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Biology

Shark Date.

Hamady LL, Natanson LJ, Skomal GB, Thorrold SR (2014) Vertebral Bomb Radiocarbon Suggests Extreme Longevity in White Sharks. PLoS ONE 9(1): e84006. Doi: 10.1371/journal.pone.00846006

Background:

White sharks (Carchrodon carcharaias) are top predators in the world’s oceans the white shark population is considered ‘vulnerable’ on the International Union for the Conservation of Nature Red List of Threatened Species and are thus protected.   Protection policies rely heavily on age estimation because it is a key component in growth rate, age of sexual maturity, and life expectancy, which are all considered when implementing the policies.

Age estimation is determined by radio carbon dating (C14). Nuclear testing in the 1950’s and 60’s left an imprint in marine organisms that is used as a tie point for determining the ages of organisms that grow by accretion. White shark vertebrae are composed of deposited paired growth bands (figure 1) that are related to age and growth, however, previous studies have yielded limited information on the periodicity that the bands grow.

The goal of this study was to determine the periodicity of growth bands in white shark vertebrae using C14, and to then use those conclusions to estimate life expectancy of white sharks.

Figure 1: Vertebrae.  Visible growth bands marked by red dots. The line represents the vertebral radius (16.6 mm) doi: 10.1371/journal.pone.00846006.g001

Figure 1: Vertebrae. Visible growth bands marked by red dots. The line represents the vertebral radius (16.6 mm) doi: 10.1371/journal.pone.00846006.g001

Methods

Eight sharks were caught in the North Atlantic ocean between 1967 and 2010 and preserved at the National Marine Fisheries Service in Narragansett, R.I.. Vertebrae samples were taken from four male and four female sharks. Vertebrae sections were sliced, photographed, under-went extensive analysis including growth bands counts and C14 dating on collagen, as well as d13C to derive patterns of large scale movement because d13C changes with latitude and distance from the coast.

Findings:

Researchers determined that the periodicity of growth band pairs in young sharks is annual based on the good alignment of band pairs and reference chronologies. As sharks mature it is hypothesized the deposition periodicity increases or the growth bands become too thin to identify, which may lead to underestimation of white shark age. Ages from band pair counts ranged from 6-35 years for females and 9-52 years for males. Radiocarbon dating estimates the average of the female white sharks is 40 years and for males is 72 years.

Age estimations almost three times greater than previous studies estimates, ranging from 19-23 years maximum age, will require protection policies to be adjusted.

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