Top Ten Sharks #4 – The Hammerheads, Family Sphyrnidae

The Great Hammerhead (Sphyrna mokarran) – the largest species in the family Sphyrnidae, the hammerhead sharks. An alien looking creature, but beautifully unique. The placement of the eyes on the sides of the ‘hammer’ are believed to improve vision (Credit: Albert kok CC-BY-SA 4.0)

Yup, you got that right. I am going to discuss an entire family of sharks. I say that, I have a couple of species in the genus Sphyrna that I want to cover and couldn’t pick one or the other. Since they are so closely related I figured it would be easier to talk about the entire family than it would be to divide the two.

I will not be talking about each species at length because I would get too engrossed in hammerhead facts and you would get too bored, but there are some aspects of some species that warrant note.

Of course the ‘hammerheads’ are named after, believe this or not, their heads, which are, believe this or not, often sort-of hammery in shape. The genus name ‘Sphyrna’ translates to ‘hammer’ in Greek. If I started a specific charity for the Marine Conservation of Hammerheads I would try to call it ‘MC Hammer’ and make the slogan ‘Stop…Hammerhead!’.

There is, as far as I am aware, no extant shark genus or species called the ‘nail shark’ but given the senses of humour biologists tend to have (we’ve got genes named after Sonic the Hedgehog, and a protein named ‘pikachurin’ after the Pokémon) I wouldn’t be surprised if one happens in the future.

The proper name for the hammer is the cephalofoil, basically ‘headwing’.

A small group of hammerheads. Hammerhead sharks are known to form large groups, named ‘schools’ during the day. Usually the smaller, younger sharks will swim nearer the surface moving down the water column getting older and larger. Why they do this is unknown, although it is believed to help them hunt during the day. At nights they often travel in smaller groups, or even alone. (Credit: baechi via Pixabay)

I suppose that’s the first question people have of the hammerheads, isn’t it? “What the fuck is that head all about?” Well, sorry to burst your biological confidence bubble but we don’t truly know! Studies and eye exams (which is better? one or two?…one?…or…two?…one or two? – but with a shark!) Suggest the unique shape and having their eyes so wide apart on the hammer improves their binocular vision and depth perception. This would obviously have sensory benefits in terms of hunting effectiveness.

The eyes in most hammerhead species are placed at the terminals of the ‘hammer’, the cephalofoil, giving them exceptional binocular vision but also a rotational vision close to 360°! Effectively they might not be able to see what’s directly in front of their nose but they’ll have a good idea from being able to scan just about everything that isn’t! It’s an incredible system that makes sense for a mostly pelagic fish that does its hunting where the sun shines. Their sight is an important sense.

Frankly, whilst some people have posited it helps with hunting and prey manipulation turning your own face into a radar dish is more likely to be sensory, especially knowing shark’s sensitive vision, smell and the high concentration of ampullae of lorenzini in the head area, it makes more sense for this to be a sensory adaptation.

Most Sphyrna species have a mouth full of sharp pin like teeth. Do you remember what that means? They likely feed on smaller, fishy prey, though they are known to eat crustaceans as well. The particular favourite appears to be rays, which could be another explanation for their unique head shape. Spreading their ampullae of lorenzini over a large area, creating a metal-detector-like surface for finding these often disguised, bottom-dwellers on the sea-floor.

The scalloped hammerhead (Sphyrna lewini) from beneath. It looks like a female (no noticable claspers by the pelvic fins) showing its mouth full of small, thin, fish-catching teeth. What a beautiful and graceful creature. (Credit: Kris Mikael Krister CC-BY-3.0)

Hammerheads are one of the most overfished sharks in our seas, placing many of their, especially larger, species on the IUCN Red List – It’s like the sex offender’s register, or the registry of members of the British National Party, you don’t want to be on this list! Effectively they are endangered, two of the larger species, the scalloped hammerhead and the Great hammerhead, critically so.

They are mostly fished for their fins, to be cooked and used in Asian cuisine. The most remarkable aspect of this is that, despite the shark being a sizeable creature of high-protein muscle, shark-fin fishers will often cut off the fins at sea and discard the rest of the shark. A process known as ‘finning’, and one that is lethal to the shark and an abhorrently wasteful practice.

Of the hammerhead species the Great hammerhead is the largest. Average total lengths are around 4.5m but they can grow up to 6m in length – BIG SHARK SIZE! Most hammerhead species are found in shallower, pelagic waters around coasts and continental shelves. This puts them in prime human-interaction territory and is one of the reasons they are so overfished. The other reason is they have a habit of schooling. That’s not to say they send their kids out to teach them, they gather in large groups, or ‘schools’ – especially the scalloped hammerhead which is particularly vulnerable to fishing due to this behaviour. It’s a lot easier to catch sharks all hanging around in a big group than ones swimming about individually.

But, we’ve talked about other sharks as by-catch before, the stuff that gets caught in nets and killed accidentally. Many of those species have seen significant decline in their numbers just by those accidents. What difference does it make when the sharks are being actively fished? In parts of their Atlantic habitats populations of scalloped hammerheads have declined 95% in the last 30 years.

95-fucking-percent!

Let me put that in some sort of perspective for you. Rounding up the estimated human population of the Earth is around 8 billion – That’s 8,000,000,000.
A 95% reduction is 400 million – or       400,000,000.
That’s the scale of population decimation we’re talking here. I’ve deliberately spaced it so you can see the difference. 95% is ridiculous.

Because I think numbers are not as easily processed by most people’s brains as much as images are (humans are a very visual species) if you run the slider all the way to the right you will see 100 silhouettes of hammerhead sharks. Move it all the way to the left and you will see a silhouette of 5 sharks. A 95% reduction in numbers. It’s stark, harrowing, sickening and sharks need our love and protection. (Original image credit: Clker-Free-Vector-Images via pixabay – graphic by me)

I think people suspect I’m just an autistic misanthropist who hates all people and loves to bash them at every opportunity. I don’t hate people. Homo sapiens has the potential to be the greatest species ever known by a significant margin and by a whole load of metrics. I hate when we don’t achieve that potential, when we break bad records. I hate that we have the intelligence to know and understand the damage we are causing our natural world but seemingly lack the shits to give about stopping it. There’s an easy answer to this problem – STOP EATING SHARK FINS!

Do you know what I’ve heard? It doesn’t even taste great. It’s just ‘considered’ a delicacy. STOP EATING SHARK FINS!

I’ve written an article on ‘Wicked Problems’ – I know they exist. This isn’t one. STOP EATING SHARK FINS! It’s that easy of a solution “Oh but there’s a whole cultural tradition of…” blah blah blah! Nah, that’s not a wicked problem that’s an arbitrary barrier. It kills a whole shark, for a piece of gristle, that doesn’t taste nice…STOP EATING SHARK FINS!

Someone wants to wrestle a whole shark, rod and line, take it in, use every part of that shark feed their family, feed their village for a week, I’m not going to stop a human doing as humans do.

This isn’t subsistance. This is industrial slaughter of a beautiful shark for profit off of one specific part (sometimes two, some fishing operations also take the oil-rich liver) of the animal that allegedly doesn’t even taste nice.

STOP EATING SHARK FINS!

It’s despicable, it has to stop. It’s the food equivalent of fur-farming and it has to stop.

A beautiful shot showing a huge school of hammerheads off Cocos Island. I’m no diver, but I’d learn to see things like this! (Credit: AndreaPivari via Pixabay)

So overfishing of the scalloped hammerhead was one of the specific hammerhead issues I wanted to talk about. By the way, the scalloped hammerheads are the ones with multiple bumps on the front of their noggin. The ‘scalloping’ of their name, like scalloped edge shears or something like that.

I’ve actually seen them! Of, I think, all of the sharks on this list they are the only ones I have seen when they were exhibited at the National SeaLife Centre in Birmingham, a simultaneously wonderful and sad occasion.

It was wonderful because getting to see these predators up close is spectacular. Nothing can quite prepare you for the unmatched majesty of a shark swimming in water. No land animal has the capability to move with such grace. There may be some birds that make flying look dull, but if they come in to land you realise gravity is cruel! But a shark swimming is something to behold. It’s like they are static and, by sheer force of will, they move the water around them.

Yes, it certainly is! But look how smooth and graceful it moves in the water. As if the water is not there. The movement of many fish is incredible but sharks have a mastery of aquadynamics matched only by the likes of marlin and tuna, they are amazing swimmers. (Credit: 60 Second Docs via Giphy)

It’s sad because, active, high metabolism, pelagic shark species do terribly in captivity. The two hammerheads that I saw died in captivity. No effort or attempt was made to move them to a larger enclosure or re-wild them, they were left to steadily decline in health, bump their heads against things, get scratched, get infected and then die. They existed solely as a tourist trap, one that I unfortunately fell into.

This is not an isolated thing – This is pretty much the pattern for most large, active, migratory, pelagic shark species in captivity. It’s why you don’t see them in captivity. They die.

Sometimes sharks are rescued from perilous situations, baby sharks have been captured, they get put in captivity for a short period (usually under a year) and then once everyone’s happy with them they get put back in the sea. If you know of any zoos, aquariums or wildlife centres that do this, then by all means support them.

However, most parks, certainly SeaLife here in the UK, did not operate under that same ethical practice. It breaks my heart.

But that’s enough sadness and human bashing, let’s talk about the other specific member of the Sphyrna genus I wanted to talk about, the bonnethead shark, Sphyrna tiburo.

A bonnethead shark (Sphyrna tiburo) unlike their larger cousins, smaller sharks like this bonnethead can often do reasonably well in captivity. As a result, you are likely to see these in aquariums. (Credit: D Ross Robertson Public Domain)

It’s a tiddler, definitely the smallest shark we are going to discuss in anything like detail although sharks can get smaller (the smallest currently known is the dwarf lanternshark – estimated size is between 10-20cm, 5-8 inches. They also have bioluminescence allowing me to use the phrase glow-in-the-dark-shark!). Maximum size of the bonnethead is an estimated 1.5m, around 5 feet, but on average they’re about 1m (3 feet).

You would think from their smaller head (they’re sometimes called spadehead or shovelhead sharks) that they would represent an ancestral species – with the head having grown over generations but analysis of mitochondrial DNA (we’ve talked about this before, the mitochondria is a little thing inside your cells, called an organelle, that deals with energy generation mostly, but it has its own unique genetic code – meaning you actually have two genomes) reveals that ancestral hammerheads probably had pretty big cephalofoils and so the bonnethead, with its smaller head, is likely the younger species.

Weirdly they’re pretty much just like any other hammerhead except for that head weirdness and one other thing.

They eat seagrasses. Stomach contents of anywhere between 10% to a whopping 62% of caught, wild specimens could be seagrass.

Another seemingly captive bonnethead, the smallest of the Sphyrnidae family but, as we’re talking about, not the least significant by any means! Their ability to sustain themselves through an omnivorous diet is not just unique to their family, but all sharks. (Credit: Yinan Chen Public Domain)

In fact, not only go they eat seagrasses, but studies demonstrate they can live on a largely seagrass diet, digest the seagrass with around a 50% efficiency (so taking about half the nutrients for it) and in lab conditions, bonnetheads fed a high-seagrass diet all gained weight at an expected rate and seemed healthy. They have all the necessary enzymes and gut microbiome (the bacteria in your gut) to obtain nutrients from seagrass.

For a long time it was presumed to be a digestive aid, a protective aid to stop the stomach getting damaged by crustacean shells, effectively people wanted to come up with every excuse for this animal not to be an omnivore but – no. All evidence seems to indicate the bonnethead obtains nutritional value from digesting seagrass.

It’s an omnivorous shark.

It is the only currently known omnivorous shark in the world.

BABY SHARK – DODO DOODOO DODO! God damn that song to hell. Here is a Juvenile bonnethead caught in the surf off of Florida (and then hopefully returned) I know I haven’t really been doing kitten-tax for the sharks because baby sharks mostly just look like tiny versions of adult sharks! (Credit: Thekohser CC-BY-SA 3.0)

Just when you thought we’d tipped the barrel, emptied it and scraped it of every last bit of diversity we could possibly get out, there’s a little shark at the bottom eating grass!

I think the bonnethead, Sphyrna tiburo, demonstrates something about sharks – about life in general – that we take for granted. In fact, I’m sure I can find a gif so I am going to let fictional mathematician and chaos theoretician Dr. Ian Malcolm take it on this one;

There’s never an end to it. Life finds a way. It is ever-changing, ever-adapting and ever surprising. We looked at a whole family of sharks today and found a variety of lifestyles among them, despite their common similarities. This is life, and a wonderful thing it is, too.

Want to dive deeper into some aquatic shark content?
Our Introduction will give you the basics of shark biology, ecology and natural history.
#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!

#3 – The aparently philosophical basking shark! – Happy just existing.
#2 – The longest living vetebrate on earth – The Greenland Shark

Or come back soon as we begin our top 3!

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.

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