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.
