Dumaresq Dam has been in existence for 115 years and is a favourite recreation spot for many residents and visitors to the town. In the summertime the reserve is filled with picnickers and water sport enthusiasts. People may not be aware that Dumaresq dam is also home to a rare freshwater jellyfish. Craspedacusta sowerbyi tends to inhabit artificial lakes, ponds and rivers, and has been recorded from all Australian states. It was first recorded in South Australia in 1950 but is now widespread and was one of the first animals to appear in Canberra’s Lake Burley Griffin.
This species seems to have mainly a subtropical to temperate distribution and belongs to the class Hydrozoa which includes the more common hydra. The polyp stage is typically cylindrical or tube-like with mouth and tentacles at one end, usually directed upwards. The other end is attached to a substrate. The free floating medusa stage is the classical jellyfish form, with an umbrella-like body. The concave or underside of the umbrella faces downwards with the mouth at the centre and tentacles hanging down from the rim. Freshwater jellyfish differ from their marine cousins and have a unique structure.
The species in Dumaresq Dam is circular, about 25 mm in diameter and almost transparent, with a whitish or greenish tinge, and umbrella shaped tentacles about 20 mm long with a whorl of string-like tentacles around their circular edge, the ring canal. The diameter of the circular canal is anywhere from 5 to 25 mm. It bears string-like solid tentacles, arranged in sets of three to seven. These tentacles help capture food and serve as a type of protection. At their base they have eyespots which are used to detect light and dark. It can move in any direction by pulsating contractions along its bell surface.
Freshwater jellyfish favour calm water rather than currents. They can be seen floating or swimming gently just below the surface of the water and sometimes ‘bloom’ on the surface in large numbers in mid or late afternoon. During the winter, the polyps contract into a resting state that are capable of surviving the cold temperatures. It is during this resting stage that they may be transported from lake to lake, perhaps during fish stocking or with the movement of water birds. When conditions are favourable they develop into the jellyfish form and continue their life cycle. Their appearance is often noted as unpredictable, being absent for several years and then suddenly reappearing. They live on zooplankton, which range in size from 0.2 to 2.0 mm in size and are known to eat mosquito larvae and fish eggs.
They are generally not favoured as a food by fish but crayfish may eat them. While they do have stinging cells, these are not known to injure swimmers.
A video of the jellyfish swimming in Dumaresq Dam can be viewed on the Save Dumaresq Dam Facebook page which has public access. More information about this species can be obtained on the site http://aquaticlife.angfaqld.org.au/Jellyfish.htm
