Identification of Potential Predators for Asari Clam Ruditapes philippinarum Using Time-lapse Camera Observations

Abstract

Predation is considered a significant factor contributing to the recently observed low survival rates of asari clam (Ruditapes philippinarum) in Japan. Longheaded eagle ray, blackhead seabream, portunid crabs, predatory gastropods, and ducks are suggested as potential predators; however, the relative significance of these predators has yet to be evaluated. In this study, we conducted 31 single-day time-lapse camera observation trials during summer at 28 stations within 12 habitats of the asari clam in Japan (ranging from temperate to subarctic regions) to determine the relative abundance of predators in each habitat. And in a trial at the Nakatsu tidal flat in southwestern Japan, where a previous study observed a low survival rate of the asari clam, the absolute abundances of different predator taxa were estimated by quantifying the underwater visibility and visible area in images. The blackhead seabream (Acanthopagrus schlegelii), a known temperate subtropical species, was identified as the most frequently observed predator. A. schlegelii was widely observed in 8 of the 12 habitats in the southwestern to central regions of Japan. The longheaded eagle ray was not observed, and portunid crabs and predatory gastropods were few in this study. At Nakatsu, a maximum of 39 individuals of A. schlegelii were observed in a single trial (via single-day images captured every 2 min.), indicating the significance of this predator relative to asari clam mortality at this site. A. schlegelii appeared on the tidal flat during high tide, and its hourly mean abundance exceeded 20 ind./100 m² during high tide after dawn. Rising seawater temperatures along Japan’s coast might increase the predation risk for asari clam posed by temperate-to-subtropical risk species.