Abstract
Symbiotic relationships between corals and invertebrates contribute significantly to coral reef biodiversity. However, their ecological functions within this ecosystem remain understudied due to limited knowledge of the interplay among lifehistory strategies, host density and condition, population variations, and mortality rates. To address this, we investigated the population dynamics of coral-dwelling gall crabs (Cryptochiridae), obligate symbionts of scleractinian corals, across four central Red Sea reefs. Combining transect surveys with a novel fate-tracking approach, we monitored 799 crabs on 517 host colonies from September 2022 to 2024. Our data revealed significant variation in host community composition, with reef-specific conditions shaping crab abundance and diversity more than cross-shelf gradients. Fate-tracking uncovered unexpectedly frequent crab colonization and extinction events and a strong preference for settling on already inhabited hosts. In 2023, a mass reef bleaching event provided a unique opportunity to assess disturbance impacts one year into our study, resulting in greater population declines on inshore reefs. Interestingly, fate-tracking showed that most sites maintained reproductively active crab populations despite bleaching, while compounded stressors at one site caused a local population collapse. Our findings underscore the complex dynamics of the relationship between cryptochirids and their coral hosts, where high reproductive output may offset the costs of host specificity and settlement requirements, thus enabling resilience to moderate disturbances. This study provides novel insights into cryptochirid ecology, revealing unexpectedly high temporal variability in their populations. The observed dynamics suggest gall crabs may occupy a functional role akin to cryptobenthic reef fish by contributing to reef energy transfer, converting host-derived resources like coral mucus into forms accessible to higher trophic levels and supplementing zooplankton communities with larvae. In light of increasing disturbances, this study highlights the need to integrate reef invertebrates into coral reef conservation strategies for preserving biodiversity and sustaining ecosystem functionality in a rapidly changing world.