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Feature

Icy Sponges

Anti-cancer agents and global warming indicators have an unlikely conjunction in Antarctic sponges.

By Dr C. N. Battershill

Three years of research into Antarctic sponges have produced some surprising results. Not only do the marine organisms contain higher levels of anti-viral agents than expected, but they can also tell us something about the increasing sedimentation rates possibly associated with global warming.

Our initial research began by examining natural chemicals produced by marine plants and animals which showed anti-viral or anti-tumour activity. Work carried out around New Zealand and in the tropics suggested that many of the chemicals which had potential therapeutic application were used by sponges and the like as natural defensive compounds against competitors or predators.

The extremely stable environment of Antarctic reefs would suggest that sponges there would not need such defensive chemicals. The water temperature is a constant -1.9oC, and it is almost always pitch black below the two metres of sea ice which is frequently covered by at least two metres of snow. There are large numbers of grazing and predatory starfish and urchins, but the benthic community, made up mainly of sponges, remains unchanged for extremely long periods.

Would Antarctic sponges have defensive chemicals if, as we hypothe-sised, they didn't appear to need them? Examining the pristine Antarctic environment was a type of "control" exercise -- we expected to draw a blank, but we didn't...

Visibility in Antarctic waters is limited only by the darkness caused by the snow and ice ceiling above and our poor eyesight. The cleared ice above the dive site acts like a vast fluorescent light, illuminating a profuse assemblage of sponges, sea squirts, and anemones. The brilliant pastel colours of many of the species were immediately impressive and rather at odds with what we knew to be normally pitch black conditions. The next feature which struck us was the weird, ethereal sound. At times the screams and booms of Weddell seal sonar were deafening -- sinuses and lungs would shake with the resonance.

Everything was big. Sea squirts as large as footballs, and anemones with discs as big as dinner plates. At the limit of our depth restriction we could see gigantic volcano sponges looming in the distance. Some were 2-3 metres tall and could hide a diver.

Surprising Results

Over the first two years, we mapped the reef floor and quantified the distribution and abundance of all the organisms present. We found initially that, as expected, they did tend to show low levels of biological activity. This supported our idea that in a stable environment where defensive chemicals are not likely to be needed, bioactive compounds would not be produced by the resident organisms.

What we didn't expect, however, was to find that even though all of the species were endemic to Antarctica, only having distant links with counterparts from warmer waters, they still maintained the ability to produce toxic chemicals if the need arose. This suggested a chemical flexibility not previously thought possible for sponges or indeed any marine organism, as many of the chemicals produced are large and complex. Most cannot be synthesised by current technology.

This finding has generated a great deal of research in biotechnology and aquaculture in New Zealand and Australia. The aim is to artificially generate quantities of compounds which have heightened anti-viral/anti-tumour activity. As far as the Antarctic community is concerned, it suggests that the ecosystem is more complex than we originally thought.

A second significant finding relates to how the Antarctic community originated. We found that many species were reproducing asexually by simply budding off pieces of tissue. Recent research in temperate New Zealand and in the tropics suggests that this is commonplace in rocky and coral reef habitats. Successful regeneration of the community is likely to be very sensitive to other environmental conditions, the most important of which is the sedimentation regime. It became apparent that in Antarctica, this sensitivity was especially keen.

Sedimentation

In recent years, there has been an increasing amount of sediment run-off from Ross Island as some glaciers recede. This could be a possible response to greenhouse warming and reduction of the ozone layer, or it could be some long term cyclical event. The impact of increased sediment run-off from land on Antarctic reef communities is of major concern, however.

It is becoming increasingly clear that reefs characterised by sponges are important habitats for other species, particularly fish which use them as nursery and feeding grounds. The interrelationships between reefs and associated fish populations are not well understood, least of all in Antarctica.

A joint NZOI/DOC study last year looked at the reefs around Pram Point, adjacent to Scott Base, where considerable glacier retraction has been observed, to identify impacts of increasing sediment run-off. The original study site at Cape Armitage was re-surveyed for comparison. At the same time it was possible to assess whether Scott Base itself was having any impact on the adjacent subtidal reef communities. No sediment related impacts were discovered, although sediment trap experiments were installed for continued monitoring.

The advantages of this research are twofold. Firstly, detailed examination of the Antarctic marine ecosystem will provide for early warning of important changes bought about by greenhouse warming. The subtidal marine habitats are likely to change noticeably first.

The second advantage is that information gained from this work will allow us to interpret how sediment run-off has affected coastal reef communities around New Zealand. The research goes hand in hand with on-going studies into the effects of Cyclone Bola, which released over one million tonnes of sediment into one river system alone to be discharged along the Gisborne coast. To do this we needed to examine in detail the patterns of distribution of the encrusting community and assess how the structure of rocky reef habitats is maintained.

The next job was to examine the interrelationships between the benthos, and associated mobile grazers and predators -- urchins, starfish and fish. It is known that the Antarctic marine ecosystem is extremely productive and supports a vast biomass of planktonic invertebrates, pelagic fishes, marine birds and mammals. Very little is known about the interdependence of higher levels of the Antarctic marine food web with the reef floor. By examining the densities of predatory species and observing their behaviour in situ, we attempted to bridge this gap in knowledge.

Results from the three seasons' research are now being assimilated. A monitoring programme has been installed which will allow for analysis of changes in the reef community structure over long periods of time. We will be in a position to comment on the possible causes of any observed change and contrast the information from the extreme Antarctic situation with that around New Zealand. We need to know urgently how continuing sedimentation of our coastal marine ecosystems is influencing associated fish stocks.

The Antarctic sponge community is truly magnificent. It remains undisturbed and the organisms present are likely to be extremely old. The reef "feels" primordial and in many ways it represents a frozen piece of natural history unassailed by human influence. It is sensitive to any disturbance -- any changes in the community structure are likely to be severe and permanent. They would certainly herald catastrophic changes in environmental conditions world wide.

By understanding the Antarctic marine environment and monitoring it, we can be in some way forewarned of any global change, but it would be much more prudent to act now to preserve this fragile environment.

Dr Chris Battershill is a marine biologist with the New Zealand Oceanographic Institute.