Host-Parasite Relationships between Gyrodactylus stellatus (Monogenea: Gyrodactylidae) and Parophrys vetulus (Pleuronectidae: English Sole) from Coastal Waters of Oregon

Abstract

Gyrodactylus stellatus Crane and Mizelle, 1967, was found on juvenile English sole (Parophrys vetulus), speckled sanddabs (Citharichthys stigmaeus) and starry flounder (Platichthys stellatus) in Yaquina Bay, Oregon. Prevalence and intensity of infections were higher on juvenile English sole than on speckled sanddabs or starry flounders. Over a 12-mo period from July 1981 to June 1982, prevalence and intensity of infection on juvenile English sole averaged 69.2% per sampling and 5.5 worms per fish, respectively. Prevalence and intensity were maximum in June (98.0% and 10.1 worms/fish) and minimum in October (37.0% and 1.7 worms/ fish). In the laboratory, intensity on juvenile English sole reached higher levels than observed on hosts in the estuary. Intensity increased for 9 wk then decreased over the next 3-wk period. Rate of G. stellatus population growth increased with temperature from 6 to 18 C, and was higher on unfed than on fed English sole. Salinity and previous experience with high infection levels did not significantly influence the rate of G. stellatus increase. English sole (Parophrys vetulus Girard, 1854) is one of the most common pleuronectid species found along the Pacific Coast of the United States and an important component of the commercial fishery (Hart, 1973). Large numbers of juvenile English sole utilize Yaquina Bay as a nursery ground during their first year of life (Olson and Pratt, 1973) and according to Olson (1978) are commonly parasitized by Gyrodactylus stellatus Crane and Mizelle, 1967. Gyrodactyliasis caused by several species of monogenetic trematodes in this genus has been reported in both freshwater and marine teleosts (Cone and Odense, 1984). This study examines the host-parasite relationship between English sole and G. stellatus. MATERIALS AND METHODS Fishes from Yaquina Bay, Oregon were collected with a 5-m otter trawl and transported to the Oregon State University Marine Science Center. Juvenile English sole (P. vetulus) to be used in experiments were held in circular tanks (87 cm in diameter with 125 L of seawater) supplied with running ultraviolet light filtered seawater (3-4 L/min) for 1-2 wk to allow feeding to begin. Experimental fish were of similar size (7.5 + 3.2 cm long) and free of abnormalities. All fish were maintained on a diet of commercial moist salmon feed. Uninfected experimental fish were obtained by treating with a 1:4,000 formalin solution for 1 hr (Putz and Hoffman, 1963). Laboratory infections were produced by placing uninfected fish in a tank with 2-3 infected fish for 1-3 days. Received 16 April 1985; revised 27 September 1985; accepted 30 September 1985. Host specificity of G. stellatus was studied ecologically by examining a variety of potential host species from Yaquina Bay and experimentally by exposing potential hosts to the parasite in the laboratory. Prevalence and intensity of infection of G. stellatus on English sole from Yaquina Bay by season and by size of fish were determined from samples taken monthly from July 1981 to June 1982. Shortly after capture, 50 fish were selected randomly and the number of Gyrodactylus present on the fins of each fish was counted under a dissecting microscope. If the catch was less than 50 fish, all were used as samples. The method by which G. stellatus transfers from one host to another was examined in an experiment that consisted of 4 treatments. In the first and second treatments infected English sole were kept separate from uninfected hosts in flowing and static water. In the third treatment, infected and uninfected fish were allowed direct contact in static water. In the last treatment infected English sole were removed from the container before introduction of uninfected sole. Each container contained 2 infected and 3 uninfected English sole. The rate at which G. stellatus increased in number on juvenile English sole (8.8 ? 1.08 cm long) under laboratory conditions and ambient temperature was determined by following the course of infection on 50 naturally infected individuals. The number of worms on pectoral, ventral and caudal fins on each fish was counted weekly for 1 yr. To determine the effect of temperature on intensity of infection, naturally infected English sole were held in circular tanks of running seawater maintained at 6, 12, or 18 C. Each treatment was replicated with 25 fish (10.3 ? 1.12 cm long) per replicate. The experiment extended for 6 wk and the number of G. stellatus per fish was counted weekly. In an experiment to determine the effect of salinity on intensity of G. stellatus infections, infected fish (10.1 + 0.62 cm long) were maintained at salinities of 14, 21 and 28%o in static water.