A test of novel function(s) for the ink of sea hares

Most sea hares (Opisthobranchia: Anaspidea) release a purple ink when physically disturbed. The ink has been hypothesized to function to excrete unwanted byproducts of metabolism, as a smoke screen, as an anti-feedant, and as a warning signal. We tested two additional potential functions: that ink is a metabolic depressant and/or a noxious or adversive sensory stimulus. When exposed to realistic concentrations of ink from Aplysia dactylomela (Rang), none of five invertebrate species (including A. dactylomela) or two fish species significantly altered their oxygen uptake, and neither of two crab species significantly altered their heart and/or scaphognathite beat rates, suggesting that ink does not function as a metabolic depressant. In contrast, although A. dactylomela did not display strong behavioural responses to ink, behaviour of seven other invertebrates and both fish species was strongly affected by ink, supporting our hypothesis that the ink functions as an irritant. Observed behavioural changes included bristle erection by fireworms, increased mucus production by an opisthobranch, reduced feeding behaviour, increased grooming behaviour, and temporary pauses in heart and scaphognathite beating by crabs, reduced and increased activity by cryptic and exposed sea urchin species, respectively, and rapid swimming by fish. Similar behavioural changes by potential predators would Likely lead to reduced predation rates on Aplysia spp. in the field. Our conclusion that ink functions as a sensory irritant is not incompatible with other hypotheses for the function of ink. (C) 1999 Elsevier Science BN. All rights reserved.