Putting things the right way around: Identification and distribution of upside-down jellyfish

An AMF/AMRI Postgraduate award helped PhD student, Claire Rowe, provide answers on why upside-down jellyfish have recently appeared in Lake Macquarie, NSW. The search for answers led her to the Gold and Sunshine Coasts in Queensland.

Cassiopea are unusual jellyfish – they spend most of their life upside-down, with their bell resting on sediment in shallow water and their oral arms extending above them. They have photosynthetic algae in their oral arms, called zooxanthellae, and this upside-down position exposes the zooxanthellae to sunlight, helping to provide the jellyfish with up to 90% of their nutritional needs. The other 10% comes from filter feeding on zooplankton including fish larvae, which they capture using stinging cells.

Cassiopea have the potential to bloom in large numbers, with densities of over 30 individuals per m² being recorded. In these high densities they may consume large amounts of zooplankton, alter oxygen levels, and release stinging cells into the water column, causing local fish species to vacate the area and irritate swimmers. This can also cause social impacts to communities that rely on fisheries and tourism.

Upside-down jellyfish are typically considered to be tropical in occurrence. In Australia, populations are predominantly in the northern regions including Darwin, Lizard Island, and Moreton Bay. However, there have been recent reports of upside-down jellyfish from temperate New South Wales, including Wallis Lake in 2009 and Lake Illawarra in 2013, and these were identified as two different species. In 2017, upside-down jellyfish were reported in Lake Macquarie, between these two NSW sites, raising further questions about which species were occurring and where had they come from.

As a PhD student studying Cassiopea jellyfish at the University of Sydney and the Australian Museum Research Institute, my research aims to determine which species of Cassiopea is occurring in order to find out where they have come from. The second aim is to understand the distribution and population dynamics of these jellyfish within the lake. This will allow predictions of when and where the jellyfish will occur in the future and assist in implementing management strategies, where necessary.

To determine which species of Cassiopea occurs in Lake Macquarie, I compared morphology and genetic sequences to specimens held in the Australian Museum collection from Wallis Lake and Lake Illawarra. My results showed that the species occurring in Lake Macquarie is the same as in Wallis Lake but not Lake Illawarra.

The species Cassiopea maremetens occurs in Moreton Bay, Queensland and, as this is the closest other known population, I wanted to see if this was the same species occurring in Lake Macquarie and Wallis Lake. The only problem was that I didn’t have any fresh specimens from Moreton Bay to compare. To be able to make the comparison, I applied for and received an AMF/AMRI Postgraduate Award which allowed me to travel up to Moreton Bay with a car full of collecting gear and collaborate with scientists at Griffith University and the Sea Jellies Aquarium at SeaWorld. Over four days, we collected as many Cassiopea as we could find, totalling seven specimens. I fixed the tip of their oral arms and their gonads in 95% ethanol for genetic study (see below), and the rest of the specimen in a formalin solution so that their structural features remained intact. Critically, I was able to obtain two specimens from the original locality that C. maremetens was described from.

When I returned to Sydney, the specimens were deposited in the Australian Museum collections and DNA sequences were compared to the population from Lake Macquarie. The results indicate that a species occurring in Moreton Bay, is the same as in Wallis Lake and Lake Macquarie, supporting the suggestion this species appears to be expanding its range south down the east coast of Australia. The next step of my project is to examine the morphological characteristics of the specimens and confirm if Cassiopea maremetens is the correct identification.

Claire Rowe, PhD student, Marine Invertebrate, Australian Museum Research Institute and the University of Sydney.