PhD defense: Julius Nielsen – Biologisk Institut - Københavns Universitet

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PhD defense: Julius Nielsen

The Greenland shark (Somniosus microcephalus): Diet, tracking and radiocarbon age estimates reveal the world’s oldest vertebrate

Supervisor: Professor John Fleng Steffensen, MBS, BIO-UCPH

Co-supervisors
Senior Scientist Rasmus Berg Hedeholm, Greenland Institute of Natural Resources
Professor Peter Bushnell, Indiana University South Bend, USA
Associate Professor Kim Præbel, UiT The Arctic University of Norway
Professor Jørgen Schou Christiansen, UiT The Arctic University of Norway

PhD Committee
Associate Professor Bent Visman, MBS, BIO-UCPH
Professor Diego Bernal, University of Massachusetts, Dartmouth, USA
Professor Kim Holland, University of Hawaii, USA

Abstract
This PhD project has aimed at investigating longevity of the Greenland shark. The largest Greenland sharks measure at least 550 cm, and ever since Poul Marinus Hansen in 1963 presented that a recaptured medium-sized Greenland shark had grown 8 cm in 16 year, longevity of the species has been subject for speculation. Conventional age determination techniques for teleost or elasmobranchs are not applicable on the Greenland shark and its longevity has thus remained a mystery for decades.

Inspired by alternative age estimation techniques applied on other sharks and whales, I have used bomb radiocarbon dating and a Bayesian calibration model to estimate longevity of the Greenland shark. The analyzed tissue stems from the eye lens nucleus – unique material which presumably reflects age 0 of the shark, as it has not undergone metabolic changes during the animal’s life. By studying 28 Greenland shark females between 81 cm and 502 cm, I estimate the oldest shark to be between 272 years and 512 years. With an estimated lifespan of at least 272 years, the Greenland shark is the longest living vertebrate animal in the world.

In order to produce these age estimates, it has been necessary to study the carbon source of the eye lens nucleus in more detail. The center of the nucleus consists of proteins and the analyzed tissue stems from the diet of the shark’s mother. From feeding ecology and satellite tracking, I have therefore investigated adult females. Sharks of this life stage mainly occupied continental shelf waters in southern Greenland at depths between 200 and 550 m and fed primarily on cod, redfish and seals. From previous investigations of predatory sharks and whales in the north Atlantic, bomb radiocarbon has been widely applied, and I argue that a similar calibration approach is valid to use on the Greenland shark.

The main aim of this thesis is to clarify the biological assumptions behind the radiocarbon dating leading to the age estimates of the Greenland shark. These age estimates rest on classical biological feeding ecology studies, chemical isotope analysis and advanced mathematical modelling. This interdisciplinary approach has been crucial for the success of the project. The thesis also illustrates how a novel cross-combination of techniques can be applied on other marine species difficult to age determine, and how the Greenland shark is unique to the arctic ecosystem. Many aspects of the basic biology of the Greenland shark remain mysterious.