Abstract:
Few fast-swimming apex fishes are classified as ‘regional endotherms’, having
evolved a relatively uncommon suite of traits (e.g. elevated body temperatures, centralised red
muscle, and thick-walled hearts) thought to facilitate a fast, predatory lifestyle. Unlike those apex
predators, Endangered basking sharks Cetorhinus maximus are massive filter-feeding planktivores
assumed to have the anatomy and physiology typical of fully ectothermic fishes. We combined
dissections of stranded specimens with biologging of free-swimming individuals and found
that basking sharks have red muscle located medially at the trunk, almost 50% compact myo -
cardium of the ventricle, and subcutaneous white muscle temperatures consistently 1.0 to 1.5°C
above ambient. Collectively, our findings suggest basking sharks are not full ectotherms, instead
sharing several traits used to define a regional endotherm, thus deviating from our current understanding
of the species and questioning the link between physiology and ecology of regionally
endothermic shark species. With successful forecasting of population dynamics and distribution
shifts often improved by accurate physiological data, our results may help explain movement
patterns of the species, which could ultimately facilitate conservation efforts.
