Eyes of Aplysia california

Dear B.W.,
One group of molluscs, the cephalopods [octopus, squid etc], have well developed sight and are clearly able to recognise shapes and colours. However the eye of Sea Hares, and all sea slugs for that matter, have some of the same basic structures as ours - a cornea, lens and retina - but they are not capable of 'sight'. Quite a lot of experimental work has been done on the eyes and nervous sytem of Sea Hares and it seems the eyes recognise changes in light intensity. Certainly if you pass a shadow over a Sea Hare in a tide pool it will respond by retracting its head, stopping its crawling or by making some other avoidance response. But they certainly can't see or recognise objects. To complicate things there seems to be some evidence that they can also sense light by some other means than their eyes. Sea Hares have rhythms of activity [circadian rhythms] often based on a 12 hour cycle. Some species are reported to actively crawl only during the day and others only at night. Some apparently only feed during the day. It seems that there is some 'clock' in the eye which acts as a timing device keeping the various body rhythms synchronised.

In brief we can say that the eye can recognise changes in light conditions and may help in regulating the Sea Hare's biological clocks. To orient themselve in nature Sea Hares rely on touch and smell. See the Fact Sheet on the Sea Hare head

Best wishes,
Bill Rudman