Gymnodoris ceylonica inhaling a Ceratosoma tenue.
Photos and video courtesy of Jim Black, John Greenamyer, Francis Pellet, and Mike Miller
Gymnodoris - the Tyrannosaurus of the deep

All members of the genus Gymnodoris are predators, most feeding of species of sea slugs. Having grown up in California, I thought Navanax was the bad boy on the block. I was wrong. Gymnodoris is heads above Navanax in the Hunger Games category. These guys are so voracious that it is unwise to put them in an aquarium or collection jar with other slugs, as these will disappear quickly. They are even cannibalistic.

What is interesting about their feeding behavior is the fact that these guys are blind and locate their prey by smell alone. This often results in an individual biting off more than it can swallow.

Our first photo below shows a Gymnodoris inornata unknowingly tackling a much larger dorid than itself. Wonder how it going to swallow that big guy. Mike Miller(Webmaster) adds that "The Gymnodoris in my appraisal of the situation seems to be "locked" on to the prey and will probably spend some time enjoying the live meal! Don't have any references for this kind of behavior but have witnessed it many times!"

Photo courtesy of John Greenamyer

These guys are so blind they don't even know when they are competing for prey.
Here we have two G. rubropapulosa doing a tug of war with a smaller dorid.

Photo courtesy of John Greenamyer

Francis Pellet has captured Gymnodoris ceylonica at its best,
herding hundreds of Stylocheilus striatus away in fear from this nasty carnivore.

The whole concept of cannibalism in sea slugs is confusing, and somewhat unknown. From the standpoint of how cannibalism affects reproduction, things get really cloudy. What does cannibalism have to do with reproduction you might ask? Well, it is very complicated and debated by scientists. Gymnodoris have been observed to eat their hermaphroditic mate during or just following copulation. The larger animal always wins this macabre battle. Now why would this benefit the survival of the sea slug? Good question. It has been hypothesized that the larger animal benefits by being well fed from this large meal and is therefore now capable of laying larger egg masses. Whether or not the combined number of eggs from two fertile parents would be greater than that of a single, well-fed individual has not been shown, for obvious reasons - the smaller guy is gone.

Some speculate that Gymnodoris may have developed cannibalism to tide them over times when no other prey species are available. Wish I had a photo of this behavior.

Dave Behrens
New Braunfels, TX
Dec., 2023
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Dave and Peg in Texas motif prior to move from
Washington to Texas

From left to right, Terry Gosliner, Angel Valdes, Dave Behrens La Jolla, Calif.

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