Reproduction
The sex of a yabby can be determined quite easily. The reproductive or genital papillae of the male crayfish are short projections on the bases of the last pair of walking legs; the female has oval openings on the bases of the third-last pair of legs. It is common (1 in 20) to find individuals with a combination of male and female openings. These ‘intersexes’ usually prove to be of one sex and can function sexually; they are rarely true hermaphrodites able to produce both eggs and sperm.
The female yabby reaches sexual maturity when about 9 to 10 centimetres long - the male when slightly smaller. (Length is measured from the tip of the rostrum - the spine between the eyes - to the end of tail fan.) Nearly all mature females spawn, but the majority of young recruited to the population are produced by the 2 year olds, as they outnumber the older age groups.
When freshwater crayfish mate, the male deposits a small packet of sperm gel on the female, near the reproductive openings. The female then passes the eggs out through the openings and across the sperm packet, during which process they become fertilised. The eggs are guided to the underside of the tail (kept cupped during egg laying), where they are fastened on to the swimmerettes (the small legs on the abdomen) and carried until they hatch. Juveniles have special hooks on their legs to allow them to cling to the hairs of the female’s swimmerettes; they moult several times before leaving the parent.
The female protects the eggs carefully. If the level of dissolved oxygen falls, she elevates her tail and fans the eggs. If the water becomes too warm, she will find a cooler place. However, because the eggs are large, and because of the time and energy she devotes to them, she can afford to produce only a few hundred compared with the hundreds of thousands of relatively minute eggs of the marine lobsters. The newly hatched young are known as ‘juveniles’; they resemble the adults and do not pass through the free-living larval stages of lobsters, prawns and many other crustaceans. The juvenile yabby is consequently better equipped for survival than the young of most of the marine crustaceans and not as vulnerable to predation.
Breeding begins in spring when the water temperature reaches 15 to 16oC. The first batch of eggs (100 to 500 eggs per individual, depending upon the size of the female) hatches 8 to 10 weeks later in early summer. As soon as the young have left (a further 3 weeks later), the female is ready to breed again. Because of the higher water temperatures in summer, the second brood takes only 3 to 4 weeks to incubate. Some females will breed three or more times during the breeding season, which, if the temperature remains high enough, can extend into autumn. In the warmer water in the west of the State, the breeding season may continue almost year around