VIP-like immunoreactivity in the intestinal tract of fish with different feeding habits.

We studied the distribution of vasoactive intestinal polypeptide immunoreactive (VIP-ir) cells and fibres in the intestine of three fish species with different feeding habits: the silver carp (Hypophthalmichthys molitrix), the goldfish (Carassius auratus), and the pumpkinseed sunfish (Lepomis gibbosus). Each species was divided into two groups: (1) fish fed once a day up until sacrifice and (2) fish fed once a day and then fasted three days before sacrifice. Immunoreactive endocrine cells and fibres were present in all three fish species. The immunoreactive cells were distributed along the entire intestinal mucosa of the carp and goldfish but were found only in the anterior intestine of the sunfish. The immunoreactive fibres were present along the entire intestinal wall, in the myenteric plexus, in the circular muscular layer, and in the connective tissue of the mucosa in all three fish species. No differences were found between the cells and fibres of normally-fed animals and the cells and fibres of fasted animals. The authors hypothesize that the different distributions of VIP-ir cells and fibres are related to the different contents of hard and indigestible matter of the fish food.