Madeline Darnell, a visitor from Georgia, USA, asks :-

How do Paua reproduce?

Norman Ragg, an aquaculture scientist at Nelson's Cawthron Institute, responded.

Paua (NZ abalone) become sexually mature at an age of 3-5 years; at this point they develop a cone-shaped reproductive organ containing orange eggs (female) or white sperm (male).

As the time to reproduce approaches, paua become sensitive to small changes in the surrounding water; these include sudden changes in temperature and free radicals caused by lightening striking the sea. These factors are generally associated with periods of storms during spring and autumn.

During the large tides associated with the new and full moon the paua begin to climb to the highest points of their underwater reefs in preparation for spawning. One male will usually spawn first on the high tide, releasing large amounts of sperm into the water through the holes in his shell.

Neighbouring paua detect the sperm and are in turn stimulated to spawn. The tide and storm conditions help the eggs and sperm to mix together to increase the chance of fertilization, which must happen within 48 hours of release. Fertilized eggs are then carried out to sea on the tide.

Eggs hatch into larvae after about a day; these 'trochophores' join the plankton where they will feed and grow for up to a month before being swept back towards the shore. The larvae that are lucky enough to reach shallow water will sink to the bottom and begin exploring, if they find suitable rocks growing the right kinds of seaweed they will settle and metamorphose into a tiny version of the adult (smaller than a pin-head). Only about 1 in a million larvae will survive to this stage!

It is important for people to understand these processes: paua are quite easy to catch by skin diving, so numbers can be rapidly reduced. If the adults are reduced to a point where there is more than 10m between individuals it is unlikely that eggs and sperm will mix, breaking the reproductive cycle, often causing the collapse of a population.