In the front is an urchin with a fedora at a rakish tilt. Bringing up the rear is a friend wearing a pork pie hat. It's trendy among aquarists (people who maintain aquaria) to dress up sea urchins as fashionable dandies. All that's necessary to look right is a 3D printed hat (although you may need additional clothing before going outside). Sea urchins habitually cover themselves with objects to protect themselves from predators and excess light, so they'll gladly take the hats.
One hobbyist named riosouza describes his own 3D printed urchin hats:
After seen my sea urchins carrying snail shells, rocks on their back I decide to read more about it.
Studies reviewed the possible reasons would be to protect themselves against predators and/or full-spectrum light source, mainly against UV rays.
Then I decided to do a quick design for a 3d printed hat, and for my surprise they absolutely love it.
Since I replace the rocks and shells from their back with the hats, they never let it go, and I was astonished to see they moving the hats towards the light source. Which leads me to the conclusion they certainly use it against excessive UV rays.