An experimental field test of host-finding mechanisms in a Caribbean gnathiid isopod

Field experiments were conducted from dusk to dawn off St. John (18° 18′ 59.32″ N, 64° 43′ 24.5″ W) and Guana Island (18° 28″ 28.31″ N, 64° 34′ 30.83″ W), Virgin Islands from June through August 2008-2010 to assess the sensory cues used by the nocturnal/crepuscular fish-parasitic gnathiid isopod, Gnathia marleyi, to locate fish hosts. Experimental traps providing both visual and olfactory cues from live French grunts (Haemulon flavioliniatum) attracted significantly more gnathiids than traps providing only visual cues or control traps (empty or with a rock), which were not significantly different from each other. In another experiment, traps providing both cues and only olfactory cues attracted significantly more gnathiids than empty control traps, but were not significantly different from each other. Our findings suggest that during nocturnal and crepuscular periods, visual cues provided by resting or slow-moving fish are not alone sufficient to attract gnathiids, while olfactory cues alone are. The traps designed for this study offer a new method of sampling free-living gnathiid isopods.