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In teleosts, the mucosal epithelial barriers represent the first line of defence against environmental challenges such as pathogens and environmental contaminants. Mucous cells (MCs) are specialised cells providing this protection through mucus production. Therefore, a better understanding of various MC quantification methods is critical to interpret MC responses. Here, we compare histological (also called traditional) quantification of MCs with a novel mucosal mapping method to understand the differences between the two methods' assessment of MC responses to parasitic infections and pollution exposure in shorthorn sculpins (Myoxocephalus scorpius). Overall, both methods distinguished between the fish from stations with different levels of pollutants and detected the links between MC responses and parasitic infection. Traditional quantification showed relationship between MC size and body size of the fish whereas mucosal mapping detected a link between MC responses and Pb level in liver. While traditional method gave numerical density, mucosal mapping gave volumetric density of the mucous cells in the mucosa. Both methods differentiated MC population in skin from those in the gills, but only mucosal mapping pointed out the consistent differences between filament and lamellar MC populations within the gills. Given the importance of mucosal barriers in fish, a better understanding of various MC quantification methods and the linkages between MC responses, somatic health and environmental stressors is highly valuable.

The influence of an acute drop in water temperature on channel catfish ( Ictalurus punctatus ) mucous cell distribution in the epidermis was examined. Channel catfish were subjected to a water temperature drop from 22 to 10° C in 24 h and sampled for histological examination of the epidermis at regular intervals. Three days after the drop in water temperature, there was a marked decrease in the number of mucous cells in the outermost (superficial) portion of the epidermis. The number of mucous cells returned to normal levels after fish were maintained at 10° C for 6 days. If catfish were subjected to the temperature drop and also challenged with Saprolegnia , mucous cells in the superficial epidermis did not recover to normal levels but continued to decline from day 3 until the fish subsequently died from saprolegniasis. These results may explain the dry skin appearance characteristic of catfish exhibiting winter saprolegniasis. Furthermore, since Saprolegnia cysts are normally shed in sloughed mucus, low temperature-induced mucus loss may explain how Saprolegnia cysts attach and infect catfish skin to result in disease. These results also reinforce the notion that an intact external mucus layer is the first line of defence against infectious disease in fish.