WITTEN*, P.E.; ROSENTHAL, H; HALL, B.K.; Institute of Marine Research, Kiel, Germany; Institute of Marine Research, Kiel, Germany; Dalhousie University, Halifax, NS, Canada: The kype of male Atlantic salmon (Salmo salar): Restart of bone development in adult animals

Maturing upstream migrating Atlantic salmon display drastic skull alterations in both sexes, most prominently, the development of a hook (kype) on the dentary in males. Morphologically, these changes were described 60 years ago but still little is known about the development and possible regression of the skeletal alterations in surviving spawners. Most remarkably, bone growth restarts in a life phase in which the animals do not feed but use energy reserves for upstream migration, gonad development, and competition at the spawning grounds. To provide a basis for understanding the skeletal alterations, we studied the growth and differentiation of the kype supporting skeleton in upstream migrating males and its regression in males returning to the sea after spawning. Despite claims in the older literature that the kype is “entirely bony” or “entirely made from connective tissue” our results show that the kype is supported by a meshwork of fast growing needle-like skeletal structures. The kype skeleton differs substantially from regular dentary bone, displaying features of bone, of cartilage, and of Sharpey fibre bone. In surviving spawners, the kype skeleton is partly resorbed and partly remodelled into regular dentary bone. The mode of kype growth (as fast as possible using as little material as possible), its tight connection to the connective tissue, and its eventual partial remodelling, support ideas that the kype functions as a secondary sexual character. Future studies will address the impact of accumulating skeletal alterations on the fitness of repetitive spawners.