Adaptation for water balance in the partial gastrointestinal tract of summer flounder

Marine teleosts continually drink and absorb water across the intestine to prevent dehydration. Surprisingly, summer flounder that are missing most of their intestine, due to necrotizing enteritis, maintain osmotic homeostasis. Here, we tested the hypothesis that this remnant gastrointestinal tract undergoes compensatory adaptation for fluid uptake. Flounder (Paralicthys dentatus) with a partial gastrointestinal tract had an emaciated liver. Moisture content of muscle however was similar to healthy cohorts with an intact gastrointestinal tract, indicative of an undisturbed osmoregulatory status. Mass-specific rates of fluid uptake across all segments of the partial gastrointestinal tract were less than or similar to rates in corresponding segments from intact flounder. In contrast, weights (percent of body mass) were doubled in stomach and partial intestine of the remnant gastrointestinal tract. Consequently, total capacity for fluid uptake (μL h− 1 g body mass− 1) was similar for both groups. The functional capacity of the remnant gastrointestinal tract was therefore of a magnitude sufficient to maintain osmoregulatory ability, further evidencing a critical role of the intestine in salt and water balance of marine teleosts.