現生種と絶滅種の収斂進化 : 単体サンゴの固着器官の進化史(2015年度日本動物分類学会シンポジウム)

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  • Convergent evolution of attachment apparatus in Rhizotrochus (Scleractinia, Flabellidae) and extinct rugose corals(PROCEDINGS, JSSZ SYMPOSIUM, 2015)
  • 現生種と絶滅種の収斂進化 : 単体サンゴの固着器官の進化史
  • ゲンセイシュ ト ゼツメツシュ ノ シュウレンシンカ : タンタイ サンゴ ノ コチャク キカン ノ シンカシ

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Marine sessile benthos living on hard substrates have evolved a variety of attachment strategies. Rhizotrochus (Scleractinia, Flabellidae) is a representative of azooxanthellate solitary scleractinians with a wide geographical distribution and unique attachment structures; it firmly attaches to hard substrates using numerous tube-like rootlets, which are extended from a corallum wall. In contrast, most sessile corals, whether extant or extinct, are attached by stereome-reinforced structures at their corallum bases. However, morphological and constructional traits of the rootlets themselves, along with their evolutionary significance, have not yet been fully resolved. Growth and developmental processes of spines in Truncatoflabellum and rootlets in Rhizotrochus suggest that these attachment structures are homologous. Both structures commonly develop from the calicular edges of walls through the transformation of wall structures, and their skeletal microstructures exhibit similar patterns of rapid accretion and thickening deposits. Taking molecular phylogeny and fossil records of flabellids into consideration, Rhizotrochus evolved from a common free-living ancestor and invaded hard-substrate habitats by the exploitation of rootlets originating from spines, which were adaptive for soft-substrates. Repeated evolution of attachment apparatuses in both scleractinian Rhizotrochus and extinct rugosans (e.g., Silurian Dokophyllum) is likely to represent evolutionary convergence. This study of attachment strategies in Rhizotrochus provides new insights into the understanding of not only the history of adaptive strategies of sessile benthos on changing substrates, but also the natures of soft tissues in extinct organisms.

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