Dory is a bit of a forgetful fish, according to the classic children's film Finding Nemo. In 2016 we will see much more of her, in the upcoming sequel Finding Dory.

"I suffer from short-term memory loss," Dory tells Nemo. "I forget things almost instantly, it runs in my family."

It's very funny and ultimately touching, but this depiction is just a little unfair. The fish Dory is based on does not have short-term memory loss. It is rather more awesome than that.

It has several names, including royal blue tang, regal tang and surgeonfish. Its scientific name is Paracanthurus hepatus.

The memory myth

Like other fish, the royal blue tang is unlikely to have a bad short-term memory. There are no studies to suggest any fish has a "three-second memory". In fact, the opposite is true.

Fish can remember things for many months. One team of researchers discovered that carp could learn to associate a certain sound with food, and then remember it up to 5 months  later.

Fish can even be trained to respond to visual illusions. This requires repeated training sessions so that they easily recognise certain shapes and environments.

No one has specifically studied the memory of the royal blue tang. But given that several fish species have good memory skills, we should expect that Dory and all her relatives will be just as good.

Healthy oceans

Royal blue tangs have an important role to play in our oceans. They are part of a family of fish called surgeonfish, which are known to feed on the algae that grows on coral reefs.

This means that they keep corals healthy. By eating the algae, they prevent it from growing out of control and choking the coral to death. So in a way, Dory really does save the day.

Changing colour 

In the films, Dory is depicted as blue and black (or very dark blue) with a yellow tail. This is accurate, but not all the time. One study found that strange things happen to this fish's colour at night.

Researchers discovered that P. hepatus can change colour at night. The blue parts of the fish appear visibly paler, due to the way that thousands of special pigments in the fish's skin reflect light. 

The researchers note that the fish becomes "rather whitish with a shade of violet". This is thought to occur because their nervous system becomes less active at night.