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Changes in digestive pH and protease activity have been determined throughout the transition from larvae to the juvenile stage in Sparus aurata in rearing conditions (from 0.04 to 100 g wet weight). Measurements of pH have been taken in the stomach and different segments along the length of the intestine using a pH microelectrode. In starved fish, the gastric pH ranged between 6.0 and 8.0 approximately, except in juveniles of intermediate size (between 1.0 and 7.0 g wet weight), which exhibited a wider pH range of 2.0-8.0. Fed fish with digestive content showed, in general, lower pH values in the stomach. A progressive decrease was observed from a pH range of 5.5-8.0 in the youngest animals (0.04 g) to a pH range of 2.0-6.2 when juveniles were approaching 1.0 g wet weight. Above this weight, the gastric pH remained constant (between 2.0 and 6.0 approximately). The pH values in the intestine ranged between 6.7 and 8.4. They were similar in the different segments and weight classes examined, and there were no significant differences between fed and starved animals. Specific acid protease activity (units per milligram soluble protein) in fed animals increased from small (0.04-1.0 g) to intermediate juveniles (1.0 and 7.0 g), but then remained similar in larger juveniles. On the contrary, specific alkaline protease activity in fed animals decreased from small to intermediate juveniles, and then remained at a similar level in larger juveniles. The results reflect a progressive transition during several months from alkaline digestion in larvae with undeveloped stomachs to the acid digestion in juveniles with fully developed stomachs. Full gastric capacity is developed in seabream juveniles of 1 g wet weight, which represents approximately 100 days post-hatching in cultured populations. Nevertheless, in the following 2.5 months, during which the intestine reaches the appropriate length, juveniles still show a transitional period in the regulatory mechanism of digestion, probably linked to the adaptation to a different feeding habit.

A study was conducted to study variations in dry weight, caloric content and different body constituents that are representative of biomass quality in fish larvae of the Senegal sole Solea senegalensis Kaup from egg fertilization to metamorphosis. Results indicate that the Senegal sole shows quick and efficient larval development. It also exhibits early functionality of the mouth and digestive tract and excellent efficiency in initial feeding.

The study of feeding behavior and prey size selection of gilthead seabream larvae reveals the preference for live prey than inert food. Excessive prey width/mouth width ratio affects this preference. Length of larvae influences the uptake of microcapsules of different sizes. Further research on larvae feeding behavior can facilitate food preparation for marine fish larvae.