Effects of the crown-of thorns starfish (Acanthaster planci) and the sea urchin (Echinometra mathai) on coral reef community structure at Sesoko Island, Okinawa, Japan

Acanthaster planci densities were estimated within a 300 x 30 m strip of outer reef flat and slope in front of the Sesoko Marine Science Center on Sesoko Island, Okinawa over a period of 10 days. The study site was searched, every day or second day on snorkel. Starfish not found out in the open were usually found by searching near fresh feeding scars.

Sea urchin densities were measured within twenty contiguous 0.25 m² quadrats at each of five randomly selected sites within the study area. The quadrats were also searched for other potentially important coral predators such as the gastropod, Drupella.

To examine Acanthaster feeding preferences, the locations of ten adult Acanthster planci were marked with a surface buoy and any nearby feeding scars were marked with crayon. Percent cover of hard and soft corals within a 5 m radius of each buoy was estimated by eye. Feeding activity of the starfish was monitored every day or second day over a nine day period and new feeding scars in the areas marked by the buoys were marked and measured. All corals (scleractinia, non-scleractinia, alcyoniids) and zoanthids >= 1 cm in diameter, within a 40 cm radius of the eaten colony were identified to genus in situ and measured. Samples of corals were collected when identification was not possible in the field. The diameter of roughly circular corals was measured, otherwise the greatest and least dimension of the projected surface was measured. The nearest distance between each colony and the prey colony was also measured.

A night dive was also conducted to determine if juvenile Acanthaster planci and other animals such as Culcita novaeguineae or Dupella, not observed during the day, were eating the small coral colonies.

Data were used to derive an index of feeding preferences in the form of the ratio of proportion eaten to proportion available, for each coral family and genus, both in terms of numbers of colonies and area ("coral cover" calculated from diameter or dimensions) of colonies.