Toxicity of formalin to bullseye puffer fish (Sphoeroides annulatus Jenyns, 1843) and its effectiveness to control ectoparasites

Abstract

First attempts at culturing Sphoeroides annulatus (bullseye puffer fish) resulted in recurrent parasitic infestations caused by the monogenea Heterobothrium ecuadori and the dinoflagellate Amyloodinium ocellatum, both of which are associated to mortality events in other cultured species. However, little information exists on the therapeutic levels to prevent and control such infections. The objective of the present study was to evaluate the safety of formalin to bullseye puffer fish and determine the efficacy of formalin to control parasitic infestations. The formalin median lethal concentration (LC50) for the bullseye puffer fish ranged from 1095 mg l−1 at 30 min and 972 mg l−1 at 60 min to 79 mg l−1 at 72 h. The formalin in vitro effective median concentration (EC50) against H. ecuadori was 225 mg l−1 at 30 min and 87 mg l−1 at 60 min. The resulting therapeutic index was 5 at 30 min and 11 at 60 min. Formalin was further tested in vivo against A. ocellatum. In comparison to the controls, 51 mg l−1 of formalin significantly reduced the number of parasites on the skin (97%) and gills (68%) of fish after 1 h of exposure, while 4 mg l−1 significantly reduced the numbers on the skin (66%) and gills (84%) after 7 h. The differences in toxicity between the bullseye puffer fish and specific parasites indicated that formalin was efficacious in the control of parasitic epizootics in the bullseye puffer fish culture.