Causative agent for dark spots in ocean surgeonfish (Acanthurus tractus)
Coral reef ecosystems provide a number of important ecological services, such as nurseries and protection from storms. This makes their health of vital importance for human populations. Past epidemics in the Caribbean involving high mortality of predominant species, such as long-spined sea urchins (Diadema antillarum) and elkhorn coral (Acropora palmata) have shown the potential of disease to fatally disrupt coral reef ecosystems already under stress. The high prevalence of an unknown disease in ocean surgeonfish (Acanthurus tractus) in the Caribbean, and its apparent ability to infect other fish, including parrotfish and other predominant grazers, is a source of concern since it affects a number of herbivorous fish that are integral to the health of the reefs. This disease is identified by the presence of black spots over the body and fins of infected fish. The number of spots can vary widely. Fin rot and lethargic behavior have been noted in fish with large numbers of spots. Bacterial cultures of swabs from healthy and dark spot epidermis, and necropsy of eight A. tractus specimens were used to attempt to identify the causative agent. This study found smaller bacterial numbers in the dark spot epidermis compared to healthy epidermis cultures, and the presence of encysting organisms embedded in the epidermis directly below black spots in body and fins of A. tractus. Additional encysting organisms were found deeper in the muscle tissue and did not produce a black spot. These encysting organisms are proposed to be digenean trematodes in the metacercariae life stage.