Abstract
There are not many specimens of Risso's dolphins kept in captivity. Compared to the bottlenose dolphins, minimal information is available regarding the physiological characteristics and needs of this species. In 2005, a 1 year old female Risso's dolphin stranded along the Italian coast of the Adriatic Sea. The calf was considered not releasable. The calf was hand reared and was introduced to a pool with several bottlenose dolphins after her clinical situation was stabilized.
Studies performed on Risso's dolphins demonstrate that in the wild this species eats almost exclusively cephalopods. During the past year, the captive animal was examined repeatedly with ultrasonography to study the anatomy and activity of the gastric compartments in this species. Tissue samples of the three different gastric compartments were obtained from bottlenose dolphins and a stranded Risso's dolphin and have been processed for histologic and immunohistochemical analysis.
The first gastric compartments of both species appear similar by ultrasound and histologic review.
Several differences are present in the second stomach (or glandular compartment). On ultrasound, the lumen of the second stomach is larger in the Risso's dolphin, while the rugal folds are smaller compared to the rugal folds in the bottlenose dolphin. Histologically, the mucosa of the second stomach in the Risso's dolphin presents differences regarding the location and relative percentages of the parietal cells and principal cells. The bottlenose dolphin appears to have fewer parietal cells when compared to the Risso's dolphin. On ultrasound examination, the echogenicity of the pyloric stomach is similar in the two species. Immunohistochemically, the Risso's dolphin has more G-cells and less D-cells than the Tursiops. The G-cells of the Risso's dolphin are larger than those of the Tursiops. On the basis of these morphophysiological observations, we conclude that Risso's dolphin is more sensitive to histamine stimulation of G cells. These results would indicate the Risso's is more sensitive to gastric acidity if fed with fish that can potentially develop histamine due to their levels of histidine.
Recent studies in cephalopods showed that these invertebrates do not develop histidine even if stored for 5 months at 4°C,1 while the fish can contain more of this amine.2 High level of histidine-histamine can lead to hypergastrinemia, followed by gastric hyperacidity and the possibility of developing gastric ulcers. This suggests that the Risso's stomach can be more susceptible to gastric ulcers if fed with fish instead of cephalopods.
References
1. Ozogul Y, Ozogul F, Olgunoglu A, Kuley E. Bacteriological and biochemical assessment of marinating cephalopods, crustaceans and gastropods during 24 weeks of storage. Int J Food Sci Nutr 2008 ;59(6):465-476.
2. Worthy GAJ. Nutritional and energetics. In: Dierauf LA, Gulland FMD, editors. CRC Handbook of Marine Mammal Medicine. CRC Press; 2001. p. 791-827.