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Salmonid fish include some of the most valued cultured fish species worldwide. Unlike most other fish, the hearts of salmonids, including Atlantic salmon and rainbow trout, have a well-developed coronary circulation. Consequently, their hearts' reliance on oxygenation through coronary arteries leaves them prone to coronary lesions, believed to precipitate myocardial ischemia. Here, we mimicked such coronary lesions by subjecting groups of juvenile rainbow trout to coronary ligation, assessing histomorphological myocardial changes associated with ischemia and scarring in the context of cardiac arrhythmias using electrocardiography (ECG). Notable ECG changes resembling myocardial ischemia-like ECG in humans, such as atrioventricular blocks and abnormal ventricular depolarization (prolonged and fragmented QRS complex), as well as repolarization (long QT interval) patterns, were observed during the acute phase of myocardial ischemia. A remarkable 100% survival rate was observed among juvenile trout subjected to coronary ligation after 24 wk. Recovery from coronary ligation occurred through adaptive ventricular remodeling, coupled with a fast cardiac revascularization response. These findings carry significant implications for understanding the mechanisms governing cardiac health in salmonid fish, a family particularly susceptible to cardiac diseases. Furthermore, our results provide valuable insights into comparative studies on the evolution, pathophysiology, and ontogeny of vertebrate cardiac repair and restoration. NEW & NOTEWORTHY Juvenile rainbow trout exhibit a remarkable capacity to recover from cardiac injury caused by myocardial ischemia. Recovery from cardiac damage occurs through adaptive ventricular remodeling, coupled with a rapid cardiac revascularization response. These findings carry significant implications for understanding the mechanisms governing cardiac health within salmonid fishes, which are particularly susceptible to cardiac diseases. [ABSTRACT FROM AUTHOR]

Parasites can have profound effects on intra‐ and interspecific interactions at the population and community levels through their influence on host behaviour, physiology and fitness. While host phenotypic changes are typically thought of in terms of established infections, parasite encounters may be sufficient to induce behavioural changes, even when no viable infections are established.Here, we use the Japanese rice fish medaka Oryzias latipes and the brain‐infecting microsporidan parasite Pseudoloma neurophilia to understand how parasite resistance influences behaviour.Although a previous study suggested that medaka are a suitable host for P. neurophilia, an eight‐week parasite exposure regime resulted in no detectable infection in our study. Both parasite‐exposed and control (no parasite exposure) medaka were tested in behavioural assays that assessed boldness, activity and sociality. We detected considerable changes in medaka behaviour following parasite exposure, with parasite‐exposed fish being more active, less bold and more social when compared to control fish.These data indicate that parasite encounters may induce behavioural alterations even in non‐susceptible hosts. In addition to established infection, individual differences in parasite exposure must also be considered in studies of host responses across ecological scales. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]