For the past few years several lakes in NSW have been subjected to a seasonal influx in Cassiopea population. Usually at home in far warmer waters, this tropical jellyfish has migrated south to live all year round.
Cassiopea are a genus of jellyfish with a very interesting way of life. Instead of floating like most jellyfish they rest their umbrella shaped bell upside down on the seafloor, with their oral arms trailing out into the water column. Living within the tissues of their tentacles are tiny zooxanthellae, a photosynthesising symbiotic algae. It is through these zooxanthellae that Cassiopea gain most of their nutrition. Just 10% of their needs are sourced by nematocysts (stinging cells) on their zooplankton-catching oral arms.
Due to their life cycle, whereby larvae can be produced in both the polyp (stationary) and medusa (free swimming) stages, blooms of Cassiopea are common. These blooms are partly responsible for Cassiopea’s reputation as a widely invasive species. They also cause adverse environmental and commercial impacts. Blooms can cause a decrease in oxygen in the water column. They can also negatively impact fish populations as more jellyfish consume fish larvae. These ecosystem changes can affect both tourism and fisheries.
Australian Cassiopea populations have been historically limited to the tropical east coast. However, recent rising ocean water temperatures have resulted in a southward shift in their habitat, to as far as the temperate Lake Illawarra on the NSW South Coast.
The AM is fortunate to have Claire Rowe on the case of Cassiopea’s expanding range. Claire has been a technical officer for Marine Invertebrates at the Australian Museum Research Institute (AMRI) since 2014, and in 2018 she began her PhD at Sydney University with co-supervision from the AMs Dr Stephen Keable and Dr Shane Ahyong. Claire is researching Cassiopea’s distribution, bloom triggers, population stability, impact, as well as species identification using both morphological and genetic analysis.
The first recorded instance of Cassiopea appearing in Lake Macquarie was in 2017. For several years following this, the population was found to have a seasonal distribution, disappearing from the region in the colder months between August to January. Were these jellyfish disappearing because they were migrating north or because they were dying due to the colder winter temperatures?
To find out if the seasonal decline in these jellyfish was related to changes in water temperature, Claire conducted a series of experiments at the Sydney Institute of Marine Science (SIMS), located in the beautiful Chowder Bay.
Historically, water temperatures in the lake have varied seasonally from 14-22 °C. Last winter, however, minimum temperatures were observed to have increased by 1-2 °C to 15-16°C. This increase of 2°C from the minimum temperature allowed Cassiopea to survive all year round in 2019. Furthermore, they were seen to spread around the lake and were documented in an increased number of sights.
During the experiment at SIMS Claire subjected Cassiopea specimens to four different treatments, to simulate both historic seasonal and climate change influenced temperatures of Lake Macquarie:
Treatment 1: Dropped from 22°C to 14°C (to simulate historic winter conditions, the minimum temperature of the lake)
Treatment 2: Dropped from 24°C to 16°C (to simulate a higher minimum temperature of the lake)
Treatment 3: Remained at 22 °C (as a control)
Treatment 4: Increased from 22°C to 28 °C (to simulate a higher maximum temperature of the lake)
Claire found that the cells in Cassiopea disintegrate at 14 °C, explaining their absence from the lake in 2017 and 2018. However, at 16°C Cassiopea were found to survive, explaining their presence in 2019, where minimum winter temperatures fell to only 15-16°C. Cassiopea remained healthy and happy at both 22°C and 28°C, which was expected due to their natural tropical habitat.
To reveal the impacts of an increasing Cassiopea population involves extensive fieldwork. During each fieldtrip measurements of water quality and temperature are taken, as well as samples of sediment and zooplankton in the water column. Initial observations of zooplankton suggest that there are less larval fish where jellyfish are present. If the Cassiopea population continue to grow, commercial and recreational fishing are likely to be affected.
Both morphological and genetic analysis (using the CO1 barcoding gene) has helped Claire determine from which population these migrating Cassiopea were hailing. The source population was discovered to be from Moreton Bay and Wallis Lake. But how did they migrate to their new southern home? One hypothesis is that in their polyp form they hitched a ride on the bottom of a boat.
Despite the extensive research already undertaken, Claire has her future work cut out for her. Her next port of call is to determine whether the Lake Macquarie population of Cassiopea are morphologically similar to other populations of the same species, due to the cryptic nature of the genus. To do this, extensive morphological and genetic analysis will take place. Claire is also hoping to garner increased community involvement in the Lake Macquarie area, so that citizen science can be used to track population growth and location.
Emma Flannery, Science Communicator, Australian Museum Research Institute