Factors Influencing the Toxicity, Detoxification and Biotransformation of Paralytic Shellfish Toxins - Reviews of Environmental Contamination and Toxicology - page 9

4 Accumulation and Depuration of PST in Bivalves

Filter feeding bivalves are known to be the principal vectors for PSP. However,
different bivalve species have different toxic accumulation rates and capacities.
For examples, M. edulis and M. californianus are known to accumulate high toxin
levels in a short time (about 2 weeks) and to produce toxicity that is 2-4 times
higher than other co-occurring species (Larocque and Cembella 1991 ; Hurst and
Gilfi llan 1997 ). The PST accumulation that occurs among bivalve species is associ-
ated with their variable tolerance to the toxins. Interspecifi c variability in resistance
may be related to the species-specifi c binding characteristics of the polypeptidic
receptor sites in the sodium channel, or to production of STX-binding proteins
(Daigo et al. 1988 ; Mahar et al. 1991 ). Bivalve species that are insensitive to PSTs
readily feed on toxic cells, and therefore accumulate toxins to extremely high levels
(Bricelj et al. 1990 ). In contrast, species sensitive to PST exhibit physiological and
behavioral responses such as shell clapping in scallops and complete shell valve
closure in mussels that enable them to reduce the feeding rate to minimize exposure
to toxic cells (Gainey and Shumway 1988 ). Purple clam was documented to attain a
maximum toxin level in ~20-24 days by feeding on toxic algae. Excess toxin in algae
was excreted as pseudofaeces and the toxin did not enter the digestive gland once the
purple clam accumulated its maximum capacity of toxin (Chou et al. 2005 ).
In sensitive bivalves, the bioaccumulation of PSTs is enhanced when the diet of
the producing organism is comprised of both toxic and nontoxic phytoplankton.
The presence of nontoxic cells stimulates the bivalves to open their valves, which,
in turn, promotes feeding on toxic cells (Bricelj et al. 1996 ). The size and body mass
of bivalves also infl uence toxin accumulation rates. The weight-specifi c toxicity of
bivalves ( ʼ g STXeq 100 g −1 ) has been shown to be inversely associated with body
size, because smaller individuals have higher cell ingestion rates per unit biomass
than do larger bivalves. The small (3-4 cm in shell length) M. edulis attained peak
toxicities twice as high as those of larger (>6 cm) mussels (Bricelj and Shumway
1998 ). The distribution among organs of toxin accumulation varied. During the
toxifi cation phase, the digestive gland or viscera was fi rst to come into contact with
toxic cells. Therefore, the digestive system absorbs and accumulates the greatest
proportion of the total body burden of the toxin. Most researchers have concluded
that bivalve viscera and hepatopancreas accumulate the majority of total toxins
(80-90%), whereas the locomotory tissue (adductor muscle, and the muscular foot)
of bivalves typically contains <3% of the PSP toxin body burden, despite its large
contribution to total body mass (Choi et al. 2003 ; Bricelj and Shumway 1998 ).
Toxicity levels vary among bivalve species from differences in both toxin reten-
tion and rates of depuration (Deeds et al. 2008 ). Most bivalve species (e.g., Eastern
oyster ( Crassostrea virginica ), Northern quahogs ( Mercenaria mercenaria ) and
green mussel ( Perna viridis )) remove PSTs relatively quickly (within a few weeks)
(Gacutan et al. 1989 ). Other bivalves species, such as Washington butterclams, sea
scallops ( Placopecten magellanicus ), and Atlantic surfclams ( Spisula solidissima )
are well known to be slow (up to 2 years) eliminators of the toxins, because the PSTs