Discussion

The data presented indicate that legs are lost by autotomy, which is regarded as a reflex response to injury. Lifting a crayfish by a leg will sometimes invoke the separation of that leg, so that autospasy may be exhibited also. Several cases of the absence of, or damage to, a part of a leg without its being autotomized indicate that, in Jasus lalandii, an injury does not always invoke autotomy. In these cases, sufficient damage must have been sustained to bring about autotomy if it is to occur. The explanation of the failure here may be that there is some fault in the reflex action such as might be expected from general shock, but, whatever the explanation, there must be a means of stopping excessive bleeding. It may be that when an injury is received the blood vessels are closed off at the point of autotomy, even if the leg is not autotomized. If this is so, necrosis would set in and the leg would be thrown off at the next moult.

In females the tendency for a leg to be lost increases from front to rear. This must be an indication of the hazards presented to the various legs during the page 35 animal's normal behaviour. The possibilities for loss are: escape from enemies, defence, feeding, or reproduction.

In escaping from an enemy, a crayfish flips its tail and darts rapidly backwards, The legs are directed anteriorly and lie close along the lower edge of the carapace. Each leg overlies, and to some extent protects, the leg in front of it. The fifth leg is the most exposed, to a lesser extent the fourth and third, so that when an enemy attacks, especially from the side, the fifth leg will be in the greatest danger. In defence, the slender nature of the fifth leg compared with the others would mean its more frequent loss. In moving over a bed of shellfish, it is possible that a leg may sometimes be trapped in a closing shell. The fifth, projecting from the body more than the others, would be most frequently lost in this way. Some hazard may be presented in reproduction, either in cleaning the abdomen prior to oviposition or in copulation. Copulation occurs when the females are soft-shelled, a time when legs would be most easily lost. This may help to account for the more frequent loss of legs in females. In cleaning the abdomen for oviposition, the female uses the chelate fifth leg. This represents a significant behaviour difference between the sexes. Since males do not carry out this abdomen-cleaning process, it appears that this behaviour in females may be an important contributing factor to the more frequent loss of the fifth leg in females than in males.

Whenever the legs are in danger, the long, slender nature of the fifth would mean its more frequent loss. Less force would be required to damage this leg. Consequently autotomy would be more readily produced in this leg than in the stouter anterior limbs. The increasing intrinsic strength of the legs from the fifth to the first and the increasing protection from the other legs are probably the greatest contributing factors to the gradient shown in the text.

In the backward escape from enemies, the crayfish cannot see obstacles behind. It must sometimes strike rocks, possibly with sufficient force to damage the fifth leg. The other legs, being cushioned by the fifth and being stronger, would not be damaged as often or as severely. Hence autotomy in these would be less frequent.

All of the above behaviour, except the cleaning of the abdomen for oviposition, would be exhibited by the male as well as by the female. Accordingly, it is difficult to see why the third, fourth, and fifth legs of a male are not lost more often than the others.

The frequency distribution of the number of amputations over the size-range, for females, is a normal curve, whereas, for males, it increases with size. No explanation can be suggested for this.

One or both antennal flagella are sometimes lost in nature and can be regenerated. A tagged specimen, released with a flagellum freshly broken off from the autospasy point, had almost completely regenerated the flagellum after one year, during which time it is considered to have undergone two moults.