Christmas tree worms have eyes that breathe, gills that see

Michael Bok says that all the worms that can be found in Christmas trees have their eyes “in a very silly place”. Of course, this is just part of their charm.

This type of marine worm called Spirobranchus giganteus is getting this holiday nickname from its gills: a wildly colored pair of tapering, spires that protrude from the very top of the worm’s buried retreat like ornamental trees. Michael Bok from the Swedish Lund University is saying that in the past, he has seen worm gills in yellow, orange, blue, red, and even stripes!

At the moment when a shadow looms, these tree gills are dropping down into the protective tube where the rest of the worm is hiding. The eyes that are here to see if there is some kind of danger coming, are laying beneath the branches. Michael Bok says that if you want to see the bright orange eyes, “you’ll just have to sneak up on them and look at them from the right angle”

That specific spot is limiting the eye in a way, and they can see only directly in front or behind.

Bok is really amazed by the way the gills eyes in these types of worms seem so improvised.

“These things are their own evolutionary tangent,” he says.

Bok has spent a lot of time exploring fan worm’s barely studied vision, and this are probably the only animals that are actually growing eyes on their gills. Fan worms actually have rudiments of a typical visual system and also like the other worms, they have patches of light-catching compounds on the segments of their bodies, and sometimes even on their tails. These kinds of visual bits may seem useless for detecting danger when the worm is hiding in tubes. That’s why the gills went completely visionary and evolution scattered them with light-catching molecules or even eyes.

The interesting part is that these nerves from the gill eyes are not going in the usual optic section of the brain. They are connected to another area that usually is not involved in vision.

Even the best of these gill-based eyes, exactly like those in the Christmas tree worm, are able to detect scary shadows. This is a must, cos, without their warning systems for predators, “they’d probably get their gills and ‘mouth’ ripped off all the time”, says Michael Bok.


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