Characterization of freshwater as a treatment for amyloodiniosis in red drum

Abstract

The ectoparasitic dinoflagellate Amyloodinium ocellatum is a problem of long standing in marine aquaria and recently has emerged as a cause of mortality also in marine food-sh-production facilities. The parasitic stage (trophont) attaches to the gills, skin, and fins of host fish such as red drum (Sciaenops ocellatus), a euryhaline teleost cultured intensively for seafood in Texas and other Gulf states. Amyloodinium ocellatum has a wide range of tolerance to salinity, but freshwater exposure is considered an effective therapy for infested fish. Bioassays were conducted to determine the intervals of time A. ocellatum-infested red drum need to spend in freshwater to result in various degrees of trophont detachment from the gills. Results resolved the time dependency of trophont detachment on freshwater exposure, but also indicated substantial effects of infestation age and of fish weight and group identity. Freshwater dips were generally effective in causing trophont dislodgment from A. ocellatum-infested red drum at an instantaneous rate of 20% per minute. Within the first 24 h of infestation by a trophont generation, however, effectiveness of a freshwater dip varied markedly with fish group. Fish groups fell into two sets, inferred to be comprised of fish with and without prior exposure to A. ocellatum. In putatively "nav̐e'' fish, freshwater dips were not effective in dislodging A. ocellatum trophonts younger than 24 h; in putatively "immune'' fish, trophonts of any age detached from the gills when exposed to freshwater. A computer simulation model was constructed to represent the A. ocellatum life cycle and the effects that freshwater exposure had on this cycle. Experimental results and information from the literature were integrated to model the joint effects of infestation age, the fish's weight, prior exposure to the parasite, and duration of freshwater exposure. The simulation model was most successful in explaining parasite load for red drum with A. ocellatum infestations older than 24 h (R² = 0.63). Model results helped answer questions about what the combination of freshwater exposure and age of trophonts contributed to removal of the parasite from red drum gills.