Printed in the Philippines ASIA LIFE SCIENCES 23(1): 65-73, 2014 The Asian International Journal of Life Sciences Received 14 December 2012; Accepted 24 July 2013 ©Rushing Water Publishers Ltd. 2014 Abundance and size structure of an Acanthaster planci population (Echinodermata: Asteroidea) in Sogod Bay, Southern Leyte, Philippines HOMER HERMES Y . DE DIOS 1* , DANILO T . DY 2 and FILIPINA B. SOTTO 2 The distribution and population density of Acanthaster planci (crown-of-thorns-starfish or COTS) in Sogod Bay, Southern Leyte, Central Philippines was assessed on July-August, 2012 using belt transects of 50-80 quadrats (5 x 5 m). Four sites were considered. Mean COTS density varied significantly among sampling sites, ranging from 0.4-5.2 starfishes/25 m 2 corresponding to 160-2,080 starfishes/ha, an indication of an outbreak. On the other hand, the mean body size of A. planci ranged from 13.7-32.4 cm, varied significantly among sampling sites and possibly belonging to different cohorts of COTS population within Sogod Bay. This suggests that outbreak could be due to progressive accumulation of COTS from multiple cohorts. At Adrian Reef Marine Sanctuary in Limasawa, COTS distributions were aggregated (Morisita’s Index of Dispersion = 2.6) and was significantly larger (32.4 cm) than the other sites. We believe that the aggregation at Adrian Reef Marine Sanctuary is a form of foraging behavior at this time of the year and not a preparation for mass spawning. Keywords: crown-of-thorns tarfish, COTS, Acanthaster planci, aggregation, Morisita’s Index of Dispersion, reefs, MPAs, spatial distribution, Sogod Bay, Southern Leyte, Philippines 1 Southern Leyte State University-Bontoc Campus, Bontoc, Southern Leyte, Philippines 2 Marine Biology Section, University of San Carlos, Cebu City 6000, Philippines *Corresponding author: e-mail - homeryray@yahoo.com