The Kype: An Assessment of Presence and Absence, Composition, Structure, and Evolution of a Spawning-related Lower Jaw Modification in Living and Fossil Salmonids

Abstract

When in their spawn-run, several salmonids (salmon, graylings, etc.) develop a kype. The kype is a spawning-related lower jaw modification composed of a connective tissue mass supported by a framework of chondroid and Sharpey-fibre bone skeletal needles. However, while previous research considered the structure and composition of the kype in Atlantic salmon, little is known of the kype in other salmonids, or indeed which other salmonid species are kype-bearing. Additionally, the evolutionary origins of the kype and of spawning-related morphological changes in its associated structures and tissues are also unknown, Physical examinations, Micro-CT assessments, literature surveys, and histological analyses of living and fossil material reveal that a kype is found only in eusalmonines (trout, salmon, and chars), with the exception of Hucho. The lack of kype in Hucho supports reassessments of the phylogenetic location and monophyly of this genus. The relocation of Hucho is also supported by genus-specific, life-history stage, and life history mode variation in the trends in lower jaw fat, epithelium, and blood vessel morphology evident in all salmonids. The combination of Sharpey-fibre and chondroid bone, tissues previously thought to be only associated with the kype skeleton, is present across Salmonidae to some extent. Establishment of the origins of these tissues requires further research as to the conditions of ancestral groups. This project represents the first time that a family-wide histological assessment of the salmonids has been done and uses both living and fossil material to do so.