Predation on sacoglossans

February 7, 2001
From: Cynthia Trowbridge

Hi Bill,

I am delighted to see that you are accumulating a page on predation on sea slugs.

A casual perusal of the slug literature indicates that there has been considerable work done on apparent defenses against predators (e.g., autotomizing cerata, lowered pH, viscous white secretions, etc.). Yet, there is a surprising lack of experimental evidence in the field or lab to test the effectiveness of the defenses.

About 10 years ago, I investigated the palatability of 4 common sacoglossan species on the Oregon coast to a suite of ecologically relevant predators including different sculpins, surfperch, shore crabs, Dungeness crabs, and nemerteans. Three small, cryptic sacoglossan species (Stiliger fuscovittatus, Alderia modesta, and Placida dendritica) were readily eaten by various fishes and crabs. In contrast, the larger, black and white sacoglossan Aplysiopsis enteromorphae was not readily eaten by predators.

Furthermore, I also tested the role of predation on the shore for Alderia modesta in a traditional predator-exclusion experiment (with exclusion cages, 2 types of cage controls, and unmanipulated controls). Predators significantly reduced densities of the slugs (relative to the exclusion controls) in my 12-day experiment in Yaquina Bay, Oregon (USA).

This study has not been widely cited, in part because it refutes the traditional - but largely untested - assumption that sea slugs are unpalatable. The role of predation is far more complex than researchers have traditionally assumed. Thank you for considering my comments.

• Trowbridge, C.D. 1994. Defensive responses and palatability of specialist herbivores: predation on N.E. Pacific ascoglossan gastropods. Marine Ecology Progress Series, 105: 61-70.


Trowbridge, C., 2001 (Feb 7) Predation on sacoglossans. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Cynthia,
I wish we could get more researchers interested in this type of study. It is a very complex set of questions. Some species appear well camouflaged and others are brightly coloured at least to our eyes. Many have nasty chemicals and at least on initial contact are avoided by fish. It seems there will be no general or simple rule that covers all species. I have suggested, when discussing chromodorids, (Rudman, 19) that defensive chemicals are probably the last line of defence. By the time the predator is experiencing your nasty secretions, your chances of survival are probably quite low.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Feb 7). Comment on Predation on sacoglossans by Cynthia Trowbridge. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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