Three new species of Potamothrix (Oligochaeta, Naididae, Tubificinae) from Fuxian Lake, the deepest lake of Yunnan Province, Southwest China

Abstract Three new species of Potamothrix Vejdovský & Mrázek, 1902 (Oligochaeta: Tubificinae), Potamothrix praeprostatus sp. n., Potamothrix paramoldaviensis sp. n. and Potamothrix parabedoti sp. n., are reported from Fuxian Lake of Yunnan Province, Southwest China. Potamothrix praeprostatusdiffers from its allies by its prostate glands joining atria in its proximal to middle portion, and spermathecal chaetae. Potamothrix paramoldaviensis is distinguishable from its allies by having penial chaeta but no penes, and differs from Potamothrix moldaviensisby its homogenous atrium. Potamothrix parabedoti is distinctive in the position of its reproductive organs, and differs from Potamothrix bedoti by its homogenous atrium. Hitherto, 34 freshwater oligochaete species have been recorded in Yunnan Province, including nine endemic species from the plateau lakes.


Introduction
The existence of unique faunae in ancient lakes of Yunnan Province, Southwest China has been recognized in several studies (Yang and Chen 1995;Sket 2000). However, our knowledge of freshwater oligochaetes in these lakes is poor; only an aberrant bran-Emended diagnosis. Hair chaetae present or absent, dorsal chaetae bifid and always pectinated, or only bifids. Ventral chaetae bifids. No coelomocytes. Vas deferens very short, entering atrium apically; atrium tubular, long. Prostate gland small, attached to proximal part of atrium by a short stalk, or no prostate gland. No ejaculatory duct. Penis with or without cuticular sheath. Spermatozeugmata present. Modified spermathecal chaetae present or absent.
Etymology. "prae" and "prostatus" are Latin for "proximal" and "prostate", respectively. The specific name refers to the prostate glands proximally attached to atria.
Distribution and habitat. Known only from its type locality, Yunnan Province, China; freshwater lake, 113 m depth, water temperature less than 14 °C, fine clay.
Remarks. According to short vasa deferentia, long tubular atria, each with a small prostate gland, and lack of ejaculatory ducts, the new species fits more closely the definition of Potamothrix Vejdovský & Mrázek, 1902 than that of any other described tubificine genus (Brinkhurst and Jamieson 1971;Finogenova and Poddubnaja 1990).
Distribution and habitat. Known only from its type locality, Yunnan Province, China; freshwater lake, 78 m depth, water temperature less than 16 °C, fine clay.
Remarks. According to very short vasa deferentia, long tubular atria each with a small prostate gland, and lack of ejaculatory ducts, the new species fits more closely the definition of Potamothrix Vejdovský & Mrázek, 1902 than that of any other described tubificine genus (Brinkhurst and Jamieson 1971;Finogenova and Poddubnaja 1990).
This new species resembles P. moldaviensis in some aspects of the male organs (Vejdovský and Mrázek 1902), e.g. the very short vasa deferentia, tubular atria, and the length ration of the vasa diferentia to the atria, and their differences are obvious. P. paramoldaviensis sp. n. has homogenous atria with prostate glands and no penes, while P. moldaviensis has tripartite atria without prostate glands, with short penes.
Distribution and habitat: Known only from its type locality, Yunnan Province, China; freshwater lake, 70-110 m depth, water temperature less than 15 °C, fine clay.
Remarks: According to short vasa deferentia, long tubular atria and lack of ejaculatory ducts, the new species fits more closely the definition of Potamothrix Vejdovský and Mrázek, 1902 than that of any other described tubificine genus (Brinkhurst and Jamieson 1971;Finogenova and Poddubnaja 1990).
The new species resembles P. bedoti (Piguet, 1913) in some aspects of reproductive organ (Vejdovský and Mrázek 1902), e.g. has a very short vasa deferentia, tubular atria without prostate gland, the length ration of vasa diferentia to atria, and the reproductive organs move to VIII-X. However, their differences are obvious. P. parabedoti sp. n., has homogenous atria with prostate glands, while P. bedoti has tripartite atria without prostate glands. In additional, the shapes of spermathecal chaetae are dissimilar in two of the species, the form is scalpel-like, and the ental part straight in P. bedoti (Timm 1970(Timm , 1999, but the ental end part is strongly curved in P. parabedoti sp. n (Fig. 3I).
The new species are distinguishable from other species from Yunnan Lakes in that of the position of their reproductive organs and the characteristic of some somatic chaetae. For instance, the reproductive organs are move to VIII-X in P. parabedoti sp. n. but that were in X-XIII in other species; the hair and pectinate bifids are absent in P. rhytipeniatus Cui & Wang, 2012 and P. paramoldaviensis sp. n., but present in other four species.
In the genus of Potamothrix, the histological structure of the epithelium of the atrium is taxonomically useful (Holmquist 1985;Finogenova and Poddubnaja 1990). According to histologically structure of the atrium, twenty previous species (Table 1) were divided into three types: (1) the 'tripartite type', which includes all the species with tripartite atrium, the short proximal part with densely granular inner epithelium layer, the long middle part with light granular inner epithelium, and the short distal part with homogenous inner layer (P. alatus, P. bavaricus, P. bedoti, P. heuscheri, P. moldaviensis, P. thermalis) (Brinkhurst and Jamieson 1971;Finogenova and Poddubnaja 1990;Milbrink and Timm 2001;Milbrink 1999;Timm 1970Timm , 1999; (2) the 'bipartite type', comprising of the members with bipartite atrium, the short proximal part with densely granular inner epithelium layer and the long distal part with light granular inner epithelium (P. caspicus, P. cekanovskajae, P. danubiali, P. hammoniensis, P. manus, P. marzeki, P. vejdovskyi) (Finogenova and Poddubnaja 1990;Hrabě 1981); and (3) the 'homogenous type', which consists of the taxa with homogenous atrium (P. tudoranceai, P. postojnae) (Šporka 1994;Brinkhurst and Wetzel 1984). Except the above mentioned 15 species, the histological of atrium of P. prespaensis, P. isochaetus, P. orientalis, and P. ochridanus was unmentioned in the original description (Brinkhurst and Jamieson 1971;Pop 1976), so that will need to be re-examined in the future. The species from Yunnan lakes except P. rhytipeniatus are part of the 'homogenous type' (Table 2).
In addition, the presence of pectinate bifid chaetae accompanied with hair chaetae in the Yunnan lake species could be a special feature, but their position is variable. For instance, the hairs and pectinate bifids begin from segments V, VI, VII, III or V, respectively in P. praeprostatus sp. n., P. scleropenis Cui & Wang, 2005, P. aductus Cui & Wang, 2012, and P. parabedoti sp. n. The position of spermathecal pores of Potamothrix always lies in lateral line; however, in species from the Yunnan lakes, they were ventral instead of lateral.
As for habitat and distribution, the five species of Potamothrix from Fuxian Lake are well adapted to low dissolved oxygen concentrations, only found in the profundal region, to water depths lower than 70 m, water temperatures less than 16 °C, and they are found in sediments always clayey and sandy. Another species, P. rhytipeniatus Cui & Wang, 2012 was found in Xingyun Lake, in water depth of about 5 m, water temperature around 18 °C, and muddy sediments.
Lastly, according to some specific features, such as hair and pectinate bifid chaetae, spermathecal pore position, atrium histological structure, and their habitat, the species from Yunnan lakes maybe one new taxonomical group, the systematic placement of which needs further confirmation from more work.