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Epigenetics can be regarded as “the study of phenomena and mechanisms that cause chromosome-bound, heritable changes to gene expression that are not dependent on changes to DNA sequence. Epigenetics then deals with gene regulatory events that can be inherited mitotically through cell proliferation, and meiotically from parents to offspring, that may also underly the emergence of new phenotypes. DNA methylation, histone modifications, and noncoding RNAs are the three main epigenetic mechanisms implicated in the regulation of gene expression in animals and plants. The epigenetic modifications have been studied broadly as they play regulatory roles in gene expression or suppression, and, consequently, in the formation of the phenotype, in response to environmental stimuli such as temperature, stress, and nutrition, as well as due to their implications in cancer, neurodegenerative, cardiovascular, and other diseases. DNA methylation is considered to be the most studied epigenetic modification in mammals and fish. This epigenetic mark has been used in the development of biomarkers, as it is characterized by its stability and ease of measurement through a wide range of methods that are constantly developed. Thus, scientific research is focusing on the development of biomarkers based on epigenetic changes that could be useful tools for disease treatment, prediction of traits of economic importance in aquaculture, but also in the fields of ecology and evolution. Recent epigenetic changes observed in fish can pave the way for their incorporation in the development of biomarkers valuable for aquaculture, exploring the genotype to phenotype ‘gap’ and contribute to the sustainability of the sector

Early life stress (ELS) shapes the way individuals cope with future situations. Animals use cognitive flexibility to cope with their ever-changing environment and this is mainly processed in forebrain areas. We investigated the performance of juvenile gilthead seabream, previously subjected to an ELS regime. ELS fish showed overall higher brain catecholaminergic (CA) signalling and lower brain derived neurotrophic factor (bdnf) and higher cfos expression in region-specific areas. All fish showed a normal cortisol and serotonergic response to acute stress. Brain dopaminergic activity and the expression of the α2Α adrenergic receptor were overall higher in the fish homologue to the lateral septum (Vv), suggesting that the Vv is important in CA system regulation. Interestingly, ELS prevented post-acute stress downregulation of the α2Α receptor in the amygdala homologue (Dm3). There was a lack of post-stress response in the β2 adrenergic receptor expression and a downregulation in bdnf in the Dm3 of ELS fish, which together indicate an allostatic overload in their stress coping ability. ELS fish showed higher neuronal activity (cfos) post-acute stress in the hippocampus homologue (Dlv) and the Dm3. Our results show clear long-term effects on limbic systems of seabream that may compromise their future coping ability to environmental challenges.

The present study aimed to compare effects of increasing chronic stress load on the stress response of European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) to identify neuroendocrine functions that regulate this response. Fish were left undisturbed (controls) or exposed to three levels of chronic stress for 3 weeks and then subjected to an acute stress test (ACT). Chronic stress impeded growth and decreased feed consumption in seabass, not in seabream. In seabass basal cortisol levels are high and increase with stress load; the response to a subsequent ACT decreases with increasing (earlier) load. Basal cortisol levels in seabream increase with the stress load, whereas the ACT induced a similar response in all groups. In seabass and seabream plasma a-MSH levels and brain stem serotonergic activity and turnover were similar and not affected by chronic stress. Species-specific molecular neuro-regional differences were seen. In-situ hybridization analysis of the early immediate gene cfos in the preoptic area showed ACT-activation in seabream; in seabass the expression level was not affected by ACT and seems constitutively high. In seabream, expression levels of telencephalic cif, crfbp, grl , and mr were downregulated; the seabass hypothalamic preoptic area showed increased expression of cif and gr1, and decreased expression of mr, and this increased the grl/mr ratio considerably. We substantiate species-specific physiological differences to stress coping between seabream and seabass at an endocrine and neuroendocrine molecular level. Seabass appear less resilient to stress, which we conclude from high basal activities of stress-related parameters and poor, or absent, responses to ACT. This comparative study reveals important aquaculture, husbandry, and welfare implications for the rearing of these species.

Embryos (morula stage) of Atlantic cod, Gadus morhua L., were collected and subjected to air exposure for 2 min. followed by recovery at ambient conditions in the rearing container. Total immunoreactive cortisol and transcription of selected stress- and apoptosis-related genes of the embryos were determined before the application of the stressor and at 0.5, 1 and 24 h post-exposure. There was no significant difference in the total cortisol levels of the fertilized eggs before and after handling stress. There was high expression level of hsp70 and sod before application of the stressor and significantly increased at 0.5 h postexposure. The expression levels of cat and gpx were weak to moderate and were not affected by the stressor. The apoptotic genes, mcl1 and NR-13 were highly expressed and significantly increased after exposure to air. Bcl-X1 and Bcl-X2 were moderately expressed in the control samples, but only the expression level of Bcl-X1significantly increased following exposure to air. Cluster analysis of the different gene expression levels indicated three categories: those genes that did not show any change in the expression levels post-air exposure; those that had low expression level in the control followed by a significant increase after air exposure; and those that had high expression levels in the control followed by a further increase in expression after air exposure. These results clearly demonstrate that there are potential molecular biomarkers of the response in cod embryos as a consequence of air exposure at a time when cortisol is not fully active

Teleost fish are more diverse than any other vertebrate group, and yet only a limited number of species are fished and farmed globally. Efforts to expand the quantity and diversity of fish produced are hampered by the extreme diversity of ontogenetic responses of fish, especially during larval development. This review looks at advances in molecular phylogeny, endocrine and nutrient influences and long-term studies of the phenotypes of commercially important fish to put the sources and consequences of this plasticity into context. This nested context of evolutionary forces of the fish-specific genome duplication, epigenetic influences, ontogenetically conserved processes like metamorphosis and cell determination is further presented in relation to how fish larvae translate the environment into somatic signals, the teleostian diversity of internal processes like sex differentiation and somatogenesis, and the long-term practical consequences of changes in timing or anthropogenic influences. This review aims to present a new baseline of knowledge of marine fish larvae which is useful to scientists, managers and producers.