In jade waters off Hong Kong's eastern shoreline, scientists are thrilled to spot a cuttlefish protecting her eggs inside an artificial, 3D-printed clay seabed helping to restore the city's fragile coral reefs.

On postcards and in the popular imagination Hong Kong is a byword for urban density, a thicket of towering skyscrapers crammed along the harbour or clinging to the vertiginous hillsides above.

Yet it is circled by a surprising array of nature and the small number of coral reefs are some of the city's best-kept secrets.

Around 84 species of coral are found in Hong Kong's waters, scientists say, more diverse than those found in the Caribbean Sea.

Most can be found on remote inlets, far from the sediment-filled Pearl River Delta and its busy shipping channels.

But like all reefs in a rapidly warming world, they are under enormous pressure.

Which is where Vriko Yu and her team of fellow marine scientists come in.

They have begun using 3D printed tiles that work as an artificial bed for corals to latch onto and thrive, with promising results.

"The first time we put down the tiles, there were a few fish around," she told AFP on a recent inspection by University of Hong Kong (HKU) researchers.

Now the artificially produced reef laid down last summer is teeming with wildlife, including the cuttlefish, something Yu described as "very, very exciting".

Hong Kong's government commissioned research into local coral ecosystems after the reefs at Hoi Ha Wan marine park were struck by bleaching and mass die-offs.

Corals are colonies of billions of living polyp invertebrates and are hugely sensitive to temperature changes.

When they get too hot, they lose their vibrant colour and die.

Repopulating a dead or damaged reef requires suitable ground for the remaining coral larvae to latch onto and build a new home -- and the printed tiles have so far proven dependable.

Tiles carrying 400 coral fragments have been laid on a 40 square-metre (430 square-foot) section of sea floor in the marine park.

"The corals now on the tiles definitely survive better than the traditional way of transplantation," said Yu, putting the success rate at around 90 percent.