Researchers in north Queensland think they may have discovered a tiny new species of seahorse.

Scientists from James Cook University (JCU) have just returned from a week-long expedition examining what lives deep beneath the ocean, about 200 kilometres off Cairns.

Expedition leader Tom Bridge says scientists were looking at corals and the creatures that live in the "twilight zone" - an area between 30 and 150 metres below the ocean's surface.

"As you go deeper and deeper you find less familiar critters," he said.

"We found a tiny little seahorse that was about four or five millimetres tall.

"Pygmy seahorses have been discovered before but I've never seen one quite that small before and no one has ever recovered one from quite that deep either."

He says scientists took samples back to the university to study.

"They'll be taken back to the Museum of Tropical Queensland here in Townsville, also the Queensland Museum in Brisbane," he said.

"We look at the skeletons and examine them that way and also take genetic samples of exactly what they are, so it's a fairly long process, especially if it isn't something that's been described before."