Feeding habits of the white-spotted boxfish Ostracion meleagris reveal a strong preference for colonial ascidians

Abstract

Boxfishes (Ostraciidae) are known to be benthivores and mainly consume sessile organisms on reefs, such as tunicates. Ascidian tunicates contain various toxins and seldom suffer from predation by reef fishes, except for boxfishes. However, the importance of ascidian tunicates has not been sufficiently examined in the feeding ecology of boxfishes. We conducted field surveys to investigate the feeding habits of the white-spotted boxfish Ostracion meleagris on the reefs of southern Japan. Photosymbiotic ascidians predominantly contributed to the boxfish diet, and observed colonies of ascidians were bitten frequently by the boxfish during the survey. During the day, the feeding boxfish moved within their home ranges in a rocky, shallow zone of the studied reefs, which harbored photosymbiotic ascidian colonies of the family Didemnidae at higher densities than in the surrounding zones, including the spawning sites that were located hundreds of meters offshore. Boxfish home ranges overlapped considerably, but intraspecific agonistic interactions rarely occurred among the females, likely due to the abundance of ascidian colonies. No other types of fish were observed to make attempts to feed on the ascidian colonies on the reef throughout the study period, except for a congener Ostracion cubicus. These results suggest that the boxfish strongly rely on the photosymbiotic ascidians as a less-competitive food resource, and the limited distribution pattern of photosymbiotic ascidians in shallow water near shore can be an important factor in promoting a spawning migration in the boxfish.