Sharks- To swim or not to swim

by Matthew Norton

The ocean is full of animals who possess incredible strength and who are capable of feats of endurance that we couldn’t hope to match, not without the help of machines. Examples include the awesome punching power of mantis shrimp, the arduous migrations made by sperm whales and the exhausting patience of a saltwater crocodile as it lies in wait below the water’s surface. Though few creatures can match the sheer power and hunting prowess of the sharks. 

These three species are prime examples of a shark’s hunting prowess. 
Bull sharks Carcharhinus leucas (top left) have awesome biting power that can rival that of the great white shark.  Whitetip reef sharks Triaenodon obesus (top right) can squeeze themselves through tight gaps, ensuring there is no escape for their prey. Common sawsharks Pristiophorus cirratus (bottom) use their saw shaped snouts to dig out prey buried in sand and slash them before consumption.

But even these powerful fish don’t spend all their time eating and making little sharks. Like many animals, they too need their down time and have no interest in burning up their energy reserves without good reason. To that end, sharks generally have a light skeleton, made mostly of cartilage, and a large liver full of oils that are lighter than water. Both of these adaptations make it easier for sharks to stay above the seafloor and out of the deep sea (unless that’s where they belong). Their skin is also covered in tiny little teeth (dermal denticles) which makes the animal more streamlined and able to swim faster.

The dermal denticles of a lemon shark Negaprion brevirostris as viewed under an electron microscope. These miniature teeth often makes the skin of a shark feel smooth if you rub your hand from head to tail, but very rough if you rub the other way.

Some sharks have gone even further in their evolution, fine tuning their biology so that they exert as little effort as possible. Sand tiger sharks (Carcharias taurus) are a great example, enhancing their buoyancy by swallowing air and holding it in their stomach. This trick allows them to hover in the water, barely moving until there is something good to eat, or they feel like stretching their fins a little. 

Sand tiger sharks are also incredibly slow at reproduction. This is due to cannibalism among the embryos until only two survivors are left, one in each uterus.  At least these surviving pups are well nourished when they eventually leave their mother.

Other species, such as the nurse shark (Ginglymostoma cirratum), settle down on the seafloor for extended rest stops. During this time, they use powerful muscles in their mouth to manually pump water through their gills so they can breathe. Should the shark be covered in sand, they also have openings just behind the eyes (called spiracles) through which they can draw in water.

Nurse sharks are nocturnal hunters, spending their nights searching coral reefs and rocky habitats for fish, shellfish and cephalopods (i.e. octopus and squid). Instead of biting into their meals, nurse sharks use powerful muscles in their mouth to suck them in. Straight out of the shell if necessary.
But during the day, many of these prey are awake, alert and tricky to catch. So the nurse sharks don’t even bother trying, instead conserving their energy as they wait for night. A time where their finely tuned senses gives the shark a significant advantage. 

For many sharks, a nap on the ground isn’t an option. They have to keep swimming to keep ramming water through their gills, otherwise they will drown. But these species can still sort of sleep by turning off one side of their brain while the other side keeps the body moving and an eye out for danger and food. 

But are any of these sharks really sleeping? Or are they just in a state of quiet wakefulness? Truth be told, we’ll probably know since the only first hand experience we have of being asleep is as human beings. Assuming that sharks sleep like we do is a flawed approach at best, as is projecting any of our thoughts and experiences onto another species, a concept known as anthropomorphisation. Nonetheless, it can help to make science more widely accessible and encourage empathy with the natural world, so long as we don’t take it too far.

From a human perspective

Whether asleep, or quietly awake, a resting shark is still a shark and they should be approached with caution and respect. Even when docile, nurse sharks have been known to retaliate against divers and snorkelers who invade their personal space. Sand tigers have also occasionally clashed with humans over a tasty fish, despite their normally sluggish behaviour. 

Such close interactions between diver and nurse shark should be avoided unless the former has a justifiable reason for approaching the shark and is taking suitable precautions.

But as large and powerful as sharks are, researchers need to get close to them, sometimes uncomfortably close, to get the data needed for wildlife management and conservation efforts. Such data is also in pretty high demand since a large number of shark species are still listed as ‘Data Deficient’ by the International Union for Conservation of Nature (IUCN). Meaning we don’t have enough information to really know if they are at risk, never mind what they might be at risk from. 

There are non-invasive methods we can use to study the lives of sharks with little to no interference. These include underwater censuses, baited cameras and environmental DNA samples, all of which can be used to estimate how many of each shark species there are in a given area. Carefully planned experiments in the wild can also reveal how sharks would behave in a given situation more accurately compared to testing their responses in an unnatural setting. 

There have been two studies, performed separately, on Caribbean reef sharks Carcharhinus perezi (left) and bull sharks Carcharhinus leucas (right) to determine how the posture and orientation of humans in the water affected their behaviour. Both found that their respective shark species were more weary of the human/s when they adopted certain postures. 

Were these experiments conducted in less natural surroundings, and with more invasive techniques, the animals would have been under more stress and their reactions would less natural. In the case of the bull shark study, the researchers chose not to fix the animals with tags (see further down the article) for this reason.

But by capturing live sharks (hopefully temporarily) we can also measure their size, determine their gender and identify the species with greater accuracy. A quick glance might not always be enough to tell apart two or more similar looking species (see image above).

Having the shark in brief custody also offers the chance for more intimate measurements through blood and tissue samples. Such samples can reveal the concentrations of hormones and heavy metal pollutants in the shark’s body, the latter of which sharks are particularly vulnerable to since they can accumulate such toxins from their prey. Admittedly, we could take more samples from the insides of dead sharks, but non-lethal samples from living specimens can still indicate what toxins they have throughout their entire body. With the added bonus of not having to wait for a corpse to wash up, or relying on being able to get there while sampling is still viable.

Captured sharks can also be fixed with tags, usually on, or around the dorsal fin, so that their movements can be tracked post-release. It’s a method that has been used on sharks for nearly a century (although fish in general have been tagged for much, much longer), although these days sharks are often fixed with satellite tags that ‘ping’ their location every time they surface. As more and more sharks are tagged, the hope is that we can identify their migration routes and seasonal hotspots, particularly for endangered and elusive species, and then protect those areas as needed.

A salmon shark Lamna ditropis with a tag in the base of the dorsal fin.

But for all the information they give us, the experience of being tagged and/or having samples taken from them is never going to be a pleasant one for the sharks. The best we can do is to carry on developing new techniques that minimise the stress and trauma inflicted on these animals. 

Fast capture techniques, such as snagging sharks by the tail, have already been tested as alternatives to more conventional techniques, such as netting and angling. But a better approach, certainly from an animal welfare perspective, would be to avoid taking sharks out of the water at all. Dart tags, attached to the end of a pole, or the bolt of a speargun, are already being used. As are biopsy probes for extracting tissue samples, both underwater and from the side of a boat, before immediately preserving them for later analysis. 

With large sharks such as the great white Carcharodon carcharias (left) and the whale shark Rhincodon typus (right) capture and restraint would be extremely difficult, not to mention stressful for the animals. This makes dart tags and biopsy probes the preferred option for everyone involved.

With all the technical challenges and ethical considerations around shark research, a top priority should be getting the very most of each set of data we get from each shark. An aim that can only be realistically achieved through collaborations between individuals and organisations from different professions and countries. The Cooperative Shark Tagging Program (CSTP) is a great example since it brings together anglers, commercial fisheries and scientists from the National Oceanic and Atmospheric Administration (NOAA). Since its inception in 1962, the program has tagged over 295,000 sharks from 33 species. 

I doubt that such a monumental feat could have been achieved without the thousands of volunteers who have contributed to this program. A true testament to what citizen science can achieve. 

Thanks for reading

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Michelle Hedstrom from Arvada. 2008. [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]. https://commons.wikimedia.org/wiki/File:Carcharhinus_perezi_bahamas2.jpg

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Matthew T Rader, https://matthewtrader.com. 2018. [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]. https://commons.wikimedia.org/wiki/File:Whale_Shark_(Rhincodon_typus)_with_open_mouth_in_La_Paz,_Mexico.jpg

All other images are public domain and do not require attribution

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