Background: Hybridization between closely related fish species can generate novel phenotypes that influence aquaculture performance. This study aimed to evaluate the morphological characteristics of hybrids between the two aquaculture-relevant flounder species Kareius bicoloratus and Platichthys stellatus using a hybrid index and a newly proposed resemblance p-value-based morphometric analysis, providing insights into hybrid resemblance patterns relative to their parental species. Methods: One-year-old individuals from the three genotype groups (K. bicoloratus, P. stellatus , and hybrid) were analyzed using a combination of traditional and trussbased morphometrics. From the full dataset, 77 morphological indices were extracted, including proportions, ratios, and angular measurements. The hybrid index was computed to quantify parental resemblance, while the delta resemblance value (ΔRV) was derived from Kruskal-Wallis test to assess statistical resemblance trends. One-way ANOVA and multiple comparison tests were used to determine statistical significance among groups. Results: Hybrid flounders exhibited a complex blend of parental and hybrid-specific traits, with morphological resemblance varying by trait category. Among the 77 morphological indices, 44 (57.1%) fell within the parental range, while 33 (42.9%) exceeded parental values, demonstrating transgressive segregation or heterosis in hybrid morphology. Morphometric resemblance patterns were trait-dependent: indices relative to total length or head length tended to resemble maternal species, whereas depth-related ratios and angular traits were more similar to father. Conclusions: The integration of H-index and ΔRV analysis provided a systematic and quantitative framework for assessing hybrid morphology, offering valuable insights into phenotypic expression of hybrids, with potential relevance to aquaculture.

The species-rich genus Aphis consists of more than 500 species, many of them host-specific on a wide range of plants, yet very similar in general appearance due to morphological resemblance toward particular morphological types. Most species have been historically clustered into four main phenotypic groups (gossypii, craccivora, fabae, and spiraecola groups). To confirm whether these morphological relationships are correlated with the genetic relationships, we compared the morphometric dataset of 28 characters measured/counted from 33 representative species with a phylogeny reconstructed from the combined dataset of four DNA regions (3,842 bp). The correlation of the rates of evolution between morphological and DNA datasets was highly significant in their diversification. Analysis of trait evolution revealed that the morphological traits found to be significant based on the ANOVA were confidently correlated with the phylogeny. The dominant patterns of trait evolution resulting in increased rates in short branches and temporally later evolution are likely suitable for the modality of Aphis speciation because they have adapted species-specifically, rapidly, and more recently for many different host plants.