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What makes the shark form optimal?

Updated: Apr 18, 2022

By Jasmine Gunton


It is commonly known that sharks have existed for a long time on Earth. They are near the top of the ocean food chain and have few predators. However, few people know that sharks appeared in the fossil record before trees [1],[2]. Sharks are thought to have existed before the formation of Saturn's rings[3]. For context, the first recognisable shark fossils appeared in the fossil record 450 million years ago during the Ordovician period [1]. The first true tree genus, archaeopteris, appeared 370 million years ago, during the Late Devonian [2]. Since the first sharks appeared, an inconceivable number of marine species have emerged and then died out. Some notable extinct marine predator species include the thalassomedon and the mosasaurus. These were giant animals, much larger than many present-day species of shark [4],[5],[6]. So what makes the shark form optimal for surviving several thousand millennia of natural disasters and mass extinctions?



Humans frequently misunderstand sharks. In film and media, sharks are commonly portrayed as indestructible killing machines, consuming every creature they encounter. Although sharks are highly specialised predators, they do have several physical features that can act as a hindrance. For example, unlike other fish, sharks cannot swim backwards due to the structure of their respiratory system[7]. In fact, certain species of shark such as the great white suffocate and die if they don’t keep moving [8]. Sharks have likely received their formidable reputation because of the occurrence of several shark attack events on humans. However, shark attacks usually occur because the shark has mistaken the human for a seal [9]. The majority of shark species prefer to hunt marine animals rather than take on a scary hairless primate. Because of this, marine biologists are able to study sharks and their unique adaptations.


Often exploited by shark scientists is the behavioural phenomenon known as ‘tonic immobility’. Essentially, if one were to flip a shark so that it was floating on its back, it would enter a trance-like state similar to hypnosis. The shark would become temporarily paralysed until it managed to flip back to a normal swimming position [10]. If a large animal were able to move a shark in such a way, then the shark would be powerless to stop its attack. Dolphin species such as the orca (yes, orcas are dolphins, not whales) use this technique to their advantage [11]. The relationship between great white sharks and orcas is especially interesting. Orcas are thought to hunt and kill great white sharks, usually consuming only their livers [12]. This behaviour is likely explained by the fact that shark livers are very fatty and contain up to 270 kilograms of meat [13]. Some studies suggest that the presence of orcas in an area drives the population of great whites away [14]. The evident domination of orcas over sharks suggests that orcas must have also existed for a very long time. However, the oceanic dolphin family has only existed for around 11 million years [15]. One tends to wonder how sharks have managed to outlive so many of their natural predators.


Since their first appearance in the Late Devonian, sharks have survived five mass extinctions [16]. After each mass extinction event the shark family diversified, filling several ecological niches [17]. The diversity of ecological niches can still be seen in modern sharks. For example, the cookiecutter shark (Isistius brasiliensis) is an ectoparasite that feeds on the tissue of large marine animals [18]. The complete opposite to the cookiecutter shark is the whale shark (Rhincodon typus), which is 33 times larger [19],[20]. However, the whale shark mainly feeds on zooplankton through a filter, similar to an actual whale [21]. The great biodiversity and range of physical adaptations seen in sharks can explain why they have survived for so long.


Another reason for sharks’ longevity can be explained by their diet. Sharks are generalist predators, meaning they have a wide range of food sources [22]. Therefore, if one prey species disappears, the shark will easily be able to find other food. Furthermore, it cannot be ignored that sharks are excellent predators. One adaptation that separates sharks from other fish species is their cartilaginous skeleton. The light cartilage tissue enables sharks to expend little energy swimming long distances [23]. Moreover, the shape of the shark and its specialised scales allow high-speed movement through the water. Indeed, the fastest known species, the mako shark (Isurus oxyrinchus), can swim up to 70 kilometres per hour [24]. This is 1.5 times higher than the fastest recorded human running speed on land [25]. Fossil evidence suggests that early sharks maintained the same tapered form as modern sharks [26]. This makes the shark form one of the most efficient for surviving in the ocean.



Just when you thought the shark couldn't get any cooler, it turns out the shark has a specialised organ that can sense electromagnetic fields. Through this adaptation, there arise two distinct benefits. Firstly, the shark can efficiently navigate long distances through the expansive ocean [27]. Secondly, sharks are able to sense the electromagnetic fields of their prey and therefore locate camouflaged benthic animals [28]. However, this is not the only sense that sharks rely on to detect prey. Like other fish, sharks possess a sensory organ known as a ‘lateral line’ across the middle of their torsos. This lateral line allows the shark to sense vibrations in the water created by small animals [29]. Essentially, if you are a fish in the ocean and you encounter a shark, it's game over.


Unfortunately, it is not other marine animals that are the sharks’ greatest predators. Instead, it is humans to blame for the steadily declining global shark population. This is mainly due to the highly popular shark fin fishing industry [30]. Consequently, many shark species are now considered endangered, and some are on the verge of extinction [31]. This issue is especially important to resolve as sharks are extremely important to their ecosystems. One of their crucial ecological roles is controlling potentially destructive fish populations [32]. If these fish populations were not predated on, they could take over certain areas and devastate the other local marine species. Sharks further influence the spatial distribution of their prey through intimidation tactics [32]. In conclusion, sharks are both very important to the marine ecosystem and a particularly hardy species. The biological and ecological importance of sharks should be more heavily prioritised when managing the global fishing industry.




References


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