Megalodon & the Dwarf Lanternshark
From the 20cm dwarf laternshark to the prehistoric 40-50 feet-long Megalodon, sharks species display wide ranges of shapes and sizes. Stories of “The Meg”, like the 2018 film of the same name, ignite curiosity in how these marine animals can reach such large sizes – and why are there so few large shark species? Researchers at Swansea University, Leibniz Institute for Evolution and Biodiversity Science, DePaul University, University of Cambridge, and Stony Brook University published a collaborative research article in the journal Evolution, to try to answer these questions.
The researchers examined the phylogenetic tree of shark species and used a phylogenetic approach of 459 taxa to identify characteristics and strategies that allow for the evolution of gigantism in sharks. They found that there are two key adaptations which must occur for a shark species to evolve to “giant” sizes: body temperature regulation (mesothermic pathway) or filter-feeding (filter-feeding pathway) capabilities.
In the mesothermic pathway, sharks adapt the capability to regulate the temperature of their internal organs, allowing them to live in a wider range of habitats, including cold or deep regions, and increase their hunting ability. This may have been the adaptation responsible for the giant size of the megalodon. However, large mesothermic sharks are also prone to a high risk of extinction as they would require large prey to meet their energetic demands. In the case of the megalodon, climate changes likely decreased prey abundance and led to their extinction. Authors of the study found that mesothermic adaptations likely arose from a single origin in the Cretaceous.
The largest shark on Earth today, the whale shark, and the slightly smaller basking shark, are examples of species with filter-feeding abilities. Their large body size is supported by an abundant prey source – plankton. Currently, these filter-feeding sharks are at risk of eating large amounts of plastic and other toxins. During the Paleogene period filter-feeding likely evolved in sharks multiple times.
Catalina Pimiento, Juan L. Cantalapiedra, Kenshu Shimada, Daniel J. Field, Jeroen B. Smaers. Evolutionary pathways toward gigantism in sharks and rays. Evolution, 2019 DOI: 10.1111/evo.13680