Donald MacRae of Otago asks :-
Whilst cleaning out a shed at Taieri we found a mushroom-shaped, shell-like object. What could it be?
Abigail Smith, a marine zoologist at the University of Otago, responded.
The photo shows a large round shell, about 12 cm in diameter, with a radiating pattern of thin walls (called septa) growing out from the centre. It is this pattern which suggests that the shell is a stony coral, probably from the family Fungiidae: the mushroom corals. A solitary coral like this one is different from the colonies of tiny polyps which make up most corals on the reef. This coral is just one huge individual.
We have a few species of mushroom corals here in New Zealand, but the largest they ever get is 3 to 4 cm in diameter, and they are never found in water shallower than 100 m. So most likely this mushroom coral was brought home from a tropical holiday or cruise somewhere in the Pacific. The family Fungiidae is found on reefs throughout the Pacific and Indian oceans.
Mushroom corals live on the hard or rubble bottom of a lagoon or reef flat. They are usually not attached, and can apparently travel short distances. Tropical species (unlike the NZ species) need to live in strong sunlight, so they are found in clear shallow water. They contain symbiotic green algae in their tissues - these algae photosynthesise and share the energy with the coral. The corals reproduce by sprouting a small polyp off to the side, which soon falls off and starts life on its own.
Today you wouldn't be easily able to bring such a coral into New Zealand, as they are covered by the Convention on International Trade of Endangered Species and travelers may not take them out of their country of origin without a special permit. Nevertheless, some species of mushroom corals are much valued in tropical aquaria.