Evolutionary history
Sharks are believed to have first appeared around 400 million years ago – 200 million years before dinosaurs. Putting time into perspective, if life on Earth was equal to one day, sharks have existed for 12 minutes, humans for one second! In the history of our planet, sharks have survived four massive extinctions and have become a strong evolutionary force by impacting the structure of marine communities. The largest predatory fish that ever existed was the Megalodon shark which means big tooth. This massive shark measured up to 18 meters, it’s size was determined by the size of their fossilized teeth – the only part that is preserved. The Megalodon ruled our oceans and preyed on whales and sea turtles. This creature became extinct around 4 million years ago due to climate change, the Earth cooled down and their nursery areas became uninhabitable.
Diversity of Sharks
Currently, more than 500 shark species exist with a great diversity of habitats and sizes. Sharks inhabit all types of ocean, from the cold Arctic sea to the warm tropics. Some sharks can be found in rivers: the bull shark has been found in Mississippi and Amazon rivers. One species of shark, the Ganges shark, is restricted to freshwater habitats of India and Bangladesh. Most sharks inhabit the superficial layer of the ocean; yet, some sharks live in depths up to 3,800 meters. These sharks glow in the perpetual darkness by having the capacity to emit light. Another shark, the Epaulette Shark, is known as a ‘walking shark’. They were given this name due to the fact they use their fins to push their bodies and walk over coral reefs.
Sharks can be as huge as whale sharks measuring up to 18 meters or as tiny as a Dwarf Lanternshark that measures just 20 cm long. Less than 20% of sharks are larger than humans, and only 10 shark species reach 4 meters long.
Life History
Life-history traits of sharks are unique among fish; they resemble mammals life traits. Sharks present K-strategy characteristic of longer size, slow growth, and long-living animals. During their life spans, parents produce fewer offspring but provide more resources. For example, the female of the great white shark reaches sexual maturity at the age of 33. Thresher sharks produce one to three offspring every two years. In the animal kingdom, sharks present extreme life-history traits. The Greenland shark has the longest known lifespan of all vertebrate species. This shark, which lives in the Arctic, could live up to 500 years. They also grow very slowly at a rate of 0.5 or 1 cm per year.
The reproduction strategy of sharks is very versatile. Some sharks lay eggs; the egg case is known as mermaid’s purse. Other sharks, like humans, produce a placenta that nestles the embryos. These are connected to their mother through an umbilical cord. Other sharks present unique behaviours: embryos feed on their sibling – only the strongest will survive, and sharks take it very seriously from an early age. This ancient fish presents one of the most extreme reproductive strategies: the frilled shark has the longest gestation period of any vertebrate, which is at least 3.5 years. Sharks do not show parental care, once the shark is born it becomes independent from parents. Yet, they present one of the highest levels in the animal kingdom of maternal investment since the female invests large amounts of energy in the offspring during pregnancy. The smooth hammerhead shark nurtures up to 50 embryos for 11 months.
A Special Sense
Sharks have a powerful sense of electroreception. They are able to detect small electrical impulses in water, and this is a better conductor than air. Sharks are the most electrically sensitive species known. The sensor that perceives electric fields, known as ampullae of Lorenzini, is located in their head, mainly on the snout. This sense is very useful because animals produce electric fields. Through this sense, sharks can detect prey that is buried under the sand. They can also feel heartbeats – they feel if humans are excited or relaxed. They may also use this sense to navigate through the ocean, by perceiving the Earth’s magnetic field, like humans use of GPS. The shape of the hammerhead enhances this sense. The great white shark travels from Australia to South Africa in a transoceanic migration that covers 10,000 km. In the open ocean, where few reference points exist, sharks locate themselves by using their sense of electroreception. They do not wander in a big open ocean; they know the precise route. They are so perfectly orientated.
Sharks: Predators and Protectors of Our Marine Ecosystems
Sharks have adapted to the marine ecosystems and have become one of the main predators. Some like the Mako shark, one of the fastest fish in the ocean, can swim up to 70 km per hour. Their skeleton made of cartilage is lighter and flexible, and their skin, made of dermal denticles, decreases drag and turbulence, allowing the shark to swim faster and quietly.
Most importantly, sharks help regulate and maintain the balance and stability of our marine ecosystems. They prevent diseases and shape the genetics of populations by eliminating sick, weak, dead and old animals. If these predators were to vastly decline in number, this would cause a trophic cascade; a flood of negative ripples through the entire ecosystem.
In Shark Bay, Australia, the main coastal ecosystem is seagrass meadows. Tiger sharks are the top predators and their prey is sea turtles and dugongs, which feed on seagrass. Tiger sharks can influence the behaviour of their prey. In their presence, turtles and dugongs can change their habitat to safer nutrient-poor seagrass habitats. As a consequence, turtles and dugongs stop foraging on seagrass when Tiger sharks are around. Such discovery has implications on the health of the marine ecosystem.
In Bermuda, the decrease in shark populations caused the extinction of seagrass meadows, since turtles overgrazed the seagrass and caused erosion.
Conclusion
Sharks were one of the first vertebrates to exist – before reptiles, birds, and mammals appeared – becoming the predators of our oceans. Through a million years of evolution, they have tried countless paths, and as a result, they are a very diverse group with different habitats, body sizes, shapes, and reproductive strategies. They are so perfectly adapted to their marine environment. They move elegantly, making their presence powerful. They are also magnificent creatures. So, the next time you think about sharks, remember how ancient, important, and misunderstood these creatures are, and never forget that the ocean is their kingdom.
Sources
Camhi MD, Fordham SV, Fowler SL. Domestic and international management for pelagic sharks. En: Camhi MD, Pikitch EK, Babcock EA, editors. Sharks of the Open Ocean: Biology, Fisheries and Conservation. Oxford, UK: Blackwell Publishing; 2008.
Bonfil et al. 2005. Transoceanic Migration, Spatial Dynamics, and Population Linkages of White Sharks. Science.
Heithaus et al. 2014. Seagrasses in the age of sea turtle conservation and shark overfishing. Front. Mar. Sci. 5.
Heithaus MR, Frid A, Wirsing AJ, Worm B. Predicting ecological consequences of marine top predator declines. Trends in Ecology and Evolution 2008; 23: 202–210.
