BACKGROUND
This study examined the morphology, histology, and histochemistry of the digestive tract of the marbled flounder (Pseudopleuronectes yokohamae), a commercially valuable species in Korea and Japan that is being considered as a new aquaculture target. The authors aimed to clarify digestive physiology and feeding habits by analyzing gross anatomy, tissue structure, mucus-secreting goblet cells, and cholecystokinin (CCK)-producing cells. Understanding these features is relevant because fish digestive tract structure varies with diet, and such knowledge may support nutritional planning and husbandry in aquaculture. Prior feeding data cited by the authors suggested that marbled flounder are benthic feeders and that their main prey includes Polychaeta (45.3%) and Gastropoda (23.2%), raising the question of whether their digestive tract more closely resembles omnivorous or carnivorous fish.
METHODS
Twenty marbled flounder juveniles were studied. Mean body length was 14.6 ± 0.5 cm and mean body weight was 56.6 ± 5.1 g. Fish were euthanized with 2-phenoxyethanol at 150 ppm, and the digestive tract from esophagus to anus was dissected. The relative length of the gut (RLG) was calculated as digestive tract length divided by standard length. For histology and histochemistry, samples were taken from nine regions: esophagus, cardiac stomach portion, fundic stomach portion, pyloric stomach portion, pyloric caeca, anterior intestine portion, mid intestine portion, posterior intestine portion, and rectum. Sections were cut at 4 μm, with 10 sections per region prepared for staining. Hematoxylin and eosin staining was used for general histology, and Alcian blue at pH 2.5 plus periodic acid-Schiff staining was used to identify mucus-secreting goblet cells. For immunohistochemistry, 4-μm paraffin sections were stained with primary antibodies against cholecystokinin-8 at 1:20,000, followed by peroxidase-conjugated secondary antibodies. Data were analyzed with one-way analysis of variance at a 95% significance level (p < 0.05), and Tukey’s honestly significant difference test was used for post hoc comparisons. Assumptions of normality and homogeneity were checked with Shapiro–Wilk and Levene’s tests, and no violation was detected (p > 0.05).
KEY RESULTS
The mean RLG was 1.54 ± 0.10 (n = 20). The digestive tract included a simple stomach and 6–9 pyloric caeca. Histologically, the digestive tract showed the usual layered organization of serosa, muscularis externa, submucosa, and muscularis mucosae. In the esophagus, the muscularis externa was thick and well developed, with an average thickness of 485 μm, while mucosal fold length averaged 1031 μm. The esophageal mucosal folds were thin, long, and regularly branched.
The stomach was divided into cardiac, fundic, and pyloric portions. Gastric glands with clear lumens were well developed, especially in the fundic portion. The thickness of the muscularis externa was greatest in the pyloric portion and then decreased through the cardiac and fundic portions: 603, 245, and 150 μm, respectively. Mucosal fold length showed the opposite trend, increasing from the pyloric to cardiac to fundic portions: 906, 1044, and 1383 μm, respectively. The authors interpreted this pattern as evidence that the stomach is suited to processing a small amount of hard food.
There were 6–9 pyloric caeca, and their serosae were extremely thin. Their muscularis externa was weakly developed at 32 μm, but mucosal folds measured 669 μm. Within the intestine, the anterior, mid, and posterior portions had similarly branched folds but differed quantitatively. The anterior intestine had a muscularis externa thickness of 57 μm and the longest intestinal mucosal folds at 630 μm. The mid intestine had a muscularis externa thickness of 50.9 μm and fold length of 486.6 μm. The posterior intestine had the thickest intestinal muscularis externa at 70.4 μm and fold length of 458 μm. The rectum showed a thicker muscularis externa at 198 μm and longer mucosal folds at 859 μm than the intestine and pyloric caeca.
Goblet cell distribution differed markedly by region. No goblet cells were identified in the stomach. Goblet cells were counted in the esophagus (5743 cells), pyloric caeca (300 cells), anterior intestine portion (650 cells), mid intestine portion (528 cells), posterior intestine portion (467 cells), and rectum (1943 cells). Their distribution was significantly higher in the esophagus, followed by the anus (p < 0.05). Goblet cells were more numerous in the intestine than in the pyloric caeca, and they decreased from the anterior to posterior intestine. However, there was no significant difference in goblet cell number between the intestine and pyloric caeca (p > 0.05).
CCK-producing cells were absent in the esophagus and stomach but present from the pyloric caeca through the rectum. Counts were pyloric caeca (65 cells), anterior intestine portion (62 cells), mid intestine portion (13 cells), posterior intestine portion (8 cells), and rectum (4 cells). These endocrine-like cells were most frequent in the pyloric caeca and anterior intestine, with a progressive decrease toward the rectum. The authors noted that this distribution pattern was very similar to that of goblet cells.
CLINICAL IMPLICATIONS
This is not a clinical study, so direct patient-care implications are limited. However, for comparative digestive biology and aquaculture practice, the work suggests that the marbled flounder should be considered functionally closer to carnivorous fish than omnivorous fish. The combination of a simple stomach, 6–9 pyloric caeca, branched mucosal folds, absence of gastric goblet cells, and concentration of CCK-producing cells in the pyloric caeca and anterior intestine supports the idea that digestion is centered in the proximal gut after gastric processing. These findings may help guide feed formulation, feeding strategies, and future physiological studies in cultured marbled flounder. The authors conclude that food digested by gastric acid likely moves into the anterior intestine and pyloric caeca, where it can effectively stimulate CCK-producing cells and coordinate digestive function. They also emphasize that further ultrastructural work is needed on goblet cells, absorptive cells, and endocrine cells.