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Those Blooming Jellyfish


 
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Associate Professor Kylie Pitt, Australian Rivers Institute, Griffith University, leads us into the remarkable world of the jellyfish - animals with one or many mouths and cells that fire tiny harpoons from the cells covering their bodies. She is joint editor of the book Jellyfish Blooms (2014).

 
Jellyfish, which are closely related to corals, evolved over 500 million years ago and are part of the phylum Cnidaria. All of the animals that are part of that phylum have cells called cnidocytes which fire microscopic toxic harpoons when brushed against. This of course means that most people have a negative view of jellyfish, having been stung by them. However, jellyfish have to be very adept at defence to survive.

 
Above: Large jellyfish surrounded by juvenile fish who are using it as a safe haven from predators. Photo by Anne Laure Clement     
They drift in the water column, often on or near the surface currents in the oceans and are therefore highly vulnerable to predators. They can’t swim particularly well so outpacing any predators is not an option, hence the large numbers of stinging cells placed around their bodies. Jellyfish are only some of a range of ocean creatures which have gelatinous body features. For example the clear, gelatinous blobs that often wash up on beaches are not jellyfish, and indeed are very primitive members of the phylum Chordata, which is the phylum that includes all vertebrates - including humans.

 

 
There is now evidence from stable isotope research into marine animals' diets that a range of marine animals eat jellyfish.    
Sunfish and sea turtles are well known predators but many other species of fish, including leatherjackets grazing on them. Despite the jellyfish stings and large proportion of water in their cells, there are some upsides to a diet including jellyfish. They often in abundance, are very slow moving and are easily digested. Indeed, the predators’ digestive systems seem to cope with the toxins. For example, turtles are known to eat box jellyfish, a particularly toxic form of the animal. It is possible that the turtle's very thick mouth skin helps withstand the toxic harpoons.

 
Jellyfish live at all depths in the water column although, as with many marine species, there is an abundance in surface waters. Most jellyfish eat zooplankton, tiny microscopic animals, as well as small fish. Some species, such as the box jellyfish, prey on shrimp and small adult fish. Some jellyfish even eat other jellyfish! The mouth (or mouths) of jellyfish are varied. Some jellyfish have one mouth, like the box jellyfish which swings its prey into that mouth which is located under the bell. Other species (the rhizostomes) have multiple tiny mouths in their tentacles which channel the tiny food particles into their gut.
Jellyfish provide important habitats for other marine creatures. Large numbers of juvenile fish, for example, are often seen swimming amongst the tentacles of the jellyfish which provide an excellent refuge from potential predators.

 
Large jellyfish concentrations of jellyfish in the ocean (called jellyfish blooms) are a normal part of the jellyfish’s life history. After male and female jellyfish have fertilised their eggs, tiny larvae are formed which swim away from the adults, settling on the sea floor and metamorphose into polyps (see left for polyp image).
When conditions are favourable the polyps will bud off tiny jellyfish, some species creating 30-40 jellyfish from one polyp. These juvenile jellyfish will swim up into the water column and, because they grow very rapidly, huge numbers can suddenly appear. There are seasonal factors to their appearance. In Moreton Bay, Brisbane, for instance they are notable over summer but scarce in winter.

 
In south-east Asia and China jellyfish are a popular food item. Even Australian Asian supermarkets sell dried jellyfish. Some 300,000-500,000 tons of jellyfish are harvested globally each year and in China, where jellyfish have been over harvested, jellyfish aquaculture projects have been put in place to restock their coastal populations.

 
Ice and snow seem a far cry from jellyfish research. However Dr Pitt's work took her to the Limfjord in Norway, a location reknowned for having been 'taken over' by jellies!

Dr Kylie Pitt was interviewed for A Question of Balance by Ruby Vincent.  Images provided by Dr Pitt. Summary text by Victor Barry, April 2012/2014.

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