Hollywood has long been dominated by horror movies about the earth being taken over by aliens from outer space, but the real-life colonisation of the planet's oceans by a seemingly unstoppable force might already be taking place.
They're silent and mysterious, but it turns out that jellyfish are the ultimate colonial agent. They reproduce rapidly and by sheer force of numbers take over entire slabs of ocean, with dire consequences for the environment and human activities such as fishing and tourism.
In South Africa, jellyfish have already demonstrated their power. During 2005 Eskom’s Koeberg nuclear power station outside Cape Town lost power because unusually high numbers of jellyfish were found in the sea water that is sucked up for cooling the nuclear reactor.
In another example close to home, a boom in jellyfish numbers in Namibia has been fingered as the reason behind the decline in the country's once highly-lucrative fishing industry.
Examples like these have led to scientists looking at jellyfish with increased interest as they seek to unlock the secrets of the species.
Although data is lacking, what appears to be emerging is that a combination of man-made factors such as eutrophication of the oceans and global warming are leading to a “back-to-the-future” scenario where jellyfish are taking over patches of ocean that then come to resemble what the oceans used to look like before fish evolved about 500-million years ago.
“It is ironic that the same activities that are driving rapid industrialisation and technological achievements are threatening to push marine ecosystems back to the future,” said Professor Mark Gibbons, a jellyfish expert in the Biodiversity and Conservation Biology (BCB) Department at the University of the Western Cape (UWC).
With surprisingly little being known about jellyfish, scientists in the BCB department have geared up their research into jellyfish with the South Africa Jellywatch project that recruits volunteers around South Africa to report on jellyfish via the website http://sajellywatch.uwc.ac.za.
What is known about jellyfish, however, shows that they have had a fascinating relationship with humans for thousands of years – both as a source of food and in medical care.
As outlined by Gibbons in a journal article co-authored with scientists from the United States, United Kingdom and Australia, jellyfish have been eaten since AD 300 in China. More than 500,000 tonnes are harvested annually for human consumption in Southeast Asia.
Considered a speciality food, jellyfish is eaten at Chinese weddings and other high-profile occasions. The drive to healthier food means renewed interest in jellyfish, as it is made up of 96% water and has no fat, cholesterol or sugar.
In medical care, traditional medicine has used jellyfish for the treatment of arthritis and ulcers, amongst other ailments, and although little research has been done into how effective this is, studies have shown that jellyfish collagen suppresses arthritis in laboratory rats.
Within the ocean ecosystem, jellyfish have been no less influential, providing food for fish and endangered species such as leatherback turtles. Jellyfish found in large numbers, points out Gibbons, are not unusual, but a cocktail of factors appear to be tipping the balance to a situation where jellyfish become dominant, completely changing the ecosystem.
It's what Gibbons and fellow scientists from around the world refer to as “the jellyfish joyride”.
But it's not the jellyfish themselves that, like the aliens in Hollywood movies, are inherently out to cause mischief for the human race. Rather, evidence suggests that the blame for an explosion in jellyfish numbers can be placed squarely on the doorstep of human excesses.
Understanding the reproductive process of jellyfish is key to understanding the reasons behind an increase in their numbers.
Reproductively, jellyfish pass through two life phases, beginning life as tiny polyps only millimetres in size with an upward facing mouth surrounded by tentacles that are either attached to something or free floating. Small jellyfish emerge from the polyp and grow by feeding on plankton.
With an increase in coastal development, the surface area available for polyp attachment from which jellyfish emerge has increased.
Overfishing is another culprit. With fish being jellyfish predators, an increase in industrial-scale fishing and thus a removal of large numbers of the main jellyfish predator is seen as one of the reasons for the explosion in numbers. Coastal eutrophication – nutrient pollution of the ocean mostly caused by human pollution – is another human-induced factor. It encourages types of phytoplankton that can lead to jellyfish blooms.
And the jellyfish blooms might also be the proverbial canary in the mine shaft when it comes to climate change. Warming oceans, says Gibbons, are potentially more favourable for jellyfish than fish and could also spread the range of more toxic jellyfish species that are usually only found in subtropical areas.
The result of all these factors is like a “pattern of falling dominos”, Gibbons says. With uncontrolled population growth, natural controls simply fail, leading to an exponential process where multiplication becomes ever more efficient and jellyfish begin to invade healthy habitats.
One of the short term implications of this, says Gibbons, lie in tourism because beaches could be invaded by stinging jellyfish. But economic activity in the form of fishing is also damaged because large numbers of jellyfish present obvious problems for trawler nets.
And because jellyfish are “voracious” predators of fish eggs and larvae, they also make it extremely difficult for fish populations to re-establish themselves – with enormous knock-on effects for the entire ecosystem. The South African Jellywatch project aims to discover more about the translucent creatures.
Walkers, divers, surfers, canoeists and fisherman from all South Africa's coastal provinces have been invited to report their observations to the website, which provides a guide to the common jellyfish, a brief introduction to jellyfish diversity and a glossary of terms. Volunteers can send in photos and obtain feedback.
“We also want to study how fast they will grow, how much food they eat, how they behave and then look at whether fish feed on them. This will help provide data on the biology that will let us understand why jellyfish are so common,” says Gibbons.
Up until now, encounters by the public with jellyfish have been limited to seeing a transparent wobbly thing washed up on the beach at low tide and wondering where it came from and how it got there. Now, as the environmental crisis surrounding the world's oceans intensifies, answering these questions has become ever-more important. – West Cape News