Seasonal changes in the liver of a non-hibernating population of water frogs, Pelophylax kl. esculentus (Anura: Ranidae)

Seasonal variation of liver glycogen, lipids and melanomacrophages were investigated in a non-hibernating population of Pelophylax kl. esculentus from Calabria by histochemical methods and computer-assisted image analysis. Twenty individuals of both sexes were sampled in a tank in Roseto Capo Spulico (Cosenza, Calabria) in four periods of the year 2016 (February, May, July, October). Portions of liver from each individual were included in paraffin for glycogen and melanomacrophages, and epoxydic resin-araldite for lipid analysis. Sections were stained with periodic acid-Schiff (PAS) for glycogen (with diastase-PAS as control) or osmium-tetroxide for lipids, or left unstained for melanomacrophages (appearing naturally black due to melanin). Image analyses were performed on 9–12 grayscale converted pictures per individual. Total areas per μm2 of glycogen, lipids and melanomacrophages, as well as counts of lipid droplets and melanomacrophages and mean area of single lipid droplets and melanomacrophages, were measured. Statistical analyses were performed by analysis of variance (ANOVA) with bootstrap resampling. Significant variation among sampling periods was found for each variable. Glycogen and lipids co-vary, with higher values observed in October–February and lower values in May–July, whereas melanomacrophages reach a peak in May and have much lower values in the other months. It is concluded that, in the absence of a hibernating period, reproduction is the main force regulating the annual cycles of reserve storing and melanin production.