Unionicola male – identification to specie impossible

Fig 10 : general view


This water mite was caught the 2 June 2011 in  Saint Martin au Laërt. I think it belong to the genus Unionicola.

Here are my observations (before looking in books for specific characters).

  • Size 0.8mm
  • Free swimming (not caught in mussel or other)
  • 2 dark dots on the posterior side of the back.
  • Y yellow on the back
  • Legs very long with robust setae.
  • Eyes dark red. 4 eyes merged in 2.
  • Genitalia blueish situated at the posterior margin of the body. Shape of 2 half moon.
  • Few acetabular plates on the exterior margin of acetabular plates but my pictures are not very good at this level of magnification (fig 90).
  • Coxae plates blueish.
  • First coxae plates almost jointed
  • Coxae plates covering  2/3 of the ventral area.

Key of Hopkins.

This key is very handy for giving some idea when you don’t know the genus of an european specie. I usually use it by browsing the figures given and after by using the written key.

Browsing the images of the Hopkins key.

  • Tiphys Ornatus. fig 53. For : some similarities for the shape of the coxae plates. Against : acetabula invisible on my pictures. Genital area more close of the last coxae plates and partially between the last coxae plates.
  • Unionicola intermedia. fig 78 + p61. For : shape of the coxae plates, position of the genitalia. Against : anchoral process of the second coxae plates invisible on my pictures, shape of the genitalia.
  • Uninicola Crassipes. Fig 77 + p61. For : eyes fused, position of the genitalia, 2 small protuberance on each site of the genitalia, general shape of the genitalia of the female (fig76). Against : 12 acetabular plates invisible on my pictures.
  • Unionicola figuralis. Fig 79 + p61. For : shape of the genitalia, half moon meetings together at the top and at the bottom. Againt : 10 acetabular plates invisibles on my pictures.
  • Neumania vernalis; Fig 84. For disposition of the genitalia. Against : we can’t say it looks like half moon.

Using the key of Hopkins.

After the first step which indicate globally the genus Unionicola, I go to the page 61 of the key to try to go deeper.

  • Unionicola crassipes male : genital plates sickle shape (OK) but not joint on the lower point. Length up to 1.5mm (much larger than the 8mm of mine).
  • Unionicola crassipes female : yellow dorsal T. On mine there is a yellow dorsal Y (fig 10).
  • Unionicola intermedia male : genital plates sick shaped (OK) each plate with 5 acetabula (maybe, see mi picture 90). Length up to 1.mm (better than crassipes on this character).
  • Unionicola intermedai female : shape of the genitalia not matching.
  • Unionicola figuralis shape :  genitalia of the fig 79 of hopkins matching but length 1.4mm a bit too larger and impossible to see 5 acetabula with the 3 forming a triangle.
  • Unionicola aculeata male : shape of the genital plate OK, Acetabula aligned along the border of the plate (seems OK).

Conclusion of this step.

As we are, with my pictures, unable to count precisely the acetabulars plates on the genital plates, we can’t identify the Unionicola with the hopkins Key. Many species match more or less our specimen even if aculatea get my preference.


Notes on the similarities and difference with the Unionicola presented by Soar & Williamson.

Unionicola crassipes.

Element in favor of Unionicola Crassipes (S&W vol 2 page 126 – 131)

  • Posterior end of the body with 2 lateral protuberance.
  • Double eyes reddish in young specimen, dark for the old ones.
  • I exist a smaller form called Unionicola Crassipes Minor by Soar himself.

Element in disfavor of Unionicola Crassipes.

  • Size bigger 0.96 given for the male.
  • Bright Yello mark T shaped (Y shaped on mine).
  • Ancoral process as long as capitulum (no achoral process on my fig 20).
  • Legs relatively thin exept the first pair (on mine the second pair is not thinner than the first).

Unionicola figuralis. S&W p131 plate 36.

Against : Genital area of male pyriform and not like my fig 80. No marked posterior pappilae as in mine and Crassipes.

Unionicola Intermedia. S&W p134 plate 36.

In favor of intermedia :

  • Length form 0.75mm to 1mm
  • First pair of leg scarcely any stouter than the other pair of legs.
  • Genital plates lunate on the male.

Against intermedia :

  • No posterior papillae.
  • The fourth segment of the fourth pair of leg of the male is not thickened and slightly crooked.
  • No hook at the anchoral process of the capitulum.

Unionicola bonzi S&W vol 2 – page 136 – plate 36.

In favor of bonzi.

  • Body length from 0.5mm to 0.7mm (as the mal is even smaller it may be too small)
  • Male : genital plates lunate, their extremities meeting together at each end of the aperture.

Against bonzi

  • Long spine like bristle not arising from papillae (papilae visible on my fig 50)
  • Distal portion of sixth segment of legs thickened, end dilated to form a large sheath for the claw.

Unionicola aculeata S&W vol 2 – page 138 – plate 36.

In favor of aculeata

  • Broadest part of the male body in line with  articulation of fourth pair of legs.
  • Color a straw yellow
  • Well marked papilla at each of the posterior-lateral corner projecting beyond body margin.
  • Pigment black of the eyes.
  • Eyes pair lying on the antero-lateral corner as to be almost visible in the ventral aspect (see my figure 30)
  • No wind like extension of the madibular groove.
  • Epimera covering rather more than half of the ventral surface.
  • Legs greyish green
  • Sixth segment of fourth pair very thin.
  • First pair of legs carying spines arising from pappilae.

Againt aculeata

  • Length up to 1.12mm
  • No mention of the Y shape of the yellow part.
  • Posterior process from the first two pairs of epimera extending over the third pair (it can be a problem of definition of my pictures).
  • Sixth segment of fourth pair of legs with a small degree of curvature : I can’t see this particularity on my pictures

Conclusion on Unionicola aculeata following Soar & Williamson.

I think it is not an aculeata as there are too many elements either not present or visible (4th pair of leg for exemple).

Unionicola gracilipalpis – S&W volume 2 – page 142 – plate 36.

This specie is the sixth and last specie described by Soar & Williamson in British Hydracarina.

Against : very small palpi, Ganital plates of male lying more on the ventral surface than towards posterior end of body.

The checklist of the water mites of france of Smit and Gerecke.

Harry Smit and Reinhard Gerecke have published this paper in the review Acarologia in 2010. I’ts a good way to check the validity of an identification made in france.

Here are the Unionicola found in France and if they were found in Pas de Calais.

  • Unionicola bonzi : only in Nord but it’s very near Pas De Calais
  • Unionicola intermedia : only in Isère
  • Unionicola crassipes : present in a lot onf french departement including Pas de Calais.
  • Unionicola figuralis : Isère and Savoie (not at all the same part of France).
  • Unionicola gracilipalpis : same distribution as figuralis.
  • Unionicola minor : found in Pas de Calais.
  • Unionicola parvula : also in Pas de Calais
  • Unionicola ypsilophora : Nord but not Pas de Calais


General remarks on the identification of Unionicolae

Remark on the size of Unionicolae

From Crowel & Davids 1979, the size of the Unionicola vary greatly depending of the age of the specimen and on their feeding behaviour. This id due to the low scelorization of he body. The identification at the specie level is then done by comparing some ratio based on characters like length of the palpi, length of the body…

The sex of the Unionicolae.

Bristle in the genitalia of the female => this unionicola is probably a male.

The identification at the specie level of Unicola.

The genus Unionicola Hadelman contain more than 40 subgenera and more than 200 species. Fortunately all these species are not occuring in the same place.


  • Additions to our knowledge of Unionicola Aculeata (Koenike) by Ernest Carroll Faust. Transactions of the American Microsccopical Society. Volume 37. 1918 (link Biodiversite Heritage Library)
  • A CHECKLIST OF THE WATER MITES OF FRANCE (ACARI: HYDRACHNIDIA) by Harry SMIT and Reinhard GERECKE – Acarologia #50 – 2010 (link Acarologia).

A lot of more pictures.

Fig 20 : first pair of coxae - swimming seate

Fig 30 : posterior coxae and genital area

Fig 40 : from behind dark dot and genital area

Fig 50 : lateral coxae plates

fig 55 : beginning of the first leg

Fig 60 : 4 eyes fused and palpi from top

Fig 65 : poor focus on the end ol palpi

Fig 70 : frontal view

Fig 80 : genital area with the 2 dark spots

Fig 90 : genitalia with 3 acetabular plates visible on the right

Fig 90-2 : genitalia 2 acetabulae visible on the left border



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