Cold seawater induces early sexual developmental stages in the BPG axis of European eel males.

BACKGROUND: The impossibility of closing the life cycle of the European eel (Anguilla anguilla) in captivity troubles the future of this critically endangered species. In addition, the European eel is a highly valued and demanded resource, thus the successful closing of its life cycle would have a substantial economic and ecological impact. With the aim of obtaining the highest gamete quality, the study of the effects of environmental factors, such as temperature, on reproductive performance may prove valuable. This is especially true for the exposure to cold water, which has been reported to improve sexual development in multiple other Actinopterygii species. RESULTS: European eel males treated with cold seawater (10 °C, T10) for 2 weeks showed an increase in the proliferation and differentiation of spermatogonial cells until the differentiated spermatogonial type A cell stage, and elevated testosterone and 11-ketotestosterone plasma levels. Transcriptomes from the tissues of the brain-pituitary-gonad (BPG) axis of T10 samples revealed a differential gene expression profile compared to the other experimental groups, with clustering in a principal component analysis and in heat maps of all differentially expressed genes. Furthermore, a functional analysis of differentially expressed genes revealed enriched gene ontology terms involved in the regulation of circadian rhythm, histone modification, meiotic nuclear division, and others. CONCLUSIONS: Cold seawater treatment had a clear effect on the activity of the BPG-axis of European eel males. In particular, our cold seawater treatment induces the synchronization and increased proliferation and differentiation of specific spermatogonial cells. In the transcriptomic results, genes related to thermoception were observed. This thermoception may have caused the observed effects through epigenetic mechanisms, since all analysed tissues further revealed differentially expressed genes involved in histone modification. The presented results support our hypothesis that a low temperature seawater treatment induces an early sexual developmental stage in European eels. This hypothesis is logical given that the average temperature experienced by eels in the early stages of their oceanic reproductive migration is highly similar to that of this cold seawater treatment. Further studies are needed to test whether a cold seawater treatment can improve the response of European eels to artificial hormonal treatment, as the results suggest.

Impacts of Early Life Stress on the Methylome and Transcriptome of Atlantic Salmon.

Exposure to environmental stressors during early-life stages can change the rate and timing of various developmental processes. Epigenetic marks affecting transcriptional regulation can be altered by such environmental stimuli. To assess how stress might affect the methylome and transcriptome in salmon, fish were treated using cold-shock and air-exposure from the eye-stage until start-feeding. The fish were either stressed prior to hatching (E), post-hatching (PH), pre- and post-hatching (EPH) or not stressed (CO). Assessing transcriptional abundances just prior to start feeding, E and PH individuals were found to have modified the expression of thousands of genes, many with important functions in developmental processes. The EPH individuals however, showed expression similar to those of CO, suggesting an adaptive response to extended periods of stress. The methylome of stressed individuals differed from that of the CO, suggesting the importance of environment in shaping methylation signatures. Through integration of methylation with transcription, we identified bases with potential regulatory functions, some 10s of kb away from the targeted genes. We then followed fish growth for an additional year. Individuals in EPH showed superior growth compared to other treatment groups, highlighting how stress can potentially have long-lasting effects on an organism's ability to adapt to environmental perturbations.

Evoked potentials in the Atlantic cod following putatively innocuous and putatively noxious electrical stimulation: a minimally invasive approach.

Aspects of peripheral and central nociception have previously been studied through recording of somatosensory evoked potentials (SEPs) to putative noxious stimuli in specific brain regions in a few freshwater fish species. In the present study, we describe a novel, minimally invasive method for recording SEPs from the central nervous system of the Atlantic cod (Gadus morhua). Cutaneous electric stimulation of the tail in 15 fish elicited SEPs at all stimulus intensities (2, 5, 10 and 20 mA) with quantitative properties corresponding to stimulus intensity. In contrast to previous fish studies, the methodological approach used in Atlantic cod in the current study uncovered a number of additional responses that could originate from multiple brain regions. Several of these responses were specific to stimulation at the highest stimulus intensities, possibly representing qualitative differences in central processing between somatosensory and nociceptive stimuli.

Growth hormone and cortisol treatment stimulate seawater tolerance in both anadromous and landlocked Arctic charr.

In a comparative experiment the effect of cortisol and growth hormone (GH) on the hypo-osmoregulatory ability of a landlocked and an anadromous strain of Arctic charr (Salvelinus alpinus) was investigated. Cortisol and GH were implanted either alone or in combination, and the fish were exposed to a 24 h seawater challenge test (SWT) on days 14 and 28 after implantation. Hypo-osmoregulatory ability, measured as plasma osmolality and chloride concentration after the SWTs, was better in the anadromous than in the landlocked strain, irrespective of treatment. However, cortisol provided a strong stimulation of hypo-osmoregualtory ability in both strains, and this stimulation seemed to be potentiated by GH in an additive manner. Improved hypo-osmoregulatory ability in GH+cortisol treated anadromous Arctic charr was accompanied by increased gill Na(+), K(+)-ATPase activity and Na(+)-K(+)-2Cl(-) cotransporter protein abundance, but no changes in gill Na(+),K(+)-ATPase alpha1a and alpha1b mRNA levels. For landlocked charr the improved hypo-osmoregulatory ability in GH+cortisol treated fish was accompanied only with an increase in gill Na(+)-K(+)-2Cl(-) cotransporter protein abundance. Hormone treatment caused an improvement of hypo-osmoregulatory ability that was of approximately the same magnitude in the landlocked as in the anadromous Arctic charr. This suggests that the lack of spontaneous development of hypo-osmoregulatory ability often seen in landlocked populations of Arctic charr may depend, at least partly, on a lack of the hormonal activation seen in anadromous populations.