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Unlike other shark species that stick to either fresh or saltwater, bull sharks (Carcharhinus leucas) manage to thrive in both. They’ve specialized their ability of osmoregulation, or the ability of an organism to maintain its body. Since all fish, fresh and saltwater alike, have to osmoregulate, bull sharks have figured out how to adjust theirs to all salinity levels. An external and internal environment is separated by a membrane where substances can move. For fish, this is skin. If the internal has more water than the external, and vice versa, water will move from the highest concentration area to the lowest. So marine animals must prevent dehydration, whereas freshwater animals must reserve their salt levels.Bull sharks have managed to balance these processes out. Urea and other salty substances normally stabilize based on the environment, which is controlled by the kidneys. By regulating the amount of salt released from the kidneys, the bull shark can adjust and balance by releasing less salt and more urea based on the water. Since people often spend time in estuaries and rivers where bull sharks lurk, encounters with them are often surprised and unexpected; Especially when they were found 2,000 miles upstream in the Amazon River! Photo credit: wirodive
Placoid scales.. Dermal denticles… Either way, they’re the same scales covering just about all elasmobranchs. I’m sure you’ve heard before rubbing your skin one way on a shark makes it smooth, but the other is rough like sand paper. These babies are the reason why! Resembling scales, they’re basically specialized teeth covering the skin, covered in a hard enamel. Like our teeth, there is an inner core of pulp, made up of connective tissues and other various things, covered by a layer of dentine, and then vitrodentine, working as the enamel in our teeth. Since they’re packed together pretty tight on the skin, and grow backwards, it has a hydrodynamic function for sharks and ensures some amount of safety from predators as well. As the elasmobranch grows, the denticles grow with it.. Unlike scales, they’ll stop growing once they reach a certain size, and will instead increase in density of the dermal denticles.  Photo credit: Sea Moon
Most sharks are cold-blooded, or poikilotherms. But the family Lamnidae have managed to adapt and became homeothermic. This means they can keep a warmer body temperature than the surrounding water, benefiting these sharks in various ways. Swimming powers are rapidly enhanced with an increase in temperature, allowing their muscle tissues to contract at an efficient rate due to chemical reactions. Unlike their poikilotherm relatives, homeothermic sharks can also integrate into colder regions without facing many consequences. The warmth produced by sharks can be credited to both their specialized circulatory systems and possibly even their liver. The liver processes chemicals, which in turn produces prodigious quantities of waste heat. The rete mirabile is a parallel network of arteries and veins that allows lamnids to be more active predators by passing the heat from warm blood in the gills to cold blood from the arteries, retaining heat.  Photo credit
Besides the Ampullae of Lorenzini, sharks have an additional sense that makes them the perfect predators. The lateral line is a canal filled with fluid, lying just under the skin. Most fish have them, but in sharks it’s found in the subcutaneous, or just under the skin. These fine tuned sensors help to detect any potential food sources. Extremely tiny hair cells line the walls of these canals, making them instrumental in sensing vibrations and movements. When a food source makes any motion, it sends out a vibration that makes contact with these tiny hairs, which in turn move and sway in the liquid. Messages then travel to the brain, providing information on the location and nature of the vibrations detected. Not only can the lateral lines detect frequencies as low as 25 hertz, but also have the ability to detect odor plumes. These are 3-dimensional structures that assist not only sharks, but also other bony fish, in finding food and mates. Even with their nostrils fully exposed and functioning, lacking the ability of using lateral lines or making them chemically impaired leads to the shark not detecting any odor plumes. Since moving animals often have odor and make turbulence, sharks will detect both at the same time, which is called eddy chemotaxis. Lateral lines are also to blame for sharks ‘bumping’ into things. It’s a way for them to taste prey without actually tasting it. Photo credit: akay
Requiem sharks (family Carcharhinidae) are possibly one of the best known families of sharks. It contains 52 species of sharks, all matching the unique characteristics that most requiems retain. Just about all of these elasmobranchs are strong swimmers, travelling either alone or in groups, although that applies for just about anything. Their ability to swim is thanks to slender, torpedo shaped bodies and rounded snouts. A perfect example of this is the Galapagos shark (Carcharhinus galapagenisis) because they are built for speed and catching darting fish. Another feature commonly seen in this family of sharks is countershading, or having darker upper bodies than their lower bodies. This makes prey see them as silhouettes rather than full body figures, and helps them in successful hunting. Their diet is varied, consisting of anything ranging from fish including other elasmobranchs, cephalopods, invertebrates, marine mammals, and even sea birds. Considering the fact that most requiems look remarkably alike, identifying each individual is often hard and takes a trained eye. It really doesn’t help much, considering half of the attacks by sharks, which in general are very rare, are caused by requiems.. I can see why, since the infamous tiger shark (Galeocerdo cuvier) belongs to this family. Photo credit: moshi moshi tea time
Sharks are often known for the fight they put up, whether in the waters or on land. They certainly don’t find being near humans extremely enjoyable when caught by fishermen, which is why elasmobranchologists and shark enthusiasts alike have found a helpful way of soothing these sharks. It’s called tonic immobility, a trance-like state caused by the release of a certain chemical stimuli to the brain which triggers a calming sensation over an animal. Some believe this is for mating, but nobody is positively sure why this occurs. Although requiem sharks are often put in this state by turning them upside down, there’s other ways of triggering it. Tiger sharks, for instance, are induced to this state by placing light hands around their snout, around where the eyes would typically line up. A bigger, badder great white goes through tonic immobility like a requiem shark, but they often fall into a deeper trance and appear less responsive. There’s a documentary called “The Whale That Ate Jaws.” It shows how tonic immobility was a weakness for these apex predators and how an orca known as CA2 took advantage of it. Witnessed in the Farallon Islands off the coast of San Francisco, it’s one of my personal favorite locations for great whites. CA2 had struck the great white from depths below, striking it and flipping it over, and then proceeded to hold the shark by the neck. She stayed in this position for 15 minutes, long enough to drown the elasmobranch. Scientists noticed a sudden drop to 0 in the population of great whites, leading them to study more into tonic immobility. They released chemicals and scents from dead sharks while the animals were in tonic immobility, and it was terrifying enough to snap them out of it, causing an automatic instinctive defense of bolting. Photo credit: marshhen
scientificillustration: n34_w1150 by BioDivLibrary on Flickr. The Plagiostomia : Sharks, skates, and raysCambridge, U.S.A. :Printed for the Museum,1913.biodiversitylibrary.org/item/25684
cey-lon: Blue Shark - Azores (by James R.D. Scott)
memuco: Smooth Hammerhead
eduardo-: Sharks, And Why They’re (Literally) So Awesome We all know that Sharks (superorder Selachimorpha) are pretty much the peak of what nature has been able to muster in terms of killing prowess and perfect adaption to one’s environment. We also know that they’re also, at least in some species, amongst the largest non-terrestrial, non-mammalian creatures that’ve ever lived. The Whale Shark, (Rhincodon typus), a species quite regularly approaching 12 metres (40 ft) in length and around 20 metric tonnes (45,000 lb), is the largest extant fish species that’s left in the oceans. But, what has made the Selachimorphs so successful throughout the years? Quite simply, they’ve had at our best estimates, 400 million years to work on it. By comparison, anatomically-modern humans came to be some 200,000 years ago, and behaviourally-modern humans have existed just a quarter of that time (50,000 years). So, right off the bat they’ve been doing it a very long time. Over that time, they have diversified into hundreds of species - around 360 of them – amongst eight different physiological Orders. The kind we’re inclined to be most associative with as sharks are the ones most regularly shown in the media; these are known as the Requiems (family Carcharhinidae). Amongst these we find some of the most adept predators that have existed in any ecosystem, each with its own special skillset that allow them to capture prey in an almost effortless manner. For example, the Spinner Shark, (Carcharhinus brevipinna) lives up to its name almost worryingly well – it literally spins vertically through the water column (and inevitably the air) and captures prey as it goes. What really sets them apart from other animals, and what makes them incredibly suited for their purpose is their sensory abilities. Their ability to physically pick up on the trademark makings of a meal is almost incomprehensibly good to us. Although many species have quite well attuned eyesight for aquatic species (the biological construction of their eyes is similar to ours), where they really come into their own is their sense of smell (which can be as good as one part per million in some species), and even more amazingly, their sensitivity to electromagnetic pulses. Food which is of interest to hunting sharks produce electromagnetic energy by the simple process of respiration and circulation, and these are able to be picked up by the Shark’s Ampullae of Lorenzini. These are jelly-filled organs which line pores around the anterior portion of the body, with the highest concentration around the forward parts of the head. Despite all of this, they are pretty much all heavily in decline. As good as they are at killing other stuff - being an apex predator calls for it - we’re significantly better at the job (if you discount the approximately 60 attacks a year on humans, the majority of which don’t actually prove to be fatal). Shark Fin Soup, general fear, general lack of understanding, ecosystem destruction, harvesting for traditional medicine and liver oil… really, we’re doing our best to wipe these guys out and have been for a veeeeeery long time. It’s kind of sad when you think about it. They’ve been here 2,000 times longer and reached the epitome of what nature has yet mustered in all manifestations, and yet we fin them for soup? Really? They are awesome, we’re just assholes. But not all Selachimorphs are even keen predators! The two largest, the previously mentioned Whale Shark, and the Basking Shark, (Cetorhinus maximus) are both pelagic filter feeders. They feed on planktons. As with Whales, it’s interesting to note that the animals which have by far the greatest physical size within the group are the ones which feed on the smallest creatures. You only wish you could be cool as a Shark. But you probably won’t ever be. Not without a few hundred million years to work on it, anyway. photo: http://www.yourstuffwork.com/2011/10/world-shark-attacks-increase.html
warbyparker: Identity Crisis by Mason Phillips.
Sawfish (Family Pristidae) and Sawsharks (Family Pristiophoridae) are commonly mistaken for each other. It’s pretty easy to do that, since they’re both really unique predators built for a gnarly kill with their intense saws for teeth. They’re both elasmobranchs, making them distantly related. The sawshark is, you guessed it, a shark! The sawfish, on the other hand, is a ray. Sawsharks also have two barbels that stick out in the middle of their saws, resembling whiskers, and happen to work like that of a catfish. They feel along the ocean floor for any potential prey, which is something saw fish can’t do. Also, sawfish have gill slits on their undersides like a typical ray, whereas sawsharks have gill slits on their side like a shark. Makes sense, eh? Saw sharks have alternating teeth sizes, making them look a bit more unkempt, whereas sawfish have a nice polished look with evenly sized teeth all around. This sucks for the tidy sawfish though, since unlike that unorganized sawshark, his teeth don’t grow back. It’s cool though, since they’re bigger than sawsharks. Measuring in from 1.4 m (4.6 ft) to 7 m (23 ft), sawsharks can barely compare at up to 1.7 m (5.6 ft) long. 
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