Innate responses to con- and heterospecific alarm chemicals in juveniles of a freshwater shrimp

Abstract

Released substances from injured conspecifics and heterospecifics (alarm chemicals) often trigger inducible defenses (e.g., behavioral, morphological, and life-history antipredator defenses). Although there are good reasons to assume that the detection of damage-release chemicals may be inherited, immediate behaviors to alarm chemicals remain to be assessed in many aquatic taxa. This study examines changes in locomotor activity in juveniles of a freshwater shrimp, Palaemon argentinus, in response to (1) distilled water; (2) macerated adult conspecifics, and (3) macerated adult heterospecifics (a closely related, non-coexisting species). We tested three sets of juveniles coming from (1) alarm cues-unexposed mothers (naïve juveniles), (2) mothers exposed to macerated conspecifics, and (3) mothers exposed to macerated heterospecifics. Juveniles were tested for changes in locomotor activity before and after the addition of a corresponding stimulus. Locomotor activity of naïve juveniles decreased in response to con- and heterospecific stimuli, while it did not change in response to distilled-water controls. Effect size, as Cohen's dRM (the standard mean difference between the pre- and the post-stimulus period using the pooled standard deviation) indicated that the reduction in locomotor activity was strong in response to conspecific stimulus (dRM = −1.521), but it was moderate in response to heterospecific stimulus (dRM = −0.682). While the responses of naïve juveniles indicate an innate recognition of alarm substances, the responses to heterospecifics likely arise from the recognition of chemicals with a molecular structure partly conserved between related species, as predicted by the phylogenetic relatedness hypothesis. Furthermore, juveniles from treated mothers behaved differently than the naïve ones, suggesting that juvenile behavioral traits were modified through embryonic experience. As the embryonic exposure to alarm cues occurs during embryogenesis, we assume that the mothers have perceived these cues also, which might have caused part of the observed developmental effects.