Tuesday, August 12, 2014

More Than a Mouth

It's officially Shark Week! Discovery Channel's toothy extravaganza began its 26th year on Sunday. Not everyone is aware that, while Discovery has an educational tint, it is an entertainment channel at its heart. So naturally, in the era of upworthy and facebook, Shark Week has evolved into what I like to call "click bait TV." You can take a look at the week's lineup here and see what I mean. Happily at the bottom of that page, if you read the comments (dangerous waters I know) you'll see people crying out for more educational programming. So to meet the demand, we're going to explore the amazing diversity of sharks!
"Heeeyy yoouuu guuuys" says the whale shark (Rhincodon typus)
Courtesy Adam Brill via Flickr 

Backing up for a second, let's talk about what a shark actually is. From a taxonomic standpoint you could say that there is no such thing as a shark, or alternatively, that skates and rays are highly modified sharks. They're all what we call elasmobranchs, or fish that have skeletons made of cartilage and 5-7 gills. There's a pretty good write up on taxonomy and the nine orders of sharks on The Shark Trust's website if you'd like to explore a little more, or haven't had much experience with classification. Amazingly those nine orders divide down into, depending on who(PDF) you ask, around 450 different species of sharks. With that many animals swimming around you can imagine they don't all look or act the same. 

Familiar Faces

Look at that adorable dorky grin
Courtesy Hermanus Backpackers via Flickr

The sharks we're most familiar with fall into one of three families (a smaller relationship within an order). These are the mackerel sharks (lamnids), requiem sharks (carcharhinids), and hammerhead sharks (sphyrnids). These three families do represent quite a lot of world's sharks, but they're famous for another reason. They tend to be the bitey-est. We can trade "you're more likely to be hurt by          " back and forth forever and it doesn't make much of a difference to people's fears, because fear isn't about logic and statistics. If it was then we would be terrified by another obligate carnivore that harms people daily and even traps them in their homes: domestic cats.(If you don't click any other link, the last one is very worth it. They call 911 on their cat)

Pictured: A merciless killing machine perfectly adapted to the eradication of local fauna, and a shark. 
Courtesy Joe Dunckly via Flickr, and my cat Einstein

The long and short of it is: yes these types of sharks can be dangerous, but they are also beautiful, fascinating, and directly benefit humans. The three families mentioned above have representatives in the the incredibly unforgiving environment of the open ocean. Mackerel sharks in particular spend much of their time over the continental shelves cruising for food. Cruising really is the best word to describe how these animals usually get around. One Great White Shark (Carcharodon carcharias) was tracked travelling in a shockingly straight line from near San Jose, California to the big island of Hawaii in about a month; probably to find a new crop of pinnipeds to eat. He/she averaged about about 4 kilometers an hour that whole trip. That's like lightly jogging from Seattle to Pittsburgh because the grocery store ran out of the brand of cheese you like.

The requiem sharks are notable even among our more well known animals by being some of the most social sharks. Reef shark "feeding frenzies" have been a staple of documentaries for years, so I imagine you won't be surprised when I tell you some sharks hang out in big groups; which by the way is called a shiver. However, Bimini Sharklab in the Bahamas has been doing research that shows juvenile lemon sharks prefer to hang out with others about their own same size and age. They also preferred being with their own kind, choosing other lemon sharks over nurse sharks. Implications of shark racism aside; it's pretty cool that these animals are so social that they'll choose to "roll deep" even when there isn't a survival incentive to do so.

Lastly among our more familiar sharks we have the hammerheads. They're the perfect animal to bridge from the old staples to the weird and wonderful we'll be talking about next. The first question on people's minds whenever they see a hammerhead is of course "What's up with his face?" The weird flattened head is called a cephalofoil (dibs on the band name) and it's a pretty spectacular piece of evolution. Widely placed eyes mean the shark has 360 degree vision in the vertical plane. This means that hammerheads can see everything above and below at all times!

  "I AM looking right at the camera!"
Courtesy Jeroen Elfferich via Flickr

The nostrils of a hammerhead are also placed far to the sides of the head, not unlike our ears, and give these guys stereo smell. More face space also means that the sharks' electrosensors can sweep the bottom, like a metal detector, for prey hiding in the sand.

Weird Uncles

Since the length of this post is starting to get away from me we'll look at just three of the really unusual sharks out there. Let's start with a coastal animal so we're still in our comfort zone and move on to the deep sea from there.

Yes you did just watch a video of a shark walking. That is the fairly tiny epaulet shark (Hemiscyllium occelatum) looking more like a salamander than shark. These little guys (never more than about 3ft long) live in shallow water and even tidepools near Australia and Indonesia. The shark has adapted the walking ability as a means of moving through water too shallow to swim in. Also, like other intertidal organisms, the epaulette shark has to cope with a lack of oxygen while in isolated pools (tiny bubbles in the water escape when the sun warms it.) So to deal with this, the epaulette shark can actually turn off some of its higher brain function to use less oxygen. That's right there are "mindless" killer sharks, they're just under 3ft  long and like to eat worms...intimidating.

Next up we have...

ARKive species - Angular rough shark (Oxynotus centrina)

 Hahahahahaha are you kidding me!? That can't be real. it looks like someone tried to glue a pompadour wig onto a baby shark. Well that my friends is the angular rough shark (Oxynotus centrina). They live fairly deep down and are not very well known because of it. We do know that they are in the dogfish (squaliformes) order and we're pretty sure they hover over the bottom slurping invertebrates out of the muck. That big sail on their back is their first dorsal fin and probably helps them keep from tipping over as they suction their prey. These animals and their cousins also frequently turn up in bottom trawls (big nets that drag along the bottom) so there is concern that we may be heavily impacting these species without knowing anything about their role in their environment.

Our final shark in this survey of diversity takes us both deep under water and back in time.

ARKive image - Frilled shark swimming

The frilled shark (Chlamydoselachus anguineas), sometimes called the eel shark, is a species that probably evolved several million years ago and has changed very little since. In fact, it and its relatives in the cowshark (Hexanchiformes) order, likely represent the transition from truly ancient sharks to the modern representatives. All the cowsharks have 6-7 gill slits with the frilled shark having frilly tufts of gill tissue that show outside the gills; hence the name. Inside the frilled sharks mouth are hundreds of multi-pronged, needle-like teeth which are helpful in holding onto the generally slippery and squishy prey that's found in the deep. Amazingly one study on developing embryos taken from dead sharks suggests they might gestate their young for 3.5 years, twice as long as an elephants!

Well that's it for my first post. I hope you enjoyed learning more about the varied types of sharks in the world. I'll leave you with this cool video of a frilled shark swimming. This one comes courtesy of one of the few shows still worth watching on shark week: Alien Sharks, which covers deep water shark species.


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