Divergent embryo responses to chemical cues in two freshwater fishes with different parental care strategies

Chemical information is one of the most important modes of communication among aquatic organisms. However, it remains unclear whether fish embryos are innately responsive to ecologically-related chemical cues from conspecifics, especially to the cues of their cohort competitors, potential caring or risky cues from parents, or chemical alarm cues from damaged embryos, and whether these response patterns differ between species that model alternative parental care strategies. Here, we examined embryo responses in heart rate and incubation performance to different conspecific chemical cues (companion embryo odours, maternal odours, maternal + companion odours, embryonic alarm cues, or water controls) in zebrafish ( Danio rerio ; no parental care) and Nile tilapia ( Oreochromis niloticus ; high parental care). Zebrafish embryos did not respond to companion embryo odours but did respond similarly to maternal odours and alarm cues consistent with detecting elevated risk levels. In contrast, Nile tilapia embryos only demonstrated significant risk responses to embryonic alarm cues. These findings indicate that embryos of both species can innately recognize alarm cues but differ in their response patterns to conspecific odours. Parental care strategies may influence intergenerational chemical communication in fish embryos, with maternal odours eliciting responses consisted and risk-averse when parental care is absent as in zebrafish but not when parental care is present as in Nile tilapia.