The ink squirted by sea hares is distasteful to fish, according to research recently published by scientists from the Georgia State University.
The study, which examined the defence mechanism of the sea hare Aplysia californica, were published recently in the journal Animal Behaviour.
The sea hare releases an ink secretion consisting of purple ink and white opaline when attacked by predators. This secretion contains feeding stimulants (amino acids), feeding deterrents, and aversive compounds.
Matthew Nusnbaum and Charles Derby examined the mechanisms by which sea hares use their ink to deter a fish predator, using the Bluehead wrasse (Thalassoma bifasciatum) as an example.
The authors carried out a live feeding assay in which they offered hungry Bluehead wrasses either an intact sea hare, or a de-inked sea hare.
They scored for the number of times the sea hare was struck by the wrasse (intensity of attack), whether the release of ink occurred and damage to the sea hare.
They then carried out a cloud assay by injecting a cloud of one of four treatments (ink+opaline, ink, opaline or sea water) between a food item (piece of shrimp) and the fish, and then scoring for whether the fish touched, captured or consumed the food, and the time taken for the fish to touch the food.
The authors then prepared food pellets that were injected with one of five substances: ink, opaline, ink + opaline, a mixture of the amino acids in ink, a mixture of the amino acids in opaline, or sea water and then offered them to the fish (pellet assay).
Lastly, the authors plugged the nostrils of some of the fishes (to inactivate their olfactory senses) and conducted the cloud and pellet assays again with these fishes.
The authors found that sea hares were attacked less intensely by the wrasse if it released ink, and that the chemical components in the ink were the ones responsible for deterring feeding (there was no significant rejection of food particles treated with opaline only).
They also found that plugging the nostrils had little effect on the fishes ultimately rejecting food treated with ink. This suggested that the ink not only smelled bad to the fishes, it also tasted bad, as the fishes with plugged nostrils initially ingested the ink-treated food, but ultimately rejected it.
In another study published in the same journal, it was found that the purple pigment in the ink (aplysioviolin) was distasteful to predatory Blue crabs (Callinectes sapidus).
For more information, see the papers: Nusnbaum, M and CD Derby (2010) Ink secretion protects sea hares by acting on the olfactory and nonolfactory chemical senses of a predatory fish. Animal Behaviour 79, pp. 1067–1076.
Also see: Kamio, M, TV Grimes, MH Hutchins, R van Dam and CD Derby (2010) The purple pigment aplysioviolin in sea hare ink deters predatory blue crabs through their chemical senses. Animal Behaviour 80, pp. 89–100.
Sheybani, A, M Nusnbaum, J Caprio and CD Derby (2009) Responses of the sea catfish Ariopsis felis to chemical defenses from the sea hare Aplysia californica. Journal of Experimental Marine Biology and Ecology 368, pp. 153–160.