十脚甲殻類幼生におけるエビ型からカニ型への進化 : 発生に伴う形態変化を基にした仮説 Morphological Evolution from Shrimp-Shaped to Crab-Shaped Larvae in the Decapod Crustaceans : a Hypothesis Based on Larval and Postlarval Development
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The fundamental body structures of shrimp-shaped and crab-shaped decapod crustaceans were compared and discussed based on larval morphology. Penaeus japonicus was selected as a representative shrimp-shaped form, since it has ancestral characteristics and eight pairs of natatory thoracic appendages. Among the crab-shaped forms, Parthenope (Platylambrus) validus was selected as a representative for tracing the morphological changes of the processes and the knobs on the carapace through the larval and postlarval development. It is suggested that in decapod zoeas the natatory appendages on the thorax show an evolutionary trend towards a reduction in the number of paired appendages. The zoeas of shrimp-shaped decapods bear three to eight pairs of natatory thoracic appendages, while two pairs of natatory thoracopods characterize the zoeas of crab-shaped decapods. The latter are undeveloped in the third to eighth thoracic segments, and the thorax is rather shortened. Consequently, the posterior edge of the carapace does not extend and remains undeveloped during the zoeal stages, and the carapace takes on a helmet-shaped form. The abdomen of the zoea of crab-shaped decapods is also undeveloped. The fundamental body structure of both the shrimp-shaped and crab-shaped decapods is completed during the zoeal stages. The zoea of the crab-shaped form is less developed than that of the shrimpshaped form in the thorax, carapace and abdomen. Therefore, it seems that the Brachura and the Penaeidae differentiated in the early ontogenetic stages.
- BENTHOS RESEARCH
BENTHOS RESEARCH 48, 13-27, 1995-02-28
JAPANESE ASSOCIATION OF BENTHOLOGY