Found on a Hong Kong beach: What lifeforms are these?

AJP reports finding lots of these on the beach on the island of Po Toi last Sunday (a favourite place for birdwatchers since it is a major place for migrants to drop in) south east of Hong Kong island.

The discs are 3-4 cm in diameter.

After some searching he identified them as the remains of Porpita porpita, the Blue Button, superficially a jellyfish, but actually a colony of hydrozoan polyps, which is found in tropical and sub-tropical seas.

Now included in the Phylum Cnidaria, in our day they were in the Coelenterata along with the ctenophores.