A captive brownbanded bamboo shark in California produced a viable egg that developed into a pup at least three and a half years after it could have mated, in what may be the longest known incident of sperm storage among sharks, according to researchers.

The extraordinary birth took place in January 2012 at the Steinhart Aquarium of the California Academy of Sciences in the US. Scientists analysed the genetics of the pup and concluded it was fertilised as a result of long-term sperm storage, with their results published in the latest issue of the journal Fish Biology.

Three female brownbanded bamboo sharks (Chiloscyllium punctatum) had been completely isolated from male sharks since 2007. Researchers reported the pup belonged to one of the three females but could not be certain which one.

Sex strategy

Sperm storage tends to occur in female animals when they are solitary and mating opportunities are infrequent. Among sharks, other species known to use this reproductive mechanism include the gummy shark, the dusky shark and the scalloped hammerhead. Many birds, reptiles and invertebrates also are known to store sperm, which can come from multiple males.

Researchers from the US and the UK wanted to ascertain whether long-term sperm storage or parthenogenesis – reproduction where fertilisation has not taken place, which is known to occur in some shark species – was behind the brownbanded bamboo shark’s birth, using genetic analysis.

“[The pup] showed genetic information not seen in any of the other individuals, which came from the father,” explained lead researcher and PhD candidate Moises Bernal, from the University of Texas at Austin’s Marine Science Institute, US.

“We were somewhat surprised by the result, as we initially suspected it was a case of parthenogenesis,” he said.

“However once we observed the genetic evidence it was clear the pup had genetic contribution from a male shark.”

The case almost doubles the record of sperm storage and successful fertilisation in a shark, with the previous longest known incident at 28 months in a species of dogfish, said Mr Bernal.

Long-term sperm storage is a strategy that lets fertilisation occur even if a female is not ovulating at the time of mating and “allows females to produce several litters from a single reproductive set”, said Mr Bernal. This was seen in the captive female sharks which continued to produce multiple egg cases in the months and years after having been around male sharks.

It is thought the species stores sperm in specialised tubes in the oviducal gland.

Brownbanded bamboo sharks reproduce by depositing egg cases which protect the eggs and developing embryos inside until they hatch. Captive individuals have been observed following a reproductive pattern of mating between July and September with females storing sperm and laying eggs from July to February the following year. Babies sport distinctive dark bands, which fade as they grow older.

Top image courtesy of Timothy Wong

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