Female sharks are known to be capable of storing sperm, but it still came as a weird and wonderful shock when an egg case laid by a brownbanded bamboo shark in Steinhart Aquarium began to show signs of healthy development. Not only were there no males in the tank, but the females had been completely isolated from males for a staggering 45 months. According to scientists, this could be the longest documented case of sperm storage for any species of shark.
Sharks have been gliding through our oceans for hundreds of millions of years, first appearing some 400-450 million years ago. Sharks have therefore had a considerable amount of time to evolve a wide range of reproductive strategies, some of which are as highly developed as some mammals. These successful reproductive adaptations have allowed sharks to withstand environmental changes, and hence contributed to their survival through major extinction events.
While sharks may display a diverse array of reproductive modes, all species practice internal fertilization. Most sharks give birth to live young, but some release eggs inside a capsule, nicknamed a mermaid’s purse, that hatch later. Several such cases had been laid by female brownbanded bamboo sharks residing in the Steinhart Aquarium, and although sharks are known to produce unfertilized eggs like hens, biologists began to wonder whether any of them could be viable.
The egg cases were transferred to an incubator by scientists from the California Academy of Sciences, where two started to show signs of embryonic development. One of the eggs failed to develop properly, but the other produced a healthy pup in 2012. The team was then faced with the task of working out how this unlikely birth could have occurred in the absence of males. The team had two hypotheses: one of the females reproduced asexually in a process known as parthenogenesis, which has been observed in several different shark species, or one of the females had stored sperm from a previous mating event.
The scientists then began the process of elimination by examining the DNA of the pup and comparing it with that of the three potential mothers residing in the aquarium. As described in the Journal of Fish Biology, they found that the baby shark showed too much genetic variation to have been the result of parthenogenesis. Furthermore, the shark had 32 alleles, or versions of a particular gene, that were not found in any of the adult females. It was therefore highly likely that this genetic material was inherited from some unknown party, probably a male that had previously been housed with the females. The most plausible explanation was therefore that one of the females had held onto the sperm for almost four years, which was the last time that they had interacted with males.
This is the first time that long-term sperm storage has been documented in this particular species of shark, although it has been recorded in other sharks. This tactic is important because it means that the female is able to reproduce even when males aren’t around, and it also promotes healthy genetic diversity that would not be possible with parthenogenesis.
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