Prevalence and pathogenicity of a heterophyid trematode infecting the gills of an endangered fish, the fountain darter, in two central Texas spring-fed rivers

Gills of 194 fountain darters Etheostoma fonticola collected from the Comal River in Texas from May 1997 through May 1998 were found to be parasitized with 8-1,524 metacercarial cysts of a heterophyid trematode tentatively id entified as Centrocestus formosanus. The intensity of infection Varied amon g three sites on the Comal River. In contrast, of 130 darters from the near by San Marcos River that were examined, only 4 (3%) were infected, and thes e had 1-2 cysts per fish. Of 2,279 Melanoides tuberculata snails from the C omal River that were examined, 139 (6.1%) were infected with the trematode. Only 1 snail in 2,241 from the San Marcos River that were examined was inf ected. The presence of metacercariae in darters was associated with flared opercula, shortened or thickened gill filaments, epithelial hyperplasia, an d engorged lamellae. The normal cartilage support of the filaments was dist orted and displaced, leading to severe deformities of filament structure. G ill damage was severe and possibly life threatening for the darters with mo re than 800 cysts per fish (9% of examined fish). We suspect that fountain darter deaths were caused by the parasite in the Comal River during this st udy.