By Paddy Burke (Sharks4Kids ambassador & PhD student at Macquarie University, @Patrick_Burke)
The ocean is filled with so many incredible creatures, incredible and bizarre. Many of these fish have evolved under similar pressures which have led them to look quite similar even if they are genetically quite distinct. This similarity in appearance or development of similar structures is what is known as homology. One great example for this can be seen in sawfish and sawsharks!
Now many of you may be saying, wait, aren’t those the same thing? The answer is no! While these two groups of fish are very similar in appearance, they actually have quite a few big differences that can make it easy to tell them apart once you know them! The differences we will cover today are their ‘saws’, general body size and the gills.
The ‘saws’ or elongated toothed rostra are the features that make these fish really quite unique. The saw is thought to be used for predation and defense in both groups. However, the actual structure of the saw is really quite different! Sawfish saws are lined with a number of teeth along both sides of their saw that vary in size and number by species but each one of these teeth are permanent! Meaning that if they happen to lose one it will never grow back. The rostral teeth of sawfish also grow from their base, like a rodent’s tooth, so that the animals can sharpen them regularly in the sand. Sawsharks on the other hand have a very different saw. Teeth can be found along both sides of the saw like in sawfish, however, sawshark teeth are replaceable! The most distinguishable feature to separate a sawfish and a sawshark saw is the presence of what are called ‘barbels’. Barbels are found only on sawshark saws and they look like a little moustache coming off the saw. This moustache is thought to have a tactile function, meaning they use them to feel around in the sand for food.
The size of fish is an easy indicator of species between these groups. Sawfish are very large fish, some reaching over 7 meters in length! While sawsharks are much smaller, averaging around 1 meter for an adult depending on species. In addition to size, it is important to know that sawfish are technically rays while sawsharks are true sharks. What that means is that in sawfish you will find their gills located on the underside of the fish. Where as in sharks they are located on the side of the head.
These fish could be considered ‘cousins’ in a way since they are both chondrichthyans, meaning they both have a skeleton made out of cartilage instead of bone. However, it is important to remember that sawfish are actually rays while sawsharks are true sharks. Sawfish may not follow the typical body plan of their relatives the stingrays, but they are more closely related to them than they are to sawsharks!
A sawshark, Pristiophorus nudipinnis resting on the substrate. See the barbels? Image by Tristan Guttridge.
A freshwater sawfish, Pristis pristis, is disentangled from a gill net in order to be tagged and released. Its teeth are evenly sized and spaced, contrary to those of sawsharks. Image by Dave Nash.