Chitin Digestion and Assimilation by Seabirds

As a structural component of crustacean exoskeletons, chitin is the most important carbohydrate in the diets of many marine carnivores. To investigate the physiological and biochemical adaptations that may enable seabirds to break down this “prey defense,” we estimated chitin digestibilities for Sooty Albatrosses (Phoebetria fusca), White-chinned Petrels (Procellaria aequinoctialis), Rockhopper Penguins (Eudyptes chrysocome), Gentoo Pen­ guins (Pygoscelis papua), King Penguins (Aptenodytes patagonicus) and Leach's Storm-Petrels (Oceanodroma leucorhoa) fed Antarctic krill (Euphausia superba). These species retain a substantial proportion (46.5 ± SD of 13.1%, 39.1 ± 4.9%, 52.8 ± 37.6%, 45.3 ± 5.6%, 84.8 ± 11.7% and 35 ± 12.2%, respectively) of ingested chitin. We also obtained preliminary estimates of chitinolytic activity in the gastric mucosae of the above six species by incubating extracts of tissue samples with a chitin substrate and measuring the production of the end product of chitin hydrolysis, N-acetyl-D-glucosamine (NAG). Chitinolytic activity (up to 5,000 11-g NAG h- 1 g-1 expressed per gram tissue) was measured from proventricular tissue and within the activity range (1,350 to 61,650 11-g NAG h- 1 g- 1) reported for eight other avian species. In order to assess the energetic and nutritional benefits of chitinolytic activity in seabirds, we studied gastrointestinal absorption of the end products of chitinolysis in Leach's Storm­ Petrels. The overall absorption efficiency of NAG and its deacetylated precursor, glucosamine (Gln), in this species was 44.0 ± 3.0% and 11.0 ± 1.9%, respectively. These absorption effi­ ciencies were significantly less than for glucose, which was absorbed with an efficiency of 90.6 ± 2.5%. No absorption of NAG and Gln occurred in the proventriculus. Overall, we showed that seabirds have a capacity to assimilate a considerable portion of the carbon and nitrogen present as chitin in the exoskeleton of their prey, but we have not demonstrated that assimilation actually occurs. The potential costs and benefits of chitin hydrolysis, as well as the absorption of the breakdown products, need to be assessed.