Feature

In the Coral Sea

Black corals not only look alike, they are alike.

Dr Karen Miller

New Zealand's marine communities are an important part of our heritage, and learning more about the biology and ecology of marine species helps us to understand, manage and protect them.

The shallow water marine habitats of Fiordland are of particular interest because they are home to an incredibly high diversity of marine species which are usually found only in deep water hundreds of metres below the surface. In the fiords, a combination of calm water and low light makes the shallow water environments habitable for many species that usually only live at depths. These deep-water emergent species include rare sponges, red corals, some fish species and the endemic black coral, Antipathes fiordensis.

The shallow water populations in the fiords provide a rare opportunity for scientists to research species that are usually very difficult to access.

At NIWA we have been studying Fiordland's black corals over the past 13 years, investigating many aspects of black coral ecology, including distribution patterns and growth rates. More recently we have been studying reproduction in black corals and trying to understand how black coral larvae are transported throughout the fiords.

Like many other coral species, black corals reproduce annually and produce free-swimming planula larvae which are planktonic for a period before they settle and metamorphose into coral polyps. Two important questions we are interested in are how far the coral larvae are transported before they settle and metamorphose, and more specifically, whether larvae are transported among the 14 fiords which indent the south-western coastline of New Zealand.

This information is important because population maintenance will depend on the recruitment of new individuals, but also because recovery following catastrophic events (both natural and man-made) will be entirely dependent on larval transport to the affected areas.

Unfortunately, black coral larvae are very small -- less than 0.2 mm long -- so it is impossible to directly track the larvae in the water column and determine how far they are transported. Instead we have been using genetic techniques to help us understand more about the dispersal of black coral larvae. By comparing the genetic make-up of black corals from different parts of Fiordland, it is possible to determine how closely related different populations are, and consequently the amount of gene flow (or larval dispersal) that occurs between them.

Close Kin

We have compared the genetic structure of black corals from 28 sites in 13 fiords throughout Fiordland as well as corals from Stewart Island.

We have found significant genetic differences between sites within the same fiord (separated by 10-15 km) which suggests there is no gene flow in the form of larval dispersal between sites. Essentially, larvae are not being transported even across distances of tens of kilometres, and there is restricted larval dispersal even within a single fiord.

At the opposite end of the scale, we have found that coral colonies separated by very small distances (tens of metres) are closely related. This suggests that many coral larvae settle very close to their parent colonies, probably within 5-10 metres.

From the results of genetic studies we have been able to conclude that black coral larvae are only transported short distances and are not dispersed between the 14 different fiords throughout Fiordland. Effectively, the black corals in each of the fiords are isolated and therefore should be managed as separate populations.

"Young" Coral

One of the surprising results from our genetic research is that the coral populations in different fiords, including populations from Stewart Island, remain genetically very similar, even though they are effectively isolated from each other.

There may be a number of explanations for this pattern, but we suspect the reason there is little genetic differentiation among the 13 fiords we studied is because the populations are geologically very young. The fiords have only existed in their current form as marine environments for approximately 12,000 years.

While this may seem like a long time, black corals colonies can live for hundreds of years, so in fact after only 12,000 years there will have been relatively few complete generations of black coral. Consequently the separate fiord populations retain genetic characteristics from when they were first colonised from the population in deep water outside the fiords.

Perhaps in another 12,000 years, after more generations of corals have passed, we will find that the black corals in the separate fiords have evolved independently and show much more genetic differentiation.

Another interesting finding of the genetic studies of A. fiordensis is the discovery of clones, or colonies with identical genotypes. This means that as well as reproducing sexually, the fiord corals are also reproducing asexually. Many coral species produce new colonies by asexual reproduction but this is the first time it has been described in a black coral. Surprisingly, some of the clones are found throughout individual fiords, which suggests that the asexual propagules are more widely dispersed than the sexually produced planulae.

By using genetics we have been able to discover much more about the biology and ecology of Fiordland's black coral communities than was otherwise possible, and our results can be used to better manage the unique fiord habitats, particularly with increasing tourism and development in the Fiordland area.

Genetic techniques are invaluable tools for marine ecologists and for conservation biology, and at NIWA we are continuing to use this technology to help us answer similar questions about a variety of marine species.

Dr Karen Miller is a research scientist with NIWA based in Wellington.