A Brown-banded bamboo shark has laid a viable egg containing a healthy pup, after a 45-month seclusion from males.
Steinhart Aquarium biologists at the California Academy of Sciences were taken aback when a shark egg case dropped by an adult bamboo shark showed signs of healthy development. The scientists had good reason for surprise: the aquarium’s female Chiloscyllium punctatum adults had spent nearly four years — 45 months — in complete isolation from males.
When one viable egg resulted in the birth of a healthy pup, Academy scientists set out to examine this unprecedented example of sharks’ long-suspected (but little-documented) ability to store sperm over long periods of time. Their results, published in the Journal of Fish Biology, mark the longest documented case of sperm storage in any species of shark, and highlight a bright bit of news for the future of wild sharks threatened by overfishing and habitat loss.
"Long-term sperm storage — where a female can delay fertilisation for months or even years after mating — is a remarkable adaptation that helps promote genetic diversity," says Dr. Luiz Rocha, Academy Curator of Ichthyology. "In contrast, asexual reproduction produces offspring with very little genetic variation. Exploring the bamboo shark’s ability to store sperm gives us hope that wild sharks can help protect their population’s genetic diversity when mates are scarce and serious threats arise."
The Academy’s scientific investigation began in 2010, when curious Academy biologist Nancy Sinai wondered whether any of the shark egg cases regularly found in the aquarium’s Shark Lagoon exhibit might be viable — many sharks are known to produce unfertilised eggs. Biologists transferred several egg cases into a separate incubator, where two of the eggs showed signs of healthy embryonic development. While one egg failed to develop successfully, the other flourished. On January 21, 2012, a healthy pup emerged from its egg and began its life at the Academy’s Steinhart Aquarium.
All the other sharks in the Lagoon exhibit are female, meaning that either a female adult shark reproduced asexually in a process called "parthenogenesis" (as has been observed in other shark species), or had stored sperm from its last mating event several years before.
Examination of DNA ruled out the asexual theory. The pup displayed genetic material absent from all three adult females, most likely inherited from its father — an unknown male from the long-ago tank at the Aquarium of the Pacific.
The ability to store sperm gives females the power to produce young regardless of whether or not they are ovulating when mating occurs, and also means a potential pup will have unique genetic contributions from both its mother and its father. This type of genetic diversity is a boon for wild populations. Diversity is critical in maintaining the health of populations — especially those pared down by overfishing, environmental threats, and a lack of potential reproductive partners.
"Questions remain," says Moisés A. Bernal, lead author of the paper. "We know that several species of sharks have reproductive tactics like storing sperm or reproducing by parthenogenesis in the absence of males, but we need to know when and how these alternate techniques are triggered. Understanding these mechanisms — and how they impact genetic diversity — could be vital for the future of shark conservation."
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