Habitat associations of fish-parasitic gnathiid isopods in a shallow reef system in the central Philippines

Gnathiid isopods are among the most common ectoparasitic organisms infesting marine fishes in benthic marine environments, and are best known in coral reef ecosystems. These micropredators are ecologically important, as they can impact their hosts directly through blood loss and tissue damage, and indirectly by transmitting blood parasites. Their abundance, and thus expected impact on hosts, varies greatly over multiple spatial scales. Most studies on gnathiids in coral reef systems have been conducted in the Caribbean and on the Great Barrier Reef, Australia, with few studies conducted in the heart of marine biodiversity, the “Coral Triangle”. This is the first field study of gnathiid ecology in the Philippines. The primary goals of this study were to characterize substrate associations and diel activity patterns of gnathiids in shallow reef areas. Gnathiid larvae were found to infest at least 28 species of hosts and were found in both seagrass and reef habitats. Within reef habitat, there was a significant difference in gnathiid abundance among coral reef substrates. Gnathiid larvae were most abundant in coral rubble substrates and least abundant on live hard coral. This is in spite of host abundance (fish) being greater near live coral. More and larger-sized gnathiids were collected from dusk to dawn compared to daytime, and gnathiids were able to find hosts using non-visual sensory cues. Our results are, thus, consistent with previous studies in the Caribbean.