A few dozen species of marine microalgae are able to synthesize toxins. In some cases these molecules can be released from the algal cell (exotoxins) and directly affect marine fauna and flora. In most cases, however, they remain intracellular (endotoxins) and it is through feeding (filter-feeders, zooplankton, etc.) that contamination of food web takes place and these toxins can potentially affect humans. Different types of toxins (neurotoxins, diarrhetic toxins) can indeed be accumulated by shellfish without any damaging effects to these animals, while some toxin families are also accumulated by fish with or without toxic effects depending on the species combination.