Abstract
Connectivity between habitats and ecological communities is a critical component of trophic structure. Coral reef systems include reef, seagrass, and mangrove habitats, and the movement of fishes is a key component of habitat connectivity among them. Fishes that undergo diel migrations between habitats are among the best-studied functional groups. Studies on their role in energetic connectivity between adjacent habitats have not considered the possible contribution of parasites. Some diel-migratory species are both highly susceptible to and disproportionately exploited by gnathiid isopods, temporary, tick-like parasites of marine fishes. By leaving the reef at night, diel-migratory fishes reduce their overall exposure to gnathiids, which are more active at night and more abundant in reef habitat. Here we show that for sites in both the Caribbean and the Great Barrier Reef, gnathiids are attached to diel-migratory fishes at the time they depart reef habitat. Because gnathiids associate temporarily with host fishes, they can be acquired by hosts in one habitat and can become dislodged and deposited in another. Field experiments in the Caribbean show that gnathiids from reef habitat dislodge in seagrass habitat, where they likely remain until their next feeding. Sequencing blood meals from free-living gnathiids in seagrass beds, where they are least abundant, shows that diel-migratory and other transient fishes are the most frequently exploited hosts, confirming that deposition of gnathiids in seagrass is facilitated mainly by migratory hosts. These findings have important implications for trophic, population-genetic, and disease connectivity involving gnathiid isopods and potentially other external parasites.