They have black skeletons of dense, horny material, covered by a thin layer of living tissue from which the polyps project.

[3] The fiords where A. fiordensis grows have nearly vertical rock walls, providing limited sites for attachment of organisms, so many other species live on these corals.

One is the snake star Astrobrachion constrictum, which perches and coils its arms tightly around the coral's branches.

This snake star is found in a number of places around New Zealand, but always in association with a black coral.

[8] It seems to be a mutualistic arrangement: coral polyps are more efficient at catching prey than the unbranched arms of a snake star; the snake star appropriates some of the prey, while also cleaning mucus off the coral and preventing epizoic organisms from settling on it.