Host species: Host fishes selected for testing were French grunt (Haemulon flavolineatum: Haemulidae), lane snapper (Lutjanus synagris: Lutjanidae), longspine squirrelfish (Holocentrus rufus: Holocentridae), and longfin damselfish (Stegastes diencaeus: Pomacentridae). French grunt and lane snapper are carnivorous, nocturnal fish that forage in seagrass and sandy areas at night and school on reefs during the day. Both species are highly and similarly susceptible to gnathiids. French grunts are locally abundant and are known to be able to attract gnathiids with only chemical cues and therefore were used as a standard for the experiments (see below). Longspine squirrelfish are nocturnal invertivores that forage at night and shelter in or near refuges on reefs during the day. Finally, longfin damselfish are diurnal herbivores that are highly territorial. Longspine squirrelfish and longfin damselfish represent moderate and low susceptibility to gnathiids, respectively.
Field collection: Juvenile Gnathia marleyi and adult host fishes were collected from John Brewers Bay, St. Thomas, United States Virgin Islands (18°20’ N, 64°58’ W) from May to August 2019 and January to March 2020. Gnathiids were collected using zooplankton light traps similar to those de- scribed by Artim and Sikkel (2016). They were held at the University of the Virgin Islands’ MacLean Marine Science Center in small colonies in 150-milliliter (ml) containers filled with filtered seawater. All host fishes were caught by free-divers using cast nets and were held in a 2800-liter tank with continuously flowing seawater, at 28 degrees C. Fish were held for no longer than 24 hours and all were returned to the original capture site.
Field experiments: To determine the ability of gnathiid isopods to detect and locate hosts using chemosensory cues and whether they used these cues to distinguish between host species, traps similar to those described in Sikkel et al. (2011) and Santos and Sikkel (2019) were used. These traps were constructed using a PVC tube (20 centimeters × 5 centimeters) with two removable, inward-facing funnels. The hole in the bottom of the funnel was sealed and four 1 centimeter diameter holes were drilled in the side of the funnels, allowing chemical cues to be released without the gnathiids being able to see the fish from outside the trap. Traps were placed haphazardly on the reef in pair-wise choice arrays. Pairings included: 1) French grunt versus empty (Control); 2) French grunt versus lane snapper (high vs high susceptibility); 3) French grunt versus longspine squirrelfish (high vs medium susceptibility); and 4) French grunt versus longfin damselfish (high vs low susceptibility). Fishes of similar size were paired for trials. The traps were placed more than 1 meter apart 1 hour before sunset and collected 1 hour after sunrise. This ensured that traps were deployed at times of peak gnathiid activity. Water from the traps was then filtered through 55-micrometer (μm) plankton mesh and fish were checked to assure no gnathiids were still feeding. The number of gnathiids in each trap was counted and all gnathiids were categorized by stage. In total, 66 pairs of traps were deployed (Control: n = 10; lane snapper: n = 15; longspine squirrelfish: n = 20; longfin damselfish: n = 21).
