Like a Virgin
Updated: Jul 8
One of the most fascinating things about the sharks, skates and rays (known collectively as Elasmobranchs) is how diverse their reproductive strategies are. 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... 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"). Even amongst groups of animals that are very closely related, we see very different methods of reproduction, which have evolved separately along the different lineages. It is dizzying! Yet, 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!
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 about rays that look like sharks and sharks that look more like rays, you can check out Kissing Cousins.
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.
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") through ovoviviparity; after copulation, the female incubates fertilised eggs, which then hatch and continue to grow inside her, before being born (Fields et al, 2015, Feldhaim et al, 2017). Or that is what we always thought...
A team of scientists working in south Florida, USA, made a fascinating find whilst they were studying smalltooth sawfish (Pristis pectinata) reproduction. The team were catching (and releasing unharmed!) these sawfish, in order to take samples for genetic analysis of their "microsatellites". Microsatellites are present in every animals' DNA. They are regions of highly repetitive genetic code, which are prone to a high mutation rate. Due to these common mutations, microsatellites can be used to investigate the genetic diversity of animals and to determine which species are related. These researchers were using sawfish microsatellites to assess how related different individuals were to each other (known as their "internal relatedness" (IR)) (Fields et al, 2015).
In normal sexual reproduction, offspring will receive 50% of their genetic material from the mother and 50% from the father. If an individual's parents are closely related to each other (siblings or cousins, for example), the internal relatedness of their offspring will be higher (IR score = closer to 1). The scientists found an internal relatedness score which suggests the smalltooth sawfish in this area of Florida are not breeding with close relatives (IR average = 0.33). This is great news because it implies they will not suffer the effects of "inbreeding" (Fields et al, 2015).
However, they also found something completely unexpected! Some individual sawfish had an IR score close to 1, suggesting that their genetic material was identical to that of their parent... Meaning they were a product of "asexual reproduction". This means that female smalltooth sawfish are, in fact, capable of breeding without a male! This is known as "facultative parthenogenesis" (Fields et al, 2015).
Facultative parthenogenesis is actually more common than you might think!
In fact, scientists think that many different sexual animals are capable of this type of asexual reproduction. For example, it has been documented in snakes, birds and sharks: white-spotted bamboo sharks (Chiloscyllium plagiosum) and zebra sharks (Stegostoma fasciatum) kept in captivity have both 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!.
The scientists theorised that these sawfish are capable of facultative parthenogenesis when they are unable to find a mate. Smalltooth sawfish have a very low population density in Florida and they are on the verge of extinction on a global scale (IUCN, 2021). In fact, every single species of sawfish is threatened with extinction - they are one of the most critically threatened groups of marine animals! (To learn more, you can check out The Rhino-Saws.) The researchers theorised that these sawfish are so sporadic in this region, that sometimes the females switch to asexual reproduction when they simply cannot find a male to mate with. It is kind of a last, desperate effort to breed (Fields et al, 2015).
Only 3.7% of the sawfish the scientists sampled in this study had such high IR values that they were considered to be the product of facultative parthenogenesis. Therefore, the researchers concluded that this method of reproduction is not common in smalltooth sawfish. In fact, they believed all five of these individuals were probably from just one single litter (Fields et al, 2015).
But, these findings do give us some cause for concern about the viability of this sawfish population for two reasons. Firstly, whilst some birds and reptiles which reproduce by facultative parthenogenesis produce viable offspring, there is evidence that many animals born this way are not capable of breeding themselves. If these sawfish are not capable of producing their own offspring, this means that these individuals will not be able to contribute to the growth of this seriously threatened population. Secondly, it suggests that the population density of these animals has reached a critically low point, where they are not capable of even finding a member of the opposite sex (Fields et al, 2015).
Therefore, 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 so steps must be taken immediately to protect them on a global scale.
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.
To learn more, you can check out my other articles on shark reproduction.
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.
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.