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.

Behavioral response to a diel photoperiodicity (500 lx for 16 h, 5 lx for 4 h and < 0.5 lx for 4 h) and phototactic characteristics in dark conditions were examined with Siberian sturgeon Acipenser baerii (Chondrostei, Actinopterygii) prelarvae. Siberian sturgeon prelarvae represented both qualitative and quantitative changes in their behavioral patterns according to different light intensities in a diel photoperiodicity. Under daylight conditions (500 lx), prelarvae displayed saltatory changes of behavioral features with ages (Day 0-Day 9) in a general order of swimming-up/drifting, swimming in the upper water column, benthic swimming with rheotaxis, schooling and postschooling behavior. Compared to daylight conditions, prelarvae tended to show more benthic performances and quantitative reductions of schooling and postschooling behaviors under dimlight conditions (5 lx). Under dark conditions (< 0.5 lx), prelarvae exhibited a fairly uniform behavioral pattern characterized by the benthic swimming across the bottom of the tank. From phototaxis tests under dark conditions, navigational responses of prelarvae to a spotlight illumination were quantitatively changed as their ages increased. The phototactic responses reached the peak on Day 2, continued until Day 4, and then gradually decreased until Day 8. A partial recovery of positive phototaxis was observed on Day 9. Data from this study suggest that the diel light cycle as well as the light intensity of each interval in the cycle should be considered as important components of a practical guide for evaluating fitness and developmental states of artificially propagated Siberian sturgeon prelarvae.