THE DEVELOPMENT OF FUNCTIONAL DIGESTIVE AND METABOLIC ORGANS IN TURBOT, SCOPHTHALMUS-MAXIMUS

The functional status of organ systems involved into the processing of exogenous food is critical for the survival and growth of fish early life stages. The present study on laboratory-reared larval turbot, Sco phthalmus maximus, provides an overview on the ontogeny of structure a nd functions involved in digestion, absorption and metabolism of nutri ents. At start of exogenous feeding the intestine of larval turbot is anatomically differentiated, with enterocytes displaying an adult-type ultrastructure and being able to process lipids. At the microvillous border of the enterocytes, enzymes of contact digestion such as aminop eptidase are found. The ultrastructure of the exocrine pancreatic cell s is fully differentiated from hatching onwards. Likewise, substantial activities of trypsin type proteases are present. A stomach anlage ex ists in first-feeding larvae; however, the stomach becomes functional (appearance of gastric glands and pepsin secretion) only during metamo rphosis. Liver parenchymal cells already display a functional ultrastr ucture during the endotrophic phase; with onset of exogenous feeding t hey develop pronounced diet-related changes of their energy stores. La rval respiration is not executed by the gills since respiratory surfac e of these structures develops only towards metamorphosis. The energy generation of larval muscle tissue depends on aerobic metabolism, wher eas glycolytic activities start to increase at metamorphosis. In concl usion, two important patterns can be recognized in the development of turbot larvae: (1) The structure/function is differentiated at hatchin g or at the onset of exogenous feeding (afterwards it experiences main ly quantitative but not qualitative growth, i.e., intestine, exocrine pancreas, liver); or (2) the structure/function is absent in larvae an d develops only during metamorphosis (i.e., gills, glycolytic muscle m etabolism, stomach).