Histological and morphological evaluations of the digestive tract and associated organs of haddock throughout post‐hatching ontogeny

At hatching, the oesophagus of haddock Melanogrammus aeglefinus lacks goblet cells, the intestine is a simple undifferentiated tube, the liver is present as a rounded mass caudal to the heart, and numerous zymogen granules are present in the pancreas. The first intestinal convolution appears at day 2, at the posterior end of the digestive tract. The oesophagus displays alcian blue and PAS positive mucus secreting cells on day 12, which become numerous by day 15. By day 18, epithelial cells of the posterior intestine show evidence of protein absorption in the form of supranuclear vacuoles. The swimbladder inflates in 50% of the larvae by day 22, although inflation rate is highly variable. By day 35, or 10 mm, a pyloric caecal ridge appears which separates the presumptive stomach, which is now showing evidence of gastric gland formation, from the intestine. This marks the beginning of digestive features characteristic of the juvenile stage.