During summer and early autumn, as Perth temperatures rise and the marine influence over the estuary increases, Brown Jellyfish increase in numbers and may form a dense layer near the water’s surface. This jellyfish does not inflict a painful sting on humans, but it is best not to touch it.
Brown Jellyfish are characterised by a bell that is usually saucer-shaped and brown with white spots. These jellyfish are brown in colour due to a dinoflagellate alga that lives in the jellyfish tissues(1). The jellyfish’s mouth is under the bell and surrounded by long stinging tentacles. These animals have a stomach cavity and reproductive organs inside the thick jelly of the bell. These jellyfish grow to a width of 50cm.
Brown Jellyfish are widespread in oceans and estuaries in the IndoPacific, in the tropical western Atlantic and in the eastern Pacific(2). They occur in Australian water from north Queensland through the south-east to the south-west. They occur in the Swan Canning Riverpark, but not other estuaries along the west coast. Although they are able to move by making pulsing movements, ocean currents and ship movements have a big influence on their distribution. They are thought to have been introduced into the Swan Canning Riverpark by ships that serviced the Swan River colony between 1829 and 18372. However, the southward movement of jellyfish with the Leeuwin Current is also a plausible explanation for their presence here.
Brown Jellyfish usually occur near the surface of oceans and estuaries where their food is most abundant. They are most common in the Swan Canning Riverpark in summer and in areas where salinity exceeds 25ppt. They are absent when surface waters are dominated by lowsalinity water after winter rainfall(2).
Brown Jellyfish feed on small planktonic algae and zooplankton (including eggs and larval fish) (1). Adult jellyfish (medusa) produce eggs and sperm that are released into the water. After fertilisation eggs develop into larvae called planula that attach to rocks. These grow into polyps that eventually release medusa when they are approximately 1mm in diameter. This usually occurs between November and December and the medusa are large enough to reproduce sexually within 6 weeks. The polyps of this species have not been located but are thought to occur in the Swan Canning Riverpark(1).
Two fish species are known to be associated with these jellyfish. The larvae of the Mosaic Leatherjacket and Trevally make use of the jellyfish for shelter, transportation and planktonic food, which is trapped and killed in the jellyfish’s stinging tissue(3). The young fish are immune to the stinging tentacles and can even eat them. This association does not appear to benefit the jellyfish, nor does it seem to do them any great harm. When they are a few centimetres long these fish leave their hosts and occupy habitat elsewhere.
Often caught in prawning nets and left on shorelines. Often played with by children. Although jellyfish cannot feel pain, they can become stressed if they are touched too much.
This is thought to be an introduced species to the Swan Canning Riverpark.
1. Brearley, A. (2005). Ernest Hodgkin’s Swanland: Estuaries and Coastal Lagoons of South-western Australia. UWA Press, Perth.
2. Rippingale, R.J. and Kelly, S.J. (1995). Reproduction and survival of Phyllorhiza punctata (Cnidaria Rhizostomeae) in a seasonally fluctuating salinity regime in Western Australia. Marine and Freshwater Research: 46, 1145-51.
3. Storrie, A and Thomson-Dans, C (2000). Discovering the Swan River and the Swan Estuary Park. Department of Conservation and Land Management, Perth.
Image by Matt Kleczkowski