Over the years I have been asked all sorts of questions about sharks, covering a broad range of pretty much everything, from; “Do sharks fart?” to “How do sharks grow?”
Let’s start with the latter, this was asked by an Ecologist friend; Heather Lyons, and is a particular favourite of mine, not least because the answer takes you on a journey of discovery on both a physiological and evolutionary level.
One of the main physiological features that sets elasmobranchs apart from other fish, is their cartilaginous as opposed to bony skeleton. The majority of the skeleton of sharks, skates and rays if we want to be precise, is covered by an outer connective tissue called the perichondrium and is characterised by a tessellated structure, composed of a shell of small, mineralised plates (tesserae), joined by intertesseral ligament overlaying a soft cartilage core (Liu, et al 2014).
But what exactly is cartilage? Cartilage is made up of specialised cells called chondrocytes, these cells are responsible for producing extracellular matrixes composed of collagen fibres, proteoglycan and elastin fibres. In teleost fish a large amount of this tissue gets replaced by bone over time by a process known as ossification.
Using human bone growth (ossification) as an example; layers of calcium and phosphate salts begin to accumulate on cartilage cells, once encased within these minerals the cartilage cells die leaving tiny pocket like holes behind. Blood vessels grow into these tiny pockets, delivering specialised cells called osteoblasts. The osteoblasts help to collect additional calcium and produce a substance full of collagen fibres, they also produce layers of cortical bone that surround the cartilage. After making the cortical bone, osteoblasts become cells called osteocytes that work to form marrow and cancellous bone inside the developing structure. Eventually, other cells known as osteoclasts make their way into the middle of developing bones. They use hydrolytic enzymes and acids to dissolve the cancellous bone and make room for more marrow.
This process raises a couple of questions for cartilaginous animals; the first question takes us back to Heather’s original query; “How do Sharks grow?” If blood vessels aren’t formed until ossification takes place, how then do the cells (chondrocytes) responsible for making cartilage gain enough nutrients to continue this process? The answer is that nutrients diffuse through from other vascularised or nutrient rich areas, in sharks the nutrients diffuse from the perichondrium (mentioned earlier), a dense irregular layer of fibrous vascularised connective tissue.
The second question this raises for elasmobranchs is; in the absence of red bone marrow (responsible for the production of red blood cells in most vertebrates), how do sharks make red blood cells? For this they use different organs namely the spleen, gonads and the leydig organ, all of which play a role in the manufacturing of erythrocytes (red blood cells) which contain haemoglobin and transport oxygen around the circulatory system.
It’s worth noting that cartilaginous skeletons have primarily been thought of as a primitive biological trait, however in 2005 palaeontologists in Western Australia found a 380 million year old fossil of the “Gogo” shark (Gogoselachus lynbeazleyae), analysis of the specimens cartilaginous microstructure confirmed that whilst the cartilage was indeed like modern shark cartilage and made up of bundles of tesserae, the matrix holding these cartilage units together actually retained a cellular structure with remnant bone cells visible. This implied that sharks most likely evolved from ancestors that had much more bone in the skeleton. The evolution of modern sharks being driven by their loss of bone, suggesting they are not as primitive as previously thought. Modern sharks most likely evolving their lighter cartilaginous skeletons in order to become faster swimmers, to evade predators and swiftly catch their prey (Long et al. 2015).
OK so now we have cleared that up, I am sure you are all desperate to know the answer to the other question…. “Do sharks fart?” Well the Tiger shark (Galeocerdo cuvier) definitely does, an individual was filmed partaking in a little “off-gassing” via its cloaca, off the coast of Beqa Island, Fiji. And whilst we are on the subject of gas you may be interested to know that Oceanic Whitetip Sharks (Carcharhinus longimanus) have been filmed burping off the coast of Egypt in the Red Sea (Ritter, 2018)….. Your welcome!
Liu X, Dean M, Youssefpour H, Summers, A, Earthman, J. (2014). Stress relaxation behaviour of tessellated cartilage from the jaws of blue sharks. Journal of the Mechanical Behaviour of Biomedical Materials. 29. 68-80.
Long J, Burrow C, Ginter M, Maisey J, Trinajstic K, et al. (2015). First shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia sheds new light on the development of tessellated calcified cartilage. PLOS ONE 10(6)
Ritter E. (2018). On the relationship between digestive gases and buoyancy in sharks. Examines in Marine Biology and Oceanography. 2(2) 1-4.