Although shark movies like Jaws and Sharknado 1, 2, 3 (the list goes on…) may portray sharks in an unrealistically menacing light, they are correct in that sharks are fast. Really fast.
But the sharks (known scientifically as elasmobranches, part of the cartilaginous fish group – which means their skeletons are made of tough, flexible tissue rather than bone) are not the only fishes on the marine Formula 1 track. Drawing up alongside them we find bony fishes like the tunas, including the marlin, the swordfish and the blue-fin tuna that we often find in supermarkets (despite their stocks crashing to dangerously unsustainable levels).
Just to give you an idea of their speed, a marlin can swim at 80 kph (50 mph). This might sound like the speed your slow-coach mum, dad, grandad or cautious friend drives at, but when you consider water is about 784 times denser than air, this turns out to be an absolutely phenomenal speed.
So I guess a good question would be what’s their secret? Some secret ‘fuel’ from their diet? A streamlined body shape and powerful, ‘engine-like’ muscles? These answers aren’t too far from the truth, but to fully answer this question we have to delve deeper into their physiology.
In fact, we have to delve into their blood. That’s right, the underlying cause of their great speed was confirmed by scientists (including Yuuki Watanabe of the National Institute of Polar Research, Japan) to be because of their relatively warm blood. Using comparisons between previously published data and new data (collected using tiny sensors placed on the dorsal fins of different fish) based on size, body temperature and their phylogenetic relatedness, the scientists found that these faster fish had significantly higher body temperatures than slower fish. 
Scientists have long known that some fish have the ability to warm their blood and body temperature to a certain extent. This has led to some confusion on how to classify these animals based on their thermoregulation.
Although they are called ectotherms (animals who obtain heat from their environment and thus their temperature usually matches that of the environment), they can also vary their body temperature. This has also earned them the status as a poikilotherm (where animals are said to have variable body temperatures) but this term is being phased out of use due to its vague and unhelpful description of many organisms.
Nevertheless, if sharks and tunas are able to warm their blood, then surely they are endotherms (animals that obtain heat from their internal metabolism) too? Well not quite.
This unique and fascinating exception can be explained by the mechanism of regional endothermy, whereby an animal is able to internally heat certain regions of their body. This allows them to maintain specific regions of their body at far higher temperatures than their external environment.
These regionally endothermic fish take advantage of the ‘waste’ metabolic heat released from their muscles to warm their blood and swimming muscle regions. This increases the rate of enzymatic reactions, gas exchange and transport in their bodies which in turn allows them to swim at far faster speeds than many comparable fish that are regular ectotherms.
They also have a neat way to prevent heat escaping from these regions called a counter-current blood flow, whereby arteries entering these regions interdigitate with veins (like locking your hands together) so that most of the heat leaving via veins is returned back via the arteries. Blue-fin tuna muscles are a good example of the effectiveness of this mechanism, staying relatively constant around 28-32°C across a range of different seawater temperatures – they can be up to 10°C greater than the sea around them. However, Salmon sharks are the masters of regional endothermy, managing to heat their muscles up to an extraordinary 20°C above sea temperatures.
Mechanisms aside, the end result is that, on average, it has been calculated that these kinds of fish swim 2.5 times faster than ‘regular’ fish. And that’s just their cruising speed…
This really is the Top Gear, marine-equivalent of Jeremy Clarkson in a Lamborghini gliding past James May on a moped. If you’re James May in this you’re going to get eaten… (Or punched either way…)
Not only can these sharks and fishes swim faster than other fish, they can also migrate further each year.
What is even more interesting is that this is what we call a case of convergent evolution – natural selection has taken place independently between these quite genetically distinct groups of fish. Fascinatingly, even very closely related fish do not share the ability to be regional endotherms like their counterparts. Indeed, as Yannis Papastamatiou of the University of the St Andrews explains, these sharks and bony fish split way back down the evolutionary tree about 450 million years ago and so this adaptation to their environment has evolved separately in both because of their similar roles in the marine ecosystem. Both sharks and tunas hunt and have similar streamlined or torpedo-shaped body plans, as well as the fact many migrate or travel long distances to hunt and breed.
Interestingly, the evolution of this regional endothermy can also be linked to how these fish breathe. In certain highly active fish, a mechanism known as ram ventilation is used to save energy, instead of pumping water manually over their gills (buccal pumping), they utilise the force of water acting against them as they swim to pass water over their gills for them to respire. Being able to swim fast enough for this to be possible is therefore very important – indeed some sharks like the Salmon shark are obligate ram ventilators that have to swim constantly in order to breathe. Some sharks also switch between buccal pumping and ram ventilation, depending on how active they are at the time.
Overall sharks and tunas give us a fascinating example of how convergent evolution has led to such amazingly well-adapted animals that truly are the apex, predatory rulers of marine ecosystems.
So if you’re an ectothermic sardine or sprat swimming out there in the ocean… watch out! Those regional endotherms are coming for you…and they’re coming fast!
Image 1 – Credit: Michael Patrick O’Neill/Alamy, 2011
Image 2 – Credit: DOUG PERRINE / HOTSPOT MEDIA
Image 3 (/Feature Image) – Credit: Zina Deretsky, National Science Foundation
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