How do eyeless inverts know when its dark?

[kilofey]

My brittle stars & urchin hide behind my rockwork all day long but as soon as the lights are off they race out into the open, they come out almost instantly so it cant be the temp change or anything... How do they know??? My hubby & I were discussing it & we couldnt come up with a plauseable answer.

 

[gemmy]

Starfish can sense light through sensors in their arms.

 

[kilofey]

Really? Wierd... So they "feel" light?

 

[saxman]

Tridacnid clams have irridipores around their mantles to sense light.