In a an image with sufficient contrast between the foreground and the background, thresholding and using the fast radial symmetry transform[1] should do the trick. I have some really old code that I wrote a few years back that does something similar. I was able to use the same algorithm for counting objects in images captured from a Neubauer chamber [2] and saved countless man hours at my university.
Disclaimer: the project is really old, and from a time when I barely knew how to code. Lots of bad coding practices et al.