A local diver took a video showing unique creatures near Tsukinoshima island in Amakusa, Kumamoto Prefecture.

Reddish creatures are forming a large swarm.

They're cavolinia uncinata, also known as sea butterflies.

They have shells as small as 5 millimeters and live in warm waters.

The man who took the video has been diving in the seas of Amakusa for more than 20 years.

Diver Mizuarai Toshio / I have never seen such a large swarm of cavolinia uncinata.

The creatures are distantly related to clione, which are also known as sea angels.

A local expert also took a look at the footage.

Undersea Aquarium Sea Donut Director Kawano Soshi  / That's a lot.

Why did they emerge in such a large number off the coast of Amakusa?

Undersea Aquarium Sea Donut Director Kawano Soshi / They could have drifted northward on a warm current, with westerly and southwesterly winds pushing them towards the coast.