Upper zone: surface down to 15 metres
Most marine organisms in the fiords live on the rock walls. In depths to 5 metres, the upper layer of fresh water restricts communities to those capable of living in a brackish environment. The zone is impoverished, dominated by green seaweed (Ulva species), mussels, barnacles (Eliminius species), shrimps (Palaemon affinis) and the small cushion star Patiriella regularis.
Below the layer of fresh water the communities are extremely diverse. Between depths of 5 and 15 metres the rock walls are encrusted with tube worms, sponges, soft corals, sea squirts and molluscs. These are food for a variety of starfish, urchins, sea snails and sea slugs. In normal circumstances the big 11-armed starfish Coscinasterias calamaria assembles in large numbers just below the fresh-water layer. When several days without rain causes the layer to recede, the starfish have access to the mussel band, and feasting begins.
It is possible to view some of Fiordland’s marine life without getting your feet wet. The Milford Deep Underwater Observatory allows visitors to descend 10.4 metres below the surface to view black corals, red corals and other marine life. The marine animals are held in special trays that are raised in the morning and lowered to depths more congenial to their occupants at the end of the day.
Lower zone: 15–40 metres deep
In the perpetual gloom of depths from 15 to 40 metres, large sponges, sea squirts, corals, hydrocorals and lampshells (brachiopods) dominate the rock walls. Black coral, usually confined to offshore islands at depths greater than 45 metres, grows in abundance in all the fiords, in colonies that are up to 5 metres tall. Underwater, the colonies are a ghostly white – it is the skeleton that is black and gives the coral its name.
A small (7.5-centimetre) white sea cucumber (Ocnus species) is common on underwater cliff faces. In a few places in Fiordland the strawberry sea cucumber Ocnus brevidentis dominates the zone just below the fresh-water layer.
Lampshells are common throughout most of Fiordland. In some areas it is estimated that they reach densities of 1,000 per square metre.
The glass sponge Symplectella rowei is found at depths of 30–50 metres, attached to submarine cliff walls in Doubtful Sound. They are the largest sponges in Fiordland, and an example of deep water emergence; glass sponges are usually found only in deep oceans or the polar regions.
Deep basins: more than 40 metres
Below 40 metres, and down to the bottom of the fiord basins (up to 450 metres), animal life is more sparse. Heart urchins and tube worms predominate to depths of 200 metres; below this, shellfish, heart urchins and crabs live in a muddy environment similar to that at depths of 1,000 metres in the open ocean.