Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2000
Publication Date: 12/1/2000
Citation: MITCHELL, A.J., SALMON, M.J., HUFFMAN, D.G., GOODWIN, A.E., BRANDT, T.M. PREVALENCE AND PATHOGENICTY OF A HETEROPHYID TREMATODE INFECTING THE GILLS OF AN ENDANGERED FISH ETHEOSTOMA FONTICOLA IN TWO CENTRAL TEXAS SPRING-FED RIVERS. JOURNAL OF AQUATIC ANIMAL HEALTH. 2000. v.12. p.283-289. Interpretive Summary: An endangered species, the fountain darter, inhabits the Comal and San Marcos Rivers near San Marcos, Texas. The survival of any endangered fish is questionable but this fishs survival is further compromised by the presence of an invading parasite. The invading parasite, a digenetic trematode, was first observed in darters collected in 1994 from the Comal River and has a complex life cycle involving snails and birds as well as fish. The trematode is carried by an exotic snail (the red-rimmed melania) that was introduced into Texas around 1960. The snail releases infective stages that penetrate the gills of the darters and in high numbers cause severe damage. As many as 1524 trematode cysts were found on a single fish about 3 cm long. All the 194 darters examined from the Comal River had light to severe trematode infections. Only 4 out of 130 (3%) fish were infected in the San Marcos River. We suspect that heavily infected fountain darters may die directly from the resulting gill damage and that fish with moderate damage may succumb in stressful conditions such as low dissolved oxygen. These conditions are known to occur in the Comal River. Although unidentified, the parasite may be an exotic trematode from India and neighboring countries that has spread to many other countries including Mexico.
Technical Abstract: Gills of 194 fountain darters Etheostoma fonticola collected from the Comal River, Texas, from May 1997 through May 1998, were found to be parasitized with 8 - 1,524 metacercarial cysts of a heterophyid trematode tentatively identified as Centrocestus formosanus. The intensity of infection varied among three Comal River sites. In contrast, of 130 darters examined from the nearby San Marcos River only 4 (3%) were infected with 1 to 2 cysts per fish. Of 2,279 snails Melanoides tuberculata examined from the Comal River, 139 (6.1%) were infected with the trematode. Only 1 snail in 2,241 examined from San Marcos River was infected. The presence of metacercariae in darters was associated with flared opercula, shortened or thickened gill filaments, epithelial hyperplasia, and engorged lamellae. The normal cartilage support of the filaments was distorted and displaced leading to severe deformities of filament structure. Gill damage was severe and possibly life- threatening for the darters with >800 cysts/fish (9% of examined fish). We suspect that fountain darter deaths were caused by the parasite in the Comal River during this study.