Hakai Magazine

to Sarawak swellshark egg
While most shark egg cases are opaque, those belonging to Sarawak swellsharks are transparent—allowing the baby inside to be seen. Photo by Cheng-Chang Lee

This Mommy Shark Has a Unique Way of Making Babies

Baby shark, doo doo doo doo doo doo.

Authored by

by David Shiffman

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Baby sharks are more than the inspiration for an annoying, albeit catchy, song—the reproductive strategies behind them are downright fascinating. From live birth to egg laying to asexual reproduction, sharks have an amazing diversity of reproductive strategies; a textbook published just last year identified 10 unique methods. Now, a new study reveals an entirely new reproductive strategy in Sarawak swellsharks. Time to rewrite the textbooks!

To understand what’s new and different about the Sarawak swellshark’s form of baby-making, it’s important to get a handle on how sharks lay eggs. Before this study, scientists knew of two ways: single oviparity, in which one or two eggs do a quick stint in the shark’s uterus, leaving it vacant for more batches of eggs over the course of the breeding season; and multiple oviparity, in which lots of eggs develop over a longer time in the uterus at once, yielding more mature embryos.

Sarawak swellsharks essentially blend those two strategies. Females produce one egg at a time, as with single oviparity, but the egg has an extended stay in utero, as with multiple oviparity. Dubbed sustained single oviparity, this results in each mother having fewer, but larger and more developed, baby shark eggs that have a better chance of survival.

Sarawak swellshark eggs are striking for another reason, too—they’re completely transparent. When the study’s lead author, Kazuhiro Nakaya, the retired marine biologist who coined the terms single and multiple oviparity, first laid eyes on them, he was completely taken aback. He’d only ever encountered egg cases, sometimes called mermaid’s purses, that were yellowish, brownish, or blackish—colors better suited for camouflage. “At first, we thought it was worth reporting just the discovery of transparent egg cases, but as we examined more specimens, we noticed that the reproductive mode was quite unique—different from all other known egg-laying sharks,” Nakaya says.

Hsuan-Ching Ho, an ichthyologist at Taiwan’s National Museum of Marine Biology and Aquarium and a coauthor of the study, says he learned of the eggs when local fishers in southwest Taiwan noticed their peculiar appearance and called up the museum. Without their help, Ho says he and other scientists may never have learned about the unique egg cases and the reproductive strategy that created them.

These textbook-rewriting discoveries have shark researchers buzzing. Christine Dudgeon, a population ecologist at the University of Queensland in Australia who was not involved in the finding, says studies like Nakaya and Ho’s show that scientists still have so much more to learn about sharks. “The diversity of approaches to reproduction in sharks highlights the beauty and complexity of nature, and just how amazing the world is,” she says.