Feeding Habits of Acanthaster spp., as Indicated by the Carbon Stable Isotope Values of Food in Their Gastric Contents and Pyloric Gut

The periodic outbreaks of Acanthaster spp. which are predators of coral, are important reasons for the degradation of coral reefs. Information on the food sources of Acanthaster spp. Would be useful for suppressing outbreaks of the species. Although field observations on the feeding habits of Acanthaster spp., suggest that they feed mainly on corals, no isotopic evidence has been provided to support this. Therefore, this study was conducted to explore the food sources of Acanthaster spp. from the perspective of stable carbon (δ13C) and nitrogen isotopes (δ15N) by analyzing the consistency of the δ13C values of food in the gastric contents and pyloric gut as well as the significance of their δ13C and δ15N values, with the goal being to attain new information for comprehensively understanding or controlling outbreaks of these predators. In March 2020, various Acanthaster spp. were collected from the reef slope of coral reefs in the Xisha Islands, at a water depth of less than 10 m. The sampled individuals were immediately placed in a -20°C refrigerator for transport to the laboratory, after which they were stored in -80°C freezer. The gastric contents and pyloric gut were extracted for measurement of δ13C and δ15N values to investigate the feeding habits of Acanthaster spp. for the first time. The collected individuals were divided into three age groups: sub-adult (10~200 mm diameter), adult (200~350 mm), and senile-adult (>350 mm). The δ13C values of the gastric contents and pyloric gut were -15.03‰~-13.00‰ and -15.21‰~-13.24‰, respectively. The average change in δ13C values (△δ13C) between the gastric contents and pyloric gut were 0.17‰, 0.20‰, and 0.04‰ for the sub-adult, adult, and senile-adult groups, respectively. The results showed the following: (1) The δ13C values of the gastric contents correlated positively with those of the pyloric gut for Acanthaster spp. of different diameters. There were no significant differences in average δ13C values between the gastric contents and pyloric gut. The same correlation trend between the gastric contents and pyloric gut, indicated a similar presentation of feeding habits. Both the gastric contents and pyloric gut can be used for food research; and (2) With increase in the Acanthaster spp. diameter and age, the δ13C values of the gastric contents and pyloric gut showed a decreasing trend. By contrast, the δ15N values of the gastric contents showed an increasing trend, indicating the variable properties of the food ingested. Specifically, the trophic level of the food tended to increase. The food trophic level was relatively low for sub-adults or individuals of a shorter diameter whereas it was relatively high for the larger and older individuals. These study findings provide new reference data for exploring the feeding habits of Acanthaster spp. at different growth phases and understanding the reasons for their outbreaks.