Dietary American Cockroach (Periplaneta americana) Chitin Influences Growth Performance, Nutrient Utilization, Gut Microbiota, and Riboflavin-Related Metabolism in Juvenile Grass Carp (Ctenopharyngodon idella).

The American cockroach (Periplaneta americana) is an emerging sustainable source of insect-based chitin, yet its physiological effects and optimal inclusion levels in herbivorous fish diets remain undefined. This study evaluated the dose-dependent effects of dietary P. americana chitin (0%, 2.5%, 5%, 10%, and 20%) on juvenile grass carp (Ctenopharyngodon idella) over a 63-day trial. Results revealed a distinct biphasic response in growth and nutrient utilization. Fish fed the 5% chitin diet exhibited the highest weight gain and specific growth rate (SGR), coinciding with the highest apparent digestibility coefficient (ADC) of crude protein (74.34%) compared with the control (72.01%). Conversely, higher inclusion levels (10% and 20%) significantly suppressed growth and reduced protein digestibility. Metagenomic reanalysis showed that the riboflavin-biosynthesis gene ribA (K01497) was significantly reduced at 2.5%-10% dietary chitin (F (4,10) = 7.78, FDR p = 0.041), while the overall riboflavin metabolism pathway, the M00125 biosynthesis module, and intestinal riboflavin levels did not differ significantly among diets. Among the inclusion levels tested, 5% produced the most favorable overall response under the present experimental conditions, balancing improved growth and protein digestibility with the negative responses observed at higher inclusion levels.