The Development of Great White Shark Embryos

Check out this fascinating article on the development of white shark embryos!
http://bio.biologists.org/content/5/9/1211

(Sato, Keiichi et al. "How Great White Sharks Nourish Their Embryos to a Large Size: Evidence of Lipid Histotrophy in Lamnoid Shark Reproduction." Biology Open 5.9 (2016): 1211-215. Web. 22 Sept. 2016. <http://bio.biologists.org/content/5/9/1211.article-info>.)

For years, scientists have wondered about the reproductive system of Carcharodon carcharias, the white shark. Matrotrophy (maternal care when in the womb) is common among sharks, and takes many forms—yolks, histotrophy (where the mother releases a “milk” into the uterus), and oophagy (where the embryos will prey upon other eggs/embryos in the uterus). The relatives of the white shark, such as the sand tiger shark’s (Carcharias taurus) reproductive phases are well known. However, little is known on what forms of matrotrophy white sharks participate in. When a pregnant white shark was caught in the waters off Okinawa, Japan, scientists had the opportunity to study the shark’s reproductive tract.

There were three embryos found in both uteruses, and they were all at the same stage of development. Of the six embryos, none of them still had their external yolk sac, but there was still evidence that the embryo had yolks at one point. In both uteruses, there was a viscous, yellow fluid that contained mostly lipids. This fluid was similar to that of the “uterine milk” some rays produce and is a form of histotrophic nutrition for the embryos. The young sharks all already had small but functional teeth. These teeth were likely used to eat the nutritional eggs released by the mother. Nutritional eggs are undeveloped eggs that are in the uterus as a nutrition source for the embryos. This is a form of oophagy. However, most of the eggs hadn’t been eaten yet, suggesting that the embryos hadn’t entered the oophagy phase of their development.

The in utero nutritional sources of white shark embryos is now better understood, due all to one individual pregnant shark. The embryos first rely on their yolks for food. Then, as the yolks run out, the mother releases a lipid-based “milk” into the uterus that provides nutrients to the young sharks. Finally, the embryos use their already sharp teeth to eat the nutritional eggs in the uterus. This is a complex process that is, as expected, similar to that of the white shark’s relatives.

Oceanic Whitetip Shark

an oceanic whitetip in the Bahamas

The Oceanic Whitetip Shark
           Carcharhinus longimanus


The oceanic whitetip shark is a majestic shark that inhabits the open oceans of the world. One of their claims to fame is the tragedy of the USS Indianapolis. When the ship was sunk on a top-secret mission, less than of a third of the men survived and many deaths were attributed to oceanic whitetip attacks. However, most of the men died from exposure to the elements rather from shark bites.



Identification:
The oceanic whitetip is a grey to brown shark, known for their long, wide pectoral fins with mottled white tips. They have wide, rounded dorsal fins that are also white tipped. Oceanic whitetips have blunt, rounded snouts and small eyes.

Classification:
Kingdom : Animalia
Phylum : Chordata
Subphylum : Vertebrata
Class : Chondrichthyes
Subclass :  Elasmobranchii
Superorder : Selachimorpha
Order : Carcharhiniformes
Family : Carcharhinidae
Genus : Carcharhinus
Species : longimanus


Similar Species and Other Family Members:
One species similar to the oceanic whitetip is the whitetip reef shark - although it is similar in name and coloration only. The whitetip reef shark (Triaenodon obesus) is a much smaller shark that lives on coral reefs and has a slender body with a very short snout. They are typically dark brown in color with bright white tipped dorsal and caudal fins.

Triaenodon obesus

Reproduction:
The oceanic whitetip is a viviparous shark, meaning it gives placental live birth, just like humans. They tend to have a small amount of pups, around 5 or so per brood.


Size:
Female oceanic whitetips tend to reach about 200 cm (6.5 ft) in length, while males tend to be slightly smaller, reaching around 190 cm (6.2 ft). It is possible that oceanic whitetips could reach up to 350 cm (11.4 ft).


Diet: 
Oceanic whitetips feed mainly on pelagic bony fish, such as tuna and dolphin fish. They also eat cephalopods and have been known to eat marine mammals - normally in the form of carrion.


Teeth & Jaws:

The upper teeth of the oceanic whitetip are broad, triangular, and serrated. The lower teeth are more narrow and pointed. 

Upper tooth

Habitat and Distribution:
Oceanic whitetips are found all over the world in warm temperate and tropical waters. They live almost exclusively in pelagic waters, far off-shore.


Conservation Status:
The oceanic whitetip used to be the most abundant pelagic shark in the world; they are now listed as vulnerable on the IUCN Red List. The sharp decline in their population is due mainly to bycatch in pelagic fishing. They also suffer from heavy overfishing and are prized by shark finners for their large fins.



sources:
The Sharks of North America, Jose I. Castro
http://www.iucnredlist.org/details/39374/0
Sharks of the World, David A. Ebert, Sarah Fowler and Leonard Compagno 
note: none of the images are mine

Shark Poster

Created by Finformation

The Scalloped Hammerhead Shark

photo credit : Rafael Pesantes



The Scalloped Hammerhead
Sphyrna lewini 


The scalloped hammerhead is a very interesting species, as are the rest of the hammerheads. It is also occasionally known as the bronze hammerhead, or the kidney-headed shark. Hammerhead sharks are not normally considered a threat to humans. According to the International Shark Attack Files, from 1580 - 2011, approximately only 15 non-fatal attacks have been recorded, and no fatal attacks have been recorded.

The famous hammer shaped head.

Identification:
The scalloped hammerhead has the infamous hammer shaped head that all hammerheads have. It can be distinguished from other hammerheads by the "scalloped" edge of their head. They have a large, triangular first dorsal fin and a smaller second dorsal fin that is close to the caudal fin. Their coloring ranges from dark olive to brownish gray with a whitish ventral side. The ventral side of the pectoral fins darken with age, sometimes even darkening to black in larger sharks.

Classification:

Kingdom : Animalia
Phylum : Chordata
Subphylum : Vertebrata
Class : Chondrichthyes
Subclass :  Elasmobranchii
Order : Carcharhiniformes
Family : Sphyrnidae
Genus : Sphyrna
Species : lewini

Similar Species and Other Family Members:
The family Sphyrnidae has eight described species, all with unique"hammer" shaped heads. Scalloped hammerheads are most commonly mistaken for the Great Hammerhead (Sphyrna mokarran). However, the great hammerhead is larger and has a shorter head with more square ends.

Reproduction:
Scalloped hammerheads are viviparous, meaning they give placental live birth, like humans do. They tend to have 12-38 pups and typically have a 9-10 month gestation period.

Size:
Scalloped hammerhead are typically between 160-200 cm (5.2-6.5 ft) with the males being smaller than females at maturity. Females can occasionally reach up to 243-365 cm (8-12 ft).

Diet: 
The scalloped hammerhead tends to eat bony fish. They also eat invertebrates and other sharks. Hammerheads in general are known for their appetite for rays, including dangerous stingrays.

Schooling scalloped hammerheads

Habitat and Distribution:
Scalloped hammerheads are found worldwide in tropical and warm temperate seas. They are unique in that they migrate and school in large groups around sea mounts and islands, most famously around the Cocos Islands and the Galapagos Islands.

Life Span:
Scalloped hammerheads are believed to live up to 35 years.

The scalloped hammerhead's large fin puts it at
great risk from shark finners. 

Conservation Status:

On the IUCN Red List, the scalloped hammerhead is listed as endangered worldwide. They are heavily fished around the world, and are commonly hunted for the shark fin soup trade due to the large size of their fins.

sources:
The Shark Handbook, Dr. Greg Skomal
http://www.iucnredlist.org/details/39341/0
Sharks of the World, David A. Ebert, Sarah Fowler and Leonard Compagno 

note: none of the images are mine, however I did draw the illustrations