Temperature is important for optimization of rearing conditions in aquaculture, especially during the critical early life history stages of fish. Here, we experimentally investigated the impact of temperature (16, 18, 20, 22 and 24°C) on thermally induced phenotypic variability, from larval hatch to first-feeding, and the linked expression of targeted genes [heat shock proteins (hsp), growth hormone (gh) and insulin-like growth factors (igf)] associated to larval performance of European eel, Anguilla anguilla. Temperature effects on larval morphology and gene expression were investigated throughout early larval development (in real time from 0 to 18 days post hatch) and at specific developmental stages (hatch, jaw/teeth formation, and first-feeding). Results showed that hatch success, yolk utilization efficiency, survival, deformities, yolk utilization, and growth rates were all significantly affected by temperature. In real time, increasing temperature from 16 to 22°C accelerated larval development, while larval gene expression patterns (hsp70, hsp90, gh and igf-1) were delayed at cold temperatures (16°C) or accelerated at warm temperatures (20–22°C). All targeted genes (hsp70, hsp90, gh, igf-1, igf-2a, igf-2b) were differentially expressed during larval development. Moreover, expression of gh was highest at 16°C during the jaw/teeth formation, and the first-feeding developmental stages, while expression of hsp90 was highest at 22°C, suggesting thermal stress. Furthermore, 24°C was shown to be deleterious (resulting in 100% mortality), while 16°C and 22°C (~50 and 90% deformities respectively) represent the lower and upper thermal tolerance limits. In conclusion, the high survival, lowest incidence of deformities at hatch, high yolk utilization efficiency, high gh and low hsp expression, suggest 18°C as the optimal temperature for offspring of European eel. Furthermore, our results suggest that the still enigmatic early life history stages of European eel may inhabit the deeper layer of the Sargasso Sea and indicate vulnerability of this critically endangered species to increasing ocean temperature.

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Factors such as gamete quality can profoundly affect fertility, but the spawning microenvironment that surrounds the spermatozoa and eggs during gamete contact has largely been neglected. In fishes, understanding these gametic interactions is crucial because each female creates a unique spawning environment by simultaneously expelling her distinct ovarian fluid (OF) along with an egg batch. In turn, OF has been shown to influence spermatozoa performance traits by modifying spermatozoa behaviors and fertilization outcomes. Here, we shed light on these gametic interactions by overviewing literature on OF and how it impacts spermatozoa performance traits. Fish OF is clear or has slight coloration and can constitute ≤10-30% of egg mass. Viscosity of the OF is ~2-to 3-fold higher than water and its pH ranges 6.2 to 8.8. Osmolality of the OF is lower in freshwater (190-322 mOsmol/kg) than marine species (289-514 mOsmol/kg). Na + (98.3-213.7 mmol/L) and Cl -(89.8-172.7 mmol/L) are predominant ions in OF, while K + (1.7-19.3 mmol/L), Mg 2+ (0.4-8.1 mmol/L), and Ca 2+ (0.5-9.7 mmol/L) ions are detected at lower concentrations. Protein levels can be high in OF and exhibit intra-and inter-species variation (54-826 mg/100mL). Fish OF also contains a series of organic components and substances that enhance and/or attract sperm towards the vicinity of an egg. OF can also differentially impact sperm based on genetic relatedness of mates, male phenotype (i.e. alternative reproductive tactics), or geographic origin. To conclude, when testing further reproductive paradigms, we suggest a shift from classic spermatozoa activation medium (water only) to more natural spawning media, which encompass OF-spermatozoa interactions.

Phenotypic plasticity occurs when a genotype produces variable phenotypes under different environments; the shapes of such responses are known as norms of reaction. The genetic scale at which reaction norms can be determined is restricted by the experimental unit that can be exposed to variable environments. This has limited their description beyond the family level in higher organisms, thus hindering our understanding of differences in plasticity at the scale of the individual. Using a three-year common-garden experiment, we quantify reaction norms in sperm performance of individual genotypes within different families of Atlantic cod ( Gadus morhua ). Cod sperm showed phenotypic plasticity in swimming performance across temperatures (3, 6, 11, and 21 °C), but the pattern of the response depended upon how long sperm had been swimming (30, 60, 120, or 180 s), i.e., plasticity in plasticity. Sperm generally swam fastest at intermediate temperatures when first assessed at 30 s after activation. However, a significant genotype × environment interaction was present, indicating inter-individual differences in phenotypic plasticity. To our knowledge, this is the first study to describe variable sperm performance across environmental conditions as a reaction norm. The results have potential theoretical, conservation, and aquaculture implications.