Sacoglossan Defenses

March 22, 2001
From: Tina Calle

I am enjoying all of the information you have provided here on this site! It has educated me into avoiding the purchase of nudibranchs - they are very beautiful but my greatest concern for any captive animal is its quality of life - sponge feeding creatures should be left to the wild where they can forage on their favorite species of sponge. I hope others are discouraged from purchasing them as well.

Now - my question is- I have read about the defense system of the Nudibranch but does the Sacoglossan slug - such as Elysia crispata - have any defenses against predatory fish such as triggers, wrasse? How do they defend themselves in the ocean?

Thanks for any information:)
Tina Calle

Calle, T., 2001 (Mar 22) Sacoglossan Defenses. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Tina,
Have a look at two general pages on the Forum which will give you a lead in to other information on the site. Try the Defensive Glands Page and the Opisthobranch Defence Mechanisms pages. Sacoglossans, like many sea slugs have two main means of defence, one is to hide and the other is to taste very nasty. Most hide, or become nearly invisible by being coloured almost identically to the green algae on which they feed. To taste inedible, many species either produce chemicals which they store in glands or sacs in their skin, or they reuse molecules they have removed from the plants on which they feed. Have a look at my message on Elysiella pusilla for an example of chemical defence. Cynthia Trowbridge also lets us know about some more examples in another message worth looking at.

Concerning the safety of Elysia crispata or other similar slug in an aquarium. It is definitely not a natural environment and animals can 'misbehave'. Although I haven't kept an aquarium with fish for many years, I wouldn't trust a wrasse or a trigger fish even if I was sitting there watching them. The problem with chemical defences is that they only work if the predator knows the potential prey tastes bad. And the usual way for it to learn is to bite or 'mouth' the hapless prey. Even if the predator finds the prey too distasteful to eat and spits it out, it is often badly damaged. In nature we assume the benefit is not for the individual who gets 'tasted' but to the remaining population of the prey animal which hopefully won't now be tasted. In an aquarium, where you may only have one or two of the prey animals, the whole system of defence, which relies on a few prey animals being sacrificed to train the predators to leave the rest alone breaks down.

I'm afraid my advice would be to keep sea slugs and carnivorous or aggressive fish in separate tanks.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Mar 22). Comment on Sacoglossan Defenses by Tina Calle. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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