Observe how the fish use their fins when swimming straight, slowing down, and turning in the aquarium. You'll notice that sometimes the pelvic fins and first dorsal fin are visible, and sometimes they're not. The first dorsal fin can be neatly tucked into a pocket on its back, which is why we call it the "secret fin." The pectoral fins also use differently when swimming straight and when turning. Observe this as well.

When you see Pacific Bluefin Tuna in an aquarium, you might be surprised by its size. However, in the wild, it's nothing like that. Pacific Bluefin Tuna are known to live for over 20 years, and when fully grown, they can reach a length of 3 meters and weigh over 400 kg. Yet, even these tuna lay eggs that are only about 1 mm in size, and the hatched offspring are only 2-3 mm long.

Tuna are well-known for being fast swimmers. But just how fast can they swim? There are various theories, but past measurements have reported speeds of up to 80 km/h. Imagine Pacific Bluefin Tuna swimming across the vast ocean at the speed of a car. Pretty cool, right?

If you look inside the mouth of Pacific Bluefin Tuna in a tank, you'll see many small, sharp teeth. Do tuna chew their food with these teeth? When you observe them eating, you'll see that they swallow their food whole. The small teeth serve to hold the food in place and prevent it from escaping.

If you look at Pacific Bluefin Tuna in an aquarium, you'll notice that every individual swims continuously with its mouth and gill cover slightly open. This is related to how tuna breathe. By swimming with their mouths and gill covers open, tuna breathe by taking in fresh seawater through their mouths and passing it through their gills. This type of breathing is also seen in fast-swimming and migratory fish such as open-ocean sharks and sardines.