Defensive responses of titan triggerfish to tiger sharks at a provisioned reef.

Titan triggerfish (Balistoides viridescens) are among the most territorial reef fishes, known for aggressively defending nests from intruders. In the Maldives' Fuvahmulah atoll, where tiger sharks (Galeocerdo cuvier) aggregate in high numbers year-round, we opportunistically documented 10 interactions between these species from February to August 2024 during daily diving operations. Video footage from experienced divers was analyzed to identify and categorize aggressive behaviors, defined as bites (rapid, forceful closure of the jaws on the shark's body) and chases (short pursuits following an aggressive display). All observed aggression was initiated by titan triggerfish, most often targeting the caudal fin of individual tiger sharks. Biting accounted for 70% of interactions, with chases comprising the remainder; over half of bites were immediately followed by a chase. Several interactions occurred near the new moon, coinciding with the species' nesting period, suggesting that many of these interactions may have been linked to breeding-season territoriality; however, the opportunistic nature of the observations precluded any formal analysis of lunar phase patterns. These behaviors likely function as risk-based defense, exploiting anatomical vulnerabilities to deter much larger predators. The high frequency of these interactions in a location with artificially dense tiger shark populations suggests that provisioning and predator aggregation may increase the likelihood of such cross-trophic encounters. By linking detailed behavioral observations with the ecological context of predator aggregation, this study highlights the defensive capabilities of titan triggerfish and raises questions about how ecotourism-driven changes in predator distribution influence the behavior of non-target reef species.