The Murray River Crayfish

When people think about aquatic animals, the first things that come to mind are big sharks, coral reefs and brightly coloured fish like Nemo or Dory. Few people actually spare a thought for the aquatic life in our Australian freshwater environments. It’s not that surprising, Australia is well known for its harsh environmental conditions and its long drought periods, neither of which you would associate with lush aquatic life. However, Australia has quite a large range of freshwater fauna- fish, turtles, semi-aquatic lizards and crayfish. In fact there are just as many native crayfish species as there are native freshwater fish species in Australia. The largest of all these to inhabit the mainland of Australia is the Murray River Crayfish (Euastacus armatus). This particular species is the second largest species of freshwater crayfish in the whole world….and I personally think, the most spectacular.

The Murray Cray is also often called a ‘Spiny Cray’ or a ‘Murray River Crayfish’ and in some parts of Australia, it’s simply called a ‘lobster’. This particular species is olive green to black in body colour and identified by their large, strong white pincers (chelae). In small individuals, the chelae are green to yellow. The body, tail and chelae are all covered in short white robust spikes. It’s probably the most heavily armoured of all Euastacus species. It can grow over 2.5kg with the carapace reaching over 17cm, but are commonly seen at 9cm to 13cm carapace. Occasionally a specimen over 3kg is caught, that’s a big cray!

The Murray Cray is freshwater and endemic to Australia. Like the name suggests, it is found in the Murray Darling River as well as the Murrumbidgee River and its tributaries. This particular cray has quite a large geographical range. It is known to occupy the warmer, lower levels of the Murray Darling Basin although they do prefer the cooler, faster flowing parts of the main river. The Murray Cray can live at altitudes from close to sea level to 700m above sea level. Many crayfish, including this one, create extensive burrows in which they live. These burrows are found in or connected to open waters or the water table and are constructed in soft stream banks.

Once upon a time individuals caught in the Murray River may have been as old as 50 years. It is highly unlikely that with the pressure of fishing, they are now able to reach this age. Sexual maturity is not reached until individuals get to 15-20cm long which is around 6 to 9 years old. Often males reach maturity sooner and at a smaller size. When sexual maturity is reached, females lay between 600-1000 eggs. There appears to be a relationship between the number of eggs carried and the size of the female; the lager the female, the more eggs that are likely to be carried. Breeding for these crustaceans will take place from late autumn to early winter. The mother will carry them around in her tail until October. During this time, females tend to hide and may lie on their back to ‘fan’ their eggs. Once the eggs have hatched, they remain attached to the female until they moult 3 times and reach their adult state (see image below).

The Murray Cray is an opportunistic omnivore. They eat loads of detritus, rotting or decaying plant and animal matter but are also considered to be active hunters. Here at Sydney Aquarium, the Murray Cray if fed a variety of foods. Just like us, animals need to keep their diet varied to ensure they get all the nutrients they need to stay healthy. Our cray gets fed white bait for a strong healthy shell (carapace), as well as bits of carrot, mussels and also black worms. In an attempt to keep her happy and occupied, I try to hide her food in different spots so she has to seek it out. However, it never takes her too long to locate the food source, especially if she is hungry.

Being a crustacean, crayfish must from time to time moult. The frequency of moults that occur in a year will determine its age. The younger the crayfish, the more frequent the moulting process. For example, younger adults will moult 4 to 5 times a year, older individuals once or twice a year.

Every morning when I head over to the freshwater section I am always keen to see what our crayfish is doing and what she has been up to overnight. I forgot to mention that they are most active at night. Quite often she will excavate a new section of the tank to make a nice new home for herself. She is quite strong and it doesn’t take much effort for her to turn the tank into a mess. Some mornings I find myself spending a large portion of time fixing things she has moved, or airlines she has destroyed.

A lid was constructed for her tank because on the odd occasion she has escaped from her exhibit and been found crawling around on the floor (to the surprise of myself and other Aquarists). Being the curious creature she is and a great climber, she liked to explore the world outside her tank. Having a hard shell however and the floor being some distance from the tank, there was concern that she may crack her shell and hurt herself on the way down. Getting her back into the tank always had its challenges. She always put up a good fight, pincers ready to grab anything that came near her. Most of the time, I would lose a net or two in my attempt to get her back in.

The main threat for these crays is over fishing. As a result, there are now restrictions in place for recreational fishers, all of which vary from state to state. In New South Wales, restrictions include size and bag limit as well as taking crayfish with ‘berries’ (berries is the term used for a crustaceans carrying eggs). Added to this is a limited season in which you can collect them (start of May to end of August). The Murray Cray is considered vulnerable in the ACT and endangered in South Australia.

Amanda 🙂