Corals are right up there with whales and turtles for popularity. Coral reefs are some of the most diverse and spectacular natural structures on earth, and Australia’s two major coral reef systems – the Great Barrier Reef in the east and Ningaloo in the west – are a drawcard for divers around the world.
Corals generally like warm temperatures – but there are some that have found a niche living in cooler climes. Sydney does get warm waters in summer of course, but in winter we can get water temps down to 13 C. It surprises many people to find that we do have quite a bit of coral in our local waters.
Corals are marine invertebrates of the class Anthozoa. Subclasses of coral include the hexacorals, which have polyps with 6-sided symmetry, and the octocorals with – you guessed it – eight-sided symmetry. Hexacorals are further divided into anemones, zoanthids (both of which are soft-bodied) and scleractinia – the hard or stony corals. These are of course the famous reef-building species that make up our tropical reef systems, but they also live in temperate waters.
Scleractinia grow a hard skeleton of calcium carbonate, which accretes over time to form the famous coral structures. The soft living tissue – comprising the polyps and joining matrix – sits in a thin layer on top. Most scleratinia have symbiotic algae – zooxanthellae – which are ejected if the coral is stressed, leading to bleaching. Some scleractinia have evolved to do without these algae, and these typically have larger polyps as they have to catch all of their own food.
Sydney has both zooxanthellate and azooxanthellate corals. The former are typically encrusting, forming large plates of tiny polyps, each with characteristic brownish tinge indicating the symbiotic algae within. The two main species in Sydney are green coral Plesiastrea versipora, and the “URG coral” Coscinarea mcneilli.
We also have azooxanthellate species. These are often solitary (one polyp grows alone) or in small clusters. Several species have been recorded over the years, including Tubastraea coccinea, Balanophyllia bairdiana and Culicia tenella.
Note that some of these are provisional IDs – to be certain of a coral species, we often require a sample to be studied under the microscope. But growth patterns and sometimes colours can be a good indication. Scientists have recorded 14 species in just the azooxanthellate scleractinians in this region. Many of these are only identifiable from their skeletons at this point – perhaps one day we will have good quality confirmed living pictures of all of them!