The Importance of Sharks by Dr Andrea Gaion

With the common name “shark” we can describe more than 400 different species of fishes belonging to the class “Chondrichthyes”, characterized by having a cartilaginous skeleton [1]. Thanks to the innumerable adaptations that can be found in this group of predators, they managed to conquer almost every ecological niche possible in the marine environment, playing a key role in controlling the balance of the whole ecosystem. Their biological and physiological adaptations have been tailored by evolution in order to cope with different challenges of living in the ocean; here are SOME of the incredible characteristics of sharks:

  • As stated above, their skeleton is made of cartilage [2], this gives them an incredible flexibility and buoyancy in the water. The weight of the skeleton is lower than a bony skeleton, therefore they need to spend less energy to swim.
  • Shark skin is covered with teeth-like structures, called dermal denticles. Each species has different denticles in shape and size but their function is common to all the shark: protection and increased swimming performance. I could experience how hard is the skin when I tried to collect skin biopsies from blue shark: I’ve tried many different strategies but even with a very sharp tip I couldn’t break through it. With regards to swimming performance, it has been demonstrated that, compared to a smooth control model, shark can swim faster (6.6%) and spend less energy (5.9%) [3].
  • The shape and dimension of teeth can vary, but sharks can regenerate their teeth many times thanks to an ancient gene they kept in their “genetic library” [4]; this could come very useful for replacing teeth lost during predation.
  • They don‘t have a swim bladder, overcoming the problem to continuously control (and spend energy) the exchange of gas from the blood to regulate the buoyancy. Sharks avoid this by relying on the high lipid (oil) content of their liver [5].
  • All the bony fishes have to cope with living in a salty environment, and as a consequence they have to drink to compensate the loss of water for osmosis. By doing that, they accumulate salt in their blood and this results in and incredible activity of actively pumping out the salt from specific cells in their gills (chloride cells). Sharks are able to “compensate” the loss of water for osmoregulation thanks to their ability to utilise nitrogenous organic compounds and inorganic ions to maintain osmotic consistency with their environment, in other words: once again they save energy [6].

If we explored all the different biological adaptations of shark species, this article would never finish. Think about the recent discovery of the remarkable longevity of Greenland shark (Somniosus microcephalus), estimated to be 392 ± 120 years [7], or the ability of female zebra shark (Stegostoma fasciatum) to “switch” to asexual reproduction: although she hadn’t been in contact with any male in three years, the shark gave birth to 3 pups [8].

Sharks are incredible creatures, characterized by incredible adaptations that allowed them to conquer innumerable ecological niches but unfortunately their numbers are in decline [9]. It has been proven that a exploited elasmobranch communities in coastal, demersal, and pelagic habitats could have dramatic consequences on the balance of the whole ecosystem [10]. It is fundamental that we continue to increase the awareness of shark roles and importance in the ecosystem;  we need to try our best to protect these remarkable animals by supporting conservation projects in any mean.

 

Cover Photo: BRIAN J. SKERRY, NATIONAL GEOGRAPHIC CREATIVE

Reference list

[1] Encyclopedia Britannica (2016) Shark. [Online] Available at: https://www.britannica.com/animal/shark. (Accessed on 26/10/2017).

[2] Dean M.N., Summers A.P. (2006) Mineralized Cartilage in the skeleton of chondrichthyan fishes. Zoology, 109: 164–168. DOI: 10.1016/j.zool.2006.03.002

[3] Wen L., Weaver J.C., Lauder G.V. (2014) Biomimetic shark skin: design, fabrication and hydrodynamic function. Journal of Experimental Biology, 217: 1656-1666. DOI: 10.1242/jeb.097097

[4] Martin K.J., Rasch L.J., Cooper R.L., Metscher B.D., Johansond Z., Fraser G.J. (2016) Sox2+ progenitors in sharks link taste development with the evolution of regenerative teeth from denticles. PNAS, 113(51): 14769–14774. DOI: 10.1073/pnas.1612354113.

[5] Phleger C.F. (1998) Buoyancy in Marine Fishes: Direct and Indirect Role of Lipids. American Zoologist, 38: 321-330.

[6] Hammerschlag N. (2006) Osmoregulation in elasmobranchs: a review for fish biologists, behaviourists and ecologists. Marine and Freshwater Behaviour and Physiology, 39(3): 209–228.

[7] Nielsen J., Hedeholm R.B., Heinemeier J., Bushnell P.G., Christiansen J.S., Olsen J. (2016) Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus). Science, 353(6300): 702-704. DOI: 10.1126/science.aaf1703

[8] Dudgeon C.L., Coulton L., Bone R., Ovenden J.R., Thomas S. (2017) Switch from sexual to parthenogenetic reproduction in a zebra shark. Scientific Reports. DOI:10.1038/srep40537.

[9] IUCN Shark Specialist Group (2014) A quarter of sharks and rays threatened with extinction. [Online] Available at: https://www.iucn.org/content/quarter-sharks-and-rays-threatened-extinction. [Accessed on: 13/11/2017].

[10] Ferretti F., Worm B., Britten G.L., Heithaus M., Lotze H.K. (2010) Patterns and ecosystem consequences of shark declines in the ocean. Ecology Letters, 13(8): 1055-71. DOI: 10.1111/j.1461-0248.2010.01489.x