Comparative morphology of shark pectoral fins

Sharks vary greatly in morphology, physiology, and ecology. Differences in
whole body shape, swimming style, and physiological parameters have
previously been linked to varied habitat uses. Along with whole body
morphology, shark pectoral fins are also previously described to vary in
both shape and skeleton; however, there are limited comparative data on
external and skeletal morphology. Further, fins were previously
categorized into two discrete groups based on the amount of skeletal
support present: (1) aplesodic, where less than half of the fin is
supported and (2) plesodic where greater than half of the fin is
supported. These discrete classifications have been used to
phylogenetically place species, though the methodology of classification
is infrequently described, and the comparisons may not be valid.
Additionally, our understanding of shark pectoral fin ecomorphology is
limited by access to samples from a broad variety of species. In this
study, we sampled fins from various families, orders, and ecological
classifications. We examined the external morphology, skeletal extent, and
cross-sectional shape of the cartilaginous elements. Using phylogenetic
comparative methods, we show that fin morphology does not differ
significantly when considering the level of relatedness between species,
suggesting there may be some mechanical constraint. We also describe a
range of skeletal extent, rather than two discrete categories. We find
that fins are shaped like hydrofoils in cross-section, supporting
hypotheses that fins may be lift producing structures in sharks. Finally,
we find that a number of morphological variables such as number of
radials, radial calcification and shape, and fin taper all correlate with
skeletal extent. Within these morphospaces, we also describe that some
orders/families tend to occupy certain areas with limited overlap. With
this study, we demonstrate that there is some mechanical constraint
limiting variations in shark pectoral fin morphology, but there are subtle
differences that appear to occur within shark groups that share close
phylogenetic relationships and similar biological parameters.