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The hearts of salmonids display remarkable plasticity, adapting to various environmental factors that influence cardiac function and demand. For instance, in response to cold temperature, the salmonid heart undergoes growth and remodeling to counterbalance the reduced contractile function associated with dropping temperatures. Alongside heart size, the distinct pyramidal shape of the wild salmonid heart is essential for optimal cardiac performance, yet the environmental drivers behind this optimal cardiac morphology remain to be fully understood. Intriguingly, farmed salmonids often have rounded, asymmetrical ventricles and misaligned bulbi from an early age. These deformities are noteworthy given that farmed salmon are often not exposed to natural cues, such as a gradual temperature increase and changing day lengths, during critical developmental stages. In this study, we investigated whether natural environmental conditions during early life stages are pivotal for proper cardiac morphology. Atlantic salmon were raised under simulated natural conditions (low temperature with a natural photoperiod; SimNat) and compared with those reared under simulated farming conditions (SimFarm). Our findings reveal that the ventricle shape and bulbus alignment in SimNat fish closely resemble those of wild salmon, while functional analyses indicate significant differences between SimNat and SimFarm hearts, suggesting diastolic dysfunction and higher cardiac workload in SimFarm hearts. These findings highlight the profound influence of environmental factors such as water temperature and photoperiod on the structural development of the salmonid heart, underscoring the importance of early environmental conditions for cardiac health.

The three–spined stickleback Gasterosteus aculeatusis an intermediate host of the tapeworm Schistocephalus solidus. Changes in predator avoidance, foraging and shoaling behaviour have been reported in sticklebacks infested with S. solidus, but the mechanisms underlying parasite–induced behavioural changes are not understood. Monoamine neurotransmitters are involved in the control of behaviour and central monoaminergic systems are sensitive to various stressors. Thus, the behavioural effects of S. solidusinfestation might be a reflection of changes in brain monoaminergic activity in the stickleback host. The concentrations of 5–hydroxytryptamine (5–HT), dopamine (DA), norepinephrine (NE) and their metabolites 5–hydroxy–indoleacetic acid (5–HIAA), homovanilic acid (HVA) and 3–methoxy–4–hydroxyphenylglycol (MHPG) were measured in the telencephalons, hypothalami and brainstems of parasitized and non–parasitized female sticklebacks held in the laboratory. The ratios of 5–HIAA:5–HT were significantly elevated in both the hypothalami and brainstems of infected sticklebacks. The concentrations of 5–HTand NE were significantly reduced in the telencephalons of infected fish as compared with controls, but there was no elevation of metabolite concentrations. The results are consistent with chronic stress in infected fish, but may also reflect other alterations of neuroendocrine status resulting from parasite infection.