Quantification of exogenous protease derived from zooplankton in the intestine of Japanese sardine (Sardinops melanotictus) larvae

It has been suggested that exogenous enzymes derived from live food contribute to digestion in fish during the larval stage; however, the extent of this contribution is unclear. Therefore, we estimated the ratio of protease activity derived from zooplankton to total protease activity in sardine larva intestine. Larvae (10 days old) were starved for 24 h (0 h postfeeding; hpf), fed on rotifers for 2 h (2 hpf) and starved again for 10 h (12 hpf). Larvae were dissected into two parts; one part contained the pancreas, and the other part contained the intestine. The samples were analyzed for total protease activity using casein as a substrate and the protein content derived from rotifers by ELISA. Rotifer protease activity per unit weight of rotifer protein was also determined. Rotifer specific protease activity was estimated from these data by multiplying the rotifer protease activity per unit weight of rotifer protein by the amount of rotifer protein found in the intestine. While the total protease activity of the pancreas did not differ between 0, 2 and 12 hpf, that of the intestine was significantly higher at 2 hpf than at 0 and 12 hpf. At 2 hpf, the protease activity derived from rotifers was estimated to be 0.60% of the total protease activity in the intestine. Therefore, the contribution of exogenous protease to digestion in sardine larvae appears to be insignificant.