Taphonomy of Crown-of-Thorns Starfish - Implications for Recognizing Ancient Population Outbreaks

A field experiment was conducted to determine whether in situ mass mortality of Acanthaster planci subsequent to a simulated outbreak would leave a recognizable signature in surficial reef sediments. Constituent particle analyses comparing sediments that received starfish carcasses to those that did not revealed that, after a four year interval, the mass mortality was reflected by elevated abundances of starfish ossicles in 1-2 mm and 2-3 mm size classes, but not in > 4 mm and 0.5-1 mm size classes. Additional field study of starfish taphonomy revealed that the abundance of starfish ossicles in surficial sediments decreases through two orders of magnitude between two weeks and four years post-mortem, while tumbling experiments suggest that the size distribution of starfish ossicles is modified by physical processes: the 4 mm class. Taphonomic biasing increased the abundance of crown-of-thorns starfish (COTS) skeletal elements in the 0.5-1 mm size fraction, while the 1-2 mm size and 2-4 mm fractions produced the most reliable signature of starfish mass mortality based on element abundance. Our results demonstrate the importance of taphonomic processes in altering the original size frequency distribution of the COTS skeleton and their potential for biasing predictions of past population levels derived from constituent particle analyses of surficial reef sediments