The term Elasmobranch is used to describe all species of sharks, skates and rays. Because (believe it or not) sharks are very closely related to "batoids" (stingrays and manta rays), moreso than they are to fish that have a similar body shape to them, like tuna. In fact, there are some sharks and batoids that are so remarkably similar that it can be difficult to tell them apart, the sawshark and sawfish, for instance. So if you see one of these animals (you can see them aquariums sometimes), how can you tell which one is which?
Sawfish and Sawsharks are NOT Closely Related
Sawsharks are truly sharks, so are assigned to the group Squalimorphii and further divided into their own order called Pristiophoriformes. However, sawfish are within the superorder Batoidea and further classified into the order Pristoidei.
That was a lot of crazy taxonomic words that I don't expect you to remember, but what it basically means is these guys are related, but actually not super closely related compared to other species. Sawfish are actually more closely related to rays, than they are to sawsharks (Compagno, 1984, Aschliman et al, 2012).
The similarities in the way these two groups look is an example of "convergent evolution" - when two species that are not closely related at all evolve to have similar features, as they occupy a similar "niche" in their respective habitats (Aschliman et al, 2012; Greenfield, 2023).
Dissimilar Saws
You could be forgiven for not being able to tell saw-fish and -sharks apart thanks to one particularly spectacular feature - their "rostrums". Both of these fish have that impressive saw projecting from the front of their faces. These might look quite similar, but in fact they are very different from each other. The rostrum of the sawfish has "rostral teeth" of all roughly equal size projecting from the side. Whereas, sawshark rostral teeth have smaller and larger sizes alternating down the length of the rostrum (Compagno, 1984, Aschliman et al, 2012).
It might be difficult to spot the relative sizes of these teeth if you are lucky enough to see one of these guys though... but fear not, there are other features that mean you will definately be able to tell these remarkable animals apart!
Size Matters
If you had a sawshark and a sawfish side-to-side, one thing that would be immediately obvious is the size of these fishes. Sawsharks are relatively small and slender, only reaching around 1.4 m total length (TL). Sawfish on the other hand can be huge! One species, the green sawfish (Pristis zijsron) are amongst the largest fish in the world, reaching at least 6 m in length (Compagno, 1984).
The Location of the Gills
Sawfish can also be easily told apart from sawsharks by looking at where the gills are. Sawsharks have gills on the side of the body, like other sharks. However, on the sawfish the gills are located on the underside of the body, like their ray relatives. So if you see a saw-like fish resting on the substrate in an aquarium, if the gills are visible, it is a shark (Compagno, 1984).
Sensory Barbels
Sawsharks also have sensory "barbels" which hang down from their face, where sawfish do not. These organs are used to detect prey hidden beneath the sand on the ocean floor. The rostrum can then be used to dig the victim up and slash it into bite-sized chunks (Compagno, 1984).
Different Places
Sawfish and sawsharks also have very different habitats, so you would never find them in the same place. Sawsharks live in deep, offshore regions, where sawfish live in coastal areas, in relatively shallow water (Compagno, 1984).
The Differences Go Deep
If you were able to look inside these two animals, you would also see many differences in their internal anatomy. Most notably, in the jaws and teeth. Sawsharks have a mouth full of small, spiked teeth, whereas, the teeth of the sawfish are flattened. This is due to their differing diets. Sawsharks target primarily small fish and squid, so their pointed teeth are handy for grasping wriggling prey. Whereas the plate-like teeth of a sawfish are ideal for grinding up crustaceans and molluscs (Compagno, 1984).
Not Equally at Risk
These fish also differ in how threatened they are by human activity. Of the 5 known species of sawfish, 2 are considered 'endangered' and 3 'critically endangered', whilst no species of sawshark which has been assessed has yet been assigned to any IUCN category (IUCN, 2020).
This is because the rostrum of the sawfish was prised as a trophy in the past, so they have all been pushed to the edge of extinction... That means a feature which made them perfectly adapted to their habitat might end up being what makes them go extinct at the hands of humans (let that sink in!). Thankfully, sawfish often do well in aquariums, so in the future, there is some hope that breeding programs will manage to successfully bolster their populations in the wild.
References
Aschliman NC, Nishida M, Miya M, Inoue JG, Rosanad KM & Naylor GJP (2012). Body plan convergence in the evolution of skates and rays (Chondrichthyes: Batoidea). Molecular Phylogenetics and Evolution, 63:1, 28-42. Access online.
Compagno LJV (1984). Sharks of the world. An annotated and illustrated catalogue of shark species known to date. FAO Fisheries Synopsis, Volume 4, Part 1 Hexanchiformes to Lamniformes, pp.130-131. Access online.
Greenfield T (2023). Pristification: Defining the convergent evolution of saws in sharks and rays (Chondrichthyes, Neoselachii). Access online.
IUCN (2020). International Union for the Conservation of Natre Red List of Threatened Species. Access online.
Jorgensen S (2013). Sharks: Ancient Predators in a Modern Sea. Firefly Books, Ontario, Canada. ISBN: 0228100801.
By Sophie A. Maycock for SharkSpeak
WILL BE LISTENING IN TO THE SEMINAR. INTERESTING.