The pygmy seahorse is only about 16 mm high - that's smaller than your fingernail!
Now they've realised the seahorses are actually adults and are fully grown.
The tiny creature, officially called Hippocampus denise, lives in and around coral in the western Pacific Ocean.
But the fragile creatures may well be at risk from divers and heavy trawling gear that can flatten their coral homes, they said.
Because they're so tiny, divers could hurt them by mistake.