Top Ten Sharks: Introduction

Possibly the most famous extant (not extinct) shark, the Great White. You can see from the base of the fin this one is tagged. Scientists and marine biologists ‘tag’ sharks so they can get an idea of their territories, their swimming patterns, their feeding grounds and their mating habits. Tagging like this helps us better understand sharks and shark behaviour so we can reduce human/shark conflicts and just find out more cool stuff about sharks. Look at it, it’s so cute! How do you not want to feed it a fat old seal and give it a hug? (Credit: Elias Levy CC-BY-2.0)

Welcome to the land of big predators with even bigger scientific names!

Do you remember our lesson from the cats?

LIFE
DOMAIN
KINGDOM
PHYLUM
CLASS
ORDER
FAMILY
GENUS
SPECIES

The basic order of taxonomic (the way we organise lifeforms) classifications. Well, like cats, sharks are still (Domain) eukaryotic, (Kingdom) animals and (Phylum) chordates, so relative to genetics they’re not too far off of cats – Even though they are miles and millions of years apart! Welcome to the weird world of biology and why any biologist who is racist is an idiot. Remember, even bananas are your brothers and sisters in genome!

Sharks though are;

Class – Chondrichthyes – The cartilaginous fish.
Subclass –  Elasmobranchii
Then, unlike with cats, there is significant divergence in orders with sharks, all of them with long, ridiculous names like Carcharhiniformes, Hexanchiformes and Squatiniformes.

So versus cats we’re dealing with a whole different kettle of fish! Badum-tish!

It’s to be expected, alongside the political views of Jacob Rees-Mogg (wish he’d get eaten by a shark…) sharks are one of the oldest extant things on the planet.

The earliest known ‘sharks’ are from around 420 million years ago, meaning they likely were evolving during the Silurian period (around 445-415 million years ago) and first started forming into what we know as ‘sharks’ today during the Devonian period, between around 420-360 million years ago.

To put this in perspective, you think dinosaurs are old, right? Dinosaurs existed between 245-65 million years ago. This gives dinosaurs a run of around, give or take, 200 million years as the ‘dominant’ form of life on Earth.

Sharks still exist and are over twice as old as dinosaurs!

A simplified diagram showing the evolution of life and how incredibly long a process it has been. Starting with the sun igniting at number 5 and counting down to 0 (present day), Life begins around 4. Sharks do not come into the picture until around 0.4, but dinosaurs arrive around 0.3 and are gone by around 0.2. Sharks…Well they’re still here! As you can see, the evolution of Homo sapiens, humans, and all we have achieved, is so recent as to be seemingly irrelevant in the scale of life, yet our impact has been profound. (Credit: LadyofHats, Public Domain)

When I first started discussing cats one of the things I described as remarkable was their ‘form’. To me this means their shapes, their senses, their minds, and the way they are adapted seemingly perfectly for the job of hunting in their niche. Cats, birds of prey and sharks are three things I can think of with designs evolution stumbled upon by accident that I don’t think a designer could get better.

I admitted even then, even against my beloved felines, sharks take the win. They seem to be the perfect form of ocean predator. Of the main predators of land, sea and air they have been around the longest and, okay, life in the seas got a good couple of hundred million years head start to life on land but the fact is there were sharks around then that if you put them in our oceans today you wouldn’t know they were extinct. They’d look just as good and hunt just as well. Evolution and speciation are inevitable, and extinction does not necessarily mean failure or flaw, merely change and adaptation.

Consider it like a medical doctor. You train and evolve to learn the basics of human medicine but then you can decide to specialise – speciate – Maybe you become a GP, maybe you go into paediatrics, maybe gynaecology, maybe (if you’re very brave or very grim) oncology. You haven’t ‘failed’ as a doctor; you’ve evolved to fit a new role, a new niche. Most of the time speciation and extinction works the same way.  

Sharks have been changing, adapting, for around 420 million years, but their basic shapes, designs and functions are little different.

So, chondrichthyesCartilaginous fish, what does that mean? Well you know how your nose and ears are sort-of hard but they’re not made of bone? Or you know those chewy little joint bits you get in chicken legs or wings if you’re a meat eater – not bone, but a nice satisfying crunch (a lot of people don’t eat it but it’s actually very nutritious). That’s cartilage and the chondrichthyes, the cartilaginous fish, basically have entire skeletons composed of the stuff.

Examples of Chondrichthyes – Cartilaginous fish, from top left – The Great White Shark, again; a Reef Manta Ray; an artists depiction of the extinct species Belantsea montana; and a spotted ratfish, a type of chimaera. The chondrichthyes are a very diverse class of animals. (Credit: Prehistoricplanes CC-BY-SA 4.0)

As a result, because the bodies can be broken down so quickly and easily and don’t fossilise as easily, we don’t have a great deal of knowledge about too many prehistoric sharks. It’s a shame because the ones we do know about are absolutely fascinating and mostly known from jaws and teeth (including the famous Megalodon).

The chondrichthyes does not just include sharks; rays, skates and sawfish make up the subclass of the Elasmobranchii, while chimaeras – also known as ghost sharks – comprise the subclass Holocephali.

For this list I am likely to stick to Elasmobranchs, the holocephali are so weird and wonderful they deserve an entry all of their own.

Elasmobranchs have no swim bladders, the gas-filled organ that helps keep many of the osteichthyes – the bony fish afloat. Instead their buoyancy is mainly owed to an oil-rich liver, and is, unfortunately, one of the main reasons for their overfishing.

There’s a lot of technical terms about their jaws we could talk about but the main things you need to know is they mostly have sharp teeth covered in serrations known as denticles, the teeth keep coming through, regrowing and renewing throughout their lives, and their skin is effectively made of teeth. The same stuff that makes teeth is what makes shark skin rough, dermal denticles – effectively skin teeth.

The Dermal Denticles of a Lemon Shark as seen under a scanning electron microscope. You can see they are effectively overlapping teeth. This is why if you brush a shark one way it feels very slick, almost silky smooth, but if you brush it the other way it can cut or graze you! Their skin is made, basically, of teeth! (Credit: Pascal Deynat/Odontobase CC-BY-SA 3.0)

Interestingly there are some species of shark where the female has a thicker, denser patch of these denticles at the nape of her neck. A grim adaptation and a testament to the often very aggressive act of sex between sharks, that it exists implies female sharks have had to evolve it to stop their necks getting ripped off during mating! Sharks are hardcore.

As far as other characteristics go colouration is a hugely important one. Bottom-dwelling sharks, angelsharks for example, are generally sandy coloured, maybe even with a few protrusions (sticky-out bits) to blend in with seaweeds or sea grasses. They are mostly ambush predators and so disguise themselves to wait for prey to come to them.

A lot of sharks, however, exhibit ventrodorsal countershading. In case you didn’t know the dorsal side is the top (as in dorsal fin) and the ventral side is the bottom. A lot of sharks are a sea-neutral blue-grey on the top, fading into a white at the bottom.

This countershading – also seen in cats, incidentally, is generally held to be an adaptation to improve hunting. A shark approaching prey from above has their white belly blend in with the sunlight piercing through the waters. A shark approaching from beneath is an aquatic shade, blending in with the water below. Whichever the direction of attack, the shark has the perfect camouflage for being one step ahead of prey.

One of the cutest and most beautiful sharks, the Blue Shark, here the photo is taken off of South Africa but they are also seasonal visitors to UK waters (and usually responsible for the ‘Great White Seen off of Cornwall‘ scares!) – They demonstrate countershading perfectly, with their brilliant grey-blue top constrasting with an almost snow-white bottom. (Credit: NOAA Fisheries West Coast Photographer: Steve Woods from a study on how ocean megafauna (including sharks) are at risk from fishing CC-BY-NC-ND 2.0)

What else can we talk about in the intro? It’s not necessarily true that a shark can’t stop moving or it will suffocate. Many sharks actually lay perfectly still in ambush on the sea floor. Even then sharks can suspend themselves in currents to maintain water flow over the gills. Gills are basically exposed lungs – Effectively they just need to keep water passing over those gills, which are rich in capillaries, very fine blood vessels that will take oxygen from the water to get it into the shark’s body. Some sharks also have spiracles – these are holes, special gills, just behind the eyes to supply oxygen directly to the eyes and brain. Shark coffee, basically.

Oh and how could I forget the ampullae of Lorenzini – I know, silly name, incredible thing. Sharks are often considered ‘stupid’, but they have very large brains, seem to have exceptional intelligence and the reason for it is the sheer amount of sensory information they have to process to do their job. Ampullae of Lorenzini are special, jelly-filled, pores in sharks that detect electric fields and currents. A lot is made of how a shark can sniff a drop of blood from X-miles away, but they have this sense to detect smaller movements, every nerve impulse is, in essence an electric current that a shark can detect. Under ideal circumstances – completely still water, no rocks – a physicist’s idea of ideal circumstances – a shark could detect a muscle twitch from miles away. Of course, turbulence of water and other interferences in electromagnetic signal make this an unrealistic scenario but you get the point.

Tryptophobes look away now! These holes are actually the pores, the Ampullae of Lorenzini, that give sharks (in this case a tiger shark) their electromagnetic sense. (Credit: Albert kok CC-BY-SA 3.0)

Sharks are more than just mindless, blood-lusting killing machines, they mostly have exceptional smell, incredible eyesight, an electromagnetic sense, a sense of pressure, a sense of hearing and they even have tastebuds! Sharks can taste!

Quint’s speech in the movie ‘Jaws’, talking about the “lifeless eyes” and how the shark “doesn’t even seem to be livin’” have, I think, coloured people’s perceptions of sharks since the 1970s! They have a reputation as mindless killers, unthinking killers, actually they just sense and appreciate the world in a different way. Very few, if any, sharks actively hunt human prey and of the attacks that do take place most of them are only fatal because a shark’s way of saying “Hello!” and checking if you’re good to eat and not a threat is to have a nibble. Against our oh-so-frail flesh it’s like razorblades through butter. Persecuting sharks for the weakness of the human epidermis is hardly a sensible reaction. Maybe we need to evolve thicker skin?

The truth is sharks are some of the most intelligent, incredible and admirable creatures on this planet, and definitely one of the most remarkable species in our oceans. Their persecution, in revenge for ‘attacks’ on humans, for their liver oils, for their fins, is all a shocking abuse of what is, to me, the cat of the sea, a cute and incredible predator that deserves our care and respect.

Snuggling Lemon Sharks – There is so much more to these maligned and misunderstood predators. Their oceanic alienness I don’t think helps. Humans, for as good at swimming and boating as we are, are not creatures of the sea and innately fear the unknown and unknowable. Sharks are so distant from us that I think it is easy to seem them as stupid, vicious, unthinking killers. Actually there is so much more them, and we underestimate the value of lifeforms, of biodiversity, to our own ecological damnation. (Credit: Willy Volk CC-BY-NC-SA 2.0)

 I hope I can shed some light on these misunderstood predators, and bring to life some of the remarkable characteristics that make them so unique, so special and so worthy of our respect and our efforts at conservation.

So join We Lack Discipline as we desperately try to narrow down a massive, diverse collection of orders, genera and species into our top ten sharks.

Want to dive deeper into some aquatic shark content?

#10 – The massive, magnificent megalodon – a prehistoric giant!
#9 – The beautiful and quick blue shark
#8 – Known for it’s spiral of jagged teeth, Helicoprion – the Buzzsaw shark!
#7 – The frilled shark, a mysterious living fossil with much to tell us about sharks past.
#6 – The Great White, Apex predator, whale scavenger, more intelligent than we thought.
#5 – The Megamouth Shark – One that tells us more about what we don’t know!
#4 – The Graceful Hammerhead family – Much overfished, and is one a veggie?
#3 – The aparently philosophical basking shark! – Happy just existing.
#2 – The longest living vetebrate on earth – The Greenland Shark

Published by Karl Anthony Mercer

An overly curious lovechild of Grumpy of the Seven Dwarfs and the kitsch pen section of Paperchase. Karl is on a mission to expose the seedy underbelly of academia, and thus making it appealing to wrong 'uns.

11 thoughts on “Top Ten Sharks: Introduction

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