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First published online December 14, 2005
Journal of Experimental Biology 209, 78-88 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01972
Packaging of chemicals in the defensive secretory glands of the sea hare Aplysia californica
1 Department of Biology, Center for Behavioral Neuroscience, and Brains and
Behavior Program, Georgia State University, Atlanta, GA 30303 USA
2 Department of Biology, University of Washington, Seattle, WA 98195
USA
3 Department of Physiology, University of Utah School of Medicine, Salt Lake
City, UT 84108 USA
* Author for correspondence (e-mail: cderby{at}gsu.edu)
Accepted 7 November 2005
Sea hares protect themselves from predatory attacks with several modes of chemical defenses. One of these is inking, which is an active release of a protective fluid upon predatory attack. In many sea hares including Aplysia californica and A. dactylomela, this fluid is a mixture of two secretions from two separate glands, usually co-released: ink, a purple fluid from the ink gland; and opaline, a white viscous secretion from the opaline gland. These two secretions are mixed in the mantle cavity and directed toward the attacking predator. Some of the chemicals in these secretions and their mechanism of action have been identified. In our study, we used western blots, immunocytochemistry, amino acid analysis, and bioassays to examine the distribution of these components: (1) an L-amino acid oxidase called escapin for A. californica and dactylomelin-P for A. dactylomela, which has antimicrobial activity but we believe its main function is in defending sea hares against predators that evoke its release; and (2) escapin's major amino acid substrates - L-lysine and L-arginine. Escapin is exclusively produced in the ink gland and is not present in any other tissues or secretions. Furthermore, escapin is only sequestered in the amber vesicles of the ink glandand not in the red-purple vesicles, which contain algal-derived chromophores that give ink its distinctive purple color. The concentration of escapin and dactylomelin-P in ink, both in the gland and after its release, is as high as 2 mg ml-1, or 30 µmol ml-1, which is well above its antimicrobial threshold. Lysine and arginine (and other amino acids) are packaged into vesicles in the ink and opaline glands, but arginine is present in ink and opaline at <1 mmol l-1 and lysine is present in ink at <1 mmol l-1 but in opaline at 65 mmol l-1. Our previous results showed that both lysine and arginine mediate escapin's bacteriostatic effects, but only lysine mediates its bactericidal effects. Given that escapin's antimicrobial effects require concentrations of lysine and/or arginine >1 mmol l-1, our data lead us to conclude that lysine in opaline is the primary natural substrate for escapin in ink. Furthermore, packaging of the enzyme escapin and its substrate lysine into two separate glands and their co-release and mixing at the time of predatory attack allows for the generation of bioactive defensive compounds from innocuous precursors at the precise time they are needed. Whether lysine and/or arginine are substrates for escapin's antipredatory functions remains to be determined.
Key words: chemical defense, ink gland, opaline gland, gastropod, sea hare, escapin, L-amino acid oxidase
This article has been cited by other articles:
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  C. D. Derby Escape by Inking and Secreting: Marine Molluscs Avoid Predators Through a Rich Array of Chemicals and Mechanisms Biol. Bull., December 1, 2007; 213(3): 274 - 289. [Abstract] [Full Text] [PDF]
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 J. S. Prince OPALINE GLAND ULTRASTRUCTURE IN APLYSIA CALIFORNICA (GASTROPODA: ANASPIDEA) J. Mollus. Stud., July 11, 2007; (2007) eym016v1. [Abstract] [Full Text] [PDF]
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