Arctic Sharks: A Story of Ice-olation and Hybridization

Genetic study finds hybridization between Greenland Sharks and Pacific Sleeper Sharks. The researchers propose a model of ‘ice-olation with migration’.

Although this blog is about hybridization in birds, I will occasionally write about other taxonomic groups. This week, I came across a genetic shark study by Ryan Walter (California State University) and colleagues in the journal Ecology and Evolution. For several reasons (see further down), I could not resist fabricating a blog post on this shark story.

 

A Shark Tale

The Simniosidae is a family of sharks known as ‘sleeper sharks because of their apparent slow swimming. The recent study focuses on two species: the Greenland Shark (Somniosus microcephalus) and the Pacific Sleeper Shark (S. pacificus). The Greenland Shark ranges from the Canadian Arctic to the south of Norway, while the Pacific Sleeper Shark occurs from the Bering Street throughout the Pacific (hence the name) into the Southern Ocean. Previous studies, based on mitochondrial DNA (mtDNA) showed that both species can be distinguished from one another despite their physical resemblance.

 

greenland shark

A Greenland Shark (Simniosus microcephalus)

 

The researchers collected samples of 247 sharks across their extensive distribution. They sequences mtDNA and several nuclear markers. The genetic data confirmed previous studies; the species could be easily kept apart. However, some individuals were a genetic mixture of both species, suggesting hybridization. Further analyses (Isolation-with-Migration models for the curious readers) confirmed this suggestion. It turned out that genes have been flowing from Pacific Sleepers Sharks into Greenland Shark populations.

To explain this findings, the authors introduce a model of ‘ice-olation with migration’. Apart from being a great play of words, this model nicely places the genetic data in a climatic context. About 2.5 million years ago, a drastic reduction in global temperatures occurred. This decline resulted in sea ice formation in polar regions. The spreading of thick sea ice in combination with the submerged mountains of the Arctic probably reduced the connectivity between different shark populations. During this isolation, the shark populations diverged genetically. In periods when the climate warmed and the ice melted, the sharks re-established contact and hybridized.

 

seal broken

Another wordplay…

 

Why this blog post?

This cool story caught my eye for several reasons:

  1. Sharks! Who doesn’t like sharks? Okay, they are not birds, but they are sharks!
  2. After my PhD, I briefly worked for the science department at a Dutch newspaper (De Volkskrant). One of my most successful stories was about the Greenland Shark which can live up to 400 years! Here is the link (in Dutch).
  3. The climatic conditions that influenced the evolutionary history of these sharks is very similar to the conditions that drove the evolution of geese. Here is a quote from one of my papers on geese: “The approximate date of diversification coincides with the beginning of a period of climatic oscillations between 3.2 and 1.9 million years ago. This period was part of a fast global cooling trend, following the closure of the Panama Seaway and the uplifting of the Tibetan Plateau around four million years ago. This resulted in the formation of permanent Northern Hemisphere ice sheets, the establishment of a circumpolar tundra belt and the emergence of temperate grasslands, which opened up new ecological niches in which new groups of animals and plants were able to spread.” Do you notice the resemblance.
  4. And of course, the great wordplay in the title (Ice-olation). Priceless!

 

References

Walter, R. P., D. Roy, N. E. Hussey, B. Stelbrink, K. M. Kovacs, C. Lydersen, B. C. McMeans, J. Svavarsson, S. T. Kessel and S. Biton Porsmoguer (2017). Origins of the Greenland shark (Somniosus microcephalus): Impacts of ice‐olation and introgression. Ecology and Evolution.

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