Abstract
A major movement to improve risk assessments includes the development and use of adverse outcome pathways (AOPs). AOPs are derived from detailed toxicological cause and effect relationships that span multiple levels of biological organization, ideally from molecular initiating events to ecosystem scale responses. For more than a decade, zebrafish have been the primary experimental platform for mechanistic work on crude oil developmental toxicity in fish. While the zebrafish model has provided the initial cardiotoxicity AOP framework, it is a tropical freshwater species, while the world’s most valuable fisheries generally spawn in marine and estuarine habitats. Therefore, cross-species extrapolations remain a source of uncertainty for ecological risk assessments. Atlantic haddock (Melanogrammus aeglefinus) provides a number of pertinent features that make it an ideal subject for fully developing crude oil AOPs for marine fish with pelagic early life history stages, including large translucent embryos that provide sufficient mass for chemical analyses, a developmental rate that allows greater resolution between time points, and a sequenced genome. Atlantic haddock thus makes an excellent bridge between zebrafish and other non-model fish with less readily available embryos. The pelagic embryos and larvae of haddock were exposed to Norwegian Sea crude oil using a sophisticated continuous flow system providing controlled delivery of mechanically dispersed oil microdroplets and corresponding water accommodated fraction. We characterized visible cardiac function and morphological phenotypes and obtained corresponding RNASeq data from 6 developmental stages from embryonic exposures, and 5 stages during larval exposure. Cardiac function defects were dose-dependent and included bradycardia and ventricular asystole in embryos and atrioventricular conduction block in larvae. Embryos displayed dose-dependent failure of cardiac looping and reduced ventricular outgrowth. Gene expression data identified changes in key genes encoding specific components regulating cardiac action potentials and excitation-contraction coupling, and identified a clear cause-effect chain linking disruption of intracellular calcium handling to signaling molecules and transcription factors regulating cardiac morphogenesis. These findings thus provide major advances in delineating key molecular events in the developmental cardiotoxicity AOP from crude oil exposure.