Baby Sharks Swim in Mom's Uterus

Analysis by Dr. Karen Shaw Becker

shark

Story at-a-glance -

  • Sharks have two uteri, and researchers discovered swimming shark embryos in three pregnant tawny nurse sharks, which they observed using special underwater ultrasound machines
  • In some cases, they observed the sharks in motion, but in others they knew the sharks moved by counting the number of embryos in each uterus
  • One shark had four embryos; sometimes there would be two in each uterus while other times all four were on one side and none were on the other
  • As for why the embryos swim around, it’s likely that they’re foraging for eggs, as some embryonic sharks survive by eating their mother’s unfertilized eggs

Sharks have two uteruses, and this is just the beginning of the unique details relating to their reproduction. Not only are there double uteri, but shark embryos swim around inside them, even swimming from one uterus to the other. It’s a phenomenon not easily observed, but researchers at the Okinawa Churaumi Aquarium in Japan have done it, not once but multiple times.

In a study aptly titled, “Locomotion Is Not a Privilege After Birth,”1 the team describes swimming shark embryos in three pregnant tawny nurse sharks, which they observed using special underwater ultrasound machines. In some cases, they observed the sharks in motion, but in others they knew the sharks moved by counting the number of embryos in each uterus.

One shark, for instance, had four embryos. Sometimes there would be two in each uterus while other times all four were on one side and none were on the other.

Sharks Likely Forage for Eggs in Utero

As for why the embryos swim around, it’s likely that they’re foraging for eggs. Some embryonic sharks survive by eating their mother’s unfertilized eggs. Toby Daly-Engel, Ph.D., a shark expert at the Florida Institute of Technology, told The Atlantic, “Instead of the embryos eating one another, they appear to be competing [for eggs]. That’s just the coolest thing ever.”2

She’s referring to sand tiger sharks, in which it’s known that the first embryos to reach a certain size consume their smaller siblings in an unsettling show of cannibalism.3 Because of this behavior, however, sand tiger sharks are up to 3 feet long when they’re born, giving them an excellent chance of survival.

The tawny nurse shark study also revealed another surprising behavior among the shark embryos, which has yet to be explained. One of the embryos stuck its head out of the mother’s cervix, then went back inside.

"Our data also showed that the cervix of the tawny nurse shark sometimes opens," the researchers noted, "and the embryo exposes its head out of the uterus through the cervix. This phenomenon is in contrast to that seen in mammals where the cervix is tightly closed until birth.”4

Sharks Develop in Different Ways

While most sharks give birth to live young, some species are oviparous, or egg-laying. In these species, pregnant female sharks lay thick, leathery eggcases, sometimes called a “mermaid’s purse,”5 that enclose the embryo, which is nourished by a yolk sac. Among the live-bearing, or viviparous, shark species, some embryos are linked to a placenta, getting nutrients from the mother, while others consume unfertilized eggs.

“It has also been suggested that the young of some viviparous species may be nourished by uterine secretions during some part of gestation, but this has not been conclusively documented,” according to the Florida Museum of Natural History.6

There is no parental care for sharks, after they’re born or eggs are laid. However, even embryonic sharks in egg cases are somewhat aware of their surroundings and will “freeze” in response to predators. In a study of embryonic bamboo sharks in later stages of development, researchers emitted electric fields intended to mimic predators.

The embryos responded by stopping their gill movements. “Despite being confined to the small space within the egg case, where they are vulnerable to predators, embryonic sharks are able to recognize dangerous stimuli and react with an innate avoidance response,” the researchers explained.7

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The Great Eggcase Hunt

Much remains a mystery about shark reproduction, which has led to an ongoing citizen science project involving eggcases. Sharks emerge from the protective pouches after several months, leaving an empty eggcase behind. Different species lay different sizes and shapes of eggcases, which often wash up on shore, particularly along the strandline, or the area of the beach where seaweed often lines up.

Sometimes, divers, snorkelers or paddle boarders may also observe eggcases in the water, sometimes camouflaged by marine organisms such as algae. The U.K.-based The Shark Trust is asking for help from swimmers and beachgoers to keep an eye out for eggcases, and if you see one, record your finding.8 To date, they’ve received reports of eggcases from 30 species in 22 countries, noting:9

“Empty eggcases can help indicate species presence and diversity. By recording your finds, you're helping us to discover more about egglaying species in our waters …

Underwater records help us pin-point exactly where sharks and skates are laying and can be linked to beach records. Learning the depth and substrate that they lay on also helps us to better understand the species … If you do spot one make a note of the location and, if possible, take a photo. Then record your finding as normal.”

Many shark species are endangered, which makes projects like this so important to further education and conservation efforts to protect these fascinating creatures.

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