30 April 2011

Horseshoe crab's eyes

The much abused horseshoe crabs (they are caught by the millions to be chopped up and used as fish bait) evolved quite an impressive set of eyes. They have them both on the top and on the sides of their carapaces as well on the bottom of their bodies.

While trying to photograph mating horseshoe crabs in Florida last week, I noticed how aware they were of approaching humans. Then I noticed what seemed to be eyes on their sides. This one was on the right side of a crab.

As you can tell, the lateral eye is a compound eye similar to the eyes of insects. The individual units, ommatidia, are visible. I don't know if these are the eyes the horseshoe crabs use to detect potential predators or if their other eyes also contribute to the task.

After a horseshoe crab dies, its hard carapace survives quite a long time; they are common objects on Florida beaches. The interesting thing is that the lateral eye is also immune to decay. Instead of turning into a depression or a hole on the side of the carapace, it retains even its compound structure. Here is the left lateral eye (~11 mm long) on the carapace of a long-dead horseshoe crab.

I don't know the function of the long, reddish-brown brow above the lateral eye.


Jason R said...

They even have eyes on their underside:

Don't forget to let the Florida Wildlife Conservation Commissin know about your sightings: http://www.surveymonkey.com/s/horseshoe_crab :)


Jason, thanks for that link. I just reported my sightings.

Beej said...

That's a stunning bit of photographic insight. Thank you. Are their eyes similar in structure to those of regular crabs (the crustaceans)?

Miss Adventure said...

I'm not going to become a serial commenter - I promise, but your eye info is absolutely gripping. i remember seeing horseshoe crab in Florida, and had no idea thay had this facility. Sadly, apat from loads of post hurricane coral, I never saw any Horseshoe carapace. Now I'm REALLY disappointed - Don't worry, It'll pass!

Anonymous said...

Hi there,

I am a geologist and I remember when I was studying trilobites that one reference noted that they had compound eyes that were formed from from organic calcite crystals. Horseshoe crabs are close relatives of this extinct group; if the structure of the compound eyes is the same, it would explain why the eyes of the horse shoe crab are not decaying.

Hope this helps! Here is a refrence http://www.etrilobite.com/?p=924

Anonymous said...

Hi there,

I am a geologist and the tirllobite is a close relative of the horshoe crab. Here is a refrence on thier calcite crystal compound eyes that may explain why they do not decompose. http://www.etrilobite.com/?p=924