The genusPotamothrix(Annelida, Oligochaeta, Tubificidae): a literature review

Abstract

INTRODUCTION The genus Potamothrix (Vejdovsky et Mrazek, 1903) is a monophyletic, well-defined group of the annelid family Tubificidae, subfamily Tubificinae. These worms are smooth-bodied and relatively fragile in comparison with other tubificids, with short postclitellar segments and conical prostomium when fixed in alcohol (Fig. 1, 1, 2). The genus is characterized by long hose-like atria, vestigial vasa deferentia, small or completely lacking prostate glands, and presence (in most species) of simple-pointed and distally grooved, often nib-shaped, spermathecal chaetae (Fig. 1, 3-9). This groove can be actually a thin-walled tube since spermathecal chaetae are supposed to operate as syringes injecting some stimulating compound into the partner's body during copulation (Cuadrado & Martinez-Ansemil 2001). The sperm in spermathecae is always organized into spermatozeugmata consisting of fertilizing and cortex cells. Potamothrix spp. are an essential component of zoobenthos in many bodies of fresh and brackish water, being a food item for fish and other predators. They have been aligned to different genera in the past while synonymy has created confusion in treating their ecology and distribution. Several species have been used as indicators of water quality and trophic state of lakes (Milbrink 1978, Lang 1998). Some of them appear to be invasive species on different continents (Milbrink 1977, Brinkhurst 1978). Inconsiderate synonymization of some species, e.g., by Brinkhurst (1963, 1971) has caused some confusion with their distribution data. Herewith a short review of taxonomy and ecology of all species of this genus is presented. [FIGURE 1 OMITTED] Note: in the descriptions of the species, the segment numbers are conventionally denoted with Roman numerals (e.g. X, XI, etc.) and the intersegmental furrows with Arabian numerals (e.g., 10/11). HISTORY OF THE GENUS The genus name (meaning 'river thread' in Greek) was coined by Vejdovsky & Mrazek (1903) for the species P. moldaviensis from the Vltava (Moldau) River; another, the commonest species was described two years earlier from the Elbe River in Hamburg by Michaelsen (1901) as Ilyodrilus hammoniensis. Many European species have been described later, particularly by Hrabe (1931, 1941, 1950). These and related species have been erroneously included into the genus Ilyodrilus Eisen, 1879 (treated also as a subgenus of the genus Tubifex Lamarck, 1816). To separate the group from the original Ilyodrilus (sensu stricto), Brinkhurst (1963) introduced the name Euilyodrilus, but rejected this later again (Brinkhurst 1971) in favour of the senior synonym Potamothrix. Holmquist (1985) revised the genus and used the names Potamothrix and Euilyodrilus for two subgenera defined by herself (Euilyodrilus with a prostate gland on atria, and Potamothrix devoid of this gland). Finogenova & Poddubnaja (1990) revised the genus again and, after a closer study, rejected these subgenera as not distinctly delimited from each other. Cui & Wang (2012b) compiled a guide to 26 known species of the genus, erroneously including also Ilyodrilus svirenkoi Lastockin, 1937. The latter was transferred into the genus Haber, with some hesitation, already by Holmquist (1979) and, decisively, by Milligan (1986). Cui & Wang (2005, 2012a) also discovered a separate, apparently monophyletic species group of this genus in ancient lakes of the Yunnan Plateau, southern China (Fig. 2, 3). In all probability, the common ancestors of the genus Potamothrix were connected with the former Tethys Ocean. The Chinese and the western (originally Ponto-Caspian) species groups were separated from each other when the Central Asian mountains arose in the Tertiary and closed the eastern portion of this ocean while the western portion persisted as the Mediterranean and Ponto-Caspian seas. The eastern species group has survived in a few isolated freshwater, mountainous lakes while the western one diversified mainly in the large and versatile Ponto-Caspian water system. …