Abstract
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.
Highlights
Compared to the open ocean, coastal areas, and intertidal zones are highly variable environments characterised by abrupt changes in water parameters
Avoidance responses of D. pugilator did not depend on pH drop (Z = 0.25, P = 0.8049) nor on stress metabolites (Z = −0.25, P = 0.7995, overall Chi-square test model fit: P < 0.0001, Table 2), but were significantly more pronounced when metabolites originated from S. aurata (69%) instead of conspecifics (26%, Z = 2.46, P = 0.0139, Figure 3 and Table 2)
We could show that short-term pH drop of a similar magnitude of that experienced within the intertidal zone, and aligned to end-of-century predicted average values (Chavez et al, 2017; Landschützer et al, 2018), had negative consequences on fitness-relevant behaviours in harbour ragworm H. diversicolor and small hermit crab D. pugilator
Summary
Compared to the open ocean, coastal areas, and intertidal zones are highly variable environments characterised by abrupt changes in water parameters. This includes fluctuations in pH beyond 0.3 units, the levels predicted for average change related to the process of ocean acidification toward the end of the century (Caldeira and Wickett, 2003; Sabine et al, 2004; IPCC, 2019). It should be mentioned that there is little literature disentangling the behavioural effects of pH drop vs. high CO2, an increasing body of recent research evidenced the role of pH-dependent altered chemical communication (Roggatz et al, 2016; Schirrmacher et al, 2020; Porteus et al, 2021; Velez et al, 2021)
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