Effects of various treatments on egg hatching of Dendromonocotyle pipinna (Monogenea: Monocotylidae) infecting the blotched fantail ray, Taeniurops meyeni, in Taiwan

Abstract

The monocotylid monogenean Dendromonocotyle pipinna infects the dorsal skin of the blotched fantail ray, Taeniurops meyeni, and is problematic for Hualien Farglory Ocean Park, an aquarium in Taiwan. Over the last 2 years, eight rays have died due to heavy infections with this parasite. In this study, we found that the epidermis of T. meyeni with attached D. pipinna was not markedly thinner but contained decreased numbers of mucous cells and numerous vacuoles. We examined the effects of temperature (from 16 to 30 degrees C in 2 degrees C increments), salinity (from 10 to 50 per thousand in 5 per thousand increments), desiccation (from 1 to 10 min) and sodium hypochlorite (from 5 to 20 ppm in 5 ppm increments) treatment on the embryonation period and hatching success of D. pipinna eggs, with the goal of disinfecting equipment used in aquaria. Temperature strongly influenced embryonation period: eggs first hatched 4 days after being laid at 30 degrees C and 16 days after being laid at 16 degrees C. However, hatching rate was not significantly influenced by incubation temperature, since the final hatching rates under the incubation temperatures tested herein were not significantly different from one another. Hyposalinity had a lethal effect on D. pipinna eggs, completely preventing the hatching of eggs cultured at 10 and 15 per thousand salinity. Hypersalinity was only partially effective, with a hatching rate close to 7% at 50 per thousand salinity. Desiccation was effective at preventing hatching, and the effectiveness increased with increasing treatment duration. The hatching rate of D. pipinna eggs was significantly decreased when incubated under desiccating conditions for even 1 min. Furthermore, a complete inhibition of hatching was achieved by desiccating eggs for 10 min. Sodium hypochlorite treatment completely prevented hatching at concentrations higher than 10 ppm after 18 h of exposure, but a concentration of 5 ppm was ineffective at preventing hatching even after 24 h of treatment. We therefore propose effective combinations of sodium hypochlorite and exposure time as a means to sterilize tanks and equipment containing D. pipinna eggs.