Chemical Composition of Inks of Diverse Marine Molluscs Suggests Convergent Chemical Defenses
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Some marine molluscs, notably sea hares, cuttlefish, squid, and octopus, release ink when attacked by predators. The sea hare Aplysia californica releases secretions from the ink gland and opaline gland that protect individuals from injury or death from predatory spiny lobsters through a combination of mechanisms that include chemical deterrence, sensory disruption, and phagomimicry. The latter two mechanisms are facilitated by millimolar concentrations of free amino acids (FAA) in sea hare ink and opaline, which stimulate the chemosensory systems of predators, ultimately leading to escape by sea hares. We hypothesize that other inking molluscs use sensory disruption and/or phagomimicry as a chemical defense. To investigate this, we examined concentrations of 21 FAA and ammonium in the defensive secretions of nine species of inking molluscs: three sea hares (Aplysia californica, Aplysia dactylomela, Aplysia juliana) and six cephalopods (cuttlefish: Sepia officinalis; squid: Loligo pealei, Lolliguncula brevis, Dosidicus gigas; octopus: Octopus vulgaris, Octopus bimaculoides). We found millimolar levels of total FAA and ammonium in these secretions, and the FAA in highest concentration were taurine, aspartic acid, glutamic acid, alanine, and lysine. Crustaceans and fish, which are major predators of these molluscs, have specific receptor systems for these FAA. Our chemical analysis supports the hypothesis that inking molluscs have the potential to use sensory disruption and/or phagomimicry as a chemical defense.
KeywordsAmino acid Ammonium Anaspidea Aplysia Aplysioidea Cephalopoda Chemoreception Cuttlefish Gastropoda Ink Octopus Opisthobranchia Phagomimicry Sea hare Sensory disruption Squid Taurine
We thank Jelle Atema (Boston University), Tom Capo (NIH/University of Miami National Resource for Aplysia Facility), William Carr (University of Florida), William Gilly (Stanford University), Arie Sitthichai Mobley and Mary Lucero (University of Utah), Richard Taylor (University of Auckland), Leigh Walsh (National Resource Center for Cephalopods), and Michiya Kamio, Ko-Chun Ko, and Shkelzen Shabani (Georgia State University) for assistance with collection of secretions. Supported by grants from NSF (IBN/9876754, 0324435, 0322773, 0614685) and the GSU Brains & Behavior Program.
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