I have found numerous eggs on the lower side of stone in fast flowing river section (Latvia, geographically central part of the Europe). What species lay these eggs? Ancylus fluviatilis (Mollusca)? Other mollusc? Caddishflies (Trichoptera)? Something else?
It would help a lot if you included a scale bar, or some way for us to know what diameter each egg sack is. Even if the image looks "life size" on your computer screen, it may be larger or smaller on other computer screens with different resolution settings.
As Brian says, a scale bar would be helpful. I agree with Andrew that they don't look like molluscs, assuming that each "lump" is an egg and not an egg mass. Can you collect some and examine them under a microscope, or culture them and watch as they develop?
I disagree with Trichoptera, but I do not know the shape of it for every species. Why not Physidae egg-mass?
@Ton, Yes, if the "lumps" are egg masses, then Physid or Planorbid egg masses are a strong possibility. I initially thought they were single eggs with scattered pigmented granules, but now I think you are right and they are egg masses. We need better photographs and scale bars.
You might think beyond invertebrates for the source of these eggs. In North America, some darters and some cyprinids lay their eggs on the underside of rocks. Consider the fishes in your area, especially cyprinids.
For darters I'd expect a single dot in each ball of gelatin. I have not seen frog egg masses like this and I'd also expect a single dot per ball. Many Physidae masses are a bit more linear with three or four eggs across the width and many more along the long axis. Trichoptera eggs are often in balls like this...but often the individual eggs line up in regular stripes across the width of the egg. Having said that, I could not eliminate Trichoptera as a possibility; they return to the water to lay and I have found molding females beneath submerged rocks next to fresh egg masses.
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