Polymerurus rhomboides (Stokes, 1887)

Most likely ID: n.a.


Synonym: n. a.


Sampling location: Purrren pond, Ulmisried, Simmelried


Phylogenetic tree: Polymerurus rhomboides



  • body slender and narrow, elongated
  • length 250–420 µm, width 50–60 µm
    head rounded or with 4 acute corners, distinct cephalion
  • pharynx cylindrical
  • hypostomium present
  • two lateral tufts of cilia, ventrally around mouth opening 2–3 paired tufts of cilia
  • 1–2 pairs of setolae, posterior ones on special scales
  • toes strongly elongated, bases completely covered with scales; distal extensions divided into 18–23 rings
  • dorsally 20–25 longitudinal rows (ventrolaterally 7–9 each) of leaf-like to teardrop-shaped smooth scales
  • ventrally 10–12 longitudinal rows of small scales, built like the dorsal ones
  • at the posterior end of the ventral side, a large, circular scale with a distal incision is located
Polymerurus rhomboides

Polymerurus rhomboides is a common species in all my sites with a layer of decomposing leaves and plant material. At low magnifications, the dorsal scales appear to form a rhombic pattern. At higher magnifications it become obvious, that the scales are leaf-shaped or teardrop-shaped. In my population I could find specimens with three elongated scales at the dorsal posterior end (s. fig. 6). It is unclear to me if this is intraspecific variation or if previous authors overlooked them. 


Fig. 1 a-d: Polymerurus rhomboides. L = 328 µm (with toes). Ventral view of a freely swimming specimen. Obj. 40 X.


Fig. 2 a-c: Polymerurus rhomboides. Three focal planes of the head. HY = hypostomium. Obj. 100 X.


Fig. 3 a-b: Polymerurus rhomboides. The dorsal scales of a freely swimming specimen (a) and of a slightly squashed specimen (b). Obj. 100 X.


Fig. 4: Polymerurus rhomboides. The ventral scales focussed from dorsal. Obj. 100 X.


Fig. 5 a-b: Polymerurus rhomboides. The segmented toes of a freely swimming specimen (a) and of a strongly squashed specimen (b). Obj. 100 X.


Fig. 6: Polymerurus rhomboides. A specimen with three elongated scales on the dorsal side at the posterior end (arrows). Obj. 100 X.