Background/Objectives: Stearoyl-coenzyme A desaturase 1 (SCD1) plays a crucial role in fatty acid metabolism. However, its roles in the feeding habit transformation of mandarin fish (Siniperca chuatsi) remain largely unknown. Methods: Juvenile mandarin fish (10.37 ± 0.54)g were trained to feed on an artificial diet and then divided into artificial diet feeders and nonfeeders according to their feed preference. Afterwards, the scd1 gene of mandarin fish (Sc-scd1) was identified and characterized, and its transcription difference was determined between S. chuatsi fed live artificial diets and those fed prey fish. Results: Our results show that Sc-scd1 coding sequence is 1002 bp long, encoding 333 amino acids. The assumed Sc-SCD1 protein lacks a signal peptide, and it contains 1 N-linked glycosylation site, 24 phosphorylation sites, 4 transmembrane structures, and 3 conserved histidine elements. We found that Sc-SCD1 exhibits a high similarity with its counterparts in other fish by multiple alignments and phylogenetic analysis. The expression level of Sc-scd1 was detected with different expression levels in all tested tissues between male and female individuals fed either live prey fish or artificial diets. Conclusions: In particular, the Sc-scd1 expression level was the highest in the liver of both male and female mandarin fish fed artificial diets, indicating that scd1 genes may be associated with feed adaption of mandarin fish. Taken together, our findings offer novel perspectives on the potential roles of scd1 in specific domestication, and they provide valuable genetic information on feeding habits for the domestication of mandarin fish.

Microplastic (MP) pollution poses a threat to wild animals, but its toxicological impact on terrestrial passerines remains unclear. To address this gap, we conducted the first systematic study investigating how microplastic particle size and dosage jointly influence gut microbiota and multi-system physiological functions in a small terrestrial bird. Eurasian tree sparrows (Passer montanus) were exposed to polystyrene microplastics (PS-MPs) of two particle sizes (0.5 and 15 μm in diameter) and two dosages (100 and 500 μg/d) via oral ingestion for 21 days. After exposure, body status, peripheral blood cell profiles, organ indices, intestinal histomorphology, oxidative stress, and barrier integrity markers displayed no significant changes compared with the control group. In the gut microbiota, large PS-MP particles significantly enhanced microbial species richness and phylogenetic diversity, and their effect was more pronounced than that of small ones. Additionally, structural alterations and distinct community compositions emerged across groups. Both particle size and dosage affected gut microbial composition and taxa abundance, with particle size exhibiting a relatively stronger effect. However, the relative abundance of the top 10 dominant phyla and predicted microbial functional profiles exhibited no significant intergroup differences. In summary, short-term PS-MP exposure primarily impacts the gut microbial structure of Eurasian tree sparrows without disrupting their key physiological functions. This suggests that the birds possess a certain buffering capacity against short-term PS-MP stress, though their long-term ecological tolerance to complex, real-world MP mixtures remains to be further investigated.