3.4.1 Uptake and elimination of DSP toxins in aquatic organisms
Diarrhoeic shellfish toxins associated with Dinophysis spp. and Prorocentrum spp. are readily accumulated by shellfish. However, little is known of the retention time of these toxins (Hallegraeff et al., 1995). A few studies described DSP toxin kinetics in bivalves under either natural or controlled laboratory conditions. Dynamics of DSP toxins were examined in juvenile and adult bay scallops by feeding cells of the epibenthic dinoflagellate Prorocentrum lima to scallops in a controlled laboratory microcosm. Analysis of DSP toxins in dinoflagellate cells and scallops tissues was performed by means of liquid-chromatography combined with ion-spray mass spectrometry (LC-MS). Juvenile and adult clearance rates were not inhibited by exposure to P. lima cells and no scallop mortalities were seen. Scallops could exceed regulatory toxin limits of 0.2 mg DSP toxin/g wet weight in less than one hour of exposure to high P. lima cell densities. Toxin saturation levels (2 mg DSP toxin/g wet weight) were attained within two days, however toxin retention was very low (under 5 percent). Although most of the total toxin body burden was associated with visceral tissue, weight-specific toxin levels were also high in gonads of adult scallops. Rapid toxin loss from gonads within the first two days of depuration indicated that the toxin was derived primarily from a labile (unbound) component within the intestinal loop section through the gonads. Detoxification of visceral tissue, however, followed a biphasic pattern of rapid toxin release within the first two days of depuration, followed by a more gradual toxin loss over a two week period, suggesting that faecal deposition may be an important mechanism for rapid release of unassimilated toxin and intact dinoflagellate cells (Bauder and Grant, 1996).
Sedmak and Fanuko (1991) also observed two phases of DSP toxin release during a decontamination phase of mussels. There is first a rapid decrease in toxin content followed by a slow decrease with the toxicity remaining above the quarantine level of 0.5 MU/g hepatopancreas. The patterns of contamination and decontamination are specific for shellfish species and do not seem to depend on the type of dinoflagellate toxin.
Toxic scallops (Patinopecten yessoensis) cultivated in tubs in which filtered and sterilized seawater was circulated, with or without supply of planktonic diatoms as feed, showed a gradual decrease of DSP during cultivation (microbial assay method). DSP decreased to 30 percent of initial value within two weeks when dense cultures of Chaetoceros septentrionelle were supplied as the feed. Relatively high toxicity scores of DSP were detected in excrement of cultivated scallops. When other diatoms such as Skeletonema costatum, Asterionella japonica, Rhabdonema spp. and Thalassiosira spp. were supplied as feed not only the toxicity but also the amounts of glycogen, free amino acids and free fatty acids decreased, causing a deterioration in quality (Van Apeldoorn et al., 1998).
During decontamination of mussels (Mytilus galloprovincialis) from Galicia in northwestern Spain for 70 days under different environmental conditions (salinity, temperature, fluorescence, light transmission), fluorescence and light transmission appeared to have the most prominent effect on depuration. In most cases, there was an inverse relation between depuration and body weight. It could not be clearly concluded whether the DSP depuration evolved following 1- or 2-compartment kinetics (Blanco et al., 1999).
In a study on the feeding behaviour of the mussel Mytilus galloprovincialis on a mussel farm in the Gulf of Trieste (Italy) during a DSP outbreak, the mussels seemed to feed selectively on dinoflagellates rather than diatoms. Further selection was observed among different dinoflagellate genera and a preference for the genus Dinophysis was particularly evident. The mussels seemed to open the thecae of Dinophysis cells and digest them more easily than other dinoflagellates (Sidari et al., 1998).
3.4.2 Shellfish containing DSP toxins
In Japan, the shellfish causing DSP were found to be the mussels Mytilus edulis and M. coruscum, the scallops Patinopecten yessoensis and Chlamys nipponesis akazara, and the short-necked clams Tapes japonica and Gomphina melaegis. Along the European Atlantic coasts, particularly M. edulis but also Ostrea sp. were contaminated with DSP toxins (Viviani, 1992).
In Japan and the Atlantic coast of Spain and France, the infestation ranges from April to September and the highest toxicity of shellfish is observed from May to August, although it may vary locally. By contrast, in Scandinavia, in February oysters have caused DSP and in October mussels have caused DSP. Data from the first DSP episode in the Adriatic Sea in 1989 indicated that the infestation period in some coastal areas ranged from May to November (Viviani, 1992).
Comparative analysis in various shellfish from one area in Japan revealed that the highest toxicity was found in blue mussels (Mytilus edulis) with less toxicity in scallops and very little in oysters. Differences in toxicity were also noted between mussels cultivated at different depths with concentrations differing by factors of two to three (Viviani, 1992). The highest toxicity was obtained in mussels from the upper level (3-6 m), whereas toxicity was reduced to half that level at 6-8 m and 8-12 m (Botana et al., 1996). OA levels of 0.63 and 4.2 mg/g hepatopancreas in adjacent mussels were reported within the same mussel growing site and levels of 0.63 and 10 mg OA/g hepatopancreas in mussels grown at different depths along the same rope (Van Apeldoorn et al., 1998).
Spanish mussels from Galician Rias contained OA as the major toxin besides less polar DSP toxins. The levels of less polar DSP toxins never exceeded the OA levels. Highest low-polar DSP levels corresponded to the highest OA levels. The authors hypothesized that the low polar DSP toxins found in the hexane layer which is usually discarded, belong to the acyl-derivatives group (Fernández et al., 1996).
Data from DSP episodes in the Adriatic Sea showed that not all species of bivalve molluscs absorbed and concentrated the enterotoxin in their tissues to the same extent, although these species were living in the same habitat infested by microalgae. In particular, Mytilus galloprovincialis, Chamelea gallina, Tapes decussata and Venus verrucosa were monitored for DSP toxins by means of the mouse bioassay and DSP was detected only in mussels, although they were drawn from the same habitat in the Adriatic Sea. This uneven distribution of DSP will have an impact on developments of sampling plans for shellfish, as part of monitoring schemes for control purposes (Viviani, 1992). In M. galloprovincialis from the northern Adriatic sea, OA and DTX1 as well as YTX were detected (Ciminiello et al., 1997). Cooking did not alter the toxicity of the contaminated shellfish but intoxication could be avoided if the digestive glands were eliminated beforehand (Viviani, 1992).
OA homologues in the alga D. fortii, the scallops Patinopecten yessoensis and the mussel Mytilus galloprovincialis, collected at the same site in Mutso Bay, Japan, were determined by liquid chromatography-fluorescence detection. Prominent toxins in scallops and mussels were DTX3 and DTX1, respectively, whereas only DTX1 was detected in D. fortii. Toxin contents in mussels were significantly higher than those in scallops indicating that mussels have a higher potential to accumulate OA homologues than scallops (Suzuki and Mitsuya, 2001).
Persistent low levels of DSP toxins were found in green mussels (Perna viridis) from the Johor Strait, Singapore. Six isomers of OA and five of DTX1 were detected and generally the levels of the isomers were higher than that of OA and DTX1. The highest concentration found was 97 ng/g mussel digestive tissue (wet wt) of an isomer of DTX1 (DTX1a). The maximum level of OA was 24 ng/g. These values were below the threshold limit for consumption (Holmes et al., 1999).
DSP was also widely distributed in different shellfish species along the Chinese coast. Twenty six out of 89 samples contained DTX1 or OA but only six samples contained levels above the regulatory limit for human consumption (20 mg/100 g soft tissue). The highest level of 84 mg/100 g was found in Perna viridis from Shenzhen (Zhou et al., 1999).
3.4.3 Other aquatic organisms containing DSP toxins
DSP toxins accumulate in mussels by plankton filter-feeding. However plankton filter-feeding is largely a non-selective process which is also used by certain fish and may therefore lead to accumulation of DSP toxins in fish. Predators can also accumulate significant amounts of toxin in only one meal given that many bivalve molluscs concentrate toxins in the digestive gland. OA may appear in predatory fish as a consequence of their preying on mussels and fish containing OA.
Cod fish in cages fed toxic mussels showed the highest concentrations of OA particularly in the liver (0.7 mg/g). Lower concentrations were noted in muscle and gonads. Whereas the mussels used for feeding showed the presence of higher concentrations of DTX1 than of OA, DTX1 was nearly absent from fish tissue. After giving non-toxic feed the OA levels disappeared in one to two months time, least rapid from testis. Analysis of wild fish (cod, sea-cat, shark and herring) caught in Scandinavian waters in January to February 1992, when OA and DTX1 content of mussels in the vicinity was low, showed no OA. No OA was found in refined cod-liver oil (Van Apeldoorn et al., 1998).
Traditionally only filter-feeding molluscs are included in monitoring programmes. Shumway (1995) stressed the importance of including also higher-order consumers (such as carnivorous gastropods and crustaceans) in routine monitoring programmes, especially in regions where non-traditional species are being harvested. There are currently no records of DSP toxins in gastropods or crustaceans, but this is, undoubtedly, only because no one has looked for them. Based on the data above it cannot be excluded that DSP toxins also accumulate in higher-order consumers.