This Mediterranean violet sea urchin (Sphaerechinus granularis) was photographed near Ugljan island, Croatia, while moving across a gold sponge (Aplysina aerophoba).
<a href="//commons.wikimedia.org/wiki/User:LinicMarino" title="User:LinicMarino">Marino Linić</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>