The Effects of Stocking Density and Food Deprivation on Mucous Cells and Lysozyme Activity in the Skin and Gills of Silver Catfish

SIMPLE SUMMARY: This study aimed to identify the regions of the skin of a South American catfish with a higher number of mucous cells. Additionally, the effects of stressful conditions that may occur in fish cultures, such as different stocking densities and food deprivation, on the number of mucous cells in the skin and gills and their impact on cutaneous mucous lysozyme activity were investigated. The sampled catfish were exposed to the conditions of high stocking density (32 kg/m(3)) and fed, high stocking density and fasted, low stocking density (2.5 kg/m(3)) and fed, and low stocking density and fasted. After 14 days, samples of cutaneous mucous and skin fragments from seven different points and the second left branchial arch were collected. The ventral point in front of the ventral fin was the portion with the highest number of mucous cells. The number of mucous cells in both the skin and gills varied based on the collection point and treatment applied. The highest lysozyme activity in the epidermal mucous was observed in fish exposed to high stocking density and fed. Stocking density and food deprivation create stressful conditions for this type of catfish, which modulate its mucosal response in the skin to each situation. ABSTRACT: This study aimed to identify the regions of the body surface of silver catfish (Rhamdia quelen) with a higher population of mucous cells in the skin. Additionally, the effects of stressful conditions, such as different stocking densities and food deprivation, on the proliferative response of mucous cells in the skin and gill epithelium and their impact on cutaneous mucous lysozyme activity were investigated. Silver catfish were divided into four experimental groups: high stocking density (32 kg/m(3)) and fed (HSD-F), high stocking density and fasted (HSD-FS), low stocking density (2.5 kg/m(3)) and fed (LSD-F), and low stocking density and fasted (LSD-FS). Fish in the fed groups received commercial feed twice a day, amounting to 1% of the tank biomass. After a 14-day experimental period, the fish were anesthetized and euthanized. Samples of cutaneous mucous and skin fragments from seven different points and the second left branchial arch were collected. Histological slides of the skin and gills were stained with PAS + Alcian Blue at pH 2.5, and the epidermal mucous lysozyme activity was assessed using the turbidimetric method. The ventral point in front of the ventral fin was found to be the optimal location for collecting cutaneous epithelia due to its higher density of mucous cells. The population of mucous cells in both the skin and gills varied based on the collection point and treatment applied. The highest lysozyme activity in the epidermal mucous was observed in fish from the HSD-F group. Overall, these findings suggest that stocking density and food deprivation create stressful conditions for silver catfish, which modulate their mucosal response to each situation.