Occurrence of Chaetogaster limnaei K . von Baer , 1927 ( Oligochaeta , Naididae ) associated with Gastropoda mollusks in horticultural channels in Southeastern Brazil

The aim of this study was to evaluate the presence of Chaetogaster limnaei K. von Baer, 1927 in four species of mollusks collected in irrigation channels of a horticultural garden in the city of Juiz de Fora, Minas Gerais State, Southeastern Brazil. The collections were made from June 2005 to January 2006. Of the four species of mollusks analysed, we could not detect the presence of C. limnaei in Pomacea lineata (Spix, 1827). There were recorded 320 specimens of C. limnaei in association with Aplexa rivalis Mastou & Rackett, 1898, Lymnaea columella Say, 1818 and Biomphalaria sp., the latter having the highest number of associated Naididae.

In the past, naturalists considered C. limnaei a true parasite, since they believed the species fed from the mucus produced by the host (Michelson, 1964).However, latter investigations showed that this species mainly feeds off diatoms, algae, protozoans and rotifers, thus being considered commensal (Grufyydd, 1965b;Semenas and Brugni, 1996).
There are few records of the presence of C. limnaei associated with aquatic mollusks in Brazil (Ruiz, 1951;Andrade and Campos, 1968;Callisto et al., 2005).Thus, the present work aimed to evaluate the presence of this Naididae in association with four species of mollusks collected in irrigation channels of a horticultural garden in the city of Juiz de Fora, Minas Gerais.

Material and Methods
The mollusks were collected along irrigation channels of a horticultural garden situated in the northwest region of the city of Juiz de Fora, Minas Gerais state (21° 39' 6" S and 43° 25' 54" W) between June 2005 and January 2006.In total, 291 mollusks were collected using small nets; from these 75 were Aplexa rivalis Mastou & Rackett, 1898, 75 Biomphalaria sp., 75 Lymnaea columella Say, 1827 and 69 Pomacea lineata (Spix, 1827).After shells were measured (diameter in B. straminea and height and width in the other species), gastropods were dissected under a stereoscopic microscope, in order to collect species of C. limnaei, which were put in formaldehyde 8% and kept in alcohol 70 °GL.Taxonomic criteria adopted by Righi (1984) and Brinkhurst and Marchese (1991) were used for species identification.
The absolute abundance, mean abundance and prevalence of C. limnaei in each species of mollusk were determined.Due to the low prevalence of this Naididae in L. columella and A. rivalis, the mean density was determined only in Biomphalaria sp..Moreover, the correlation degree between the area of the shell of Biomphalaria sp. and the prevalence of C. limnaei was evaluated.The Chaetogaster specimens were deposited at the Laboratory of Benthic Invertebrates at the Universidade Federal de Juiz de Fora (LIB-UFJF).

Results and Discussion
In the 291 mollusks analysed, 320 specimens of C. limanei were recorded.Of the four species of mollusks analysed, the presence of C. limnaei was not detected only in P. lineata.Biomphalaria sp., with 316 worms, was the species of mollusk with the highest number of associated specimens (Table 1).
C. limnaei showed higher prevalence in Biomphalaria sp.(56%), and a lower one in A. rivalis and L. columella, both with 2.7%.This result may reflect a true difference in host specificity, such as that shown by Buse (1974) for species of British gastropods.
The highest mean density (18.37 specimens/cm 2 ) of C. limnaei in Biomphalaria sp. was observed in August 2005.In this month, the density varied between 1.85 ind.cm -2  It was observed that C. limnaei was present in a higher number in Biomphalaria sp. with its shell area ranging from 0.60 to 0.69 cm (n = 10) and from 0.80 to 0.89 cm (n = 9) making a total of 25.3% of the mollusks associated with C. limnaei.This indicates that there was no correlation between host size and the presence of this Naididae (P > 0.05).A similar result was obtained by Conn et al. (1995) with the bivalves Dreissena polymorpha Pallas, 1771 and Dreissena bugensis Andrusov, 1897.However, Andrade and Campos (1968) detected a positive relation between the size of B. straminea and the prevalence of C. limanei.
The number of C. limnaei present in Biomphalaria sp.ranged from 1 to 27 specimens, and a higher number of mollusks with 1 to 3 organisms (Table 2) was observed.The maximum number of C. limnaei found was close to that found by Andrade and Campos (1968), in which the maximum number was 23 worms per B. Straminea; however, it is quite inferior to 70 parasites/host found by Gruffydd (1965b).
In conclusion, these results attest the ability of Chaetogaster limnaei to use different organisms that are present in an aquatic environment as substrate, and also they show the preference of this Naididae for Gastropods of the genus Biomphalaria.

Table 1 .
Total abundance, mean abundance and standard error of C. limnaei in three species of mollusks collected in irrigation channels of a horticultural garden (Juiz de Fora -MG) between June 2005 and January 2006.