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Parasites play significant roles in the function of ecosystems and can make up a large proportion of overall biomass. Yet, fundamental aspects of their ecology are often understudied relative to other organisms. Gnathiid isopods are the primary ectoparasites of fishes in coral reef ecosystems. While some studies have investigated their host-detection capabilities, the means by which they seek hosts are largely unknown. Gnathiids are benthic and live on the bottom, and all collection efforts involving live hosts have thus far involved traps set at or near the reef substrate. We investigated the distance gnathiids will travel vertically in the water column to attach to a fish host. The majority of gnathiids collected were at or less than 1 m above the reef substrate, and gnathiids were collected in serially diminishing numbers at 2 and 3 m above it. No gnathiids were collected from any fish hosts set more than 3.5 m above the reef substrate. Results suggest that gnathiids will actively seek host fishes in situ and will travel further from their benthic habitat than previously known. By swimming into the water column, gnathiids can exploit larger, highly-mobile fishes, which can serve as a major source of dispersal.

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