Bluefin tuna are predators in their ecosystem and they are also a source of food for larger predators, including humans. They act as hosts for at least 72 parasites. These include: Euryphorus brachypterus, found in branchial cavities, Brachiella thynni, found on the fins, Pennella filosa, which inserts itself into the flesh of the fish, Pseudocycnus appendiculatus, found on the gill filaments, and Caligus bonito and C. productus which are found on the surface of the body and the wall of the branchial cavities.

They are also mutualists with other tuna species because tunas school in groups of similar sizes and mixed species, rather than groups of the same species.

Mutualist Species:

• other tuna species (Thunnus)

Commensal/Parasitic Species:

• Copepoda (copepods)
• Trematoda (tissue flukes)
• Monogenea (gillworms)
• Cestoda (tapeworms)
• Nematoda (roundworms)
• Acanthocephala (spiny-headed worms)
• Digenea (flukes)
• Euryphorus brachypterus
• Brachiella thynni
• Pennella filosa
• Pseudocycnus appendiculatus
• Caligus bonito
• Caligus productus