How do hermit crabs "see" in the dark?

[anniehorsekrab]

I considered this an anatomy question because whatever part of their body they use to "see" in the dark would be their anatomy, right? Anyway, I was thinking they might have night-vision, but now I'm thinking that it's probably more of an antenna thing, where they feel around with their antennae. My 3 PP's are always the most active at night, but it is pitch dark in my room, so how exactly DO they see at night?

[emmac350]

Crabs don't exactly have eyes the way you and I do. They have compound eyes, like insects. Have you ever looked through a "bug eye" toy that makes you see a bunch of whatever you're looking at? If so, that's more like what your crabs see.

In order to see in the dark, our crabs have their ommatidia (the cells that make up the compound eye) organized in such a way as to maximize light intake. Their eyes are better at picking up movements (you would see it too if you had hundreds of separate pictures to see it in!) than objects most of the time. Sight at night, however, isn't necessarily their primary sense - our crabs' antennae help them find their way around. The long pair are used mostly for touching things in their path to determine what it is. They're kind of like how blind people use their canes - it feels in front of them to make sure it's safe. The inner, bent pair are the smelling and tasting ones. They are how crabs find the food they eat.

Sources:

http://www.enature.com/expert/expert_sh ... onID=16069
http://en.wikipedia.org/wiki/Ommatidium
http://en.wikipedia.org/wiki/Arthropod_eye
http://www.thecrabbagepatch.com/crabanatomy.html

[anniehorsekrab]

Thanks for the great reply! I read all of it and now have a much better understanding.