The experimental design was unusually rigorous for a commercial-scale comparison. Juvenile seabream from a commercial hatchery in İzmir were divided into two groups based on geographic origin: one of Atlantic Ocean ancestry and one from the Aegean–Mediterranean region. Roughly 80,000 fish were placed in each of six net cages, three replicate cages per group, each cage twelve meters in diameter and depth. Initial mean body weights were nearly identical, at 4.24 grams for the Atlantic group and 4.28 grams for the Mediterranean group, ensuring that any later divergence would stem from biology rather than starting conditions. The trial ran for 457 days, spanning more than a full year of seasonal temperature swings in the bay.

Feeding followed a standard commercial protocol. The fish received a gilthead seabream diet produced by a commercial manufacturer, with pellet sizes ranging from one to six millimeters as the fish grew. Crude protein and lipid ratios shifted from 52:18 in the smallest pellets to 42:20 in the largest, matching the changing nutritional demands of growing juveniles. Fish were fed three times daily, with feeding rates adjusted according to fish size and water temperature. Water quality remained within optimal ranges throughout: mean temperature was 19.53 degrees Celsius, dissolved oxygen averaged 6.54 milligrams per liter, salinity 37.49 parts per thousand, and pH 8.15. These stable conditions minimized environmental confounding, a critical consideration when comparing genetically distinct stocks.

Monthly sampling was extensive. Four hundred fish per cage, 1,200 per group per event, were anesthetized with phenoxyethanol, weighed to the nearest 0.01 gram, and measured for total length. Fifty fish per cage were dissected monthly to determine hepatosomatic and viscerosomatic indices, which measure the relative sizes of the liver and visceral organs against total body weight. Liver samples were fixed, sectioned at five micrometers, and stained with Sudan Black to visualize lipid accumulation. The sampling design was grounded in a power analysis targeting the detection of a ten percent difference in mean body weight with eighty percent statistical power, a level of rigor rarely seen in commercial aquaculture trials.

By the end of the experiment, the Atlantic group had reached a mean final weight of 415.32 grams, compared with 374.03 grams for the Mediterranean group. The Atlantic fish also showed a higher specific growth rate, 0.89 versus 0.74 percent per day, and a better feed conversion ratio, 1.79 versus 1.92, meaning they converted feed into body mass more efficiently. Survival rates were statistically indistinguishable, at 88.0 percent for Atlantic and 85.5 percent for Mediterranean fish. Condition factor, hepatosomatic index, and viscerosomatic index showed no meaningful differences between groups, suggesting that overall body shape and organ proportions were similar even as growth rates diverged.

Here lies the study’s most important methodological lesson. The initial analyses, conducted with conventional analysis of variance, suggested statistically significant differences in final weight, specific growth rate, and feed conversion ratio, all with p-values below 0.05. However, when the researchers applied a False Discovery Rate correction using the Benjamini–Hochberg procedure, designed to account for the many comparisons made across multiple traits and time points, none of these differences remained statistically significant. Adjusted p-values were 0.151 for final weight, 0.294 for specific growth rate, and 0.112 for feed conversion ratio. The authors are transparent about this: the consistent direction of the trends may reflect genuine biological differences, but random variation cannot be ruled out, and the findings should be treated as indicative rather than definitive.

Histological examination of liver tissue added a subtle but intriguing dimension. By the end of the trial, the Mediterranean group showed more pronounced lipid accumulation in liver sections than the Atlantic group, though the difference was not statistically significant in the physiological indices. Hepatic steatosis, or fat accumulation in the liver, is a well-documented phenomenon in farmed gilthead seabream, typically linked to dietary lipid levels and energy balance. Because both groups received identical feed, the researchers suggest that the observed differences may reflect not only dietary effects but also interactions between genetic background and lipid metabolism, a possibility that could have important implications for feed formulation and fish health management in commercial operations.

The genetic analysis revealed a rich picture of population structure. Using five microsatellite loci and RAPD-PCR markers, the researchers found high within-population diversity in both groups. The number of alleles per locus ranged from six to sixteen in the Mediterranean population and eight to fourteen in the Atlantic population, with expected heterozygosity values consistently high across loci, ranging from 0.672 to 0.903 in the Mediterranean group and 0.710 to 0.880 in the Atlantic group. Pairwise genetic differentiation, measured by F_ST values, ranged from –0.010 to 0.045, indicating low-to-moderate but statistically relevant structuring between the two origins. Both populations were in Hardy–Weinberg equilibrium, suggesting genetic stability in these farmed stocks.

RAPD analysis reinforced the picture of distinct but not deeply divergent lineages. The mean within-population genetic similarity index was 0.583 for the Atlantic group and 0.662 for the Mediterranean group, while the between-population similarity was 0.499, confirming that individuals within each group were more genetically similar to each other than to members of the other group. The genetic distance between populations was 0.354, notably higher than the 0.245 reported in earlier Turkish broodstock comparisons, underscoring the influence of broader geographic separation and environmental adaptation on genetic structuring. Notably, the Atlantic group exhibited higher genetic variability overall, a trait often associated with greater adaptability, disease resistance, and growth efficiency.

The study’s conclusions are measured but forward-looking. Atlantic-origin seabream showed a consistent trend toward better growth performance and feed conversion under the tested conditions, and they carried higher genetic variability, making them potentially attractive candidates for broodstock selection. However, the lack of statistical significance after multiple-testing correction means these findings remain preliminary. The researchers call for multi-environmental studies, higher-resolution genomic tools such as SNP panels and whole-genome sequencing, and multi-generational comparisons to determine whether the observed trends reflect true genetic advantages relevant to aquaculture. For an industry increasingly focused on sustainability, feed efficiency, and selective breeding, this work provides a valuable foundation, even as it reminds us that in biology, as in statistics, the most interesting answers often lie just beyond the reach of a single experiment.

Subject of Research: Atlantic gilthead seabream show growth edge over Mediterranean cousins in 15-month cage trial

Article Title: Growth, feed efficiency, and genetic diversity in two farmed populations of gilthead seabream, Sparus aurata ( Actinopterygii , Eupercaria , Sparidae ), of Atlantic and Mediterranean origin

Article References: Sert, C. S., Korkut, A. Y., Gamsiz, K., Karahan, B., Özcan-Gökçek, E., Gurkan, S., Engin, S., Tolon, M. T., Karacalar, U., & Tokaç, A. (2026). Growth, feed efficiency, and genetic diversity in two farmed populations of gilthead seabream, Sparus aurata (Actinopterygii, Eupercaria, Sparidae), of Atlantic and Mediterranean origin. Acta Ichthyologica et Piscatoria, 56, 223-235. https://doi.org/10.3897/aiep.56.188432

Image Credits: AI Generated

DOI: 10.3897/aiep.56.188432

Keywords: aquaculture, gilthead seabream, genetics, growth performance, feed efficiency, Sparus aurata, selective breeding, population genetics, Growth, feed, efficiency, genetic

The research, led by Andreas Tsipourlianos and Katerina A. Moutou of the University of Thessaly in Greece, together with João C. R. Cardoso and Deborah M. Power of the University of the Algarve in Portugal, focused on two fish species of enormous commercial importance in Mediterranean aquaculture: the gilthead seabream (Sparus aurata) and the European seabass (Dicentrarchus labrax). These species are evolutionarily close relatives, yet they occupy ecologically distinct niches, making them an ideal comparative pair for dissecting how genome history and ecological lifestyle interact to shape the genetic architecture of core metabolism. Both species also possess well-annotated reference genomes, a prerequisite for the kind of rigorous comparative genomics the team undertook.

Teleost fishes carry a particularly rich evolutionary legacy in their DNA. Like all vertebrates, their ancestors experienced two rounds of whole-genome duplication deep in evolutionary time. Then, roughly 350 million years ago, the lineage that gave rise to modern teleosts underwent a third, teleost-specific genome duplication. These events doubled and redoubled the genetic raw material available to fish ancestors, creating thousands of duplicate gene pairs. Most duplicates are eventually lost, silenced by mutation, or repurposed for new functions. But some are retained, and understanding why certain duplicates persist while others vanish is one of the central questions in genome evolution. OXPHOS genes, with their tight dosage requirements and dual-genome coordination, represent an especially demanding test case for theories of duplicate retention.

Using comparative genomics across euteleost lineages, the researchers identified 23 multi-copy OXPHOS gene families in the gilthead seabream and 21 in the European seabass. This means that for a substantial number of the genes encoding the respiratory machinery, both fish carry more than one copy, or paralogue, in their nuclear genomes. Critically, the team was able to trace the origin of most of these duplicated families back to the teleost-specific genome duplication, demonstrating that this ancient genomic upheaval left a durable imprint on one of the most conserved metabolic pathways in biology. The finding challenges any assumption that core energy genes are immune to the effects of genome doubling.

Why would an organism keep two copies of a gene whose product must be produced in precise stoichiometric proportions to assemble a functional respiratory complex? The researchers tested a hypothesis that has gained traction in evolutionary biology: duplicate retention reflects a balance between dosage constraints, which favor keeping both copies active at reduced levels to maintain the correct overall output, and functional divergence, which allows one copy to specialize in a new context, tissue, or developmental stage. To examine this balance in action, the team turned to a life stage where energy demand is at its most extreme: early larval development.

Fish larvae are biological sprinters. Within days of hatching, they must grow rapidly, develop organs, begin swimming and feeding, and reorganize their metabolism from the yolk-dependent state of the embryo to the self-fueling physiology of a free-living organism. Mitochondrial energy production is central to every one of these transitions, and any disruption to OXPHOS function during this window can be lethal. This makes early development an ideal natural experiment for asking whether duplicated OXPHOS genes do the same work or different work.

The transcriptomics analysis revealed that paralogous OXPHOS genes do not behave uniformly. Some paralogues showed stable, coordinated expression patterns across development, consistent with the dosage-balance model: both copies contribute to maintaining the required output of the respiratory complexes. Others displayed a strikingly different behavior, being expressed only at specific developmental stages, suggesting that they have acquired stage-specific regulatory roles. This split personality among duplicates, with some copies serving as dosage partners and others as developmental specialists, provides direct evidence that functional diversification has shaped the OXPHOS repertoire of these fish since the teleost genome duplication.

The implications extend beyond evolutionary theory. Aquaculture is one of the fastest-growing food production sectors in the world, and gilthead seabream and European seabass are cornerstone species of Mediterranean fish farming. Larval survival is a persistent bottleneck in hatchery production, and energy metabolism is a key determinant of whether a larva successfully navigates the vulnerable early stages of life. By mapping which OXPHOS paralogues are deployed at which developmental moments, the study lays a molecular foundation for understanding, and potentially improving, larval performance under farming conditions. Genes that are switched on during critical developmental transitions could serve as markers of metabolic health or as targets for nutritional and environmental optimization.

The work also speaks to a broader question in biology: how do the mitochondrial and nuclear genomes, which are inherited in different ways and evolve at different rates, maintain their intricate partnership across hundreds of millions of years? Duplicated nuclear OXPHOS genes add another layer of complexity to this coevolutionary dance. If one nuclear copy diverges in function or expression, the mitochondrial components with which it interacts must remain compatible. The retention patterns documented in seabream and seabass suggest that this negotiation has produced a flexible but carefully balanced system, one in which redundancy provides resilience and specialization provides developmental precision.

Funding for the study came from the European Union through the H2020 PerformFISH project, which aims to integrate innovative approaches for competitive and sustainable performance across the Mediterranean aquaculture value chain, along with Portuguese national funds from the Foundation for Science and Technology. Larval samples were supplied by the Hellenic Centre for Marine Research in Crete, whose certified aquaculture facilities enabled the controlled developmental work underpinning the transcriptomic analysis. As the first comprehensive survey of OXPHOS paralogue evolution in euteleosts, the study opens a window onto how ancient genome doublings continue to echo through the metabolism of modern fish, and it suggests that the duplicated genes left behind by those events are not evolutionary leftovers but active, functionally relevant players in the energy economy of development. For a pathway as fundamental as oxidative phosphorylation, that flexibility may be exactly what allowed teleosts, the most diverse group of vertebrates, to radiate into nearly every aquatic habitat on the planet.

Subject of Research: Evolution and retention of duplicated oxidative phosphorylation genes in euteleost fishes

Article Title: Evolution and retention of oxidative phosphorylation paralogues in euteleosts: insights from gilthead seabream and European seabass

Article References: Tsipourlianos, A., Cardoso, J. C. R., Angelakopoulos, R., Kotoula, A., Power, D. M., Mamuris, Z., & Moutou, K. A. (2026). Evolution and retention of oxidative phosphorylation paralogues in euteleosts: insights from gilthead seabream and European seabass. BMC Genomics. https://doi.org/10.1186/s12864-026-13338-x

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13338-x

Keywords: oxidative phosphorylation, OXPHOS, gene duplication, teleost genome duplication, gilthead seabream, European seabass, mitochondria, larval development, comparative genomics, transcriptomics, gene retention, aquaculture