HCA: Hermit Crab Association

You can tell what something can see by the specral sensitivity of the cells in their eyes.If coenobita are like other crustaceans, they probably see around the low 500nm wavelength (green) and the mid 300nm wavelength (UV). The UV would let them see day/night variation and shadow. The green probably alerts them to food and shelter.Also, since terrestrial crabs developed from marine crabs, and red light is filtered by the ocean below a few dozen meters, it makes sense that they would only see in green, and not in red.Conclusions I can draw from this: UV light is important to regular their day/night cycle, and blacklights shouldn't be used at night. And the addition of fake green foilage to the crabitat may stimulate activity.

Infrared light would be OK at night, because they can't see it, so it would look like night to them.Infrared is a way to eminate heat without eminating visible light.

Cool! You always have the most interesting info Jedi! Thanks for sharing it with us.I do have a question about the red infrared light at night. If they can't see it and there is no other lights on, wouldn't they need some type of moon light to see at night?

I don't think night vision is important for crabs.There's always a little light, even if all the lights are off in the house, eventually your eyes get accustomed to the low light and you can see.But crabs are more dependent on their sense of touch and smell than vision, most likely. Otherwise they wouldn't walk off counters in broad daylight.

Yeah, but I think the walking off stuff just comes from their absent/to poor depth perception. I know I read they have compact eyes, like a fly. But I've also read a few posts here and there about how people say their crabbies seem to avoid the color red. Like I seem to remember someone using a red cd case to block off their molters. The other crabbies seem to avoid that particular colored one more than others. Granted this is all pretty much hearsay and may just be circumstantial, but it's still enough to give me pause. I absolutely believe their sense of smell, and probably sound is much better than their sight, but I'm still not sure what their sight is. I mean, they are different from marine hermies. They grew modified gills, for one thing! Plus, if you look at marine hermies eyes, they just look different from land hermies. Appearances can be deceiving, but superficially at least, they do seem different. I guess I'm just reserving judgement.

i have used both moonglow bulbs and infarred bulbs. the only differnce i really notice is that the crabs dont see me from outside the tank with the red light. with the moonglow if i got too close, even with all other lights off, the would sometimes see me. with the red bulb, i can sit practically face pressed against the glass and they dont notice. my thinking is they cant see red at all. of course, this is just my experience.

Perhaps red light and red objects are perceived differently. That would make sense.

quote:Originally posted by KittyCaller:Perhaps red light and red objects are perceived differently. That would make sense. That does make sense. I wonder if they can't see things that are red? It all is coming clrarer now that my crabs hated the new red climbing net that i got them. I just took it out of the tank yesterday because they never used it. I thought it was because it had holly sewn on it but they love the other climbing vines I have. I am thinking now that they didn't like it because it was red. I have a dark green one ordered now to replace it. Plus they have all but worn out the natural brown one that I have. It has big holes in it! I am going to try and make it into climbing ladders.

JMT - what exactly do you mean by "black lights"? The disco type that makes some colors glow? or the moon-glo bulbs that look black?gk

By "black lights" I mean the disco things.Hermit crabs have apposition type compound eyes.Apposition type compound eyes seem to get their depth perception from eye and head motion. I.e,. they interpolate depth from parallax motion. But this of course isn't nearly as good as our binocular vision.

I have a few questions regarding hermies' perception of color:Some species of LHC's are fairly brightly colored. Why bother with the red and purple coloration if they can't see it? Now, there are many animals with very bright coloration who are color blind. It is thought that these colors are warnings to other creature, mainly predators.Also, LHC's tend to be pretty nocturnal. Noturnal animals don't tend to have very good color vision. This has to do with the relative concentration of "Rod" and "Cone" cells in the eyes. One type sees color well but very little light. The other is great for low light vision, but dosen't perceive color well.We have eyes that are designed to see color well, so our night vision is poor. On the other hand, cats have eyes designed to see in low light, so they're pretty much color blind.As far as depth perception goes: Most climbing animals have pretty good depth perception. of course, most of these animals don't have four walking legs and two pincers, either. From what I've seen, LHC's climb by feel. This suggests that they don't need very good eyesight to climb.Doc

The more I think about it, the weirder the subject becomes. On one hand, I tend to agree with those who say that hermies are color blind.Even on my darkest crab, who is almost completely maroon, there are purple markings. These markings occur in areas that are white or cream on other crabs. I'm beginning to think that the purple color on caribbean LHC's is actually just a resonse to sunlight. Just as melanin is for humans. The older the crab, the more sunlight it has received and the darker it becomes, with purple areas becoming maroon and whiter areas becoming purple.But there's something else to consider.Some arthropods see into the ultraviolet. This is why the gentle pastels on some flowers turn into distinct dark-and-light patterns when seen through a UV filter. The distinct pattern attracts insects from a greater distance. Could it be possible that LHC's see ultraviolet wavelengths as well? They're land arthropods just like bees and wasps, both of whom have similar UV vision.Another posibility is that LHC's actually locate food with UV vision. It has been a long-standing theory that carrion-eating birds can spot food at great distances because the plume of gasses that decaying animals give off actually reflects UV light. Vultures, being able to see UV, can pick up these plumes at distances greater than their sense of smell alone would allow. Could LHC's have developed an ability to sense UV light as a means of both finding food and identifying patterns on each other that we can't see?Doc

It is thought that these colors are warnings to other creature, mainly predators.The animals that have bright warning colors are either venomous, or are harmless but have markings similar to their dangerous kin to fool predators into thinking they're venomous too. One such imitator would be the scarlet king snake which has the same color bands as the dangerous coral snake but in a different formation. We all know that rhyme "Red on black, friend to Jack./Red on yellow, kill a fellow."I don't think hermit crabs are trying to scare predators off with bright colors. Imitation is a great tactic for otherwise defenseless creatures, but the hermit crab seems to have plenty of protection already. It has a hard exo and a shell to hide in. A predator would have a hard time getting at it! Why they have different colors? I don't know, but as far as my knowledge in evolution goes... Not every feature of an animal has a role in survival. Sometimes, it just happened and circumstances just allowed it to stay that way.That really got my noodle cooking. Thanks for the thoughts!