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Nursery Rhyme

One of the greatest mysteries about sharks that prevails to this day is the way that they breed... In is very uncommon to witness sharks copulating and for many species we know very little about how sharks find and choose a partner, where they mate and where they give birth. But as technology is developing, scientists are finding new ways to learn about shark reproduction. We now even have electronic tags which can be implanted inside a female shark's uterus! So how does this work? How can you fit a shark with a tag like this? And what can these tags helps us to learn?


Mangrove forests are critical nursery habitats for many species of sharks and rays (Image Credit: Rodolfo Asar / Shutterstock)

Small Fish in a Big Pond

Shark matings are incredibly rare to witness. Even if scientists are lucky enough to see the magical moment in person, when the sharks then move off, they are so free-ranging and broadly distributed, that we are then left with so many questions... where do they go after mating? How long are they pregnant (known as "gravid")? And where do they then give birth?

We do know that sharks commonly use "nursery habitats". These are specific locations - maybe an estuary, an enclosed bay or a mangrove forest - where neonate and juvenile sharks live until they have grown sufficiently to be able to move out into the big wide world. Adults never live in these habitats (this is known as "size segregation"), but year after year young sharks can be found growing up independently of their parents in exactly the same spot (Heupel et al, 2007).


Young lemon sharks remain in their nursery habitat amongst the roots of mangroves for several years (Image Credit: Anita Kainrath / Shutterstock)

Maternity Ward

Scientists have been able to hazard an educated guess at the location of "parturition sites" (where sharks give birth), based on where we find nursery habitats, as we can presume that they are likely nearby each other (Heupel et al, 2007).


For some species of sharks though, we don't even know the location of the nursery. In this case we can narrow it down by considering where we commonly find juvenile sharks. If we find a strip of coastline that only ever hosts young sharks and there are never any larger individuals around, save for the odd pregnant female, we can safely extrapolate that they must be pupping nearby (Heupel et al, 2007).


It's thought that this pregnant bull shark gave birth somewhere near Playa del Carmen, Mexico (Image Credit: Steve Barze / Shutterstock)

Time After Time

For other species we know a little bit more about their potential parturition sites because of a phenomenon known as "philopatry". Sharks are famous for this ability. They are able to return back to a very specific site, year after year, even after extended, transoceanic migrations. Sometimes they will literally home back to an exact spot - in a certain bay for instance - years later. For example, in some tagging studies, individuals have been resighted subsequent years within a mile of the place they were recorded the year before! (Klein et al, 2019).


It is remarkable! Especially when you compare this to human beings, who (if they are anything like me) can't even remember where they parked their car 20 minutes ago!


In some species, like lemon sharks (Negaprion brevirostris), females will return to the same sites again and again, with remarkable accuracy, to give birth to their young (Klein et al, 2019).


Female blacktip reef sharks (Carcharhinus melanopterus) return to the site of their own birth (aka "natal site") to give birth to their young. They will go back to this specific spot, even if alternative, equally suitable habitats are close by (Mourier & Planes, 2013).


In French Polynesia, blacktip reef sharks rarely migrate to another island and most mothers give birth at their home island (Image Credit: Damsea / Shutterstock)

Tag, You're It!

With the development of new technologies we have been able to learn a lot more about where sharks give birth. Electronic tags have been used extensively to study how sharks move around the oceans. Sometimes these tags are internal, but it is more common that they are external - clamped to the shark's fin for example. For more info check out Tag You're It.


A study using a vaginal implant transmitter has allowed us to identify where a female tiger shark gave birth off the coast of the USA (Image Credit: Albert Kok / WikimediaCommons)

Scientists are now developing internal devices which have previously been used for terrestrial animals, like elephants. As the name might suggest, these vaginal implant transmitters (or VITs for short) can be placed inside an animal's uterus. As the device will come out at the same time the pups do - Bingo! - they may instantly show us the location of birthing sites (Sulikowski & Hammerschlag, 2023).


As engineers have now managed to make these devices waterproof, satellite linked and much smaller, they have been able to be deployed successfully for a female tiger shark (Galeocerdo cuvier) and a scalloped hammerhead (Sphyrna lewini). Scientists inserted the device through the "cloacal opening" and into one of the uteri (sharks actually have two) without harming the unborn pups.

Scientists managed to successfully fit a female scalloped hammerhead shark with an intrauterine tag to track where she went to give birth (Image Credit: Barry Peters / WikimediaCommons)

After a little wait, thanks to these new birth-alert-tags (BATs), we now know that these individuals gave birth to their pups off the coast of the USA (Sulikowski & Hammerschlag, 2023).


As these devices continue to be developed and become more ready available, they could become a very powerful tool for scientists studying reproduction in other species of sharks. If we can identify more shark parturition sites and nursery habitats, we can ensure these critical habitats are protected. This will be especially important for conserving threatened species (Heupel et al, 2007; Sulikowski & Hammerschlag, 2023).




References

Heupel MR, Carlson JK & Simpfendorfer CA (2007). Shark nursery areas: Concepts, definition, characterization and assumptions. Marine Ecology Progress Series, 337, 287–297. Access online.


Klein JD, Bester-van der Merwe AE, Dicken ML, Mmonwa KL & Teske PR (2019). Reproductive philopatry in a coastal shark drives age-related population structure. Marine Biology, 166, 26. Access online.


Mourier J & Planes S (2013). Direct genetic evidence for reproductive philopatry and associated fine-scale migrations in female blacktip reef sharks (Carcharhinus melanopterus) in French Polynesia. Molecular Ecology, 22, 201–214. Access online.


Oliver SP & Bicskos Kaszo AE (2015). A pelagic thresher shark (Alopias pelagicus) gives birth at a cleaning station in the Philippines. Coral Reefs, 34:17. Access online.


Sulikowski JA & Hammerschlag N (2023). A novel intrauterine satellite transmitter to identify parturition in large sharks. Science Advances, 9, eadd6340. Access online.



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