The Christmas tree worm, Spirobranchus giganteus, has some flashy gills. While the worms bury themselves in coral, only their colorful fir-shaped radioles—appendages they use for respiration and eating—are visible. They also make the world visible to the worm: That's where their eyes are located.

Their bright-orange eyes are hidden inside the creature's gills. If you look very closely, you can see them tucked away in the darkest-colored part of the animal like Christmas presents with neon wrapping paper.


Nick Hobgood via Wikimedia Commons // CC BY-SA 3.0

 
The Christmas tree worm’s obscured peepers might be the only example of eyes located on gills in the animal world, as marine biologist Michael Bok told ScienceNews. Unfortunately for the worm, eyes shrouded by branch-like gills can be limiting. The Christmas tree worm can only see what’s directly in front or behind it, and when a shadowy predator passes by, the worm sucks its radioles down out of sight. Bok calls the worm’s eye position “really silly.” But, since most other fan worms don’t even have eyes at all, the Christmas tree worm is actually pretty far ahead in the visual acuity game.

Scuba divers from the Indo-Pacific to the Caribbean are fond of spotting the creatures on coral reefs, but they have to maintain a little distance lest they spook the worms into hiding. Earlier this year, scientists from UCLA studying this hiding behavior among Christmas tree worms in Mo’orea, French Polynesia, found [PDF] that the animals don't seem to find strength in numbers: The more worms in an area, the more time individual worms spent hidden in the coral. Conversely, worms with only a neighbor or two spent more time in the open. It's possible that a lot of eye-catching radioles in one place make the worms far too conspicuous to their predators, which include sea urchins, sea stars, flounders, and stingrays.