Development of the digestive capacity in larvae of haddock ( Melanogrammus aeglefinus) and Atlantic cod ( Gadus morhua)

Using biochemical and molecular biological techniques, we describe the expression of several key digestive enzymes throughout the ontogeny of larval haddock and Atlantic cod. The pattern of activity of general proteases (GP), trypsin-like enzymes (TLE), pepsin-like enzymes (PLE), general lipase (GL), bile salt-activated lipase (BAL), alkaline phosphatase (AP) and α-amylase were studied in larvae of both species using colourometric techniques. All enzymes were detected as early as hatch, except for α-amylase. Activity of GP generally increased from 0 to 144 degree-days (DD), decreased until 333 DD and then increased again in the oldest larvae of both species. Activity of TLE in haddock was high at hatch and generally showed an inverse pattern to that of Atlantic cod. Using zymography, we detected serine proteases, particularly trypsin, in the protein digestion of both species. Using RT-PCR, trypsinogen transcripts were detected as early as hatch. For both species, in situ hybridization analysis showed localization of trypsinogen expression to the pancreas during larval development. Although activity of PLE was detected as early as hatch in both species, pepsinogen transcripts were detected by RT-PCR only in the oldest larvae sampled, and after the gastric glands were identified morphologically. Atlantic cod BAL activity showed no significant differences over time. Activity of GL remained constant over time in larvae of both species. Levels of AP activity in haddock larvae showed no significant differences from hatch to 333 DD, except at 66 and 401 DD when there was a significant increase compared to other time points. In Atlantic cod, AP activity remained relatively constant until 476 DD, when a significant increase was detected. α-Amylase activity could not be detected in either species, except at 43 and 60 DD for Atlantic cod and 144 DD for haddock when large numbers of live prey were present in the gut. The estimated contribution of Pavlova lutheri-enriched rotifers to the total larval digestion was negligible, except for α-amylase, for which their contribution was estimated to be 100%. Our results suggest that both Atlantic cod and haddock larvae are capable of digesting protein and lipids at the time of mouth opening and that they have a limited capacity to digest carbohydrates.