Feedback on Tentacles & Clam In Desert Landscape

This is my first render in Blender, and while I’m mostly satisfied, I think it needs some more polish.

The thing that bothers me the most are the shadows.
The shadows on the sand to the right of the clam are striped? The bottom of the clam looks very flat, but I’m unable to force shadows there.

I have a single source of light (sun) and render in Eevee.

Any other suggestions?

Edit: the site resized the image to 1080, so the stripes are not as visible. Attaching detail below.
2021-10-10 3 Pearl of the Desert shadow detail