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A Study of the Marine Spiny Crayfish Jasus lalandii (Milne-Edwards) Including Accounts of Autotomy and Autospasy

Autotomy and Autospasy

Autotomy and Autospasy

It is well known that autotomy (Fredericq, 1883), autospasy (Pieron, 1907), and autotilly (Wood, 1932) occur widely in Crustacea. Wood (1932) gives the following definitions after the above-mentioned workers: autotomy is the reflex severance of a part from the body by the animal itself; autospasy, the separation of an appendage from the body at a predetermined locus of weakness when pulled by an outside agent; autotilly, the rending of an appendage from the body at a predetermined locus of weakness by the mouth-parts, claws, or other legs of the animal itself.

Wood (1932) discovered that Palinurus vulgaris exhibits autotomy in all of the legs, and the antennae exhibit autospasy. Jasus lalandii, belonging in the same family (Palinuridae), would be expected to show similar phenomena. That it does so is shown by the following:—

(i)

Crayfish are sometimes found with one or more legs missing. These have separated from a constant point which is the suture between the basipodite and ischiopodite. Freshly caught crayfish, struggling on the decks of the fishing boats, often throw off their legs without any traction on their own part or on the part of an outside agent. This is autotomy.

If a crayfish is lifted by a leg, the leg will sometimes be released. This indicates that the legs may exhibit autospasy also.

Some specimens are found with only a part of a leg missing or with some part of a leg crushed. Since such legs are not autotomized, damage to a leg does not invariably lead to autotomy.

(ii)

Crayfish are sometimes found with one or both of the antennal flagella missing. In each case, the flagellum has been separated from a constant point near the base. If a crayfish is held by an antennal flagellum, traction is produced by flipping the tail, and the flagellum breaks off from the constant point near the base. Some specimens have been found with only a part of the flagellum missing. In these cases, the flagellum, although damaged, has not been thrown off. These two observations indicate that the antennal flagella are lost by autospasy.

(iii)

No autotilly is reported in Palinurus vulgaris and no opportunity has presented itself for the study of autotilly in Jasus lalandii. It does, however, seem improbable that the mouth-parts are capable of removing legs because the legs and mouth-parts are too far separated and the mouth-parts seem hardly strong enough. It is possible that a leg may be torn off by the action of other legs, especially of the stronger anterior ones, but the absence of any chelation would make such a process rather difficult.

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Much work has been carried out and many theories proposed to account for the mechanism of autotomy. This account discusses an aspect which appears to be untouched—namely, the relative tendencies of the various legs to be lost.

The data presented were obtained from a study of 1,937 crayfish caught near Wellington during 1947 and 1943. Of these, 889 were males and 1,048 were females. Only "total" amputations are considered. A "total" amputation is defined as the absence of all that part of a leg which is distal to the autotomy point. The presence of a regenerated leg is not considered as an amputation. Consequently, all of the amputations considered must have been received since the last, or certainly a recent, moult. Amputations received during capture are considered to be "unnatural," and are not included in the data. These are indicated by clean, flesh-coloured scars on the stumps, whereas older (and considered to be "natural") amputations are indicated by dark-coloured scars with thick, chitinous membranes over them.

Inclusion of regenerating legs as representing old amputations led to unreasonable results, such as a difference between opposite sides of the body, which is unacceptable in a bilaterally symmetrical animal. The difficulty of establishing criteria for the difference between a completely or almost completely regenerated leg and a leg which has never been amputated is thought to be the cause of this skewing effect.

Of the 1,937 specimens examined, 191 had one or more legs missing. Of these, 65 were males and 126 females. Since multiple amputations are common, the 191 specimens had a total of 250 legs absent. The males had 91 of these and the females had 159. Table 1 gives the figures.

Table 1
Sex Number examined Number with amputations Total number of amputations
Male 889 65 91
Female 1,048 126 159
Both 1,937 191 250

More females than males have amputations (P = probability factor, is less than 0.01)—that is, the tendency to leg amputations is greater in females than in males. The total number of legs missing is greater in females than in males (P less than 0.01)—that is, a female tends to lose more legs than a male.

As is to be expected in a bilaterally symmetrical animal, there is no difference in the frequencies of autotomy on either side of the body in either sex.

Since there is no difference between opposite sides of the body, the frequencies of amputations on both sides have been totalled and the number of positions where amputation can occur is considered to be five. Table 2 gives the frequency of absence of each leg.

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Table 2
Leg No. Males Females
1 18 11
2 17 11
3 19 38
4 13 42
5 24 57

See Fig. 14. The frequencies are plotted as percentages of all the amputations found in that sex. In females, leg five is not lost more often than leg four (P less than 0.2). Leg four is not lost more often than leg three (P less than 0.7), but legs three, four, and five are lost more often than legs one and two (P much less than 0.01). Hence there is a grouping of legs in the female—the last three legs are lost more often than the first two.

In males, all five legs show approximately equal tendencies to loss. Application of the chi-squared test to legs four and five gives a value of P only slightly greater than 0.05. Accordingly, it is possible that leg five is lost more often than the others.

Leg five is lost more often in females than in males (P less than 0.01). Leg four is lost more often in females than in males (P less than 0.01). Leg three is not lost more often in one sex, but, since P is only slightly greater than 0.05, a larger sample might show that this leg is lost more often in females. Leg two is not lost more often in one sex (P is less than 0.2). Leg one is not lost more often in one sex (P is less than 0.2).

Crayfish are sometimes found with more than one leg missing. Table 3 gives the frequencies of specimens which lack the various numbers of legs. The frequencies are expressed as the percentages of all the specimens of that sex which have amputations.

Table 3
Number of leys missing Males % Females %
1 81.6 79.4
2 4.6 13.5
3 6.2 4.8
4 3.1 0.8
5 1.6
6
7
8
9 1.6
10

Of all the males which have amputations, 16.2 per cent. have more than one amputation; 19 per cent. of the females which have amputations have more than one. Absence of more than four legs is uncommon.

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Fig. 14: Frequency of Amputations; Fig 15: Proportions of Crayfish of Each Size Group

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Females in the size-group 28.1 cm. to 30 cm. have the greatest number of amputations. The frequency of amputation falls off to either side of this group (see Fig. 15). The number of amputations in males increases with size. In the graph, each plot is the number of amputations in the size-group as the percentage of all the specimens examined in that size-group. The numbers marked on each plot are the numbers of specimens examined in each size-group.