Black coral ~ Antipathella fiordensis

The spectacular black coral, Antipathella fiordensis, is a species of tree-like colonial coral endemic to the fiords of New Zealand.

Black coral is actually white in appearance, the moniker black coming from the hard skeleton consisting of protein and chitin that lies under- neath the living tissues. The living polyps that cover the skeleton are tiny, less than 1 cm in diameter, and these filter out the plankton from the water column, the same as sea pens, anemones and other coral relatives.

Black corals are slow-growing and long-lived. Colonies live up to 300 years old! Being so old and delicate means they are very susceptible to disturbance; they stand little chance of surviving from anchors, pots and trawling. In recognition of this all black corals are fully
protected by law under the 1953 Wildlife Act. Fortunately, the sheer-sided underwater walls of the fiords provide some natural protection – not really a suitable place to anchor or place fish pots.

Another unique environmental condition of the fiords is their freshwater layer. Due to the steep mountainous terrain topside, and the gargantuan 8m of annual rainfall, the tannin-stained freshwater running into the fiords creates a dark, brackish layer that simulates deep water. Consequently, a diver will encounter black coral living as shallow as four or five metres. Compare this to other regions of New Zealand, where other black corals are not found at recreational diving depths.

These flexible “trees” can be up to 3m tall, and as such are quite a feature of the underwater environment, providing habitat and shelter for many other species.

One of their common allies is the snake star Astrobranchion constrictum, which lives mutualistically on the coral. The black coral is more adept at catching plankton prey from the surrounding water than the snake star which benefits from it. In return, it is thought that the snake star cleans off mucous and prevents other organisms from settling and growing on the black coral.

8 facts about black coral

1. The skeleton underneath the living tissue is black
   
2. Covered in small 6-tentacled polyps less than 1-cm in diameter
   
3. Does not contain zooanthellae like tropical corals
   
4. Protected by law since 1953
   
5. Lives in extremely shallow depths in Fiordland
   
6. Becomes more common and shallower the further into the fiords you go
   
7. The mutualistic snake star Astrobranchion constrictum lives in its branches
   
8. Can live up to 300 years