Eiseniella tetraedra tetraedra (Savigny, 1826)

Description – External – Body length 20 – 80 mm, diameter 1.5 – 2.5 mm, 65 – 100 segments. Colour brown or yellowish -brown. Head epilobous, first dorsal pore in 4/5. Setae closely paired, setal arrangement after the clitellum:  Clitellum ring–shaped but less glandular on the ventral side, extends on segments 22, 23 – 26, 27. Tubercles on 23, 24 – 25, 26. Male pores great, usually on 13 but sometimes they are on 11, 12, 14 or 15 and often asymmetrically arranged. Nephridial pores alternates irregularly between b and above d. The body has a characteristic quadrangular shape behind the clitellum, with setae on the edges.

Internal – Dissepiments 7/8 – 11/12 thickened. Crop in 15–16. Gizzard small, confined into one segment in 17. Two pairs of testes free in 10, 11, and four pairs of seminal vesicles in 9 – 12. Receptaculae seminis two pairs in 9/10 – 10/11 open above setal line cd. Calciferous glands with diverticula in 10. Excretory system holonephridial with sac –shaped nephridial bladders. The cross section of longitudinal muscle layer are of pinnate type.

Remarks – Eis. tetraedra is a widely introduced peregrine species with different parthenogenetic morphs. A number of different forms and varieties has been described, and some of them has subsequently been regarded as subspecies (Zicsi 1982) differing only in the position of male pores. The subspecies Eis. t. tetraedra, Eis. t. intermedia and Eis. t. hercynia have the male pore on 13, 14, and 15 respectively. In parthenogenetic populations the location of the male pore has no reproductive value, and this might be the reason of the high variability recorded.  Distinguishing subspecies exclusively on the basis of such a variable feature unjustifiable, furthermore the proposed subspecies simply do not meet the requirement of a subspecies, namely the separate distribution.

Ecology – Eiseniella tetraedra is a limicolous species showing an overwhelming preference for damp habitats. It occurs in ponds, lakes, streams, rivers springs etc.

Distribution – It is of Palaearctic origin but has been widely introduced all over the world.