What are these dotted ball shaped things I circled in yellow in the image?
I thought they may be sea urchin skeletons. Though the things in the image may be too many and too large?
They are in a mural in Brighton, England. The subject is “Protecting Costa Rican Wildlife”, so I guess they are of some native species there.
Jellyfish maybe?
Yes they look like sea urchin. However, the live ones look like the black shapes.
Urchin shells
To be more specific on the terminology they are sea urchin tests: https://en.wikipedia.org/wiki/Test_(biology)
Fun fact: we call them “sea hedgehogs” in Hungarian.