FIRST REPORT OF LETHAL EFFECTS OF COSMOPOLITAN HARMFUL ALGA, Dinophysis caudata, ON PACIFIC OYSTER, Crassostrea gigas

Basti Leila¹, Ryoji Matsuchima², Toshiyuki Suzuki², Satoshi Nagai².

¹ Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan.

² National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Kanazawa, Kanagawa 236-8648, Japan.

e-mail: lbasti1@kaiyodai.ac.jp

Key words: Crassostrea gigas, mortality, pathology, oxidative stress, diarrhetic shellfish poisoning (DSP), Dinophysis caudata

A dozen species of the cosmopolitan microalgal genera, Dinophysis, produce okadaic acid (OA) and/or its derivatives, dinophysistoxins (DTXs) which are responsible for the widespread intoxication in humans, the diarrheic shellfish poisoning (DSP). The toxins OA and DTXs are inhibitors of serine/threonine protein phosphatases and cause rapid onset of gastrointestinal symptoms in humans following the consumption of shellfish, typically bivalve molluscs that have accumulated the toxins of Dinophysis. Several strains of Dinophysis also produce another set of lipophilic toxins, pectenotoxins (PTXs) which accumulate in shellfish tissues but do not produce symptoms in human, although some variants are acutely toxic by intraperitonial injection and are interesting candidates for toxicological and pharmacological studies.

Bivalves have been known to accumulate OA, DTXs and PTXs without any apparent physiological effect, leading to long closures of shellfish beds and consequent economic loss for related fisheries and aquaculture industries. Therefore, the kinetics of toxication/detoxification has become a subject of research interest following the recent breakthrough in the culture of Dinophysis spp. In an attempt to shed light on such interactions, we conducted laboratory-controlled feeding experiments of Pacific oyster on D. caudata. However, mortality of oysters occurred. The research has since been reoriented to study the mechanism of lethality of Dinophysis in Pacific oysters.

Two sets of experiments were conducted in which adult oysters were fed cultures of D. caudata for one week, at different daily rations. The mortality rates were followed daily and at the end of the feeding experiments, surviving oysters were either preserved whole for histopathological studies, or the gills and digestive tracts were immediately dissected and preserved at –80° C for oxidative stress analyses.

Nine organs were examined for pathological alterations. The activity of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), gluthatione-S-transferase (GST), and gluthatione peroxidase (GPx) were assessed based on colorimetric assays, and measured spectrophotometrically. Several pathologies were observed and differential up- and down-regulations of the antioxidant enzymes were found. This is the first study to show that a cosmopolitan species of Dinophysis is lethal to Pacific oysters with potential impacts on related fisheries and aquaculture.