No abstract

Heat alters biology from molecular to ecological levels, but may also have unknown indirect effects. This includes the novel concept that animals exposed to abiotic stress can induce stress in naive receivers. Here, we provide a comprehensive picture of the molecular signatures of this process, by integrating multi-omic and phenotypic data. In individual zebrafish embryos, repeated heat peaks elicited both a molecular response and a burst of accelerated growth followed by a growth slow-down in concert with reduced responses to novel stimuli. Metabolomes of the media of heat treated vs. untreated embryos revealed candidate stress metabolites including sulphur-containing compounds and lipids. These stress metabolites elicited transcriptomic changes in naive receivers related to immune response, extracellular signalling, glycosaminoglycan/keratan sulphate, and lipid metabolism. Consequently, non heat-exposed receivers (exposed to stress metabolites only) experienced accelerated catch-up growth in concert with reduced swimming performance. The combination of heat and stress metabolites accelerated development the most, mediated by apelin signalling. Our results prove the concept of indirect heat-induced stress propagation towards naive receivers, inducing phenotypes comparable to those resulting from direct heat exposure, but utilising distinct molecular pathways. Group-exposing a non-laboratory zebrafish line, we independently confirm that the glycosaminoglycan biosynthesis-related gene chs1, and the mucus glycoprotein gene prg4a, functionally connected to the candidate stress metabolite classes sugars and phosphocholine, are differentially expressed in receivers. This hints at production of Schreckstoff-like cues in receivers, leading to further stress propagation within groups, which may have ecological and animal welfare implications for aquatic populations in a changing climate.Significance StatementAquatic animals utilise chemicals to mediate adaptive behaviours. For instance, predated fish release chemical cues that elicit antipredatory responses in naive receivers. But whether abiotic factors such as heat likewise alter chemical communication has received little focus. Here, we uncover a novel dimension of chemical communication — heat-stressed donors can induce stress in naive receivers. We show that heat activates molecular stress responses, leading to the release of distinct stress metabolite classes into the environment. These stress metabolites alter the transcriptome of receivers, resulting in faster development and hypoactivity. Heat combined with stress metabolites had the largest effect, highlighting that abiotic stress, experienced both directly and indirectly, can alter chemical communication and affect embryonic development.Graphical AbstractHighlightsWe elucidate the mechanism for a novel dimension of the heat stress response — chemical communication from heat-stressed donors that induces stress in naive receivers — constituting a positive feedback loopRepeated heat stress induces a cellular and cortisol stress response and alters the phenotype of zebrafish embryosHeat-stressed embryos release stress metabolites enriched in lipids and sulphur-containing organo-oxygen compoundsIn combination, heat and stress metabolites induced 47% distinct differentially expressed genes, with many related to organ developmentThese stress metabolites alter the transcriptome and induce both faster development and hypoactivity in naive receivers, a similar response to that of heat stress itself

Studies on pH stress in marine animals typically focus on direct or species-specific aspects. We here test the hypothesis that a drop to pH = 7.6 indirectly affects the intra- and interspecific interactions of benthic invertebrates by means of chemical communication. We recorded fitness-relevant behaviours of small hermit crabs Diogenes pugilator, green shore crabs Carcinus maenas, and harbour ragworms Hediste diversicolor in response to short-term pH drop, and to putative stress metabolites released by conspecifics or gilt-head sea bream Sparus aurata during 30 min of acute pH drop. Not only did acute pH drop itself impair time to find a food cue in small hermit crabs and burrowing in harbour ragworms, but similar effects were observed under exposure to pH drop-induced stress metabolites. Stress metabolites from S. aurata, but not its regular control metabolites, also induced avoidance responses in all recipient species. Here, we confirm that a short-term abrupt pH drop, an abiotic stressor, has the capacity to trigger the release of metabolites which induce behavioural responses in conspecific and heterospecific individuals, which can be interpreted as a behavioural cost. Our findings that stress responses can be indirectly propagated through means of chemical communication warrant further research to confirm the effect size of the behavioural impairments caused by stress metabolites and to characterise their chemical nature.

Studies on pH stress in marine animals typically focus on direct or species-specific aspects. We here test the hypothesis that a drop to pH = 7.6 indirectly affects the intra- and interspecific interactions of benthic invertebrates by means of chemical communication. We recorded fitness-relevant behaviours of small hermit crabs Diogenes pugilator, green shore crabs Carcinus maenas, and harbour ragworms Hediste diversicolor in response to short-term pH drop, and to putative stress metabolites released by conspecifics or gilt-head sea bream Sparus aurata during 30 minutes of acute pH drop. Not only did acute pH drop itself impair time to find a food cue in small hermit crabs and burrowing in harbour ragworms, but similar effects were observed under exposure to pH drop-induced stress metabolites. Stress metabolites from S. aurata, but not its regular control metabolites, also induced avoidance responses in all recipient species. Here, we confirm that a short-term abrupt pH drop, an abiotic stressor, has the capacity to trigger the release of metabolites which induce behavioural responses in conspecific and heterospecific individuals, which can be interpreted as a behavioural cost. Our findings that stress responses can be indirectly propagated through means of chemical communication warrant further research to confirm the effect size of the behavioural impairments caused by stress metabolites and to characterise their chemical nature.

and is available as an interactive resource at: www.cassandra-genes.org. A network cytoscape session with the vertebrate network as well as the ClueGo results is available at doi:10.6084/m9.figshare.15010872. Code is provided in the Supplementary Materials as Supplementary Appendix 1.