Identification of Pre-Settling Behavior Leading the Planula Larva to Settlement in the Coral Acropora.

Abstract

Settlement, the transition from planktonic life to sedentary life, is a critical phase in the life cycle of corals, and is an important factor for population dynamics influencing new recruitment. However, previous studies on coral metamorphosis and settlement have overlooked actual processes from swimming to settlement on substrates. In this study, we discovered unique behavior of planula larvae of the coral Acropora that led to settlement on substrates covered by biofilms and we thus termed "pre-settling behavior". Pre-settling behavior consisted of three distinct steps in the sequence: (1) random crawling, (2) circular crawling, and (3) upright pausing. These steps were thought to correspond to the search for, evaluation of, and confirmation of appropriate settlement positions, respectively. Crawling speed and trajectories were analyzed, and their features are described. Since settlement occurs as adhesion of the aboral side to the substrate, it is conceivable that upright pausing is essential for metamorphosis to settled polyps as normal development. Frequent abortion of pre-settling behavior at any step to return to free swimming provides a view of repeated trials to seek settlement positions. Circular crawling in the counterclockwise direction was performed by forward movement with body curvature which determined the circle's radius, together with clockwise body spinning without forming helical trajectories. This movement suggests that changes of body curvature positions are synchronized with spinning to produce smooth, circular trajectories. The discovery of pre-settling behavior may contribute to understanding of settlement mechanisms and to coral seedlings production by elevating the settlement efficiency.