SANTINI, F; Museum Nationalle d’Histoire Naturelle, Paris: Fish fins, morphological modularity and evo-devo

Ocean sunfishes (molas) are a small clade of pelagic fishes, closely related to pufferfishes. Molas are very active swimmers, in spite of the fact that they lack the spiny dorsal fin, the pelvic fins and possess a very modified structure (the psudocaudal fin) in place of the caudal fin found in almost all teleost fishes. The homology of the pseudocaudal fin (or clavus) has been at the center of a long-standing debate among fish comparative morphologists, due to the fact that its bones have been homologized with those of the soft dorsal- and anal-fin, thus suggesting that it might be composed of these median fins that have migrated around the posterior section of the body, while the muscles of this structure have been considered homologous to those of the caudal fin. With this presentation I intend to briefly discuss the evidence for the competing hypotheses that the pseudocaudal fin of the molas is homologous to the caudal fin of other teleost fishes (as the myological data seem to suggest) or that is composed by bony elements of the soft dorsal and anal fins, and that hence the posterior part of the body of molas does not develop (as the osteological data seem to suggest); or that both hypotheses might be correct. If the osteological elements of the clavus are homologous to those of the dorsal and anal fins, and the muscles are homologous to those of the caudal fin of pufferfish, this would be a clear example of morphological modularity, in that the osteological and myological systems in these fishes are under the control of independent genetic mechanisms. The implications of this scenario for evo-devo will be discussed.