A Swell Shark Surprise

Researchers and staff at the National Aquarium were among the first to discover a remarkable capability in swell sharks: asexual reproduction.

Published July 26, 2018

Swell sharks, a type of catshark, are benthic—or bottom-dwelling—sharks found in the eastern Pacific Ocean, from the central California coast to the coast of Chile. Their name comes from their ability to swallow water—or air, when exposed to it—to make themselves appear larger.

Swell Shark

Believe it or not, this isn’t even the most interesting aspect of this species. In 2014, when swell shark pups began to hatch from egg cases at our Animal Care Center, staff was shocked. After all, the female swell shark in our care hadn’t been in contact with a male swell shark in three years.

So how did this happen?

In some species of sharks, females have been known to hold onto sperm from their male counterparts in specialized glands for years, waiting for the right moment and conditions to produce fertilized eggs. However, that couldn’t explain the phenomenon. Genetic analysis revealed only maternal alleles were present in the DNA of the pups at the National Aquarium–no paternal alleles were present.

Instead, our staff determined this was the first-ever documented instance of facultative automictic parthenogenesis–the ability to grow embryos without paternal contribution—in swell sharks! Previously, this has been documented in several other shark species, including blacktips, zebra sharks and hammerheads.

In honor of Shark Week, Ink Dwell is launching their shark-inspired line of prints, cards and more. A portion of the proceeds will benefit the National Aquarium’s mission! Take a look at their limited edition items here!

Founded in 2012 by artist Jane Kim and journalist Thayer Walker, Ink Dwell studio makes art that inspires people to love and protect the natural world. Merging classical techniques of science illustration with modern fine art, we create public and private commissions, fine art, illustrations, and exhibitions, including the mural in Living Seashore!

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