Morphological Evolution of the Caprellidea (Crustacea) : Degeneration and Reacquisition of Pereopods and Abdominal Segments

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  • Ito Atsushi
    Graduate School of Life and Environmental Sciences, University of Tsukuba
  • Aoki Masakazu N.
    Shimoda Marine Research Center, University of Tsukuba:(Present office)Graduate School of Agrucultural Science, Tohoku University
  • Yahata Kensuke
    Graduate School of Life and Environmental Sciences, University of Tsukuba
  • Wada Hiroshi
    Graduate School of Life and Environmental Sciences, University of Tsukuba

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Other Title
  • ワレカラ亜目(甲殻類)の形態進化 : 付属肢・体節の退化と再獲得のメカニズム
  • ワレカラ アモク(コウカクルイ)ノ ケイタイ シンカ : フゾクシ ・ タイセツ ノ タイカ ト サイカクトク ノ メカニズム

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Abstract

Caprellidea have undergone a complicated morphological evolution. Most caprellid families exhibit strongly degenerated third and fourth pereopods (walking thoracic limbs) and abdomens. However, a caprellid family, Caprogammaridae, has developed abdominal segments with swimming appendages, whereas another family, Phtisicidae, possesses well-developed third and fourth pereopods. We conducted molecular phylogenetic analyses of caprellid families and ancestral state reconstruction. The analyses suggested that third and fourth pereopods in Phtisicidae and the abdomen in Caprogammaridae might be regained. The embryonic development of Caprella scaura was observed in order to understand the developmental and genetic mechanisms involved in the complicated morphological evolution of Caprellidea. In the embryonic development of C. scaura, distinct abdominal segments did not appear. In addition, elongation and segmentation of limb buds did not occur in the third and fourth pereonites (free thoracic segments), although the primordial gills appeared. These observations indicate that the telopods were not formed in the third and fourth pereonites. In order to confirm this hypothesis at the molecular level, gene expression analyses of Distal-less was performed and it was observed that Dll was not expressed in the third and fourth pereonites. This is the first molecular phylogenetic and developmental study focusing on the morphological evolution of Caprellidea.

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