The Scientific Side Of Sharks

Sharks aren't always dangerous. Encompassing a group of over 500 different species, sharks are an incredibly diverse group of fish and belong to the Class Chondrichthyes. While 143 of these are currently listed as vulnerable to critically endangered by the IUCN - mostly attributed to overfishing - most are absolutely no threat to humans. They are opportunistic feeders, with the majority hunting for small fish and invertebrates in saltwater environments like the sea. In fact, the biggest reason for why we occasionally see shark attacks in Florida, Hawaii, Western Australia and other shark-ridden coastlines is their wariness of humans.

According to neurobiologists at Macquarie University, aggression towards humans in sharks is driven mainly by two factors: how agitated they are, varying with factors such as sea temperature and hunger; and that humans have an uncanny likeness to seals. Or at least from a shark's perspective. All sharks are widely known to be colourblind, for one, with poor eyesight in accompaniment. Add the murkiness that comes from trying to see surface objects from five metres underwater, and suddenly it makes sense that the silhouttes of seals, dolphins and humans would look practically identical to them. Bear in mind that the biggest number of shark attacks have come from great whites (Carcharodon carcharias), which are famous for changing their diet as they grow up to begin snacking on seals.

Before recognising a human as prey, a typical great white shark will usually make a cursory inspection to find out what you are, exactly. They do this by nibbling on you. Unfortunately for the surfer at the receiving end, however, the nibble is coming from the powerful jaws of a great white shark and often results in something like a shorn-off limb. The ensuing panic can either scare the shark off, or irritate them enough to fight you for their territory. (Poor shark; you stressed it out!) Either way, it is probably better for humans and wild sharks to stay away from each other. Alas, recent reports have shown the increasingly dire impact that global warming has on sharks and their habitats. As the oceans warm, fish and prey either die or migrate away, forcing our cartilaginous friends to move themselves. Sometimes, this can cause them to enter human-populated shores. Alongside the human population growth, researchers speculate that global warming is one of the reasons that the world saw a rise in shark attacks in 2021, after the pandemic lockdowns had brought them to a trough.

Including great whites, there are only 13 species of sharks that are confirmed to have bitten people over ten times in history. Some renown ones are the bull shark (Carcharhinus leucas), the tiger shark (Galeocerdo cuvier), and the blacktip shark (Carcharhinus limbatus). Many others haven't attacked humans even once. Since their group is so large, sharks can also differ from being 20 centimetres long (like the dwarf lanternshark) up to the 18-metre whale sharks in the Atlantic. Further traits differing between them include numbers of teeth; growth and maturation rates; reproduction methods and offspring quantities; and of course their diet. But what exactly allows this variation?

Shark Depths

One interesting study on shark variation was recently conducted by a team of researchers at the University of Florida. It involved the collective analysis of all the shark species in the Northeastern Pacific (NEP), a pelagic region (i.e., belonging to the open sea) characterised by a multitude of ocean currents, but principally by the unswerving California Current and the rotating Central Pacific Gyre to the west of it. Alongside a number of oceanic seamounts that allow fish of different underwater depths to mix together, for example by the Hawaiian Archipelago, these phenomena make up the perfect environment to hold as many shark species of different habitats in the same space. Consequently, the team decided to sample the NEP for its biodiversity to see how different macroecological gradients were related to the kinds of sharks present. These included latitudinal, thermal, carbon source and bathymetric (underwater depth) gradients.

Previously, shark biologists had widely thought that thermal and latitudinal changes in the ocean were the greatest factors behind shark variation. Theoretically, were this the case, sharks in the equator should vary drastically from those in the North Atlantic. While this is not necessarily wrong - you just have to see the differences between equatorial whale sharks and Greenland sharks to believe it true - the team at the University of Florida obtained some challenging results. In particular, they found that groups of sharks sharing similar functional traits were separated not so much by their latitude in the ocean, but by how far they were underwater. (Note that deeper waters typically have colder temperatures, so the thermal gradient hypothesis could very well play into this.) In other words, the more related the sharks were, the more often they were grouped at the same bathymetric strata in the ocean.

Some major features shared by closely related sharks include their reproductive method, growth rates, diet, and dental morphology. In the aforementioned study, for example, one functional group was made of the following species: the basking shark (Cetorhinus maximus), the goblin shark (Mitsukurina owstoni) and the megamouth (Megachasma pelagios). I like to call them the lubbers. All three consume zooplankton, tiny or immature animals drifting around the ocean waters, including small shrimp or insects - though goblin sharks also eat larger teleost fish, crabs, squids and even a few octopuses. Like most sharks, they are ovoviviparous, meaning that their eggs hatch inside the mother to give birth to live pups. Compared to other sharks, however, the lubbers tend to be born pretty darn big. As oophagous animals, their developing embryos feed on the surrounding eggs while still in their mother's uterus. In this way, the embryos basically consume their potential siblings. Although admittedly very grotesque, this does allow the pups to measure up to 2 metres in length at birth! (Still gruesome, though.)

Another shared trait in the lubbers is their teeth. More specifically, because the number of teeth varies in practically all shark species, they share the way their teeth look. Both megamouths and basking sharks have small, hooked teeth (up to 1,500 teeth in the latter!) to prevent prey from escaping once inside their mouth. Comparatively, goblin sharks diverge somewhat, with only around 100 teeth that are a bit longer and more fang-like relative to their size. They are still hooked, though - because, really, who would want their food running away? Now, I should note that the lubbers differ substantially in size. Goblin sharks typically grow up to 4 metres in length, for instance, while basking sharks can sometimes measure double that. Nevertheless, their similarities mean that they often appear around the same strata underwater. Considered deepwater animals, these sharks are usually observed between 200-2,000 metres beneath surface level, in the meso- and bathypelagic zones.

When we look at them genetically, of course, all sharks are pretty close to each other. In fact, amongst marine creatures, sharks display some of the most conserved genomes and least species richness (the number of different species in an ecological community) that we know of. Nowadays, some marine biologists even fear that their dwindling populations are starting to inbreed, which could decrease their genetic diversity over time and potentially up the frequency of heritable diseases in sharks. This kind of evolution is referred to as genetic drift, and it occurs mostly in small populations. Given their long gestation periods and slow growth rates, shark populations can take years to recover from human threats, making them all the more susceptible to genetic drift. Therefore, according to some marine biologists, it is vital that fisheries and shark conservation centres genetically sequence their sharks to try to promote further variation between them.

Fortunately, evolution did not leave sharks alone. If we zoom out a bit, so to speak, you'll find that the chondrichthyans do not just consist of sharks. They are split up into two subclasses: the Holocephali (made up entirely of fish in the Order Chimaeriformes, such as the spookfish) and the Elasmobranchii. Comprised of sharks, rays and skates, the elasmobranchs have partitioned quite a following in the scientific community. Involved in ecological studies and fields like developmental biology, there is much we can discover from studying them - not the least because of their fascinatingly unique anatomy.

Of Fins And Cartilage

In contrast to other fish, most elasmobranchs are covered in tough skin and an outer layer of tooth-like scales known as dermal denticles. These are arranged and stacked to be sharp in one direction and rounded on the other, so that, if you were to stroke a shark for some reason, you would either feel a very smooth sensation or be left with a very scraped hand. Researchers believe that oral teeth in elasmobranchs may have even evolved from their denticles (or perhaps the other way around).

The denticles are not the most curious part of a shark's body, however. I mentioned this briefly at the start of this post, but sharks and other chondrichtyans have cartilaginous endoskeletons. Although their embryonic growth involves a skeletal element typical in vertebrates - the notochord, which may be thought of as a precursor to the spine - elasmobranchs do not develop the rigid bones common in vertebrates, but instead obtain a skeleton made exclusively of cartilage. Being strong yet light and flexible, cartilage is composed of fibrous biomolecules like collagen and polysaccharides hidden under a sheet of calcified chips, and it is what gives sharks the ability to move swiftly in water without expending much energy on it. Cartilage also lets them float more easily, acting as part of why they can grow larger than other fish (hence the enormity of whale sharks) - all of which is crucial for sharks to catch prey.

Importantly, the cartilaginous skeleton normally starts growing from the aforementioned hexagonal chips, called tesserae. Using the round stingray Urobatus halleri as a model organism, past research has further demonstrated an intricacy behind tesseral anatomy. We now know that the joints between tesserae form convoluted surfaces of attachment fibres, the articulations of which are supported by wheel-like spokes that radiate from tesseral centres and lengthen with the organism's maturation. How this helps shape the developing cartilage underneath is still being studied, but such findings are still of much use in comprehending embryonic development and tissue regeneration. (One day, perhaps we could even build our own cartilaginous structures with 3D bioprinting?)

Inside, elasmobranchs sport five to seven gills, each with hundreds of lamellae placed on several gill filaments to increase their surface area. As the lamellae contain capillaries that have blood flowing in one direction, any water flowing into the animal's mouth moves in the opposite direction to the blood. In turn, the resulting concentration gradient between the two causes the water to exchange dissolved oxygen with the gases in deoxygenated blood. The 'used-up' water then flows back out of the fish through its gill openings, which are the famous slits synonymous with sharks and rays (like the ones from movie posters!). Because of this, most pelagic sharks are pretty much obligate swimmers; if they don't move forward and take in water, they suffocate. To avoid tragedy, elasmobranchs use their fins to propel them forward (for example with their caudal fin at the back) and prevent them from yawing/rotating without meaning to.

All in all, I find that sharks and their cousins are not so scary after all. In fact, if you look at them at just the right angle, you might even think they're cute!

References

• Tanaka, K. R., et al (2021). North Pacific warming shifts the juvenile range of a marine apex predator. Scientific Reports 11:3373. https://doi.org/10.1038/s41598-021-82424-9
• Chapman, B. K. & McPhee, D. (2016). Global shark attack hotspots: Identifying underlying factors behind increased unprovoked shark bite incidence. Ocean & Coastal Management 133:72-84. Retrieved from https://doi.org/10.1016/j.ocecoaman.2016.09.010
• Siders, Z. A. et al (2022). Functional and phylogenetic diversity of sharks in the Northeastern Pacific. Journal of Biogeography. Retrieved from https://doi.org/10.1111/jbi.14383
• Domingues, R. R., Hilsdorf, A. W. S. & Gadig, O. B. F. (2018). The importance of considering genetic diversity in shark and ray conservation policies. Conservation Genetics 19:501–525. Retrieved from https://doi.org/10.1007/s10592-017-1038-3
• Seidel, R., Lyons, K., Blumer, M., Zaslansky, P., Fratzl, P., Weaver, J. C., & Dean, M. N. (2016). Ultrastructural and developmental features of the tessellated endoskeleton of elasmobranchs (sharks and rays). Journal of Anatomy 229(5):681–702. Retrieved from https://doi.org/10.1111/joa.12508