Prevalence and intensity of pathologies induced by the toxic dinoflagellate, Heterocapsa circularisquama, in the Mediterranean mussel, Mytilus galloprovincialis

Abstract

The harmful dinoflagellate, Heterocapsa circularisquama, has been causing mass mortalities of bivalve molluscs in Japan, at relatively low cell densities. Although several studies have been conducted to determine the toxicity mechanisms, the specific cause of death is still unclear. In a previous study, in our laboratory, it was shown that H. circularisquama (103cellsml−1) caused extensive cytotoxicity in the gills of short-neck clams, Ruditapes philippinarum. In the present study, Mediterranean mussels, Mytilus galloprovincialis, were exposed to H. circularisquama at four cell densities (5, 50, 500, 103cellsml−1), three temperatures (15, 20, and 25°C), and three exposure durations (3, 24, and 48h), and the pathologies in nine organs (gills, labial palps, mantle, hepatopancreas, stomach, intestines, exhalant siphon, adductor muscles, and foot) were assessed.Foot, adductor muscles, and exhalent siphons of mussels were not affected; however, 16 inflammatory (hemocytic infiltration and aggregation, diapedesis, hyperplasia, hypertrophy, edema, melanization, and firbrosis) and degenerative (thrombus, thrombosed edema, cilia matting and exfoliation, epithelial desquamation, atrophy, and necrosis) pathologies were identified in the gills, labial palps, mantle, hepatopancreas, stomach, and intestines. The total prevalence and total intensity of pathology in each individual mussel, and the prevalence and intensity of pathology in each organ increased significantly with increased cell density, exposure duration, and temperature. The prevalence of pathology was the highest in gills, followed by the prevalence in labial palps, mantle, stomach, and intestines. Pathology was least prevalent in the hepatopancreas. The intensity of pathology was the highest in the gills, followed by the labial palps and mantle, the stomach and intestines, and the hepatopancreas.This detailed quantitative histopathological study demonstrates that exposure to H. circularisquama induces a broad cytotoxic effect in six vital organs, even at low density (5cellsml−1) and low temperature (15°C), but not in muscular organs. Combining cell density, time, and duration of exposure, the organ most affected by the harmful alga was the gill, followed by the labial palps and mantle, the stomach and intestines, and the hepatopancreas. The results of this pathological analysis show that exposure to H. ciruclarisquama severely affects the gills, the labial palps, and mantle thereby interfering with particle clearance and sorting, cleansing, and respiration, but also affects the stomach, intestines, and hepatopancreas, altering the digestive processes and possibly detoxification pathways, if mussels are able to detoxify the toxins of H. circularisquama. In the most severe cases, bivalves would most likely have died as a result of combined severe alterations of the vital functions, failure of tissue repair, and moderate to heavy hemorrhaging in both the external organs and the digestive organs concomitantly with light to moderate alterations in the detoxifying processes.