Without a detailed analysis, there are three probable answer (although it may be more of them): 1 / pollen is one of the species of flowering currently,  2 / ichtioplkankton, 3 / egg (a shape typical) one of the benthic organisms of the estuary. However, share the following information: where and how the material obtained in which season, which is the size (in microns), and how and where the sample was preserved. This information will allow the elimination of the found object from the "wonders of nature" and correct its qualitative assessment using the appropriate keys to determining the species hydrobionts.

I  have already seen this in the stomach shrimp of yhe mediterranean sea, it seems to be pieces of radiolarians or coccolithophoridae

radiolarian?

Do you have a scale bar for those images?

The porous nature of it reminds of a foraminifera test; however, as it looks to only be a fragment, I wouldn't worry about identification and enumeration unless there were many in your samples. Good luck!