Chemical defence in Elysiella pusilla

May 6, 2000
From: Bill Rudman

Here is the summary of an interesting paper (Paul, V.J. & van Alstyne, K.L.,1988) on the ability of Elysiella pusilla [as Elysia halimedae] to convert and use chemicals from its algal food for its own defence. This is a continuing theme throughout the sea slugs and that in itself is fascinating for it suggests that retaining chemicals for defensive purposes has evolved independently, many times amongst the opisthobranchs.

Sacoglossans are specialized herbivores of tropical green seaweeds and other seaweeds (diatoms, red algae, seagrasses). In this study, they investigated the feeding relationship between the sacoglossan Elysiella pusilla and its preferred food, the seaweed Halimeda macroloba Decaisne, in Guam. The toxic diterpenoid metabolites of many species of Halimeda have been shown to act as feeding deterrents toward nonspecialized grazers such as herbivorous fishes. However, E. pusilla feeds only on species of Halimeda and prefers young plant growth, which contains the highest concentrations of the Halimeda compounds. Their observations suggest that E. pusilla modifies the major diterpenoid from H. macroloba and sequesters and stores this modified compound for its own defense. When irritated, E. pusilla secretes large amounts of mucus containing this defensive compound. The compound occurs in E. pusilla as approximately 7% of the whole animal dry mass. Similar high concentrations of the compound are found in the egg masses of E. pusilla. In the study, they isolated and determined the structure of the sacoglossan compound and tested the compound in field assays on carnivorous and herbivorous fishes on Guam. This compound has a significant feeding deterrent effect at naturally occurring concentrations toward potential predators. Thus they show that E. pusilla is adapted to feed on toxic algae and utilizes the algal chemical defenses for its own defense against predation.

Reference:
Paul, V.J. & van Alstyne, K.L. (1988) Use of ingested algal diterpenoids by Elysiella halimedae Macnae (Opisthobranchia: Ascoglossa) as antipredator defenses. Journal of Experimental Marine Biology and Ecology, 119: 15-29.

Rudman, W.B., 2000 (May 6) Chemical defence in Elysiella pusilla. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/2361

Copyright © Australian Museum, 1998-2010