A team of researchers from the School of Life Sciences has discovered a stickleback fish that successfully delivered offspring from eggs that were fertilized while they were still inside of her. This represents the first-ever discovery of internal fertilization and the development of babies inside of a species that normally lays eggs.

The experts collected the fish, which was nicknamed “Mary,” during an expedition to the Outer Hebrides to gather wild sticklebacks. The three-spined stickleback, Gasterosteus aculeatus, is a small fish that can be found in the Northern Hemisphere in both salt and freshwater.

Typically, the male sticklebacks build nests and attract females to lay their eggs inside by performing a zig-zagging dance routine. The male chases the female away, fertilizes the cluster, and then guards and nurtures the eggs for around two weeks until they hatch.

Mary was close to death when her unusual pregnancy was noticed, and the researchers made the decision to put her to sleep and deliver the near-complete embryos by Caesarean section. The team successfully delivered 54 embryos, which hatched and grew to adulthood. The experts found that the offspring could be bred in normal aquarium conditions.

“We were astounded at what we found when we examined Mary in our lab in the Outer Hebrides,” said Dr. Laura Dean. “She looked like an ordinary egg-bound fish so we couldn’t believe it when we found she had almost completely developed embryos inside her ovaries. This is pretty much unheard of in an egg-laying species. The embryos were perfectly healthy, not deformed in any way, and most have gone on to live a normal adult lifespan.”

This abnormal type of reproduction in fish can occur in three possible ways. The first possibility was that the fish cloned herself, and the second was that may be hermaphroditic with both male and female sex organs.

“We were able to rule these two possibilities out because, in parthenogenesis, her offspring would have been genetically identical to the mother, or, if hermaphroditism, they would have only had versions of genes that she had, with no genetic input from anywhere else. So, we did some very simple genetic testing on the offspring and found that they had versions of genes that Mary didn’t have and so must have had a father,” said Dr. Dean.

“Our theory is that somehow sperm had got into the fish, fertilized the eggs and they developed into normal two-parent embryos. What has probably happened is that she has gone into a nest to lay her eggs where another female had already laid her eggs which had been covered with sperm by a male stickleback. Somehow some of the sperm in the nest must have got into Mary, presumably through her egg tube, and fertilized the eggs inside her but she never laid the eggs.”

“Although this almost accidental find revealed a vanishingly rare phenomenon, it might help us to understand a really important change that has happened throughout the tree of life,” said Professor Andrew MacColl. “Most animals lay eggs, but some (including almost all mammals, but few fish) retain their eggs inside and give birth to live young. Although this appears to be a difficult thing to achieve in evolution, this one little fish seems to have got there almost by itself!”

The researchers are now actively looking for more female sticklebacks who display the same type of pregnancy. They are exploring the idea that this phenomenon may be an indication of an evolutionary or genetic change in the reproductive mechanism of the species.

The study is published in the journal Scientific Reports.

By Chrissy Sexton, Earth.com Staff Writer

Mary’s offspring, Image © Dr. Laura Dean