Ampullary organs of Acipenser naccarii sturgeons were examined by optical and electronic microscopy (transmission electron microscopy and scanning electron microscopy) from hatching until 1 month later when the juvenile phase is completely established. It was observed that, when A. naccarii begins to feed actively, the ultrastructural characteristics of ampullary organs already correspond to those of adult animals. These organs may, therefore, be functional and, together with taste buds, facilitate food search after exhaustion of yolk sac food reserves. Mature ampullary organs of A. naccarii are formed by an ampulla that communicates with the exterior by means of a short channel. These ampullae correspond to the sensory portion of these receptors and are formed by two cell types: receptor cells and support cells. Receptor cells present a kinocilium on their free surface and establish ribbon synapses with axon nerve endings that arise from the underlying conjunctive tissue. Support cells enclose receptor cells, bear stereocilia and occasional cilia, and are of a secretory nature. The mucus associated with ampullary organs mainly comprises neutral mucopolysaccharides, whereas mucopolysaccharides are usually acid in other fish groups.

The present work examines the temporal appearance and degree of activity of digestive enzymes of pancreatic and intestinal origin (amylase as well as neutral and alkaline proteases) together with the evolution of the ontogenic development of the intestine and pancreas during the first month of free-living (30 days post-hatch-[dph]) of the sturgeon Acipenser naccarii. In addition, the influence of exogenous live feeding with Artemia salina on the detection of digestive enzymes was studied. Shown are that in the first life stages of the A. naccarii, from the time of fertilisation to the juvenile stage, the following events related to its digestive physiology occurred: digestive enzymes were detected in the embryo stage within the egg, presumably related to the hatching gland; in the free-living embryo stage, opening of the mouth was at 10 dpf and protease and amylase digestive activities occurred due to an early differentiation of the pancreas and intestine (12-13 dpf). At this stage, digestive enzymatic activities are partly owing to the live feed offered before disappearance of the yolk reserves (14 dpf); the expulsion of the melanin plug and the appearance of a continuity of the digestion tract (16 dpf) together with the disappearance of the yolk reserves (17 dpf) mark the passage from the free-living embryo phase to the juvenile stage; finally, from the first month after hatching in this sturgeon species, there is a stabilisation in digestive amylase and protease activities together with a fully developed digestive structure. The contribution of enzymes from prey in the detection of enzymatic activities determined in the fish was beyond doubt; therefore these exogenous enzymes must also have a certain physiological importance. Subsequent work will be needed to clarify the importance of digestive processes of the exogenous enzyme pool from live feed as well as to assess the possibility of shortening the weaning period.

This study describes the stomach development in the sturgeon Acipenser naccarii, focusing on its histoenzymatic and ultrastructural characteristics from hatching until the end of the juvenile phase, 1 month later. The study aims to assist the aquaculturist in identifying the best timing for the start of feeding and to select the appropriate food for each stage of A. naccarii development. At hatching, the anterior portion of the digestive system consists of a yolk sac with a large central yolk mass. The yolk mass is enclosed by differentiating endodermal epithelium. The most significant ultrastructural characteristics of the epithelium are the numerous supranuclear vacuoles containing protein and lipid yolk granules, dilated intercellular spaces, and filiform cytoplasmic projections related to the uptake of reserve material on the apical surface. Newly hatched eleutheroembryos show optimal enzymatic activities at a pH of 1.5 and 3, which is probably related to the presence of hatching glands and the digestion of yolk components. During the free-embryo phase, the yolk sac epithelium undergoes a series of histological, ultrastructural, and functional modifications to form a cecal stomach with anatomical characteristics similar to those of juvenile specimens. Gastric glands emerge between 6 and 7 days post-hatching (PH), although the absence of secretion granules in their oxyntic-peptic cells indicates that the stomach is not yet functional. During the following days (the exotrophic period), development of the tubular-vesicular network is observed, and secreting granules occur in the cytoplasm of oxyntic-peptic cells until around day 35 after fertilization (day 27 PH), when acid protease enzymatic activity is again detected. These histological and enzymatic changes and the greater number and size of oxyntic-peptic cell secretion granules appear to be related to the gastric compartment ontogeny reaching functionality at this developmental stage.

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