Lobster attack induces sensitization in the sea hare, Aplysia californica

Abstract

Studies of the neural mechanisms of learning, especially of sensitization, have benefitted from extensive research on the model species, Aplysia californica (hereafter Aplysia). Considering this volume of literature on mechanisms, it is surprising that our understanding of the ecological context of sensitization in Aplysia is completely lacking. Indeed, the widespread use of strong electric shock to induce sensitization (an enhancement of withdrawal reflexes following noxious stimulation) is completely unnatural and leaves unanswered the question of whether this simple form of learning has any ecological relevance. We hypothesized that sublethal attack by a co-occurring predator, the spiny lobster, Panulirus interruptus, might be a natural sensitizing stimulus. We tested reflex withdrawal of the tail-mantle and head of individual Aplysia before and after attack by lobsters. Lobster attack significantly increased the amplitude of both reflexes, with a temporal onset that closely matched that observed with electric shock. This result suggests that electric shock may indeed mimic at least one naturally occurring sensitizing stimulus, suggesting, for the first time, an ecological context for this well studied form of learning.

Figure 1.

A single lobster attack causes sensitization in head withdrawal reflex of Aplysia. Shown is the duration of head withdrawal (sec) before (−10 and −5 min) and after (5, 30 and 60 min) attack. Asterisks refer to repeated-measures t tests (*p < 0.05, **p < 0.01, ***p < 0.005, ****p < 0.001). The probability values that span posttest experimental and control histograms indicate two-sample t tests of withdrawal difference scores (posttest − pretest). NS, Not significant.

Figure 2.

Lobster attack causes sensitization in tail-mantle withdrawal reflex. Shown is the duration of tail-mantle withdrawal (sec) before (−10 and −5 min) and after (5, 30, and 60 min) attack. Asterisks refer to repeated-measures t tests (*p < 0.05, **p < 0.01, ***p < 0.005, ****p < 0.001). The probability values that span posttest experimental and control histograms indicate two-sample t tests of withdrawal difference scores (posttest − pretest). NS, Not significant.