Differential dynamics of dinophysistoxins and pectenotoxins between blue mussel and common cockle: a phenomenon originating from the complex toxin profile of Dinophysis acuta

Abstract

Different toxin profiles of dinophysistoxins and pectenotoxins have been reported before between blue mussel and other bivalve species, such as common cockle, razor clam, clams, etc. Comparison of toxins present in plankton in mussel growing areas and in cockle growing areas, respectively, showed there was no particular incidence of dinophysistoxin-2 (DTX2) in plankton from mussel growing areas that could account for the higher percentage of DTX2 in relation to okadaic acid (OA) found in mussels; or of pectenotoxin-2 in cockle growing areas that could explain the higher levels of pectenotoxin-2 seco acid (PTX2sa) found in cockles. A detoxification experiment between mussels and cockles showed the higher percentage of DTX2 in mussels was due to slower elimination of this toxin in relation to OA; while the lower levels of PTX2sa were due to quicker elimination by mussels than by cockles. The slower elimination of DTX2 explains why in late summer and autumn this toxin gradually accumulate in mussels throughout the entire coast, while other bivalves species have a lower percentage of DTX2, very close to the 3:2 OA:DTX2 ratio found in natural plankton assemblages when Dinophysis acuta predominates. In the clam Donax spp., DTX2 concentration also tends to build up in relation to OA, this being made up predominantly by free DTX2 while esterified DTX2 is found only in trace levels (similarly to what is found in mussel for DTX2). We hypothesise that the esterified forms of OA and DTX2 are more easily eliminated than the free forms, by all shellfish species. The free forms are more difficult to eliminate. This is particularly notable in these two species that present a very low conversion of DTX2 into acyl esters. The high pool of free toxins is partially responsible for these two species (mussel and Donax clams) being the sentinel species for DSP contamination throughout the Portuguese coast. Esters of OA and DTX2 were found in a plankton sample where D. acuta was the predominant toxic species found. The nature of the esters remains to be elucidated. The boiling of these DTX2 esters seems to favour the rearrangement of the parent molecule to the DTX2 isomer, DTX2i, recoverable after alkaline hydrolysis. The isomerization was also observed with DTX2 esters present in mussel, but thus not appear to occur with the same extent with free DTX2.