A Morphohistological and Histochemical study of hatchery-reared European hake, Merluccius merluccius (Linnaeus, 1758), during the lecito-exotrophic larval phase

The larval development of reared European hake, Merluccius merluccius (Linnaeus, 1758), during the lecithotrophic phase, from hatching until 5 days post-hatching (dph), and throughout the endo-exotrophic feeding phase (6-10 dph) was studied by histology and histochemistry. Many crucial morphological, cellular and tissular changes were observed during both feeding phases, mostly those related to digestive and visual ontogenetic events, such as differentiation of buccopharyngeal cavity and eye development (at hatching); pigmentation and differentiation of cone-photoreceptors (4 dph); opening of the mouth and anus, appearance of intestinal valves (5-6 dph); presence of buccopharyngo-esophageal (5-6 dph) and intestinal (9 dph) mucous cells; folding of intestinal mucosa (6 dph); development of regional specific digestive musculature (6 dph); typical structure and functionality of the liver (sinusoids, biliary and pancreatic ducts, glycogen, protein and lipid reserves) (4-6 dph); and characteristic acinar distribution pattern of eosinophilic zymogen granules of the exocrine pancreas (6 dph). Between 9 and 10 dph, the hake larvae showed evident signs of lipid absorption within enterocytes of the anterior intestinal region and a remarkable process of pynocitosis and intracellular digestion was detected in the posterior intestine (supranuclear inclusions or acidophilic protein vesicles). In hake larvae at 10 dph, a proliferation of renal tubules, spleen differentiation and gill development, as well as the presence of the first thyroid follicle, were clearly distinguished. At this time, stomach gastric gland differentiation was not detected and endocrine pancreas and gill lamellae were not evidenced. However, and interestingly, swim bladder and eyes (developing rods) were well differentiated in larval development from 9 dph onwards. In summary, in European hake larval development during the endo-exogenous feeding phase and especially at 9 to 10 dph, most systems, organs and tissues were well differentiated. Particularly, digestive and visual systems were developed and physiologically functional some days before the start of the exogenous feeding phase.

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