Seeing the light

[capncapo]

Dr. Ron,

I tried to find this info in your articles but either missed it or was just too lazy to look harder.

How do creatures like sea stars "see" light when it's present? I assume that they have some form of photoreceptors but where?

Also, would the same hold true for other echinoderms, like urchins, or do they "see" differently?

Thank you, sir.

[rshimek]

Hi,

The story is pretty complicated, actually. Asteroids have "eye spots" at the end of their arms or rays. See the image below (the dark spot in the center at the end of the ray is tie eyespot).

http://reefcentral.com/gallery/data/500/41Eyespot_at_end_of_asteroid_ray_1_r.jpg

The eyespot is pigmented and ennervated, but it is essentially impossible to get a reaction out of the star by varying light intensity on the spot. Some people think the spot is glandular and produces hormones in response to light. If the hormones were, for example, produced in the light, and broken down in the dark, the eyepsots could be a way to assess changes in day length. When the hormones would reach a certain threshold level, it might trigger the star to spawn or otherwise change its behavior. However, this is all supposition.

Sea urchins also have defined "ocular" plates, surrounding the anius, with similar eyespots. These animals may respond to illumination with rather rapid locomotion either into or out of shady spots.

Recent work indicates that some (maybe all) brittle stars have the capability to respond to light through transparent skeletal plates located all over their bodies.

Not much work has been done in this regard with either cukes or crinoids, so no guesses here.

Lots of animals seem to have general photosensitivity of their epidermal tissues and echinoderms are rather the epitome of this. Keep in mind as well, these animals are essentially brainless. Therefore, they can't really process the complicated images sent by most eyespots or eyes, so it is hard to tell what triggers responses.

In other words, a lot of work needs to be done with Echinodermata as a whole in this regard. :D