Joel Harrison Gayford, a marine biologist at James Cook University, in Australia, who specializes in research focused on chondrichthyan evolution and ecology, has published a in the journal Royal Society Open Science regarding two unusual forms of reproduction in chondrichthyans and possible reasons for them.

Prior research has shown that chondrichthyans, which include sharks, rays, chimaeras and skates, have existed much longer than most other creatures on Earth. This tenure, Gayford notes, has allowed them to develop a wide variety of reproductive methods.

Unlike most other animals that have evolved a single means of reproducing, different chondrichthyans have wildly different ways of reproducing. Some lay eggs, for example, while others have live births, and some almost always produce twins鈥攐ne species in Australia delivers its babies with an attached placenta.

In this new effort, Gayford focuses on two unique ways that some chondrichthyans reproduce: The first is via facultative parthenogenesis, where no males are involved, and the other is when females lay eggs where there are multiple embryos per egg case (MEPE).

Gayford notes that females producing offspring without assistance from males is not unheard of in other species and is relatively common in sharks, though it seems that sharks can choose when to do so. MEPE is less common. What is not known, he notes, is what evolutionary advantage is conferred by these two methods of reproduction.

Having multiple embryos in an egg case means less work and energy expenditure for the mother, he points out, but it also suggests more risk for the embryos. If the egg case is found and eaten, she will lose all her offspring in a single gulp. He suggests more work is required to better understand the advantage for the species.

Facultative parthenogenesis, he suggests, is a little easier to appreciate, though still not completely understood. Female sharks have been known to use it even when males are around, suggesting they feel replicating themselves would produce better offspring than the available male gene pool.

Gayford suggests that learning more about why chondrichthyans have evolved so many ways to reproduce, and why some have developed in such unique ways, could help to better understand long-term evolution in general.

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