Cherax - Freshwater Crayfish
The everyday or common name yabby (yabbie) is loosely used for several species of freshwater crayfish in central and eastern Australia. The common name yabby is derived from the aboriginal term yabber, which was used by wandering tribes to describe the native crayfish from central Australia. The generic name Cherax is thought to be a misspelling of the Greek word 'charax', meaning a pointed stake - a thing that scratches.
The Cherax genus has the most widespread distribution of any crayfish genus in Australia. They are found in all mainland states and territories and also in New Guinea and the Aru Islands. They occur in lakes, swamps, billabongs, farm dams, irrigation canals and bore drains, and also in slow, muddy rivers and creeks. They are especially hardy and can survive years of drought by burrowing, later emerging during wet periods to feed, breed and migrate. As the ground dries up they burrow, following the falling water table, and seal the burrow entrance with an earthen plug. In a small, moist chamber at the bottom, they enter a state resembling suspended animation, its bodily functions (respiration, pulse and digestion) practically ceasing. This adaptation is called aestivation, not hibernation. They can remain like this for some years.
The body of crayfish may be broadly divided into two sections; the abdomen (tail) and the cephalothorax (head). The head, and other internal organs, is protected by the carapace and armoured at the front with a strong, pointed rostrum. The major sensory organs for crayfish are the large feelers or antennae and the finer, more central feelers known as antennules. The eyes, although quite prominent in the head, are of little use in the murky environment in which they live. Thus, the antennae and antennules act in place of the poor eyesight as touch and taste sensors, locating potential food as well as sensing changes in water quality parameters such as temperature and salinity.
The abdomen is divided up into six segments that are individually encased in hard shell. A flexible membrane joining each segment allows them to move relatively unhindered. Appendages located on the underside of abdominal segments two to five are known as pleopods (or swimmerets). These are very important for female crayfish as the edge of each pleopod is lined with fine hairs, or setae, to which they attach their eggs. The appendages found on the sixth segment are larger than the pleopods and are referred to as uropods. These, along with two central flaps and the terminal abdominal segment (the telson), form the tail flap that is used to create the thrust that moves the crayfish quickly through the water. Females also utilise the tail flap as a device to protect the eggs during incubation, forming a temporary "brood chamber" by folding the tail under her body. Swimming is restricted to backward movement resulting from rapid flexion of the entire abdomen (the abdominal pleopods have little locomotor function). This backward movement is widely recognised as a salient crayfish behavioural trait.
Cherax are bottom-dwelling opportunistic scavengers, and a large part of their diet consists of decaying leaves and other plant detritus. Cherax can be quite aggressive towards one another and recently moulted animals can be cannibalised by other crayfish. Their life cycle is very different from their marine relatives. In the marine environment, newly hatched eggs are released into the sea as planktonic larvae, whereas freshwater crayfish hatchlings continue to develop on the swimmerets under the female's abdomen. The swimmerets move gently to provide a constant supply of well-aerated water, which is necessary for the survival of the developing young. When the developing larvae have absorbed their yolk sacs, they leave the female's protection and commence a free-living mode of life.
Cherax form the most speciose group of crayfish inhabiting freshwaters within the Australia and New Guinea. Nevertheless, relatively little is known about the biology and ecology of the majority of species. A review of the literature currently available also highlights a very large gap in our knowledge of many species. There are some species where there is a considerable amount of information available while there are other species where there is little or no information available. In addition, there are specific gaps in the information available in otherwise well documented species. As well as a number of species that are in need of additional research, information on their distribution and natural habitat conditions is limited.
Several studies addressing the taxonomy of Cherax species in Australia have described new species or synonymised others which have resulted in some confusion and disagreement regarding the status of certain taxa. Identifying Cherax species is a lot easier if you have all species present. Many people are unaware that so many species are commonly found in Australian waters. Cherax species from different regions can often be quite different in appearance. There is even evidence that specimens from within the same waterway can look quite different. However, based on past revisions and new descriptions the following species are currently recognised as being in Australia:
Classifications are based largely after Riek (1969), Austin (1996) and Munasinghe et al. (2004).
In New Guinea, the Cherax genus contains around 21 species that unambiguously define areas of endemism south of the island's central divide, and contains certain lacustrine endemics, particularly in the Paniai Lakes and Lake Kutubu. The taxonomy of and number of species is the subject of continued debate amongst taxonomists. However, many species are being imported into the European and Japanese aquarium trade. Most of these species are being collected in New Guinea (mostly from western New Guinea) and many of them are new to science.
Keeping & Caring
Cherax are probably best suited to a species only tank. Although there will always be some reports of successfully keeping varied combinations of crayfish and fish, etc. together. Nevertheless, the aquarium structure should be designed so that the aquarium inhabitants can avoid each other if they wish. It is important particularly when keeping freshwater crayfish that the decorative material such as rocks, driftwood, caves and aquatic plants provides hiding places, as many of these creatures react aggressively if they constantly meet other inhabitants of the aquarium. Whilst some species will roam the aquarium during the day and can be easily observed, most activity and feeding occur at night or under low light conditions.
Crayfish do not grow in the same way as fish, but have to moult to change their size. Each moult is an enormously decisive and dangerous experience in the life of these creatures, since for a period of time ranging from hours to days; their otherwise protective shell is soft, leaving them practically helpless against hungry predators, which, in the case of Cherax, include their own species! Caves and crevices are therefore vital as they provide hiding places.
Cherax are extremely tolerant of a number of environmental variables and water quality is generally not considered to be a critical factor, within certain limits. Although acidic waters are tolerated by some species; most species are rarely found in waters with a pH below 7.00. Cherax can tolerate a wide range of temperatures, but the optimum level is around 22-30°C, with lethal limits estimated to be around 9-10°C and 34-35°C. Water temperature is the key factor controlling metabolic rate and the moulting cycle and growth.
Cherax are opportunistic and omnivorous scavengers; feeding on whatever animal and plant material they can locate using chemosensory means. Their diet largely consists of pieces of organic detritus consumed while browsing on the stream or lake bottom. In billabongs they have been found to feed mainly on macrophytes, but also consumed eucalyptus leaf litter, filamentous algae, and animal material.
Munasinghe D.H.N., C.P. Burridge and C.M. Austin (2004) The systematics of freshwater crayfish of the genus Cherax Erichson (Decapoda: Parastacidae) in eastern Australia re-examined using nucleotide sequences from 12S rRNA and 16S rRNA genes. Invertebrate Systematics 18: 215-225.
Riek E.F. (1969) The Australian freshwater crayfish (Crustacea: Decapoda: Parastacidae), with the descriptions of new species. Australian Journal of Zoology 17: 855-918.
Adrian R. Tappin