LAUSR

Abstract The objective of this research was to determine the circadian cycle of basal digestive enzymatic activity of the freshwater prawn Macrobrachium tenellum and the effect exerted by the photoperiod and molting cycle on digestive enzymatic activity. Juvenile prawns of M . tenellum (0.61 ± 0.21 g) were collected from a natural affluent in the area of  Puerto Vallarta, Jalisco, Mexico. For basal digestive enzymatic activity, five prawns were randomly sampled from of a stack of 200 organisms in fasting conditions every hour over a period of 24 h under natural photoperiod (13:11 h; light: dark); for digestive enzymatic activity under different photoperiod conditions, five organisms were randomly sampled every two hours during a 24 h period after eight days of pre-acclimatization under three photoperiod treatments (0:24 h, 13:11 h, and 24:0 h); and for the effect of molt cycle on the digestive enzymatic activity, 15 prawns from each molting stage (premolt, D; intermolt, C; postmolt, B) were sampled during one-hour span (08:00–09:00 h). All sampled prawns were sacrificed by immersion in ice cold water and stored at − 20 °C until use for subsequent biochemical analysis. Enzymatic extract of the complete organisms was obtained to determine specific activity of general protease, trypsin, chymotrypsin, amylase, and lipase. To conclude, the circadian cycle of digestive enzymatic activity in M . tenellum for the general proteases, lipases, and amylases was biphasic at 08:00 and 20:00 h with a fluctuation of less activity throughout the day, which could be affected by the alteration of the natural photoperiod, modifying enzyme concentrations and basal peaks; likewise, the concentration of digestive enzymes was different in each of the molt cycle phases, showing the highest concentration of general proteases, trypsin, chymotrypsin, and lipases during postmolt (B). This is the first study on digestive enzymatic activity of M . tenellum juveniles, creating the basis to clarify its digestive physiology and contributing to nutritional knowledge of the species.