Life on a reef

The Leigh Marine Laboratory is located 100 km north of Auckland on the edge of the Cape Rodney-Ōkakari Point Marine Reserve (commonly known as the Goat Island Marine Reserve). The reserve was created in 1975 and was the first marine reserve in New Zealand. Since its creation, the reserve has been monitored and the marine wildlife studied.

The reserve contains a number of different marine habitats, from rocky shores to deep reefs. Each habitat supports a range of sea creatures uniquely adapted to the specific conditions. The scientists featured in this context are particularly focused on the reef environment that is home to such species as kina, shrimps, a variety of fish, kelp and many more. Each has its home on the reef, in a particular place, and all contribute to a dynamic ecosystem.

Reef zones

Many different things affect reef habitats – wave action, light level, currents, rock structure, water temperature and salinity. Of these, wave action and light level are the most important, and it is common to use these two factors to divide the reef into several zones.

At the top of the water surface, there are strong currents and lots of light – this is the intertidal (or littoral) zone. This is the region between the high tide mark and the low tide mark. Plants and animals in this zone will only be covered with water when the tide is in. This makes it an ideal habitat for crabs, barnacles and sea anemones.

Below the low tide mark is the infralittoral zone, which is home to seaweeds, kelp forests and reef fish. Some of the noisiest species of the reef – shrimp and kina – live in this middle zone. The edge of this zone is marked by light. When it becomes too dark for kelp to survive, the circalittoral zone starts. This is home to sponges, sea squirts and corals.

Exploring the infralittoral zone

Under the low tide mark, but where light can still penetrate through the water, the reef habitat is alive with species.

Just below the water surface is a type of seaweed called bladder weed (includes flapjack and wide weed). Around Goat Island, this seaweed grows to a depth of around 6m under the low tide mark. Bladder weed attracts fish such as the parore (Girella tricuspidata), which is able to change colour to suit the environment from a black/blue and white pinstripe to white, black, olive and a blotchy mix of all these.

Parore graze on small algae that grow on the bigger plant-like seaweeds, providing a cleaning service that allows the seaweed to flourish. This area of the reef is also inhabited by fish such as the silver drummer (Kyphosus sydneyanus), marblefish (Aplodactylus arctidens) and kelpfish (Chironemus marmoratus).

Below the forests of bladder weed are the kina, often in a zone with no large seaweed – referred to as the barren zone. Kina are voracious eaters of kelp, particularly young kelp, and are known to decimate kelp forests if left unchecked. The natural predator of kina is snapper. In many areas, overfishing has left too few snapper to control kina populations, and the kelp forests are disappearing. Around Goat Island, the marine reserve has helped the populations of snapper rebound. This is keeping the kina numbers in balance and kelp can rejuvenate.

The barren zone is also home to creatures such as starfish (or sea stars) and sponges as well as fish such as leatherjackets (Parika scaber), spotties (Notolabrus celidotus) and moki (Cheilodactylus spectabilis).

Kelp extends beyond the barren zone, down to depths as far as light can travel. Around Goat Island, this may be 18 m or more. Species such as snails, small crustaceans (shrimps and crabs), butterfish (Odax pullus) and wrasse (Notolabrus fucicola) can be found down to this level.

The circalittoral zone

Below the kelp is the deep reef. It is here that the brightly coloured corals grow. Rocky ledges and holes provide an ideal habitat for species such as lobsters, shrimps and octopuses.

From here, the light disappears altogether and the reef begins to merge with the sea floor and the habitats change again. The reef with its rocky edges and swirling currents becomes sand and darkness.