The Titan Triggerfish (Balistoides viridescens - Bloch and Schneider, 1801): An Ecosystem Engineer in a Feedback Loop

Abstract

Coral reef biodiversity relies on a dynamic balance between destruction and renewal. In this dynamic, the titan triggerfish (Balistoides viridescens) plays a paradoxical role—visibly damaging reefs by breaking coral rock, yet enhancing structural integrity by preying on bioeroding bivalves that undermine reef frameworks. This investigation evaluates B. viridescens as an ecological engineer in a feedback loop, examining how its foraging influences reef structure. Titan feeding and the resulting bioerosion were observed in various locations, including Lakshadweep, Indonesia, and many locations in the Maldives. An exploratory study was conducted on Kunfunadhoo Reef in Baa Atoll, Maldives. Surveys quantified coral cover, bivalve density, triggerfish abundance, and bioerosion rates. Coral mortality in 1998 produced abundant substrate for boring bivalve invasion, and by 2004, erosion of dead branching and tabulate corals increased the exposure of coral rock colonized by boring bivalves—a preferred prey of B. viridescens. While titan bioerosion rivaled that of individual scarids, parrotfish collectively eroded far more substrate. Although titan bioerosion may facilitate reef renewal by clearing weakened substrate, its benefits depend on CaCO₃ accretion exceeding erosion—a balance increasingly threatened by ocean warming and acidification. This study explores the titan's dual role in reef erosion and construction, and indicates the need for broader spatial and temporal assessments.