Thunnus is a genus of ocean-dwelling ray-finned bony fish from the Scombridae (Mackerel) family. More specifically, Thunnus is one of five genera which comprise the Thunnini tribe – a tribe that is collectively (and famously) known as the tunas. Also called the true tunas or real tunas, Thunnus consists of eight species of tuna (more than half of the overall tribe), divided into two subgenera. The word Thunnus is the Middle Latin form of the Ancient Greek: θύννος (thýnnos) "tunny-fish" – which is in turn derived from θύνω (thynō), "to rush; to dart". The first written use of the word was by Homer.^{[citation needed]}

Their coloring, metallic blue on top and shimmering silver-white on the bottom, helps camouflage them from above and below. They can grow to 15 feet long and weigh over 1,000 pounds, and can swim up to 50 miles per hour when pursuing prey. Atlantic bluefin tunas are warm-blooded, which is a rare trait among fish, and are comfortable in the cold waters. Bluefin fish are found in Newfoundland and Iceland, as well as the tropical waters of the Gulf of Mexico and the Mediterranean Sea, where they go each year to spawn.