Romina B. Ituarte, Maria G. Vazquez, and Claudia C. Bas

Most aquatic animals use infochemicals from both conspecifics and heterospecifics to assess local predation risks and enhance predator detection. Released substances from injured conspecifics and other species (chemical alarm cues) are reliable cues to indicate an imminent danger in a specific habitat and, often mediate the development of inducible defences. Amphibian and fish embryos have shown to acquire this information while at the embryonic stage of development, in relation to the developing nervous system and sensory development. Except for Daphnia, so far there is no information on chemically-mediated responses to alarm cues in embryos of other crustacean groups. We tested whether embryo exposure to chemical cues simulating predation on conspecifics, or heterospecifics (a closely related, non-coexisting species) or a mixture of both cues alters embryonic developmental time, size and morphology of the first larval instar in Palaemon argentinus (Crustacea: Decapoda). Embryonic exposure to chemical alarm cues from conspecifics shortened the embryonic developmental time and elicited larger larvae with longer rostrum. Rostrum length of the first larval instar changed independently of their size, thus elongated rostra can be considered a defensive feature. Embryonic developmental time was not altered by chemical alarm cues from either heterospecifics or the mixed cues treatments; however, embryonic exposure to those cues caused larger larvae compared to distilled water control. Chemically-induced morphological plasticity in larvae in response to con- and heterospecifics suggests that alarm cues are conserved in palaemonids shrimps, providing embryos with an innate recognition of heterospecific alarm cues as predicted by the phylogenetic relatedness hypothesis.

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