Crocodile Shark

Pseudocarcharias kamoharai

Our final Shark of the week is the Crocodile Shark, a small Mackerel Shark  that can be found in tropical  oceans all around the world. They are the smallest members of their Order, which also contains giants like the Great White and Megamouth Sharks.

Crocodile Sharks grow to lengths of up to 3ft, and can be identified by their slender bodies, large eyes, long gill slits and small dorsal fin. Their large eyes help them to hunt at night, and they also posses a very large liver (up to 1/5 their entire body weight) that helps them to maintain a very neutral buoyancy. The species undergoes daily migrations, moving from deep waters during the day to more shallow hunting grounds at night.

Crocodile Sharks are another one of those interesting Ovoviviparous species. And even more intriguing is that they have litters of 4 pups... two in each uterus. This is a trait not especially common in sharks-- most others have only one pup per uterus.

Because of their small size, Crocodile Sharks are not generally considered dangerous to humans. Man-made structures however? That's a while different story! Back in 1985 AT&T laid down the first deep sea fiber optic cable. The cable was shorting out, and expensive repairs were needed to fix it. Turns out that a bunch of Crocodile Sharks were attracted to the new installation and were biting it!

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IUCN Status : Near Threatened
Location : Tropical Oceans Worldwide
Size : Length up to 3.3ft (1m)

Classification : Phylum : Chordata -- Class : Chondrichthyes -- Order : Lamniformes
Family : Pseudocarchariidae -- Genus : Pseudocarcharias-- Species : P. kamoharai

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Greenland Shark

Somniosus microcephalus

Today's Shark is one of my all time favorites-- they are just so unique! Greenland Sharks live farther north than any other Shark species in existence, swimming in the cold waters of Greenland, Iceland, and the North Atlantic. They can also grow to lengths over 20ft, allowing them to rival the Great White in size.

Greenland Sharks are such unusual creatures. While many other Sharks species are quick moving, these icy giants swim very very slowly (only around 1mph!). They also spend a great deal of time not moving at all-- another name for these guys is "Sleeper Shark" since they are so frequently inactive.

Don't let their slowness fool you! Greenland Sharks are still predators! They feed primarily on fish, but are also able to snatch up unsuspecting sea mammals as well. Horse, Polar Bear, and Reindeer remains have all been found in Greenland Shark stomachs, though how much of that was carrion is not quite known.

Greenland Sharks are solitary, but that doesn't mean that they are alone! They have a really bizarre relationship with a tiny crustacean known as Ommatokoita elongata. These copepods feed on the Sharks' eyes. The Sharks have impaired vision, since their corneal tissue is being gobbled up, but they don't use their eyesight all that much anyway. Besides, the partnership isn't completely one-sided. The parasites are bioluminescent, a trait that helps the Sharks to attract food!

The lifespan of the Greenland Shark is still a bit mysterious. Know one really knows how long they can live or when they reach maturity, but we do know that they grow very slow. A tagged Shark was re-captured 16 years after its original identification, and had only grown 8cm in that time-span (from 2.62m to 2.7m).

And before I leave you with a video of one of these guys, one final fact-- Greenland Shark skin is toxic! But that doesn't keep people from eating it. The skin is considered a delicacy in Greenland and Iceland, and must be repeatedly boiled in order to be safe for consumption.

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IUCN Status : Near Threatened
Location : North Atlantic
Size : Length up to 21ft (6.4m)

Classification : Phylum : Chordata -- Class : Chondrichthyes -- Order : Squaliformes
Family : Somniosidae -- Genus : Somniosus -- Species : S. microcephalus

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Cladoselache

Cladoselache species

Today's Shark is the largest one that we have talked about so far, and it is also the oldest. Cladoselache is the name of a genus containing eight different Shark species, all of which lived during the Devonian Period, around 370 million years ago.

Sharks have been around for hundreds of millions of years, and the Cladoselache Sharks are some of the best known of the ancient species. This is because many of their fossils were very well preserved in a formation near Lake Erie. Some of these fossils are so detailed that we know their stomach contents!

These extinct sea creatures are interesting not only because they lived a very, very long time ago, but also because they had some interesting anatomical features. For one, they almost completely lacked scales. The only scales it had were found on the tips of the fins, and around the mouth and eyes. Secondly, these Sharks lacked "claspers"-- the reproductive organs that are found not only in modern sharks, but in many other ancient species as well. Scientists are still unsure how they specifically reproduced. Finally, the Cladoselache Sharks had smooth, blunt teeth. Rather than tear apart their prey, they grasped it and swallowed it whole.

Cladoselache Sharks were probably very fast and agile, based on their body shape, dorsal fins, and large tail fin. They were high speed predators who also needed to be quick in order to avoid becoming prey themselves. Sharks weren't the only large predators swimming the seas-- this was the time of the 30ft Dunkleosteus as well!

There are no close relatives of the Cladoselache Sharks left today. All other genera within the family are extinct, and that family was the only one within the entire order.

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Status : Extinct for around 370 million years
Location : North America
Size : Length up to 6ft (1.8m)

Classification : Phylum : Chordata -- Class : Chondrichthyes -- Order : †Cladoselachiformes
Family : †Cladoselachidae -- Genus : †Cladoselache

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Horn Shark

Heterodontus francisci

Today's Shark is a very interesting looking species-- it has a very blunt head, prominent ridges over its eyes, two tall dorsal fins with spines, and a sandy colored body covered in dark spots. It also happens to be on the smaller side, though not as small as yesterday's Shark. Horn Sharks can reach lengths of around 3ft.

You can find Horn Sharks swimming off of the western coast of North America, in a range that extends from Monterey to Baja. As juveniles they live in deeper sandy flats, and as adults they move closer to shore, swimming near reefs as shallow as 2m!

The adult Horn Sharks have small ranges, and actually return to the same shelter after each night of hunting (they are nocturnal). They feed in small fish and a large variety of benthic invertebrates, including squid, crabs, and sea urchins. Interestingly, Horn Sharks aren't the greatest swimmers, and are quite slow and clumsy. They often use their large pectoral fins to pull themselves along.

Horn Sharks are one of those cool Shark species that shows some sort of care towards their offspring. After mating takes place in December or January, the female will spend 2-3 months laying her eggs two at a time (layings are spaced 11-14 days apart). Each egg measures 4-5in long, is conical in shape and has spiraled flanges around it. The females will take the eggs and wedge them into crevices so that they are protected from predators (other sharks will often just lay their eggs and leave). The eggs take 6-9 months to hatch.

Horns Sharks are not normally dangerous to humans, though bites have happened due to harassment. The Sharks are kept and bred in captivity, and are sometimes hunted for their spines (which are made into jewelry).

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IUCN Status : Data Deficient
Location : Western North America
Size : Length up to 3.3ft (1m)

Classification : Phylum : Chordata -- Class : Chondrichthyes -- Order : Heterodontiformes
Family : Heterodontidae -- Genus : Heterodontus -- Species : H. francisci

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Dwarf Lanternshark

Etmopterus perryi

The 25th Annual Shark Week starts today, so obviously we need to learn about a Shark! Sharks have been very popular on Animal A Day, and we've learned about loads of them in the past (a full list can be found here). But we haven't yet talked about the smallest of all the sharks-- the Dwarf Lanternshark.

Dwarf Lantersharks reach mature lengths of around 7in, and pregnant females can grow a small bit larger, topping out around 8in. They are an ovoviviparous species, and give birth to 2-3 pups at a time.

Not only are these Sharks very small, but they are also quite mysterious. They have only been found in a small section of the Caribbean, off the coat of Colombia and Venezuela. They swim at depths of between 900-1,400ft (274-426m), and are rarely caught or observed.

So little is known about the biology and population size of these tiny sharks that they are listed as "Data Deficient." They have no economic value in the fishing industry and there are no conservation measures being taken.

So unfortunately there isn't a whole plethora of information out there regarding this particular shark, but that's isn't all that surprising considering its small size and tiny range. Stay tuned for a few more sharks during the rest of the week!

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IUCN Status : Data Deficient
Location : Caribbean
Size : Length up to 7in (18cm)

Classification : Phylum : Chordata -- Class : Chondrichthyes -- Order : Squaliformes
Family : Etmopteridae -- Genus : Etmopterus -- Species : E. perryi

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