The histology and mucus histochemistry of the pleuronectid post-gastric alimentary canal was examined using light and electron microscopy. Distinct differences in goblet cell mucus histochemistry were observed between species, with the two closest taxonomic species, the winter flounder and the yellowtail flounder showing the most diversity and the halibut showing regional variation. Numbers of goblet cells within post-gastric regions did not differ significantly between species, but were significantly different between regions within species increasing toward the rectum. The post-gastric region was divisible into two areas based upon the ultrastructural features of lipid digestion and absorption in the intestine and pyloric caeca, and of exogenous protein in the rectum. The combination of species-specific histochemical differences in mucus and general histological and ultrastructural differences within the post-gastric regions between these species suggest a correlation between lumenal environmental conditions/histology and natural prey preference. 1996 The Fisheries Society of the British Isles

The histological development and mucous histochemistry of the alimentary tract in larval yellowtail flounder were studied using light microscopy. Samples were taken when the larvae were first offered food at 3 days post-hatch, then at 7, 10, 29, 36, and 46 days post-hatch, at which time they were metamorphosing. Regional partitioning of the digestive tract into the buccal cavity, pharynx, oesophagus, post-oesophageal swelling (PES), intestine, and rectum was complete by day 10. Goblet cells were present only in the buccal cavity, pharynx and intestine by day 7, but increased in number and distribution as development continued. By day 29, the posterior zone of the oesophagus had a marked increase in goblet cell density and mucosal folding. At the transition from oesophagus to PES/stomach stratified epithelium with goblet cells changed abruptly to a columnar epithelium with no goblet cells. Multicellular glands in the PES of 36-day larvae allowed it to be defined as a stomach. The distinct brush border of columnar epithelium and the presence of goblet cells characterize the intestine and rectum. All goblet cells throughout the digestive tract were strongly positive for acid mucins as was the luminal layer of the stratified epithelia lining the buccal cavity, pharynx and oesophagus. The PES/stomach epithelium stained weakly for neutral mucins. No mucin staining was associated with the gastric glandular epithelium. The brush borders of the intestine and rectum were strongly positive for combinations of neutral and acid mucins.

Atlantic halibut (Hippoglossus hippoglossus) collected off Newfoundland first mature at about 80 cm fork length (FL) for males and about 115–120 cm FL for females. Captive Newfoundland halibut did not release milt or eggs or have detectable levels of estradiol-17β or 11-ketotestosterone until exceeding 80 cm (males) and 115–120 cm (females). Estradiol-17β and testosterone increased to highest levels in females during gonadal recrudescence before spawning. Lower levels were observed in spawning fish. Vitellogenin (VTG) levels were highest in spawning fish. A sudden drop in estradiol-17β and VTG preceded release of the first batch of eggs. Estradiol-17β, testosterone, and VTG fluctuated with release of successive batches of eggs. Male halibut started to mature during fall and early winter, as indicated by increased testosterone and 11-ketotestosterone and abdominal swelling. Milt was first released in January and February when testosterone and 11-ketotestosterone were near maximum levels. Hence, rising levels of plasma sex steroids and VTG in fall indicate that reproductive activity is underway 1–2 mo before any noticeable swelling of the abdomen. Individual maturing halibut can be sexed by rising levels of estradiol-17β and VTG (females) and 11-ketotestosterone (males) in late fall and early winter.