Vertebral morphology in the tail-whipping common thresher shark, Alopias vulpinus

Thresher sharks (Alopias spp.) are characterized by an elongated,
scythe-like caudal fin that is used in tail-whipping, a behavior where the
tail is thrown overhead to stun prey. Tail-whipping is performed via
extreme dorsoventral bending of the vertebral column and is dramatically
different from lateral oscillatory motion used for swimming. Previous work
has examined thresher shark vertebral morphology and mechanical properties
but in the context of swimming loads. Our goal was to assess centra
morphometrics and microarchitecture for variations that may support
extreme dorsoventral bending. We examined anterior and posterior body
vertebrae from an embryo, 5 juvenile, and 4 adult thresher sharks using
micro-computed tomography. We used principal component and landmark
analyses to examine variables influencing vertebral morphology and mineral
arrangement, respectively. We found that morphology and microstructure
significantly varied across body regions and ontogeny. We hypothesize that
anterior body vertebrae increase stability, while posterior body vertebrae
support the caudal fin. Vertebral size and quantity of mineral structures
(lamellae and nodes) increased across ontogeny, suggesting vertebrae adapt
over development to support a larger body and tail. Based on our results,
we hypothesize that thresher shark vertebrae vary in morphometrics and
mineralization (amount and arrangement) supporting the mechanical needs
for tail-whipping.