Hammerhead sharks are known for their unique head shape and researchers are fascinated by the potential advantages that their wide heads may give them. While all shark species have a knack for detecting the scent of prey with their powerful noses, some researchers have wondered whether the shape of the hammerhead could mean that picking up scent is easier for these species, since the space between the nares (shark nostrils) could help them to figure out where a scent is located. Hammerhead species also have specialized grooves in front of the nares which could help them to detect faint scents in larger volumes of water. While previous work has suggested that hammerheads are no more sensitive to scent than other sharks, there is still much to learn about how the sense of smell is shaped in this extraordinary species. Lauren Simonitis and Christopher Marshall from Texas A&M University, USA, investigated the olfactory organs of bonnethead sharks, a hammerhead species with a slightly narrower head than the typical hammerhead, to understand better how this system picks up scents when water flows over it.
The researchers collected four bonnethead sharks from local fishers for testing. They took detailed photos using a scanning electron microscope of the tiny ribbings, called lamellae, which cover the scent detecting olfactory bulbs and interact with the water to allow a shark to sense a scent. They also studied the internal structure of the olfactory bulbs using a light microscope to view thin slices of the olfactory bulb. They measured how much of the olfactory bulb was made up of tissues that can sense scent and compared the shape of the lamellae across different areas of the olfactory bulb, since previous studies had revealed that water flow rates vary across the bulb.
The research duo found that the bonnethead shark's olfactory bulb is similar to that of other shark species, but they lack structures known as true olfactory knobs (swollen structures on scent reception cells) that occur in other non-hammerhead sharks, such as spiny dogfish. They also found that bonnethead sharks have larger lamellae in areas where the water flow over the olfactory bulb is highest and smaller lamellae in areas where the water flow is slower. The areas with high water flow tend to be the most scent-sensitive tissues, with more folds in the lamellae, which could help the lamellae to physically withstand powerful water flows while picking up fainter scents from the fast-flowing fluid.
These results tell us that while many of the structures in the shark olfactory systems are similar across different species, there are important variations that can affect their sense of smell. Looking within the olfactory bulb, we know that water flow rates can vary in different areas and see that the shape of the lamellae and the area of sensory tissue are correlated with these differences. The increased sensitivity in these areas is essential for the sharks to detect dilute scents carried in fast-flowing water, just like it is harder for us humans to smell something outside when it is windy compared to the stagnant odours in an unaired room.