First of all: That is a really nice and impressive image as it is now.
But I have to agree with others. The mushroom and the snail doesn’t look photoreal for several reasons.
Some time ago Andrew Price said something about the basic principles of photorealism in one of his tutorials. And he mentioned accurate modelling as the first and most important one. And with respect to this it’s mainly the shell of the snail that doesn’t looks natural to me. Also the area where the shell blends with the body of the snail needs some redefinition. The modelling of the mushroom looks very natural too me, although I couldn’t tell what kind of mushroom that is - but that doesn’t mean anything, because I’m not an expert at this:)
The second area that could be enhanced are the materials of the snail and the mushroom. At the moment they don’t look like a snail’s or a mushroom’s tissue. I know that this is really difficult. But this is one of the rare cases where I’d really recommend the use of subsurface scattering. Especially for the snail. And you may want to test mixing some transparency into its shader. The mushroom could benefit also from SSS.
And then a tiny little subtle touch of sun on the ground could make the lighting a bit more interesting and it would blend better with the background plate, which clearly shows a sunny day.
Just my humble thoughts. But you should be really proud of what you achieved so far!