Yabby Frequently Asked Questions - Yabby Biology
What is a yabby?
Yabbies are a decapod (10 legged) crustacean like marron, rock lobsters and prawns. Crustacea belong to the very large group of jointed leg animals (arthropods) which includes insects, spiders and scorpions. These animals are peculiar in having their skeleton on the outside of their soft body, as a hard calcified shell or exoskeleton.
What is our yabbie's true name; I've heard that there are lots of different species of yabbies?
The everyday, or common, name yabbie (from an aboriginal word) is loosely used for several species of small freshwater crayfish in eastern Australia. The true, or scientific, name for our Yabby in WA is Cherax albidus. The first name is the group, or genus, of rather similar crayfish to which yabbies belong (over 30 species in Australia and PNG, including marron, koonacs and gilgies in WA). The generic name, Cherax, is thought to be a mispelling of the Greek word Charax, meaning a pointed stake - a thing that scratches. The second name (no capital letter) is the particular species, the white Yabby which was first named as Cherax albidus by a Victorian museum taxonomist, Dr Ellen Clark, in 1936. Our yabbies were introduced from the Wimmera farming district in western Victoria - the exact spot is known, Miram swamp near Nihil. It is an interesting coincidence that the species name for our yabbies is derived from the same Latin word ("albus" = white) which is used to describe the reflection of sunlight from our white clay dams ("albido"), where white yabbies are so successful.
I've seen different coloured yabbies?
Yabbies can vary considerably in overall colour and in the intensity of their shell colour patterns; some of this variation is due to age, but it is mostly camouflage to match the colour of their background. The shell and the female's deep green eggs are coloured by pigments obtained by eating plants. Juvenile yabbies from muddy dams are normally very bland in appearance (the "white yabbie"); older yabbies are olive-greenish and more strongly marked. Darker, dirtier yabbies are ones that are not growing by making a new shell frequently; very clean, pale, often pinkish, juveniles have just cast off their old shell. Yabbies from clear, very clean waters tend to be bluish; those from green water, quite greenish; and those from tannin-coloured water, brownish.
Where do yabbies come from originally?
Australia and southern PNG have a rich freshwater crayfish fauna of over a 100 different species and there are a few similar species in NZ, South America and Madagascar. These are a different family (the Parastacidae) from freshwater crayfish in the northern hemisphere (the Astacidae in Europe and the Cambaridae in North America and eastern Asia). All these crayfish evolved from ancient prototype marine lobsters. This move to freshwater happened separately in the two hemispheres of the earth more than a hundred million years ago when Australia, India, Africa and Australia-NZ were together, attached to Antarctica as the giant southern continent called Gwondanaland. India and Africa broke off first to drift north before freshwater crayfish could spread from the original freshwater site (SE Australia) which was invaded from the sea, so none are found in these countries. A few crayfish spread to South America, NZ and Madagascar before these countries split off and drifted north. As Australia drifted north later, to ultimately collide with Asia, the climate changed, particularly from cold to hot and wet to dry. Although WA is now separated from eastern Australia by a desert barrier, at one time it was linked by a giant inland lake. The genus most adaptable to these changes was Cherax, which evolved and spread to the west to give the marron (Cherax tenuimanus), northwards the red claw (Cherax quadricarinatus) in sub-tropical Qld and over Torres Strait (dry at one time) to evolve more than a dozen Cherax species now found in strait islands and southern PNG.
Undoubtedly, the most adaptable species of Cherax is the common eastern states Yabby or Cherax destructor (species-named because it burrows sideways through soft irrigation channel banks and lets the water out). It is found in a very wide range of habitats: from hot waterholes in central Australia, to cold lakes and permanent streams near Mt Kosciusko in the Great Dividing Ranges and from southern Victoria and SA to southern NT and SE Qld. It has a number of closely related sister species of yabbies, such as our white yabbie. Some taxonomists are arguing whether or not our Yabby is really a different species. In any case, yabbies are remarkable animals.
What's different about the yabbie's lifestyle?
Biologically, yabbies are adapted to making sure that some of them survive an extreme drought to start a new population when surface water returns. They do this by burrowing down to the water table when surface water dries up. Drought for a crayfish usually means the regular annual cycle of drying out of surface water in swamps, pools and small creeks after winter. Our native species of koonacs and gilgies live in these "ephemeral" habitats; they breed in their burrows in late summer and emerge to feed and grow during winter and early spring. The famous red swamp crawfish, Procambarus clarkii, in Louisiana has the same lifestyle. Yabbies are different; they breed during spring and summer and grow best during summer and little during winter. Yabbies seem to be adapted to the long term drought of the outback lasting several years. When rain does fall (often as summer cyclonic downpours), surface water lasts for several years. The few surviving yabbies are biologically adapted to start the rapid build up to a large population number before the surface water disappears again. Females are very fecund (large number of eggs per spawning) and can spawn several times in succession. Baby yabbies grow very rapidly and sexual maturity occurs in females at a relatively small size and in their first year. Yabbies have young populations; most yabbies are in their first and second years of life. Yabby population explosions ensure a very large number of burrowers when the water eventually dries up, so that some will survive a long drought.
What happens when yabbies are put in a farm dam that doesn't dry up?
They go through the same population explosion as in the wild and reach very high numbers, particularly of juveniles, in a year, or two. In crayfish, and many other animals and plants, growth is closely related to their density (number per unit area) or degree of crowding. As density increases, growth slows dramatically and survival at moulting (see below) is poor, so the throughput to larger (marketable) sizes is low. However, breeding does not seem to slow. This overpopulation, or crowding, is currently seen as the central problem to be overcome in Yabby farming, so as to increase production of larger sized yabbies in a dam.
What stock management methods are there?
The most obvious method is frequent, heavy harvesting so as to relieve the population pressure. Most farmers probably don't harvest enough. Of course, the effort of (too) frequent harvesting needs to be balanced against diminishing catch. Harvesting at monthly to six weekly intervals is practiced. The other harvesting factor is the sizes removed. In 1991, the emphasis in marketing was for harvesting yabbies larger than 50 grams and smaller yabbies were returned to the dam or used to stock new dams. Although the value of crayfish per kilogram increases with their individual size, markets have been found for yabbies down to 30 grams and recently down to 20 grams (the size when females start to breed).
A more drastic method is to kill a poor performing (stunted) stock at the end of autummn with poison, of the synthetic pyrethroid type. These poisons, freshly applied, are extremely toxic to crustacea, but are biodegradeable within a few days in the dam and no residues can be detected subsequently in the dam or in yabbies restocked in the dam. (Dams should not be treated during summer when the water layers do not mix or during winter while a dam is overflowing and poison can be carried to natural water-courses). The dam is restocked before spring with 20-30 kilograms of juveniles, of less than 20 grams individual weights, in the case of a "2000 cubic yard" dam. We have researched this method, because a number of farmers have been using it. The benefit of poisoning and restocking only lasts for a year or so before the dam is overpopulated again. A refinement is to restock with males only. There is no doubt that this method works and, in theory, poisoning does not have to be repeated if there is no breeding. But there is some argument about the effort involved in sorting out females, without mistakes, and in the promotion of poisoning and use of all males, as general industry practices. Many leading Yabby farmers advise against poisoning and would prefer that we do not mention it. However, you're bound to hear about it, so the topic is best discussed, here. We agree that adverse publicity about use of poison can have bad consequences in food markets, even though we've proven that no chemical residues are present in the restocked yabbies.
A number of other methods are, at least, theoretically possible; some are impractical or expensive. We're researching methods over the next few years. Not all Yabby dams are overpopulated- just most of them- so there's a clue for research there.
With a hard shell, how do yabbies grow?
The general process of growth is called moulting and involves a cycle of moult stages (A-D), which has to be repeated many times through their life. Periodically, they make a new, but soft, shell under the old hard one (stage D). Then, laying on its side, the Yabby breaks out of the old shell at the join on top between head and tail- an involved contortion called ecdysis. Amazingly, this involves all parts of the external shell (and some fore and rear gut lining): eyes, gills, legs,...). The new exposed soft shell is expanded quickly by drinking water (land insects take in air) (stage A), and then hardens so the Yabby can get mobile (stage B). During the subsequent intermoult (stage C) the Yabby feeds and replaces the water with soft body growth. In early stage C the Yabby is hungriest and most readily caught in a baited trap.
Yabbies often lose legs by fighting; they can completely replace a leg, gradually through 3 or 4 moult cycles.
Most people call the actual shedding of the shell "moulting", but this stage of the whole moult cycle is more correctly called "ecdysis".
Do they die often during moulting?
Yabbies are very vulnerable during moulting; most crayfish deaths occur while they're sluggish just before or after ecdysis or defenseless at ecdysis - they get stuck trying to emerge from the old shell or they're attacked by other crayfish or a predator. A Yabby on its side in shallow water is usually moulting; sometimes what you see is the old empty shell; if not, if you wait a few minutes the "new" Yabby will emerge. Don't disturb or handle an ecdysing yabbie; they can't breathe at this stage and any delay means they run out of puff before finishing. Small crayfish ecdyse in a few minutes but very big ones can take 20 minutes, or longer.
How many times do they have to moult to get to, say, 50 grams?
About 20 times. Yabbies put on up to 50% of their weight at each moult. Small ones moult at weekly or shorter intervals and the interval becomes longer, a month or more, as they get to 50 grams but it only takes them about 4 moults to get from 10 to 50 grams. Slow growth means longer intervals between ecdysis, so if you don't see many freshly ecdysed (very clean shells) yabbies in your traps, the stock isn't growing much. The reasons could be time of year (water temperature), crowding and/or lack of food.
Yabbies are cold-blooded aren't they; how does this affect growth?
Yes, their feeding activity and, therefore, growth are very dependent upon the water temperature. They are called poikilotherms because their body temperature varies with the temperature of their environment (we're homoiotherms, with a constant, and high, body temperature). The annual season for Yabby growth starts in September and ends in May. The yabbie harvesting season with baited traps follows the same annual pattern because capture is dependent upon the feeding activity of the yabbie. Temperature values favouring growth are given in another answer.
How fast do yabbies grow?
Crayfish, and fish, don't have fixed growth rates so there's no single, simple answer. Given that spawning females don't grow, if a group of yabbies is placed in a tank of water at their optimum temperature for growth and fed a highly nutritional diet to excess, growth (and survival) will still depend very much on their density, since they interact behaviourally, with larger ones dominating smaller ones. Probably the best practical measure of growth and survival in a particular dam is the annual total weight of marketable yabbies a farmer can harvest. We know that this quantity varies considerably from dam to dam.
What are the names of the different outside body parts of yabbies; how do you tell male from female?
Yabbies and other arthropods evolved from marine worm-like animals with a simpler, more uniform body of 20 similar, articulated segments, each with a pair of double- branched "legs" (biramous appendages). (Inside these worms had a gut and nerve cord running from the mouth to the tail end. At the mouth end the nerve cord was barely enlarged, the primitive head-brain.) In yabbies these segments and appendages can still be identified, but many segments are highly modified and fused and different pairs of appendages are also highly modified for particular purposes. Some of the scientific names for body parts and their purposes or functions, which you may care to know to understand this animal, are mentioned in the following brief description.
The front half of a Yabby is called the cephalothorax (head-chest) with 14 segments (6 head, 8 thorax). The first pairs of appendages are the sensory feelers for smell and touch called antennae and antennules, followed by various, small mouthparts used for feeding( maxillae, maxillipeds, mandibles,..). Protected under the large plate-like sides (branchiostegites) of the shell cover (carapace) are the external aquatic gills (branchiae) in a water-filled gill (branchial) chamber. Inside the chamber, a special plate-like appendage (scaphognaphite) circulates water forward through the gills. The first, large pair of legs (chelipeds) has big, powerful claws or chelae, which grow disproportionately larger in adult males and are used for threatening other yabbies, if not for actual fighting. The second and third legs are walking legs and have tiny, sensitive (touch and taste) chela which are used to feed on minute organic particles. In females the third or centre pair of legs, each has a round egg opening at the base, connected to an ovary. The remaining two pairs of non-clawed (non-chelate) legs are used for walking, cleaning and spawning. On the bases of the last pair, the male has twin papillae, connected to his testes, used to fix a sperm packet (spermatophore) on the female at mating.
The rear part (abdomen) of a yabbie is still largely articulated segments (6). The last segment has an articulated extension (telson) which is the centre flap of the tail fan; the two other flaps (uropods) are the last pairs of appendages. The abdominal flesh, we like to eat, is muscle used to flap the tail fan for escape swimming, backwards. The upper part (tergum) of each segment of the abdominal shell extends down the sides and these side plates (pleura) are disproportionately enlarged in adult females to protect the eggs carried under the abdomen. Under the abdomen (sternum), four segments have pairs of biramous swimmerets or pleopods. At spawning the female glues her fertilized eggs to one branch of each pleopod (exopodite); in adult females this branch is broader and flatter with long fine hairs or setae; the other branch (endopodite) helps to oar a flow of well-oxygenated water over the incubating eggs. After the eggs hatch, the baby yabbies hang head down from the pleopodal setae, using special snap-hooks found at these attached stages on their most posterior (back) two pairs of legs. Interestingly, all northern hemisphere baby crayfish hang tail down, using the claws on their first pair of legs.
What are the most noticeable internal parts?
Its quite cruel to break open a live Yabby and a very messy way to investigate its internals. Take a cooked (boiled) Yabby in the usual eating grasp with both hands and flex it open at the join between the cephalo-thorax and abdomen, but without breaking it in two. Lift the carapace back towards the head. If you've got a mature female, the first thing seen, on top, is the red-orange, bilobed ovaries; in the uncooked state these are dark green. In males the white testes are harder to find but can be traced along the highly coiled spermatophore tubes (vasa deferentia) leading from the insides of the bases of the last pair of legs. Underneath the gonads (lift and cut them out with tweezers and scissors) is a large organ called the hepatopancreas, joined to the gut; this is the yabbie's liver and is usually mustard coloured. In other countries with tiny crayfish they don't waste the head part like we do, but suck out this so-called "mustard". The other major organ, nearer the mouth, is the large round dark stomach, usually filled with fine particles. It contains hard grinding teeth called the gastric mill (these teeth and the inside lining of the gut in front of them are shed at ecdysis along with the outer shell). On either side of the stomach wall is a large diameter plate. These plates thicken with stored calcium extracted from the outside shell (exoskeleton) through moult stage D leading up to ecdysis; after ecdysis, through stages B and early C, the white buttons (gastroliths) are used up to calcify the new exoskeleton. (These indigestible crayfish buttons are often found on banks and under trees near clearwater crayfish habitats when shag predation is prevalent).
If you now crack and remove the tail shell in the usual way, the thinner top layer of abdominal flesh can be peeled back. If you've boiled up a freshly caught yabbie, the hind gut will then be obvious as a black strip, usually called, wrongly, the "vein". Most people like to tediously remove this rather offensive looking (faecal) piece of gut before eating the tail meat (muscle). An important part of Yabby processing before marketing is to purge out the dark food waste from the gut by starving the yabbies in clean water for several days.
How do yabbies breed (spawn)?
The male turns the female on her back and deposits a sperm packet (spermatophore) on her shell between the last two pairs of legs. This packet is black in rock lobsters and called a "tarspot"; we often (wrongly) call the marron and yabby packet a tarspot, though it's whitish in these species. The female then curls her tail under to form a chamber and picks open the male's "spermspot" to mix sperm with her eggs extruded from the openings on her middle pair of legs. She attaches the fertilized eggs onto the long fine setae on her abdominal pleopods with a glue called glar. The eggs are green at first and then black; later, the developing baby yabbie (embryo) can be seen as a yellowish spot near the egg surface. When the eggs hatch, the first stage baby Yabby is still undeveloped, but has a large amount of yellow yolk in its oversized cephalothorax to feed on. After a moult, the next stage is more developed with less yolk left. The well developed third stage, now definitely yabbie-like in appearance, remains attached but can walk off the female to learn to feed. The attached young hang onto their mother's pleopods head down, using special snap-hooks in these stages on their back pairs of legs. After a third moult, the juveniles are truly independent of their mother, who often spawns again within a short time.
Spawning females don't feed much and, of course, they can't shed their shells to grow (i.e., ecdyse). They have been described as " shy and retiring" in nursing their eggs and, then, hatched young. The female spawners you'll actually catch in baited traps are a low proportion of what's in a dam. Fair numbers with early green eggs are caught, fewer with black eggs and very few with attached young. Some recent mothers we've recaught (last caught and marked as newly berried)after release of young, are shell-stained, indicating they've been hiding in burrows.
When do they spawn?
Yabby spawning (Cherax destructor) has been studied in a laboratory in South Australia where they spawned repeatedly under controlled "summer" conditions of long daylength (14 or more hours of daylength or photoperiod) and high water temperature (20oC or higher). Field observations in the east, indicate spring to autummn spawning, depending on a localitie's temperatures. Cherax albidus spawning in our study dam at Pingelly starts in September as the temperature in shallow water rises to about 14-15oC; spawning may start earlier farther north and later in the Great Southern. Young from the spring spawning are released by December and many females then repeat spawning with a shorter incubation time, due to higher water temperatures, and release young by February. We have not seen a later, or third, wave of late summer spawning.
What sizes are mature males and females?
Size at maturity is not fixed in fish or crustacea and usually varies between individuals and under different growth conditions. Spawning females in the stunted stock we've been studying at Pingelly are predominantly in the 20-40 gram weight range, with very few (just) below 20 grams. Females just below 20 grams examined in late winter-early spring have well developed eggs in their ovaries, but can't spawn until they undergo a moult that widens their tail shell (see previous answer on external body parts). Many measurements have shown that tail broadening occurs in females in this dam at 20 grams or so; mating trials at our Waterman laboratory have confirmed that mature females can be reliably identified by this easily seen external characteristic.
Males appear to mature at a larger size than females. But which sizes of males actually mate with females may be more a question of behavioural size dominance, like bulls and rams, than potential ability. Since females spend most of the growing season spawning, and therefore not growing, males of the same age grow on to much larger sizes (50 grams and larger). Over 40 grams the claws on males start to become disproportionately larger than those of females and smaller males, and this change continues to exaggerate the claws to a massive size in males over 100 grams. As in many animals, one male crayfish can mate with many females and it is likely that the largest male yabbies dominate smaller males and do most of the mating. Our mating trials showed that males from a dam did not mate immediately with mature females only slightly smaller than them, unlike big-clawed males. However, after some weeks these smaller-clawed males did mate.
What are the ages of yabbies in dams?
Crayfish can't be aged like fish because they don't have any permanent hard parts (scales, otoliths, spines, bones) to form growth rings. External tags attached to the shell are lost at ecdysis; an external tag planted in the flesh at the join between cephlothorax and abdomen, and not lost at ecdysis, is used for large rock lobsters but is not successful for small crayfish. Crayfish have to be killed to retrieve magnetic internal tags or to use a new microscopical chemical ageing method on brains.
We follow growth of yabbies in our study dam, and estimate their total numbers, by punching marks on their uropods and pleura which correspond to weight grade category and month of capture, resp.. These marks last through several ecdyses. At Christmas, you'll need a mosquito net to catch the tiny, newly released young-of-the-year from the spring spawning. Most of the yabbies caught in traps then are from the two spawnings in the previous year (one year olds) and they range from 10 grams (tail-enders of the summer spawning) to 40 or 50 grams (males from the spring spawning); there are relatively few two year old yabbies; mostly very large males, easily identified by their much darker colour and punch marks from the previous year. We believe that our dam is fairly typical, but some other dam stocks may be different.
How many eggs do females have?
The number of eggs increases with size of female crayfish. If the numbers of eggs in the ovaries are counted, this "potential fecundity" can be related to female size by an equation. However, the "actual fecundity" or number of eggs or hatched young on the tail of a female is always less than the potential fecundity and for yabbies the number is extremely variable for females of a given size. Some researchers have found no relationship to female size; we do have an equation with rather wide limits. Most berried females have from 200 to 400 eggs.
Females in one of my dams have only a few, dead looking eggs under their tails; what does this mean?
If most females have a large, healthy (shiny green or black) bunch of eggs, we would not worry about the occasional poor mother. However, if many females in a dam are poorly berried, this means that oxygen conditions in the dam water are very bad (see later answers).
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