Multiorgan histopathological assessment reveals gill-predominant changes in yellowtail (Seriola quinqueradiata) during acute exposure to the Chattonella marina complex

Red tides caused by the raphidophyte Chattonella marina complex (hereafter Chattonella ) remain a threat to aquaculture despite current mitigation efforts. Although gill injury during Chattonella exposure has been reported, whether overt histopathological abnormalities occur in major extrabranchial organs during the acute preterminal phase remains unclear. We examined the physiological and histological responses of yellowtail ( Seriola quinqueradiata ) at 60 min after the start of acute Chattonella exposure. Physiological disturbance was first assessed based on blood partial oxygen pressure ( p O 2 ), plasma pH, and plasma Na + , K + , and Ca 2+ concentrations, followed by systematic histological examination of the gills, heart, liver, kidney, head kidney, spleen, intestine, and stomach, including organs reported to be affected by exposure to other harmful algae. Exposed fish showed decreased blood p O 2 , lower measured plasma pH, and increased measured plasma Na + , K + , and Ca 2+ concentrations. Under the present acute exposure conditions, overt hematoxylin and eosin (H&E)-detectable abnormalities were gill-predominant and characterized by epithelial detachment at the base of the secondary lamellae, whereas no overt H&E-detectable abnormalities were identified in the other organs examined at this sampling point. These findings indicate a distinctive acute response profile to Chattonella exposure, characterized by altered blood parameters and gill-predominant histopathology without overt H&E-detectable abnormalities in the major extrabranchial organs examined.