Patterns of Infestation of Fish by Parasitic Gnathiid Isopods and Monogeneans in the Virgin Islands

For the past four years, we have studied the infestation dynamics of parasitic larval gnathiid isopods on St. John reefs throughout the 24 h diel cycle. Gnathiid infestation on caged damselfish peaked strongly at dawn, remained low during the remainder of the day, and increased again at night, until about midnight. Gnathiids were largely inactive during the pre-dawn period. Peak loads on fish retrieved at dawn were the highest reported in any study thus far. The dawn peak consisted almost exclusively of individuals from the smallest size class, whereas nocturnal activity consisted almost exclusively of individuals of the largest size class. Fish placed in PVC tubes with small funnel openings and deployed on the reef overnight had virtually no gnathiids. Thus, gnathiids appear to locate hosts using visual cues. Because of the high rates of infestation at night and dawn, reduction of parasite infestation may play an important role in the selection of nocturnal and crepuscular shelter holes and settlement sites by reef fishes, and appears to explain the dawn peak in visits to cleaning stations.

During the past year 2005-2006, we quantified parasitic monogenean loads from the skin of two species of surgeonfish collected at four sites in the U.S. and British Virgin Islands. Only 3% of the 90 ocean surgeonfish (Acanthurus bahianus) collected were infected, compared with nearly 55% of similar-sized blue tang (A. coeruleus) collected from the same sites. Loads among A. coeruleus varied significantly among sites. The highest loads occurring on fish collected from the three shallow bay sites, especially off Guana Island, BVI, while virtually none of the fish collected from Flat Cay were infected. Preliminary experiments suggest that differences in susceptibility explain the strong between-species difference, while local environmental factors are the most likely explanation for the among site differences. In addition, we appear to have solved the almost 80-year old mystery of the origin of the type material of Neobenedenia melleni (MacCallum, 1927) and use that information to restore the species Neobenedenia pargueraensis Dyer, Williams and Bunkley-Williams, 1992, as the Atlantic representative of the N. melleni species complex. We are currently conducting experiments to determine whether Caribbean cleaner shrimps remove monogeneans from fish hosts.
Presenter: Paul C. Sikkel

Ph.D. Oregon State University 1992

Assistant Professor, University of the Virgin Islands 1994-1997

Current Affiliation: Research Assistant Professor, Dept. of Biology, Murray

State University, Murray, KY 42071 and Visiting Assistant Professor Biology

Program, Centre College, Danville, KY. 40422. E-mail: