Bachelor Thesis

Plastic responses to food limitation in the pyloric caeca of Acanthaster planci (Linnaeus, 1758)

Since the 1950´s scientists have noted a remarkable number of A. planci migrating through numerous valuable reefs and leaving devastating damage behind. Even though the cause of A. planci outbreaks is only vaguely understood, the sudden disappearance of the animals after an infestation is a special riddle. Scientists assume that the starfish continue migrating to the next living coral reef, covering large distances without any predictable food sources on their way. This leads to the further assumption that the animals are able to withstand long periods without any food. If so, the animals would be capable of storing energy during feeding, absorbing these when food is limited. It has been proven that other species of the echinoderms are able to store energy reserves in their pyloric caeca. In this study the pyloric caeca of A. planci have been examined over a time period of seven months, observing the pyloric caeca of the starfish during food availability and absence. The animals were exposed to a period of food availability (102 days), consisting of artificial food and different kind of corals, such as Discosoma sp., Protopalythoa sp., Sacrophyton, and vice versa to a period of total food absence (105 days). In a time interval of about six weeks the animals were scanned by MRI and the pyloric caeca of each animal were analyzed and visualized three-dimensionally with the software Amira®, which allows to measure the volume of the pyloric caeca. During food availability the pyloric caeca showed a significant growth but during food limitation a significant decrease in volume, reflecting plasticity in morphology. This shows the capability of A. planci to store energy reserves in their pyloric caeca and translocate these reserves when required during food limitation. The ability of A. planci to mobilize their energy reservoir while covering considerable distances leads to the conclusion, that these starfish are able to migrate from reef to reef.