Water mites is a group of about 5000 species of mites. Scientific name of them is Hydrachnidia or Hydrachnidia or Hydracarina or Hydrachnellae. They are usually very small and this is one of them. I found it in my freshwater aquarium on Helisoma anceps. It is so small that I had to use lens to verify, that the small dot on the shell is a water mite. The water mite is in the centre of the photo. It is on the mantle edge near the aperture. Detail cropped from the previous photo: You can compare the size of the water mite with two Ferrissia fragilis snails on the top of the shell of Helisoma anceps. Both of these freshwater snail species are of North American origin and both belong to the same family Planorbidae. Ferrisia fragilis can reach 3.2 mm maximum shell length, but I think my snails are a bit smaller. The water mite is about 0.5 mm or smaller. It was actively moving on the snail. The dot in the centre of the photo is the water mite: I do not know the origin of the water mite. I found only one in my aquarium meantime.
Helisoma anceps lays eggs in gelatinous translucent clusters.
This cluster is taken shortly after eggs laying. I think it is about the second or the third day. There are seen five small dot-like embryos:
This is yet another cluster of nine eggs shortly before hatching. Eight hours later all snail were hatched. The length of this whole cluster is 4 mm and it is attached to a stem of a common freshwater plant from the genus Hydrocotyle. There are seen shells of all nine embryos and there are seen heads with eyes of three snails in the first row:
Helisoma anceps, synonym: Planorbella anceps, is a freshwater snail from family Planorbidae. It is native to North America. It is sometimes kept in aquaria in Europe.
Planorbidae are breathing free air and their breathing pore is called pneumostome (it is the same as in land snails). The pneumostome is on the right side of the body, but it is close to the median line (= to the center).
The very large pneumostome is visible on this photo of juvenile Helisoma anceps:
Additionally Planorbidae have another breathing organ, that is used for breathing underwater. It is called pseudobranch and it is placed on the left side of the body. It evolved by extending of the anal region and therefore anus is always close to the pseudobranch. The pad-like pseudobranch of an adult Helisoma anceps is about 2 mm wide and 1 mm high. You can see the pseudobranch easily, if you will watch to the left side of your planorbid snails crawling in the water.
The following two photos shows the extended pseudobranch of the same specimen of juvenile Helisoma anceps: