Comprehensive investigation on an extraordinary-sized ocean sunfish (Mola mola L., 1758) stranded along the North Adriatic coastline of Italy

The ocean sunfish ( Mola mola , Linnaeus 1758) is a rare, pelagic, cosmopolitan species of negligible commercial interest and subject to unintended bycatch. It is a little-known species, and strandings are rare; we benefited from the stranding of an adult female to conduct a multidisciplinary investigation (toxicology, parasitology, microbiology, pathology) that evaluated multiple findings. Trace element concentrations (in fish tissues and parasites), organic compounds, and microplastics were analyzed. Trace elements were present at low levels in the animal’s tissues and at higher concentrations in the parasites (i.e., Zn: 0.056 mg/kg in liver, 0.074 mg/kg in muscle, and 0.322 mg/kg in cestodes). The unexpected detection of pyrethroids was one of the most significant toxicological findings. Microbiological analyses and histological investigations did not yield significant findings correlated to the cause of death. A severe parasitic infection was otherwise detected in the intestine (over 3000 adult cestodes). Morphologic and molecular analyses of helminths identified cestodes belonging to the Triaenophoridae family, and adult trematodes of the family Accacoeliidae. The severe gastrointestinal parasitic burden observed might have plausibly contributed to a state of general weakening. It may have exerted a subtractive effect on essential elements, which ultimately might have contributed to the animal’s defedation.