Sperchon setiger (Thor, 1898) – Identification uncertain.

Fig 10 Sperchon (?) setiger dorsal

Here is a water mites of 1.2 mm length found the 5th of July 2011 in the mouth of the stream of Kerescars in Brittany. According Smit & Gerecke (2010), it is the first water mite reported from Finistère and even from Brittany.

His behaviour in my aquarium is rather different from the other water mites I’ve encountered up to now. Unlike the other, it doesn’t swimm freely in the water or on the bottom of the aquarium but it only walk slowly on the bottom of the aquarium.

My observations.

  • Length 1.2mm
  • Width 1mm.
  • Color a light pinkish when seen in my aquarium. A straw yellow on my pictures.
  • Outline oval with the anterior part narrower.
  • Coxae plates and legs blueish.
  • Third and fourth coxae plates relatively small
  • A red mark between the gnathosoma and the genital area (fig 20).
  • Conical process on the flexor surface of the second segment of the palpi (fig 40).
  • Gnathosoma distally elongated.
  • End of the legs yellowish
  • 2 sclerites with a shape of very bold comma on the dorsal (fig 10)
  • 3 pairs of acetabulae, the first two pair elongated, the last pair almost round (fig 30)
  • The genital flaps are capable of a certain contraction (see the rollover at the end of the fig 70).
  • Body flattened and more flattened at the anterior side(fig 60)
  • Genital area not extending beyond the posterior margin of the fourth pair of epimera.

Notes taken during my process of identification.

  • The look of the skin, the dermal glands, the shape of the coxae plates, the shape of the genital area tend to indicate a specie of the genus Sperchon.
  • A rapid study of the plates in Soar & Williamson give as possible specie Sperchon denticulatus female (fig 9 on plate XII) and Sperchon mirus (fig 5 on plate XIII).
  • On the plates of S&W the size of the genital area looks bigger than mine on Sperchon denticulatus.
  • For the Sperchon mirus, the proportion of the genital area is good but I can’t see on my pictures the margin shown on the anterior side of the third coxae plates.
  • The description of Sperchon setiger deserve a further enquiry (length, colour, sclerites, description of the acetabula…).
  • According to the size of Sperchon in Gerecke 2010, it can be : a female of Sperchon setiger, a female of Sperchon mutilus, or maybe a female of Sperchon thienemanni.
  • It is a little broader than  Sperchon thienemanni and the colour of Thienemanni orange to red while mine is more yellowish. May I exclude Thienemanni ?
  • Sperchon mutilus female match for the size but it is not known as present in western europe.

Other specimen caught the same day.

The same day (4 june 2011), i’ve caught 3 specimen of this Sperchon. My pictures are less good for the other but they are very similar. The size are rather variable but all match with the size given in Gerecke 2010.

  • The smaller is 1mm long and 0.9mm wide
  • The medium (shown on this article) is 1.2mm long and 1mm wide
  • The larger is 1.3mm long and 1mm wide.



  • Gerecke, R. (Ed.) (2010). Chelicerata: Acari II. Süßwasserfauna von Mitteleuropa 7/2(2). Spektrum Akademischer Verlag, Hei

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