A newborn brownbanded bamboo shark (Chiloscyllium punctatum) hatched in the lagoon tank at Steinhart Aquarium in San Francisco—almost four years after its mother was last in contact with a male! This finding is almost twice the previous sperm storage record of 843 days for sharks, which was held by the chain catsharks (Scyliorhinus rotifer). This amazing new discovery has added a new wrinkle to the already complicated field of shark reproductive biology.
As a group, sharks have unusual and diverse reproductive strategies. Unlike many species of bony fish, which reproduce by releasing clouds of egg and sperm into the water, sharks and their cartilaginous relatives employ internal fertilization, as mammals do. Although sharks like the brownbanded bamboo are oviparous (lay eggs), many species are viviparous and give live birth. Still others have a combination of these strategies called ovoviviparity, in which eggs hatch inside the mother’s body. Sand tiger sharks famously demonstrate intrauterine cannibalism, in which one embryo eats all the others and only one pup per uterus is born.
Along with insects, birds and even some mammals, a few species of sharks have been known to exhibit long-term sperm storage, keeping sperm in specialized glands in the body for a long while after copulation, says study author Luiz Rocha, assistant curator and Follett Chair of Ichthyology at the California Academy of Sciences. “The advantages are many; for example, females can store sperm until they have viable eggs ready to be fertilized, eliminating the need to copulate when the female is ovulating. Females can also save the sperm to be used at a time when they can’t find a male, which is what happened here.”
Rocha said that some insect species can store sperm from a few different males and choose which sperm they want to use to reproduce. Sharks can’t choose sperm from among different partners, but multiple paternity has been observed in litters of several shark species. This means that if a female mates with four males, it’s possible that pups fathered by each male can be born in the same litter.
Some species of sharks are also known to demonstrate parthenogenesis, in which the mother gives birth to genetic replicas of herself without involving a male at all. This feat was first discovered when bonnethead sharks (Sphyma tiburo) at the Henry Doorly Zoo in Omaha produced newborns, even though the tank held no males. In fact, the California Academy of Sciences research team initially suspected parthenogenesis accounted for the hatching of the brownbanded bamboo shark at the Steinhart Aquarium, until they found the genetic contribution of a second shark in the newborn’s DNA. Because the pup was a genetic combination of the mother and another shark, parthenogenesis could not have occurred, and a male had to have been involved at some point. “This study opens up future questions of when and how shark species use alternate techniques, such as sperm storage and parthenogenesis, to produce offspring in the absence of males,” says lead author Moises Bernal, a PhD candidate at The University of Texas at Austin Marine Science Institute. “Understanding how these mechanisms are triggered is vital for conservation of dwindling shark populations.” Parthenogenesis has to date only been observed in captive sharks but appears to be triggered by the lack of males. If observed in the wild, it could provide further evidence of severely declining shark populations.
Knowledge of shark reproductive strategies is important for devising effective fisheries management policies for these animals. Unlike many species of commercially harvested bony fish—which reach reproductive maturity after a few years and can produce tens or hundreds of thousands of eggs each year—sharks reproduce only relatively late in life and have very few offspring at a time. These traits mean that their populations cannot recover as quickly from overfishing pressure, thereby fishing quotas must be lower than for a similarly sized population of bony fish.