How and Why Is the Whale Shark the World's Largest Fish?

Abstract

The whale shark is the largest extant fish species and likely the largest fish to have ever lived, yet its closest relatives are bottom-dwelling carpet sharks of unremarkable size. So, what led the whale shark to become the world’s largest fish? The answer may lie in the combination of a planktivorous lifestyle and relatively slow metabolism inherited from their ancestors. The apparent absence in whale sharks of adaptations for partial endothermy, which occur in many other large pelagic species and likely incur significant energetic cost, may provide a release from some constraints on their size. But why are whale sharks not larger, achieving the same gargantuan body sizes as the baleen whales? The authors speculate that the flexible cartilaginous skeleton of whale sharks may place biomechanical limits on their body size. Alternatively, the relatively low food density of the tropical oceans, combined with the energetic costs of filter feeding, may constitute a maximum size threshold past which it is impossible to harvest enough energy to sustain such enormous bulk. Hypotheses surrounding the extraordinary gigantism of whale sharks need rigorous testing, but in the meantime, conservationists should exploit body size bias to improve the conservation plight of this endangered species.