This article is written by Andy Lewis of Tevene'i Marine based in Australia. Tevene'i Marine is a Coral Reef Ecology Consultancy and Ecotourism business established by Dr. Andrew Lewis and Daniela Lewis, graduates of the Department of Marine Biology at James Cook University. All images by Roger Steene.

Figure 1: A Soft Coral (Scleronepthya spp)

Biological Background

Soft Corals are a conspicuous and colourful component of coral reefs throughout the Indo-Pacific. They are colonial animals and members of the Phylum Cnidaria, which includes the Jellyfish, Hard (Scleractinian) Corals, Hydroids, and Anemones. Soft corals, along with Gorgonian or Fan corals, are grouped together in the Sub-Class Octocorallia, which reflects the fact that the polyps that make up the colony usually have 8 tentacles surrounding the polyp mouth.

As their name suggests, the colony is usually soft and fleshy, and they have no hard internal skeleton of calcium carbonate like the reef-building Scleractinian corals. Hard skeletons provide support and structure for the colonies of the Scleractinians, and also give the polyps a day-time refuge from fishes and other predators. Soft corals have no such skeletal protection, so how do they avoid being eaten?

Like many other soft-bodied reef animals, soft corals avoid predation by storing toxic chemical compounds in their tissues. This makes them highly unpalatable or even poisonous to most potential predators. We call these chemicals "secondary metabolites", because they are not involved in the primary metabolic functions of the organism. The secondary metabolites found in soft corals come from a range of different chemical "families", but chemicals called terpenes are probably the most common. We have quite a good understanding of the types of chemicals found in Soft Corals because many of them show promise as anti-bacterial, anti-fungal, or anti-cancer drugs, so pharmaceutical companies and marine chemists have spent considerable time and effort screening and evaluating the chemicals found in soft corals on the Great Barrier Reef.

Amongst the Soft Corals, there is a range of fragilities of the colony tissue, and this corresponds quite well with the concentration and toxicity of the secondary metabolites found in the coral. In general, the more fragile species have the most toxic chemicals. For example, the Scleronepthya spp. in the main image above has a relatively rigid colony, and if you look closely at the main branches, you can see numerous calcium carbonate spicules inside the tissue of the branches. A close-up of these spines is shown below.

Figure 2: Calcium carbonate spicules in the branch tip of a Neptheid soft coral

These confer some physical protection from predation, so the coral needs to expend less energy on producing secondary metabolites. In contrast, the colonies of Sarcophyton spp. and Clavularia spp. shown below are much more delicate, and these species have a higher concentration of toxic chemicals in their tissues to deter predators.

Soft Corals are not totally without predators though - for each species, there is usually one or two species of predators that have evolved resistance to their toxins, and can therefore feed on the coral unharmed. The beautiful white egg cowrie shell, the butterfly fish Chaetodon melannotus, and the damselfish Neoglyphidodon melas are some known soft coral predators, although we are still unsure of exactly how each of these species is able to resist being poisoned by the coral.

For more information on the distribution of Soft Corals on the Great Barrier Reef, browse the AIMS website and have a look at the Soft Coral Atlas by Dr. Katharina Fabricius and Dr. Glenn De'Ath at:

http://www.aims.gov.au/pages/research/soft-corals/soft-corals00.html