all other components of benthos community in Barra Lake.We also expect that reporting its occurrence in Brazil and its distinctive features may contribute to finding it in other localities, and to get a deeper knowledge of its ecological requirements.

Table 2 .
Sediment fractions (sand, silt and clay), percent organic matter and textural classification of sediment at site 4, Barra Lake, Middle Rio Doce Valley, Minas Gerais State, Brazil, where individuals of Dero (Aulophorus) bimagnasetus were found.

Table 3 .
Length, width and biovolume of four individuals of Dero (Aulophorus) bimagnasetus found in the sediment collected at site 4, littoral region of Barra Lake, Middle Rio Doce Valley, Minas Gerais State, Brazil.SD: standard deviation.