Drs. José I Castro, Keiichi Sato, and Ashby B. Bodine, have discovered a new method of embryonic nourishment in sharks. The tiger shark secretes a nutritive liquid which fills the egg sacs where the embryos are growing, permitting higher numbers of offspring to attain a large size. Their paper, entitled "A novel mode of embryonic nutrition in the tiger shark, (Galeocerdo cuvier)," has just been published in the journal Marine Biology Research.
It explains how many reproductive adaptations have evolved across the different families of sharks to ensure that the young are as independent as possible at birth. This takes time, which is why shark gestation periods are long, usually a year or two. Plenty of nourishment is also required for the embryos to attain a good size.
Some sharks have large eggs with rich yolks, while others have a placental connection not unlike mammals. In some sharks, the needed nourishment is provided by extra eggs, while in other cases, it is extra siblings that are eaten. So many strategies have evolved over the eons, that it is not possible to classify them all simply. There may be special adaptations in any given shark species that are different from those found in its close relatives.
The tiger shark is the only species in its genus, and the biggest of the large shark family Carcharhinidae, which includes the requiem sharks of tropical and subtropical seas. It is also the only one, of all the sharks in that family, to nourish its off-spring without a placental connection between the foetus and mother.
But researchers had noted that there was not enough nourishment in the egg yolks of the tiger shark to sustain its embryos up to the large size of 80 to 89 cm, which they attain prior to birth, and that during foetal development, the egg cases fill with about a litre of a clear, yellowish fluid.
Castro suspected that this liquid must provide the missing nourishment, and he and his team have proven his hypothesis to be correct. Their experiment demonstrated that this liquid, which they have termed, embryotrophe (literally “embryo food”) is the major source of nutrition for tiger shark embryos. The weight gains they found for the fertilized eggs as they grew into term embryos, were as high as 2119%. An unexpected finding is that proteins are not the main source of energy in this rich liquid; Castro will publish its full analysis in a future paper. He explains, "Whenever we study one shark species in detail, we find that they are doing things we had not expected....and they differ from closely related species."
The researchers propose the term embryotrophy for this new reproductive process.
Tiger shark embryos lie in their egg cases in separated compartments, and are arranged perpendicular to the axis of the mother's body, rather than parallel to it, as is the case in all the other species of the Carcharhinidae family. The embryotrophe appears to be secreted by the uterine lining, and ingested orally by the embryos, which was suggested by the finding of partly digested food in their intestines, and that some of them had inverted stomachs. Tiger shark foetal nourishment represents a case of matrotrophy—or feeding from the mother.
This reproductive strategy is apparently unique to the tiger shark, which, according to the fossil record, evolved early in the very long evolutionary history of sharks. Researchers speculate that it may be due to the loss of the placenta somewhere along the way. It may be an adaptation for producing larger broods of large embryos in the great pelagic species. Pelagic sharks use oceanic regions as nurseries, and tend to have more offspring than their coastal cousins—seventy to eighty pups for the tiger shark, and blue shark, for example, and three to four hundred for the whale shark. Their off-spring are smaller than those born to the coastal sharks, who have fewer pups, of a larger size, and use shallow nursery areas to birth.
Castro says, "Most of human evolution took place in the last 4 million years. Imagine what Mother Nature can do in 100 times that time span--400 million years--that is how long the evolution of sharks has lasted. That is why they are such amazing creatures."