A Developmental Perspective on Evolutionary Innovation in the Radula of the Predatory Neogastropod Family Muricidae

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bauplan, heterochrony, muricid, ontogeny, rachidian

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The neogastropod family Muricidae includes a diverse set of radular bauplane, including a beaked, three-dimensional form, a flattened-pentacusped form, and a third “dagger” type in which the central rachidian cusp is massive and elongate. Examination of the radular ontogenies of representatives of five muricid subfamilies reveals that several species undergo changes in radular form during ontogeny on a scale comparable to the evolutionary differences between higher taxa. The species Concholepas concholepas(Bruguière, 1789) (Rapaninae) and Trophon geversianus (Pallas, 1774) (Trophoninae) begin ontogeny with a three-dimensional rachidian characteristic of the Ocenebrinae or Muricopsinae but end with the dagger rachidian typical of their respective subfamilies. Young individuals of Vitularia salebrosa (King and Broderip, 1832) (Muricopsinae?) also have a three-dimensional rachidian but shift to a double-dagger morphology by adulthood. Chicoreus (Phyllonotus) pomum (Gmelin, 1791) (Muricinae) has a typical flattened muricine rachidian as an adult but possesses a “buccinoid”-like rachidian just after hatching. Urosalpinx cinerea (Say, 1822) (Ocenebrinae), was unique among the species examined in exhibiting no ontogenetic changes in radular form. The occurrence of two radular bauplane within the same individual snail during ontogeny suggests great potential for rapid, convergent evolution of adult features through simple changes in developmental timing. A three-dimensional rachidian, for example, could be retained into adulthood through paedomorphosis in any lineage possessing the three-dimensional-to-dagger ontogeny. Systematic assignments of muricids based solely on radular features should be reexamined.

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American Malacological Bulletin, v. 23, issue 1, p. 17-32