Sand Tiger

An introduction to the Sand tiger shark, Carcharias taurus,  by Camilla Christensen

The sand tiger shark (grey nurse shark or spotted ragged-tooth shark), Carcharias taurus, has a grey or brownish colour, sometimes with darker spots occurring on the upper part of the body [1-3]. It has a ferocious appearance, with spear shaped teeth and a large body (up to 320 cm) [1,4]. Previously the species was wrongly accused of being a ´man-eater´, but it is in fact a rather docile shark. Their diet consists of teleost fishes, smaller sharks, rays, squids and crustaceans [1,3]. The docile behaviour is a quality that makes it a popular display species for scuba divers and in public aquari-ums [2,3]. However, the reproduction strategy of the species reveals a gruesome phenomenon. After fertilization, multiple embryos develop in each of the two uteri, but only two pups are born (one from each uterus). This is due to the phenomenon called intra-uterine cannibalism or oviph-agy, which refers to that the largest hatched embryo, consumes the others. With a biennially cycle of reproduction, sand tiger sharks produce only one offspring per year. Consequently, the low productivity of the sand tiger shark along with other life history characteristics such as longevity and late sexual maturity [4] makes it a species of concern and it is argued to be vulnerable to pressures [2,5].

The sand tiger shark has a cosmopolitan distribution and occur in the Indian, Pacific and Atlantic oceans, in sub-tropical to temperate inshore waters (15-25 m depth) [1,2]. On a global scale, sand tiger sharks are currently classified as ´vulnerable´ on the ICUN red list. Whereas, the subpopula-tions of the east coast of Australia and of the southwest Atlantic are classified as ´critically endan-gered´ [6]. Studies of population trends in these populations have revealed population declines [2,7,8], and worldwide studies show low migration rates between current populations and high population differentiation between populations from the northern and southern hemisphere [9]. Thus, to some extent, we have information of the current population structure and condition, but this information is mainly focused on the populations found in Australian waters.

Through Project Genojaws we will improve the current knowledge of the effective population size, the population genetic structure and connectivity of the sand tiger shark on a global and temporal scale. Information of these population genetic parameters can be obtained by comparing the genetic composition between contemporary and historical DNA samples for a large number of coding genes. Such retrospective analysis, can ultimately help us infer knowledge about the extent of the decline and the adaptive potential of the populations and will help inform and improve man-agement efforts worldwide.

 

References

[1] Pollard, D.A., Smith, M.P.L. and Smith, A.K., 1996. The biology and conservation status of the grey nurse shark (Carcharias taurus Rafinesque 1810) in New South Wales, Australia Aquatic Conservation: Mairne and freshwater ecosystems 6, pp. 1-20.

[2] Otway, N.M. and Parker, P.C., 2000. The biology, ecology, distribution and abundance, and identification of marine protected areas for the conservation of threatened grey nurse sharks in south east Australian waters., pp. Final Report Series No. 19. NSW Fisheries Office of Conservation, Sydney, NSW, p. 132.

[3] Ebert, D.A. and Stehmann, M.F.W., 2013. Sharks, batoids and chimaeras of the North Atlantic. FAO Species Catalogue for Fishery Purposes, 7.

[4] Goldman, K.J., Branstetter, S. and Musick, J.A., 2006. A re-examination of the age and growth of sand tiger sharks, Carcharias taurus, in the western North Atlantic: the importance of ageing protocols and use of multiple back-calculation techniques. Environmental Biology of Fishes, 77, pp. 241–252.

[5] Gilmore, R.G., Dodrill, J.W. and Linley, P.A., 1983. Reproduction and embryonic development of the sand tiger shark, Odontaspis taurus (Rafinesque). Fishery Bulletin, 81(2), pp. 201-225.

[6] Pollard, D. and Smith, A., 2009. Carcharias taurus. The IUCN Red List of Threatened Species 2009: e.T3854A10132481.

[7] Lucifora, L.O., Menni, R.C. and Escalante, A.H., 2002. Reproductive ecology and abundance of the sand tiger shark, Carcharias taurus, from the southwestern Atlantic. Journal of Marine Science, 59, pp. 553–561.

[8] Otway, N.M., Bradshaw, C.J.A. and Harcourt, R.G., 2004. Estimating the rate of quasi-extinction of the Australian grey nurse shark (Carcharias taurus) population using deterministic age- and stage-classified models. Biological Conservation, 119, pp. 341-350.

[9] Ahohen, H., Harcourt, R.G. and Stow, A.J., 2009. Nuclear and mitochondrial DNA reveals isolation of imperilled grey nurse shark populations (Carcharias taurus). Molecular ecology 18, pp. 4409-4421.