Autotomy and regeneration

Autotomy refers to the process by which animals shed a part of their body, either as a result of damage or as a voluntary (spontaneous) act. It involves breaking part of the skeleton, whether internal or external, as described in detail by McVean (1982). In decapod crustacea there is a specific fracture plane for each appendage that can be au-totomised. The process of autotomy occurs at the base of the ischial joint with the involvement of the levator muscles which, through their acute contraction, allows the preformed breakage plane to separate.

In M. rosenbergii, the most frequently autotomised appendages are the second pereopods (chelipeds). As discussed earlier, the length ofthe male chelipeds is one ofthe characteristics of dominance within a population. When the male chelae reach a critical length in relation to the body length, the male voluntarily sheds both chelae at moult, and with the following moult a cycle of chelae regeneration starts. According to Schmalbach et al. (1984), the critical point above which autotomy can take place is a body length to chelae length ratio of 1:2.8. These authors suggested that this autotomic behaviour could be of great advantage as it leads to a succession of dominant males and that this rotation would increase the genetic variability within a specific population. It could however be simply a function of the difficulties of moulting such extreme chelae.

Following autotomy, epidermal cells located on the periphery of the resulting stump enlarge and migrate. After several weeks, a papilla is formed as a result of a blastema erupting through the scab. The regenerated appendage is enclosed within a protective sheath which is released at ecdysis (Skinner 1985).

There is a close relationship between regeneration and moulting, so that regeneration only occurs during the proecdysial period. Thus an animal missing a single limb does not form the new one until it passes through the next moult cycle. However, the onset of preparations for moulting and intensive regeneration may occur much sooner in animals missing a large number of appendages (Skinner 1985).

The occurrence of antennule regeneration replacing the eyestalk (known as heteromorphosis) has been reported in cultured M. rosenbergii (Nevin & Malecha 1991). They may be functionally similar to normal antennules and may respond to mechanical and olfactory stimulation. This hypothesis is corroborated by Mellon et al. (1989) who demonstrated that the sensory axons of heteromorphic crayfish antennules are connected with the olfactory lobe of the brain.

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