Black-spot syndrome in Caribbean fishes linked to trematode parasite infection (Scaphanocephalus expansus)

Abstract

Despite the evidence that diseases have increased in marine taxa, parasites remain underrepresented in studies of marine ecology. Recently, observations of black-spot syndrome (BSS) in Caribbean fishes, especially ocean surgeonfishes, Acanthurus tractus (Poey, 1860), have been reported, although its cause(s) has remained conjectural. We investigated the etiology of BSS and whether the pathology was functionally and consistently associated with particular infections. By examining the patterns of BSS among reef fishes in Bonaire, we show that observed dermal spots are the encysted stages (metacercariae) of the heterophyid trematode Scaphanocephalus expansus (Creplin, 1842). Metacercariae were detected within ten species of fishes, with loads ranging from 1–315 per fish. These represent the first published accounts of the genus Scaphanocephalus in the Caribbean and are all new host records. Molecular analysis of sequences identified the infection as a heterophyid, while morphological examinations and comparisons with archival specimens confirmed the species identity as S. expansus. Field-based estimates of BSS by scuba divers and subsequent video analysis correlated positively with parasite counts from necropsy. No such correlation was observed between BSS and metacercariae of a second trematode genus, Bucephalus. The consistent link between host burdens of S. expansus and BSS highlights the potential for low-cost and low-impact assessments in fish populations, including the use of images from citizen scientists and public databases. Taken together, these results help establish the foundation for future investigations into the pathology, geographic distribution, life cycle, and ecological consequences of BSS.