Glands in sea hares secrete two compounds that protect the organism from predators by reacting together to create an unpalatable mixture of hydrogen peroxide and organic chemicals.

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Sea snails and their cousins, sea hares, are extremely vulnerable creatures due to their soft bodies and limited maneuverability. To compensate for their natural disadvantages, sea hares have evolved complex chemical defenses including inks like those found in squid and octopodes. The inks are complex mixtures with remarkable properties. The secretion is composed of two viscous fluids, ink and opaline, that are produced and stored in separate glands. Independently, these two substances have characteristic functions; ink is an unpalatable substance that fish avoid consuming while opaline is not. However, when combined, enzymes in ink catalyze the breakdown of amino acids in opaline to produce new organic compounds and hydrogen peroxide, both of which are unpalatable to predatory fish and are even antimicrobial.

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"[S]ea hares, are soft-bodied and slow-moving benthic snails that live in many marine habitats. [but] No predator is known to make a regular meal of them...Sea hares would be highly vulnerable to predators if not for the possession of a variety of defenses." (Nusnbaum and Derby 2010:282)

"Inking is an active chemical defense that is used as a late line of deterrence during attacks. Sea hare ink secretion is a sticky, purple mixture of the products of two glands: ink, a product of the ink gland, is a deep purple color; opaline, a product of the opaline gland, is white and highly viscous...Ink secretion reduces predation...through a variety of mechanisms including unpalatability, sensory disruption, and phagomimicry...Ink, but not opaline, from A. [Aplysia] californica is unpalatable to the sea catfish Ariopsis felis. In fact, opaline and its amino acid fraction are appetitive to sea catfish, suggesting that opaline might contribute to the effect of the ink secretion through sensory disruption or phagomimicry." (Nusnbaum and Derby 2010:283)

"[I]nk or ink + opaline cause significant rejection of otherwise palatable food." (Nusnbaum and Derby 2010:286)

"When combined, ink and opaline form a more persistent, sticky secretion than ink alone...escapin in ink is mixed with high concentrations of L-lysine and L-arginine in opaline...[and forms] a mixture of lysine intermediate products and H2O2 [that makes food unpalatable]...This mixture of lysine intermediate products and H2O2 is also responsible for the secretion’s powerful bactericidal effect...The molecular identities of the compounds accounting for most of the unpalatability of ink to any predatory species are mostly unknown, though the purple pigment aplysioviolin has recently been identified as being effective against both invertebrate and fish predators." (Nusnbaum and Derby 2010:289)

Journal article
Effects of sea hare ink secretion and its escapin-generated components on a variety of predatory fishes

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California SeahareAplysia californicaSpecies

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