Long-term effects of elevated pCO2 levels on the expression of Chaoborus-induced defences in Daphnia pulex

Abstract

Increased carbon dioxide from fossil fuel combustion results in an enrichment of CO₂ in the global carbon cycle. Recent evidence indicates that rising atmospheric CO₂ impacts the partial pressure of carbon dioxide (pCO₂) in freshwaters. This affects freshwater biota by disrupting chemical communication between predator and prey. One such well-described predator-prey interaction is the phantom midge larva Chaoborus preying on the freshwater crustacean Daphnia pulex. To counter Chaoborus predation, D. pulex develops defensive neckteeth in response to chemical cues. The strength of neckteeth expression is reduced when D. pulex experience elevated pCO₂ levels. This is discussed to directly impair predator perception and results in reduced defence expression. However, it is not known whether there are also long-term effects associated with continuous elevated pCO₂. Here, we investigated the effect of long-term exposure of D. pulex to elevated pCO₂ levels in a life-table experiment over three generations. Using a flow-through system, we continuously exposed D. pulex to cues released by the predatory larva Chaoborus and control or elevated pCO₂ levels. We determined morphological defence expression in the 2^nd juvenile instar and the number of neonates as a measure for life-history traits over three successive generations. We detected that elevated pCO₂ significantly reduces the expression of predator-induced morphological defences (i.e. neckteeth) and life-history parameters (i.e. number of neonates) in successive generations. Our data clearly show that at least three generations become more vulnerable to predation without indications of transgenerational acclimation. As Daphnia is a keystone grazer of freshwater ecosystems, this may destabilise population growth rates. In conclusion, long-term effects of pCO₂-induced reduction of predator-induced plasticity may significantly affect trophic interactions.

Keywords: inducible defences; long-term pCO(2) effects; transgenerational acclimation.