Corals of Sydney – Part 2: Soft Corals

In our previous article on Corals of Sydney I discussed hard corals or Scleractinia. In this article I’ll describe the soft corals, or octocorals. These are colonial animals with eight tentacles on the polyps, typically growing on a soft rather than hard skeleton.

Soft corals have a lot in common with hard corals – they both have stinging polyps which want to get up into the water column to feed – but in the case of soft corals they build flexible structures rather than hard calcareous ones. Some soft corals use a hydraulic system, inflating when the currents are strong enough to feed.

The local endemic species Dendronephthyastralis does this; see this one uninflated:

and another one inflated:

Other soft corals build a more rigid structure, often using material like our fingernails.

Sea fans and whips, or gorgonians, do this:

Carijoa, a very successful genus around the world, common in Sydney at places like Shiprock, goes one further and enjoys a symbiotic relationship with an orange sponge which coats and protects the central structure on which the polyps grow:

You may have noticed another interesting gorgonian endemic on dives, the southern sea fan Sphaerokodisis australis:

Then we have the sea pens. these also have a hydraulic system to pump themselves up into the current to catch prey:

But, they fall prey themselves to a large sea slug predator, Armina, which you will often find buried in the sand at the base of the sea pen:

You can also find encrusting soft corals in Sydney, like Erythropodium hicksoni:

And there is the common but unremarkable soft coral Capnella sp:

In short, it’s not just the tropics that have a diversity of corals! Whilst our reef structure in Sydney is rock not coral, we have plenty of interesting corals growing on the substrate: