List of Accepted Abstracts

Fish fins are essential for postural control and movement in the water. In particular, the caudal fin is known to generate a driving force during swimming and to have evolved a highly diverse morphological structure across a wide range of lineages. However, the molecular genetic mechanisms underlying this diversification are not well understood. To elucidate these mechanisms, we use the three-spined stickleback, Gasterosteus aculeatus, as a model. The three-spined stickleback is originally a marine fish species but has repeatedly colonized different freshwater habitats such as lakes, streams, and ponds, resulting in adaptive phenotypic diversification. Because these habitats vary in flow velocity and depth, we expected to see diversification in caudal fins between populations. Morphological analysis revealed the interspecific variation in the branching pattern of caudal fin rays in sticklebacks. In particular, a freshwater population inhabiting mainland of Japan had a significantly higher rate of second branching of the caudal fin rays than other marine and freshwater populations. The second branching may provide more support for the fin membrane, allowing water to be pushed out more efficiently. Furthermore, this freshwater population has reduced defensive structures against predators such as armor plates and spines. Thus, the increased rate of second branching might be involved in the escape strategy by burst swimming rather than using weakened defensive traits. In addition, to investigate when the second branching occurs in this freshwater population, we analyzed the branching pattern of caudal fin rays from juvenile to adult stages in the natural habitat. Second branching was found in individuals approximately larger than 35 mm in size, which is expected to be around one year old. Further molecular analysis at this stage may reveal the genes and mutations responsible for the diversification of caudal fins and escape behavior in response to predators.