The occurrence of new phytoplankton species in a coastal area
may be explained by the import of shellfish containing whole live algal
cells in their digestive tracts. Indeed, shellfish containing toxic algal
cells can induce both primary contaminations in safe areas (initially free
from toxic microalgae), and secondary contaminations of other shellfish
growing in the same area via the faeces of the imported animals. To mitigate
this problem, shellfish need to be placed in a separate holding tank and
their intestinal content purged. For a deeper understanding of the risks
associated with transferring contaminated shellfish, oysters (Crassostrea gigas) and mussels
(Mytilus edulis) were purposely fed either Alexandrium minutum or A. catenella (Dinophyceae) or Pseudo-nitzschia calliantha (Bacillariophyceae) toxic algae
for 2 h. They were then transferred into individual tanks where they were
continuously fed with a non-toxic alga, Tetraselmis suecica. Biodeposit production, faeces
composition, and filtration rates were monitored for the shellfish over a
6-h period. The effect of temperature differences and different initial
toxic algae concentrations were compared. This study revealed a relationship
between temperature and cell lysis in the oyster digestive tract. It also
indicated that toxic algae concentration did not seem to influence gut
passage time in oysters, while a significant effect was observed in mussels,
and confirmed the existence of a difference between oyster and mussel
feeding patterns.