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

Most sea hares (Opisthobranchia: Anaspidea) release a purple ink when physi cally disturbed. The ink has been hypothesized to function to excrete unwan ted byproducts of metabolism, as a smoke screen, as an anti-feedant, and as a warning signal. We tested two additional potential functions: that ink i s a metabolic depressant and/or a noxious or adversive sensory stimulus. Wh en exposed to realistic concentrations of ink from Aplysia dactylomela (Ran g), none of five invertebrate species (including A. dactylomela) or two fis h species significantly altered their oxygen uptake, and neither of two cra b species significantly altered their heart and/or scaphognathite beat rate s, suggesting that ink does not function as a metabolic depressant. In cont rast, although A. dactylomela did not display strong behavioural responses to ink, behaviour of seven other invertebrates and both fish species was st rongly affected by ink, supporting our hypothesis that the ink functions as an irritant. Observed behavioural changes included bristle erection by fir eworms, increased mucus production by an opisthobranch, reduced feeding beh aviour, increased grooming behaviour, and temporary pauses in heart and sca phognathite beating by crabs, reduced and increased activity by cryptic and exposed sea urchin species, respectively, and rapid swimming by fish. Simi lar behavioural changes by potential predators would Likely lead to reduced predation rates on Aplysia spp. in the field. Our conclusion that ink func tions as a sensory irritant is not incompatible with other hypotheses for t he function of ink. (C) 1999 Elsevier Science BN. All rights reserved.