Watching sea slugs moving it appears that they do not have a clue where they are going. They just go into whatever direction until they touch something with their tentacles. Then they turn.
So do sea slugs have eyes?
Other slugs turn when you get in front of them with a camera. Does that mean they can actually see the diver with a camera and quickly turn not to get blinded by the flashes?
Look at the picture below, this slug definitely does have eyes, right?
So, yes, sea slugs do have eyes, all of them. Some are visable, some cannot be made out as easily. Luckily they do which gives many of them a cute face we humans can associate with and there is slugs that look absolutely adorable like sheep or bunnies.
But can they actually see?
The cells of the eye tissue are sensitive to light and a primitive optic nerve transmits the information to the brain. This allows them to make out shadows to help them to know if a predator is near and to give them a basic idea about the surrounding. However, that is about it. They cannot actually see. As the sea slug expert Bill Rudman states: „It seems a shame with all their beautiful shapes and colours that they can’t see each other.“ (1)
The eyes are embedded in the skin and not on a stalk like we know it from their relatives on land. For some species the eyes are even further under the skin covered by an unpigmented patch that allows light to pass through to the light sensitive cells.
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While it is easy to see the eyes for sea hares and supsucking slugs it is harder to make out the eye for others as they are integrated in the color patterns.
(1) Rudman, W.B., 2000. Comment on Do nudibranchs have eyes? In Sea Slug Forum. Australian Museum, Sydney. Retriefed from http://www.seaslugforum.net/find/2377 on July 19, 2016
(2) Behrens, David W. 2005. Nudibranch Behavior. New World Publications, Jacksonville. ISBN 1-878348-41-8