Check out this fascinating article on the development of white shark embryos!
(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.