One of the most fascinating things about the sharks, skates and rays (known collectively as Elasmobranchs) is how diverse their reproductive strategies are. Whilst all sharks are rays breed through sexual reproduction, every single species has evolved a unique tactic, which makes every species completely unique. Amongst the myriad, there may even be other reproductive strategies that scientists are only just discovering! For example, recently researchers have learned that not all elasmobranchs need to reproduce sexually! In fact, some species of sawfish are capable of virgin births! So how do sawfishes breed asexually? And why do they do it?
Let's Talk About (Shark) Sex, Baby
All elasmobranchs breed through sexual reproduction with internal fertilisation. This means a female and a male come together and copulate in order to produce offspring which are genetically related to both parents bUt this is really the only similarity amongst all the species...
Where Skates lay eggs (known as "oviparity"), many sharks and rays give birth to live young ("viviparity"), and many elasmobranchs have internal eggs which hatch inside the mother before birth ("ovoviviparity"). To learn more, you can check out my other articles on shark reproduction. Even amongst groups of elasmobranchs that are very closely related, we see very different methods of reproduction, which have evolved separately along the different lineages. It is dizzying!
But recent discoveries have shown us there is still a lot to learn about elasmobranch reproduction...
Enter The Sawfishes
Sawfishes (Pristidae) are a family of batoids from the order Rhinopristiformes. Despite looking quite like sharks, with their bulky body and strong tail, they are in fact more closely related to rays and skates. Their closest relatives include the wedgefishes (Family Rhinidae) and guitarfishes (Family Rhinobatidae) (Aschliman, 2011). To learn more you can check out The Rhino-Saws.
Sawfish are very different to their other ray relatives, as they have an iconic extended "rostrum" with awesome "rostral teeth". Often called a saw, this appendage is used for digging prey out of sandy substrates and for slashing meals into small, bite-size pieces (Aschliman, 2011).
Sawfish live primarily in shallow, coastal habitats. They are capable of surviving in both salt and fresh water (this is known as being "euryhaline"), so are also found in brackish estuarine regions. They feed primarily on fish, crustaceans and molluscs (Aschliman, 2011; Feldhaim et al, 2017; Fields et al, 2015).
Sawfishes Give Birth to Live Young
Sawfishes have a distinctive breeding season, in which males and females come together to mate. During this period, both sexes will copulate with multiple partners (this is known as "polygamous mating"), in order to maximise their chances of having a healthy litter of offspring. Female sawfishes breed every other year (known as a "biennial reproductive cycle"). Sawsharks are ovoviviparous, meaning that after copulation, the female incubates the eggs, which then hatch and continue to grow inside her, before being born (Feldhaim et al, 2017; Fields et al, 2015). Or that is what we always thought...
Sawfish Can Breed by Asexual Reproduction
A team of scientists working in south Florida, USA, made a fascinating discovery whilst they were studying smalltooth sawfish (Pristis pectinata) reproduction. The team were taking samples from wild sawfish, to be used for genetic analysis. Namely they were looking at "microsatellites" - regions of highly repetitive DNA, which are prone to a high mutation rate. Microsatellites are found in the DNA of all animals, so they can be used to investigate the genetic diversity of a population and to determine which animals are more closely related (known as their "internal relatedness" or IR for short) (Fields et al, 2015).
The researchers found something completely unexpected! Some individual smalltooth sawfish had an IR score close to 1. This would mean that their DNA was almost identical to that of their parent. This only happens when you have a single parent or a clone... So it means that female smalltooth sawfish are capable of breeding without mating with a male! This is known as "facultative parthenogenesis" (Fields et al, 2015).
Strong, Independant Women
Only 3.7% of the sawfish that the scientists sampled in this study had such high IR values that they were considered to be the product of facultative parthenogenesis. So asexual reproduction is certainly not common in smalltooth sawfish Fields et al, 2015).
But in the animal world as a whole, facultative parthenogenesis is actually more common than you might think! In fact, scientists think that many different sexual animals, including snakes and birds, are capable of this type of asexual reproduction. And white-spotted bamboo sharks (Chiloscyllium plagiosum), zebra sharks (Stegostoma fasciatum), bonnethead sharks (Sphyrna tiburo), blacktips (Carcharhinus limbatus) and whitetip reef sharks (Triaenodon obesus) which are kept in captivity, have been known to reproduce via facultative parthenogenesis (Fields et al, 2015; Dudgeon et al, 2017). To learn more, you can read Don't Need No man!.
Sawfishes Are Struggling to Find Mates in the Wild
The fact that sawfish in Florida are switching to asexual reproduction is a great concern. The researchers behind the study worried that these sawfish are only capable of facultative parthenogenesis when they are unable to find a mate. It is kind of a last, desperate effort to breed. This suggests that sawfish are so sporadic in this region that it is rare for two mature animals to find each other Fields et al, 2015).
Smalltooth sawfish have a very low population density in Florida and they are on the verge of extinction on a global scale. In fact, every single species of sawfish is threatened with extinction! They are one of the most critically threatened groups of marine animals in the world (IUCN, 2021).
Virgin Births Could Further Threaten Endangered Sawfish
So... is it good news that at least these sawfish are finding some way to breed? Sadly, the answer is no!
Whilst some birds and reptiles which reproduce by facultative parthenogenesis can produce viable offspring, there is evidence that many animals born this way are not capable of breeding themselves. If these young sawfish are not able to produce their own offspring, this means that these individuals will not be able to contribute to the growth of this seriously threatened population. So whilst facultative parthenogenesis might cause a short term boost, is not sustainable for population growth in the long-run Fields et al, 2015).
Whilst this study is very cool and really fascinating, it should also set off serious alarm bells! Smalltooth sawfish seem to be on the verge of complete collapse in Florida and steps must be taken immediately to protect them!
If you would like to contribute to the conservation of sawfish, you can donate to the Shark Trust and contribute to their See A Saw campaign. You can also stay up-to-date with sawfish conservation initiatives through the Sawfish Conservation Society's regular newsletters.
References
Aschliman NC (2011). The batoid tree of life: Recovering the patterns and timing of the evolution of skates, rays and allies (Chondrichthyes: Batoidea). PhD Degree Thesis, Florida State University, USA. Access online.
Dudgeon CL, Coulton L, Bone R, Ovenden JR & Thomas S (2017). Switch from sexual to parthenogenetic reproduction in a zebra shark. Scientific Reports,7:40537, DOI: 10.1038/srep40537. Access online.
IUCN (2021). The International Union for the Conservation of Nature Red list of Threatened Species. Access Online.
Feldheim KA, Andrew T. Fields AT, Chapman DD, Scharer RM & Poulakis GR (2017). Insights into reproduction and behavior of the smalltooth sawfish Pristis pectinata. Endangered Species Resaerch, 34, 463–471. Access online.
Fields AT, Feldheim KA, Poulakis GR & Chapman DD (2015). Facultative parthenogenesis in a critically endangered wild vertebrate. Current Biology, 25:11, PR446-R447. Access online.
By Sophie A. Maycock for SharkSpeak
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