Regeneration and sex-biased transformation of the sexually dimorphic pincer claw in adult snapping shrimps

Abstract

Adult snapping shrimps Alpheus heterochelis have paired asymmetric claws, a large snapper which is similar in the sexes, and a much smaller pincer which is sexually dimorphic. The male pincer is slightly more hypertrophied compared to the female and therefore intermediate in size between the female pincer and the snapper. This has led to the suggestion that female pincer, male pincer and snapper represent a continuum in claw development. To test this possibility we examined regenerating snappers and found them to pass through a pincer-like stage in both sexes. In males they transiently possess the pincer-characteristic setose fringe. As well, the regenerating closer muscle shows a band of fast fibers reminiscent of the pincer closer muscle in both sexes. Since the male pincer more closely resembles the snapper in external and internal morphology than the female pincer, its transformation to a snapper may be more easily precipitated. To test this possibility we cut the snapper dactyl and this triggered transformation of the pincer to a snapper in 84% of male shrimps but only in 28% of female shrimps. During the intermolt in which the snapper dactyl is cut, axon numbers on the pincer side exceeded those on the snapper side in males but not in females. Since this shift in axon numbers favoring the pincer side occurs well before the pincer transforms to a snapper at the next molt it may signal transformation. J. Exp. Zool. 279:356–366, 1997. © 1997 Wiley-Liss, Inc.