Prevalence and intensity of pathologies induced by the toxic dinoflagellate, Heterocapsa circularisquama, in the Mediterranean mussel, Mytilus galloprovincialis.

•Histopahtology in Mediterranean mussels induced by harmful alga H. circualrisquama.•Quantitative density-, time-, and temperature-dependent toxicity in six vital organs.•Extensive cytotoxicity resulting in necrosis and atrophy, coupled with hemorrhaging.•Sequence of toxicity: gills>labial palps=mantle>stomach=intestines>hepatopancreas.•Compromised respiration, feeding, digestion, and detoxification leading to death.•Cell contact-dependent toxicity, but other mechanism(s) also implicated.