Comparative Study of the Gastric Mucosa of Risso's Dolphin (Grampus griseus) and Bottlenose Dolphin (Tursiops truncatus): A Key to Manage the Diet in Captive Conditions

Simple Summary The bottlenose dolphin (Tursiops truncatus) is the most represented species housed in zoological settings in Western countries, while Risso's dolphin (Grampus griseus) is usually kept under human care as a result of the rehabilitation process in rescue centers. Although they belong to the same family, Delphinidae, these two species present remarkably different feeding habits. Risso's dolphin is strictly teutophagous, and its diet is particularly based on cephalopods such as mesopelagic squid, which is poor in histamine. The bottlenose dolphin is mainly ichthyophagous and primarily prefers blue fish, which is rich in histamine as a degradation product and stimulates the parietal cells of the stomach to secrete hydrochloric acid (HCl). These different eating habits are usually correlated to habitat, behavior and physiological needs related to the specific morpho-functional conditions of the species. Despite these dietary differences, in captivity, the two species are fed in the same way through the administration of several kilograms of bony fish (such as capelin or herring) per day. While this diet appears to be optimal for the bottlenose dolphin, Risso's dolphin appears to be more sensitive, presenting gastrointestinal discomfort when ingesting bony fish for a few consecutive days. To histologically evaluate the gastric compartments of Risso's (Grampus griseus) and bottlenose dolphins (Tursiops truncatus) and provide suggestions for the diet of Risso's dolphins in captivity, we examined 12 stomachs from both species. While slight differences in keratinization were observed in the forestomach, significant differences came to light in the second stomach's mucosa. At this level, in Risso's dolphin, the principal cells are markedly reduced in size and located externally to the parietal cells, not interspersed between them, compared to bottlenose dolphins; differences were also observed in the structure and concentration of the parietal and principal cells of the gastric body glands (p < 0.0001). The quantitative results of G- and D-cell counts in the gastric mucosa show a clear difference, with a higher concentration of G cells in the mucosa of Risso's dolphin (t = 7.334; p < 0.0001) and a higher level of D cells in bottlenose dolphin mucosa (t = 3.123; p = 0.0049). These results suggest that parietal cells undergo greater stimulation by gastrin produced by G cells, with greater acid secretion in G. griseus. Further studies are needed to understand whether an inappropriate diet could lead to severe clinical signs due to gastric acidity in Risso's dolphin.