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4 November 2019
Jellyfish - indicators of ocean change?

(Published - 4 November 2019)

If you were asked to name the first five marine animals that came to mind, what would you say? We usually hear dolphins, seals or sharks. But how many of you would include jellyfish? A growing number of people around the world would.

In fact, we are hoping about 120 of these other interested people will be descending on Cape Town for a few days this month to celebrate the 6th International Jellyfish Blooms Symposium. This conference, which brings together experts to discuss the latest advances in knowledge and understanding about jellyfish blooms, is held every three years.

The University of the Western Cape (UWC), in collaboration with Iziko Museums and the Two Oceans Aquarium will host the event, while the scientific programme will be guided by an international panel of enthusiasts.

But why the interest in jellyfish? Aside from their intrinsic, aesthetic and cultural value, jellyfish are also a bellwether in a potentially changing ocean. Most jellyfish, certainly the ones we routinely come across on the beach, are only present in the sea for relatively short periods of time. They make episodic appearances. This reflects their bipartisan life-history.

Polyps, which measure just a couple of millimetres in size and look like sea anemones, are easily overlooked, being hidden amongst seaweeds below water, and they reproduce asexually – either budding off new polyps or, when the time is right and the environment is favourable, splitting to release medusae, or jellyfish. These jellyfish then float off into the plankton where they grow and eventually reproduce sexually. The product of fertilisation is rarely a new jellyfish, but rather the progenitor of a new polyp, which then settles on the seafloor to renew the cycle.

In most temperate environments at least, the baby jellies are released over winter so they are in place when spring comes around with its abundant food supply. Adult jellyfish reproduce towards the end of summer and then die, at about the same time as their food supply dwindles.

It has been suggested that jellyfish may be hanging around for longer periods than was previously the case, and that they may be appearing in greater numbers than before. This has a number of implications, because jellyfish in large numbers cause problems for people. They clog the filters that are fitted to the intake pumps of coastal power and desalination plants, which inevitably causes chaos.

The nuclear-powered aircraft carrier, USS Ronald Reagan, was brought to a standstill off Australia because of jellyfish in 2006, and the Chinese navy is investing in jellyfish-shredders to clear the path for warships. When jellyfish are caught with finfish, it’s goodbye to profits. When jellyfish bloom off tourist beaches, local businesses close shop as their presence is hazardous to bathers.

It is clear that when jellyfish occur in large numbers for long periods, there is cause for concern. Unfortunately, rigorous historical data against which it would be possible to track change are mostly absent. That said, there does seem to be some places where jellyfish are more abundant now than they once were.

Given the impact of human beings on our natural world, and on the ocean in particular, it is natural to assume a link between increasing jellyfish abundance and anthropogenic change. And, in this light, it should be realised that jellyfish have a number of pre-adaptations to the changes we have caused that would allow them to increase in numbers.

For example, cultural eutrophication (our dumping of phosphorous-rich wastes into the sea via rivers, sewage and storm-water drains) impacts water clarity, the size structure of the food web (promotes small and not big plankton) and the concentration of life-giving oxygen. These changes are not good for most of the types of fish we eat, as these generally use vision to see large-sized prey. They also need high concentrations of oxygen to support their active lifestyles.

Jellyfish, by contrast, don’t use visual cues to find and capture prey. They can eat small prey items and they can thrive under low oxygen conditions. Inevitably then, jellyfish have the capacity to survive and flourish in such environments whereas commercially exploitable fish do not. Examples of other pre-adaptations by jellyfish in the context of global climate change and temperature increases are coastal sprawl and overfishing. It should also be remembered that in most human-impacted ecosystems, several of these stressors might be witnessed together. 

Thus, some coastal embayments may be affected by cultural eutrophication, urban sprawl and climate change – long having lost their native fish to overexploitation. 

But the jury is still out as to whether these are actually the cause of the global increases, if they are indeed increases. These topics and many others will be under the spotlight at the symposium, which will have a development focus. The emphasis on development applies across all themes of the single-session meeting, from technology and observation, through theory to models. Human capacity development is vitally important, especially in Africa, and an additional workshop will be held for young scientists and students from across the continent, tutored by some of the international delegates.

Funding for this latter initiative has been provided by the National Research Foundation and will use material collected by the Ecosystem Approach to Fisheries-Nansen Programme of the Food and Agriculture Organization of the United Nations.

Jellyfish are beautiful and ethereal creatures and feature as products of our mythologies. They appear in our art and literature and they are part of our cuisine. Yet, despite this, they are also mysterious and may yet be harbingers of change.

Professor Mark Gibbons is from the Department of Biodiversity and Conservation Biology at the University of the Western Cape.