The Curious Case of
the Shark Egg Case
On a cold, Autumn day in 2017, Sarah Tempesta, one of the aquarists responsible for sharks at the New England Aquarium, noticed an egg case in the shark tank. The Science of Sharks is home to three shark species— Halmahera walking sharks (Hemiscyllium halmahera), epaulette sharks (Hemiscyllium ocellatum), and coral catsharks (Atelomycterus marmoratus). Pleasantly surprised by her discovery, she collected the eggs, hoping they would be Halmahera sharks, a species only discovered in 2013.
When the eggs hatched after five months, the baby sharks were not the recently discovered Halmahera, but epaulette sharks that had lived in an all-female environment for more than five years. The discovery begged the question…
How did an epaulette shark lay eggs when there were no males nearby?
Female sharks actually lay egg cases all the time, regardless of if there are males nearby, but there was something special about this case. It was viable, there was a living embryo inside.
It’s easy to see why sharks have captured the attention of the world. Every one of the 440 species of shark has evolved to help these animals survive in the wild; this includes reproductive adaptations.
Sharks reproduce three ways: live birth (viviparity), release of egg cases (oviparity), or a combination of the two, where the egg cases hatch inside the mother’s body and she then gives birth (ovoviviparity). Most sharks give live birth to their young via that third method.
Epaullete sharks and Halmahera sharks reproduce using oviparity. When compared to bony fish, which spawn to fertilize in the water column, oviparity enhances the survival of the larvae and offers protection from predators. These fertilized eggs have a sufficient amount of yolk to support the embryo’s development. When the eggs hatch, the fully developed miniature shark is ready to find food and thrive.
For the case of the mysterious eggs at the New England Aquarium, it’s important to know that female sharks have been known to store sperm for almost three years after a sexual encounter. But, in this case, sperm storage was ruled out as the female epaulette sharks had no contact with a male in more than five years. Our scientists had only one other explanation: the mother shark cloned herself in a process known as parthenogenesis.
Parthenogenesis is when an animal produces offspring without fertilization—there is no fusion of the sperm and egg. Instead, a female produces offspring using only her own genetics. No mate needed! This is very different from self-fertilization, during which the organism has both male and female sexual organs (a hermaphrodite). Parthenogenesis, often called “virgin birth,” is common in invertebrates such as mites and bees, but has been observed only in a few reptiles, birds, bony fishes, and a few shark species like bonnethead sharks, whitespotted bamboo sharks, and zebra sharks.
It is hard to find out whether parthenogenesis takes place in the wild because both the parent and the daughter DNA is required for analysis, and shark parents do not remain with their young after birth. A single known case of parthenogenesis in the wild has been documented and it’s in the smalltooth sawfish, a critically endangered ray species found in the Atlantic.
(In case you missed it: Anna our green anaconda gave birth via parthenogenesis this spring).
This baby is still too small to take a blood test to confirm parthenogenesis, but we currently have five epaulette babies at the Aquarium suspected of being born via parthenogenesis!
These baby sharks live behind the scenes, but you can come visit all the other amazing sharks in the Science of Sharks exhibit today.