From co‐occurrence to temporal coupling in a coral defender guild

Coral‐dwelling decapods can improve host fitness by defending corals against corallivores and sediment accumulation, yet the temporal dynamics linking co‐resident defenders remain poorly understood. Guard crabs Trapezia and snapping shrimp Alpheus provide complementary benefits to pocilloporid corals, and previous, largely cross‐sectional work has reported behavioral accommodation, non‐random co‐occurrence, and effects on host‐associated community assembly, patterns that suggest their association reflects facilitation instead of independent use of shared habitat. Whether this association persists as coupled dynamics through time, however, remains untested. Using repeated observations of tracked colonies spanning up to six years and three months in the northern Red Sea, we tested whether these taxa exhibit non‐random co‐occurrence and linked population dynamics within pocilloporid hosts. Across two coral species, Trapezia was more common than Alpheus , and the presence of shrimp was strongly associated with that of crabs. In colonies of Pocillopora favosa (n = 35, 733 observations), Alpheus abundance increased with Trapezia abundance, and mean abundances per colony of the two taxa were positively correlated, indicating persistent demographic coupling over time. While larger colonies held more symbionts, this abundance coupling was not fully explained by colony size. Temporal peaks in abundance were often synchronized, producing temporary assemblages that exceeded the typical pair occupancy expected for these symbionts. In colonies of Stylophora pistillata (n = 40, 710 observations), shrimp were rare, but when present, they occurred only in colonies inhabited by crabs. Our results reveal that the within‐host dynamics of co‐resident decapod mutualists are more tightly linked than previously recognized, consistent with facilitative interactions mediated through a shared host. Because such defenders can enhance host condition, their sustained co‐residence may have broader relevance for the persistence of the corals that structure these reefs and their associated assemblages.