What brings a tropical jellyfish to the temperate waters off New South Wales?
Cassiopea cf. maremetens lying on sand in upside-down position
Photographer: Stephen Keable © Australian Museum
Australian Museum Research Institute scientists have just published the first records from temperate eastern Australia of the jellyfish Cassiopea, which is usually found in more tropical climates. The sudden appearance of these jellyfish in large numbers (blooms) is significant. Elsewhere, some jellyfish, including species of Cassiopea, are considered invasive. These jellyfish have the potential to act negatively on the local marine environment, in addition to impacting the public and commercial use of waterways when congregating in large numbers.
Sightings of large numbers of unusual jellyfish reported by concerned residents have been confirmed after specimens were obtained and identified as Cassiopea. A puzzling part of this equation was the location of the sightings: Wallis Lake and Lake Illawarra, both of which are widely separated across New South Wales. These specimens have since been lodged in the Museum collections and additional field observations were subsequently made. The two new localities represent southern range extensions of the genus by approximately 600 km to 900 km from previously known locations in southern Queensland.
Cassiopea are also known as upside-down jellyfish, and both the common and scientific names derive from its unusual life-style. In Greek mythology, the queen Cassiopeia was punished for boasting of her beauty by being placed in the heavens as a constellation representing her on a throne. As the stars in the constellation turn around the North Celestial Pole, she is tipped upside-down. The jellyfish is distinctive because it is commonly found lying on sediments in shallow water with the bell, or “umbrella,” of the body on the bottom, and the feeding arms extending above, making it appear to be upside-down. This behaviour provides sunlight to micro-algae living in the tissue (endosymbionts known as zooxanthellae, similar to those occurring in corals) which provide nutrients to the host. This is in addition to the nourishment received from the jellyfish’s diet of food particles from the water column. To capture small prey, stinging cells are present, a feature common to other jellyfish species. The severity of stings to humans from Cassiopea has been variably reported from mild or absent to severe. Cassiopea can also swim when disturbed by current or wave action, pulsing the bell to move through the water.
In Lake Illawarra, Cassiopea has been reported in large numbers only once, where they seemingly disappeared after a few months, whereas Wallis Lake reports have shown large numbers of Cassiopea to have been present over several consecutive seasons. However, recent observations show the numbers of individuals can fluctuate strongly. The jellyfish is just one stage (the medusa) of five in the lifecycle of this organism. Other phases include a small polyp stage fixed to other objects (e.g. fallen mangrove leaves) and a larval stage. When conditions are favourable, the polyp has the ability to divide and produce many jellyfish, so there may be considerable unseen dormant potential for further outbreaks.
The appearance of these jellyfish in New South Wales coastal lakes could be due to normal environmental variations driving changes in species’ ranges. However, Cassiopea are not considered to have strong natural dispersal, and given the widely separated occurrences from other records, it is likely human influences are involved. This may include shipping (through biofouling or ballast water release), intentional or unintentional direct release, or via “live rock” from aquaria (rock containing small invertebrates and algae used in fish tanks). Both lakes have considerable recreational and other domestic vessel traffic, and Lake Illawarra is close to a major population centre, Wollongong, and commercial port, Port Kembla. Increased water temperatures associated with global warming, or a variation of the East Australian Current bringing tropical waters from the north, could also be involved.
Adding to the mystery is the observation that the specimens from Lake Illawarra are different from those of Wallis Lake; so separate reasons may be involved for the occurrences in these two areas. Identification of the jellyfish is presently difficult, so plans for follow up study include genetic comparison and reference to further museum specimens. Tracking changes in the current populations that are present, and relating this to environmental variables is also of interest, particularly in determining any impacts the jellyfish may have on an ecological and recreational level. This type of tracking requires regular observation, so there are plans to return the study back to where it began, as a citizen science project utilising the surveillance of local residents.
Stephen Keable, Collection Manager, AMRI
Shane Ahyong, Research Scientist, AMRI
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