The fibre composition of the closer muscle in the paired asymmetric (snapper and pincer) chelipeds of the snapping shrimp, Alpheus heterochelis (Say), was determined in the pristine and in the regenerated conditions. In the pristine condition, the snapper muscle has exclusively slow fibres in both male and female shrimps. The pincer muscle has a central band of fast muscle sandwiched by slow muscle. This band of fast muscle is proportionately and significantly larger in the female than in the male (36% compared to 21%), thus demonstrating a subtle form of sexual asymmetry.
The chelipeds were induced to regenerate following their removal within 24 h of a moult. During the intervening intermoult, limb buds formed which, at the next moult, transformed into newly regenerated chelipeds. In these regenerated chelipeds the snapper muscle was usually entirely composed of slow fibres as in the pristine muscle. In a few animals, however, a scattering of fast fibres occurred in the central region; a condition reminiscent of the pincer muscle. The regenerated pincer muscle resembled its pristine counterpart in having a central band of fast fibres sandwiched by slow fibres. In male shrimps, the proportion of fast fibres in the regenerated muscle was similar to that in the pristine muscle. In females, however, the proportion of fast muscle was significantly smaller in the regenerated than in the pristine condition (21 % compared to 36 %). Since a closer muscle with a central band of fast fibres sandwiched by slow is found during regeneration in pincer and occasionally snapper chelipeds, such a pattern represents an early developmental stage which is being recapitulated during regeneration. There is no evidence in Alpheus heterochelis that the pincer muscle initially regenerates exclusively fast fibres as was reported for A. californiensis.