Role of the cymothoid isopod, Nerocila bivittata in spread of Photobacterium damselae subsp piscicida to Thinlip mullet (Chelon ramada)

Cymothoid isopods are among the most devastating parasitic crustaceans in mariculture worldwide. A private marine fish farm in Ismailia province, Egypt suffered mass mortalities in cultured Chelon ramada during the late summer of 2022. The present study aims to reveal the role of the isopod, Nerocila bivittate (N. bivitata) in spread of the evolving prolonged spread of the devastating bacterial disease, Photobacterium damselae subsp piscicida (P. dmselae subsp piscicida) to Chelon ramada. This study was carried out on one hundred and fifty C. ramada collected during different phases of mortalities. Moribund fish showed signs of respiratory distress, erratic movements and poor appetite. Infested fish gills were pale anemic with excessive mucus secretion. Nerocila bivittata was isolated from the gill chamber, buccal cavity, caudal, and dorsal fins of infested fish. The infestation percentage was 26.5 % where gills of infested fish were the most affected tissue. The bipolar bodies of the P. damselae subsp piscicida were remarkably detected within the isopod hemolymph using transmission electron microscope. Further, P. damselae subsp piscida was isolated from both N. bivitata. Both P. damelae subsp piscida and N. bivitata were molecularly confirmed utilizing the 16S ribosomal RNA. In conclusion, N. bivittata infestation is responsible for high economic losses due to fish mortalities, reduced growth rates and increased susceptibility to devastating prolonged bacterial infections.