Seeking sun-baking, bottom-dwelling, upside-down jellyfish

Monitoring the invasive upside-down jellyfish (Cassiopea) in Lake Macquarie.

For my PhD, co-supervised at the Australian Museum and Sydney University, I’m studying the upside-down jellyfish (genus Cassiopea). These are unusual creatures that receive their common name due to a bottom-dwelling lifestyle. Cassiopea can swim but spend most of their time lying on sediment in shallow water, with the bell of the body resting on the sea-floor, and the feeding arms extending above, giving the appearance of being upside-down. This unusual behaviour optimises energy produced by photosynthetic algae, called zooxanthellae, living in the tissue of the jellyfish. Zooxanthellae provide the jellyfish with up to 90% of their nutritional needs, with the rest coming from zooplankton that are captured using their stinging cells (nematocysts).

During their lifecycle, upside-down jellyfish alternate between a larval, polyp and medusa (adult) stage. The medusa stage is the one readily recognised as a jellyfish, and the larvae are free swimming, settling on various hard surfaces to form polyps that are fixed to these substrates. During the polyp stage, when the conditions are right, many new jellyfish can be produced asexually from a single individual, through budding and strobilation. Many larvae and the resulting polyps can also be produced by the medusa stage so there is a high potential for population explosions (blooms) of jellyfish.

Upside-down jellyfish blooms have been recorded to reach up to 30 individuals per square metre and individuals can grow over 20 centimetres in diameter. Blooms can potentially impact the surrounding ecosystem. Likely influences include decreasing the oxygen content of the water, and shifting the structure of the food web through competing for resources and consuming larvae of local species. Swimmers can also be stung by the jellyfish (if this happens apply an ice pack to reduce pain and swelling, don’t use freshwater or vinegar as this will cause more stinging cells to be triggered). These negative consequences can have a flow on impact to commercial activities such as fisheries and tourism.

Upside-down jellyfish are considered globally invasive. In Eastern Australia their natural distribution has typically been considered to be restricted to tropical waters. However, recently they appear to have been expanding their range south along the east coast of Australia, into places previously considered temperate areas. New southern records were recognised in New South Wales in Lake Illawarra and Wallis Lake during 2013 to 2014, with at least two species involved. Additional New South Wales records were documented from Lake Macquarie in 2017.

As part of my PhD project, I aim to provide baseline information on the presence of Cassiopea in Lake Macquarie, and its potential impacts, informing effective management strategies. Information on the distribution, seasonality and environmental factors triggering blooms will be used to infer whether the population is increasing, stable, or declining.

I’ll also be using the Australian Museum collections to help identify which species of Cassiopea occurs in Lake Macquarie by comparing morphological characteristics to other specimens from various localities. Genetic analysis of specimens will also be undertaken to assist in distinguishing the species. Identifying which species of Cassiopea is occurring in Lake Macquarie will help determine where they may have come from and how they arrived in the new location.

Preliminary specimen analysis suggests the population in Lake Macquarie is closely related to a species occurring in Moreton Bay and the Gold Coast, just to the north of New South Wales, but is also part of widespread lineage that was originally described from the Red Sea. This lineage has also been found in the Mediterranean Sea, Florida, Bermuda and Brazil. I’ll be expanding this part of the project and examining more material over the next year to narrow down the most likely source population.

From fieldwork to date I have found Cassiopea at 7 sites within Lake Macquarie, including Kilaben Creek, Karignan Creek, Lake Petite, Mangrove Gully, Morisset Park, Myuna Bay and Wyee Point. I’ve also received photos confirming their presence in Pensioner Channel near Swansea. Since May 2018, six field trips have been completed at these sites and it seems that the jellyfish are seasonal, disappearing in August and returning in January. This raises questions about why the jellyfish only occur in certain months, and what factors cause them to disappear. To answer this, I will continue to complete field trips twice a season to count Cassiopea and take water quality, and other environmental measurements.

How you can get involved:

To assist me in monitoring upside-down jellyfish in Lake Macquarie, and other nearby coastal lakes, I have set up a citizen science program and hope to involve the local community.

Upside-down jellyfish are usually noticed as pale round patches occurring on the bottom in shallow water. They can generally be distinguished from other types of jellyfish through their flattened bell that allows them to easily rest on the sediment. They also have eight tentacle-like feeding ‘arms’ that extend above the bell of the jellyfish while they are resting on the sediment. The arms are branched, generally splayed out in a radiating pattern and typically green/ grey/ blue in colour due to the zooxanthellae within the tissue. Upside-down jellyfish are capable of swimming, however are rarely seen doing so unless disturbed (usually by wind, currents or other water turbulence).

If you identify an upside-down jellyfish in the Lake Macquarie region, take a photo of it (without touching it, as they do have a mild sting) and visit BioCollect Upside-down Jellyfish in Lake Macquarie to upload the photo, along with the date and GPS coordinates of where you saw the jellyfish. I’ll follow up to confirm any sightings from there.

Claire Rowe, AMRI Research Assistant and PhD Candidate

Dr Stephen Keable, Marine Invertebrates Collection Manager

Further reading:

• https://biocollect.ala.org.au/ala/project/index/95235e3b-65b1-4452-bf48-53c2b2157e5f
• https://australian.museum/learn/animals/jellyfish/upside-down-jellyfish/
• Keable, S.J. and Ahyong, S.T., 2016. First records of the invasive “upside-down jellyfish”, Cassiopea (Cnidaria: Scyphozoa: Rhizostomeae: Cassiopeidae), from coastal lakes of New South Wales, Australia. Rec. Aust. Mus, 68, pp.23-30.
• Keable, S.J. and Ahyong S.T., 2016. Flipside of the upside-down jellyfish. https://australian.museum/blog-archive/amri-news/flipside-of-the-upside-down/