The daily rhythm: exploring the influence of diel vertical migration patterns on bone density variation in lanternfishes (Teleostei: Myctophidae)

Lanternfishes are among the most diverse and abundant deep ocean fishes in the world and play significant roles in marine food webs by linking lower and higher trophic levels. Most lanternfish species perform diel vertical migration (DVM) to feed in the upper open ocean at night and remain in deeper layers during the day to avoid predators. While DVM is often known as an adaptive behavior in lanternfishes, morphological constraints associated with this behavior remain poorly understood. Here, we investigated the potential morphological traits associated with DVM by comparing lanternfish species that migrate and those that do not migrate. We compared the average relative bone density of three different bone structures that are important to their body movement and feeding behaviors: the 11th vertebra, the cleithrum, and the lower jaw. We hypothesized that migrants have a denser cleithrum to support their migration, while the average densities of the 11th vertebra and the lower jaw would not vary. We performed micro-computed tomography (micro-CT) scans on 34 specimens of 24 species and included two standards of known hydroxyapatite concentration (phantoms) to approximate voxel brightness to bone density. We found evidence that all the lanternfish species in this study shared similar patterns of variation across the three bones, where the lower jaw was significantly denser than the other bones assessed. There was no significant relationship between bone density variation and DVM behavior. Our results reveal complex relationships between DVM and bone density that have not previously been described, and lanternfishes might possess morphological changes beyond the three structures investigated in this thesis. This study expands on our limited understanding of DVM in lanternfishes and contributes to the growing body of work that explores their diversity and evolution.