New study reveals link between shark fins and environment

Study synopsis

Sharks vary significantly in their shapes, their physiology, and how they use their environment. Observing how sharks use their habitats has previously linked differences in swimming style and body shape.

Shark pectoral fins have previously been grounds to classify shark species taxonomically. Shark literature contains many hypotheses on how fin shape affects shark swimming. However, until recently, there have been no datasets that compare the external and internal forms of a shark fin.

In shark science, fins are categorized into two groups based on how much skeletal support is present:

1.) Aplesodic: where less than half of the fin is supported skeletally.

2.) Plesodic: where greater than half of the fin is supported skeletally.

Sarah L. Hoffmann, Thaddaeus J. Buser, and Marianne E. Porter find that the methodology of this type of classification is infrequently described. In their study at Florida Atlantic University, this team sampled fins from 18 species, six families, and three orders. The sharks studied were also grouped into five ecomorphotype classifications.

Ecomorphotype: Any morphological modification caused by, or related to, specific ecological conditions.

Morphospaces: Spaces describing and relating organismal phenotypes (traits resulting in how genotypes interact with the environment). They play a central role in morphometrics, the statistical description of biological forms.

Hoffman, Buser, and Porter examined the external shapes, the extent of skeletal support, and the cross‐sectional structure of individual cartilaginous elements. You can read the full study yourself here. Below is a summary of their findings.

Results

Hoffman, Buser, and Porter found that the fin shape does not differ significantly between different ecological types, suggesting there may be some external factors that make the shark populations resistant to evolutionary change.

However, the study also shows that the internal anatomy of the studied shark fins does vary significantly between ecomorphotypes, especially the extent and distribution of skeletal support. Their findings suggest that the surface-level similarity of fin shapes across ecomorphotypes may fail to justify a difference in function.

Finally, the team found variables that may correlate with the differences in skeletal support. Variables such as the number of radials, radial calcification (hardened by deposition of or conversion into calcium compounds) and shape, and fin taper all correlate with how much skeletal support was present.

Hoffman, Buser, and Porter demonstrate some mechanical constraints limiting external differences in the shape of shark pectoral fins. However, they found multiple variations in skeletal anatomy that occurred within ecomorphotypes, which they propose may affect function.