Persistent low concentrations of diarrhetic shellfish toxins in green mussels Perna viridis from the Johor Strait, Singapore:: first record of diarrhetic shellfish toxins from South-East Asia

Liquid chromatography-selected reaction monitoring mass spectrometry was used to identify and quantify Diarrhetic Shellfish Poisoning (DSP) toxins for the first time from tropical shellfish. Persistent, low concentrations of okadaic acid, 6 isomers of okadaic acid, and 5 isomers of dinophysistoxin-1 (DTX-1) were detected from green mussels Perna viridis from 3 sites in the Johor Strait, Singapore, between October 1995 and December 1997. Isomers of okadaic acid and DTX-1 generally occurred in higher concentrations than okadaic acid. The highest concentration of any single DSP toxin detected from Singapore shellfish was 97 ng g(-1) mussel digestive tissue (wet weight) of an isomer of DTX-1 (DTX-1a). The maximum concentration of okadaic acid detected was 24 ng g(-1) digestive tissue. These concentrations are well below the generally recommended limit for consumption of DSP toxins for humans (similar to 1 pg toxin g(-1) digestive tissue). Naturally contaminated mussels rapidly depurated okadaic acid when held in a laboratory aquarium; however, okadaic acid and some isomers of okadaic acid and DTX-1 could still be detected (<1 ng g(-1)) after 32 d. Phytoplankton samples from the Johor Strait were examined for the origin of the DSP toxins found in green mussels. Four species of dinophysoid dinoflagellates were found, with Dinophysis caudata the most frequent and abundant species; although, cell densities did not exceed 5 cells l(-1). Five hundred D, caudata were micropipetted from phytoplankton samples and analysed for DSP toxins. Okadaic acid was detected (7 x 10(-14) g cell(-1)) from D. caudata but no isomers of okadaic acid, DTX-1 or isomers of DTX-1. D. caudata were found in the gut contents of Singapore green mussels and probably contribute to the contamination of these shellfish; however, other as yet unidentified sources of DSP toxins may exist in the Johor Strait.