Hormonal control of tryptic enzyme activity in Atlantic cod larvae (Gadus morhua): Involvement of cholecystokinin during ontogeny and diurnal rhythm

• We describe the ontogenetic development of CCK and tryptic enzyme activity in larval cod
• CCK is known to play a key role in regulating digestive processes
• CCK concentrations increased during ontogeny suggesting a growing role in regulating digestive processes
• A short-term experiment reveals a feedback mechanism between CCK and tryptic enzyme activity
• Cod larvae have limited regulatory and digestive capacity to handle several meals in a short period

The ontogenetic development of the gut hormone cholecystokinin (CCK) and the key proteolytic enzyme trypsin was described in Atlantic cod larvae (Gadus morhua) from first-feeding until 38 days post first-feeding (dpff). CCK is known to play a major role in the endocrine control of digestive processes in mammals and adult fish, but its regulatory role in the larval stages of marine fish is largely unknown. Only small amounts of CCK were found in the body (excluding head) in cod larvae at first-feeding, but CCK levels increased exponentially with development, suggesting a more pronounced role of CCK during ontogeny. Tryptic enzyme activity increased slightly until a standard length of ca. 8 mm (approx. 33 days dpff) with a significant increase in larvae larger than 8 mm standard length, indicating limited digestive capacity in the early stages. To entangle the short-term feedback mechanism between CCK and tryptic enzyme activity, we conducted a 12 hour feeding experiment at 21 dpff. Cod larvae receiving only algae revealed a noticeable response in tryptic enzyme activity within two hours in the morning, whereas larvae fed algae and rotifers at the same time showed a slightly delayed response up to four hours. Tryptic enzyme activity remained low in the group receiving only algae as well as the two fed groups in the afternoon. No reaction in tryptic enzyme activity was observed in larvae that received a second meal of rotifers in the afternoon, indicating limited regulatory and digestive capacity to handle several meals in a short period. CCK levels remained relatively constant throughout the day but increased in the afternoon in all three groups when tryptic enzyme activity was low, suggesting that a negative feedback mechanism between CCK and tryptic enzyme activity is present in larval cod at least from 21 dpff.