Biological studies on parasitic isopod, caecognathia coralliophila (gnathiidae) infesting cultured marine fish

The distribution and impacts of many parasitic isopods to natural and culture fish species have been extensively studied in many parts of the world. Nevertheless, such information is lacking in Malaysia especially the parasitic crustacean isopod from the family Gnathiidae. Recently, an infestation of the parasite occurred in one of the hatcheries in Sabah. It affected the broodstock of tiger grouper, Epinephelus fuscoguttatus, Napoleon wrasse, Cheilinus undulatus and Asian seabass, Lates calcarifer. Such infestation has been blamed for the series in spawning failures of the fish broodstocks in the hatchery. The parasites were collected from the broodstock tanks in that particular hatchery and subjected to a thorough systematic morphological examination by the use of light and electron microscopes. The result confirmed that the isopod parasite was belonging to the member of Caecognathia coral/iophila (Gnathiidae). In addition to species identification, this study was also able to significantly contribute the description of new morphological features of the adult gnathiid which were never described previously, even in the holotype specimen. Investigation on its life cycle of the gnathiid was also conducted with the aim to determine which stage of the gnathiid is parasitic. It was noted that the C coralliophila undergone three main parasitic larval stages which are zuphea 1 and praniza 1; zuphea 2 and praniza 2; zuphea 3 and praniza 3 before finally malted into male and female adult. All stages of zuphea were noted dependent to fish blood for survival and malting while the praniza stages were dormant. In vitro experiment was conducted to determine the sensitivity of the parasitic larvae to chemicals that are approved and widely used in aquaculture which included hydrogen peroxide, formalin, copper sulphate and trichlorfon. The larvae were exposed to different concentrations of each chemical for 10, 20, 30, 60 min and 24 h. The result showed that trichlorfon of 0.2 ppm concentration was able to eliminate all parasites within 24h exposure. Following the result, an in vivo sensitivity test of the organophosphate at concentrations 0.2 ppm and 3.2 ppm to host fish, tiger grouper (Epinephe/us fuscoguttatus) was conducted at 24h and 60 min, respectively. It was noted that all fish survived through the trichlorfon challenge. This shows that trichlorfon can be used to treat gnathiid infestation from the member of C coralliophila in marine fish aquaculture. However, one cannot rely much on the use of chemical such as trichlorfon to treat parasitic isopod infestation as it might bring other negative impacts that yet to known to the fish, consumer and environment. Hence, further studies on alternative prevention and treatment of gnathiid infestation in marine aquaculture facilities which are environmentalfriendly, and harmless to fish and consumer are the way forward in the fish health and diseases management program for marine aquaculture in Malaysia