How well can sharks smell?

Sharks sudden appearance at shipwrecks has historically credited them with a sniffy supersense. We wanted to find out whether new research into exactly how smell reaches a sharks nostrils confirmed the legend so Chris Smith spoke to Dr Jonathan Cox from the University of Bath to find out more.

Chris -  So, tell us about your work then.  You actually started with a shark in a tank, but it wasn't a live one?

Jonathan - That's right. It was a plastic shark or rather a shark's head that we'd made a model of as a result of getting a real shark's head from the Natural History Museum into a CT scanner and getting the scanned data and turning into a plastic model which we then stuck into a tank in an engineering department.

Chris - So, you take the real shark, scan it to get very high resolution, ultrastructure or what makes up that head, all of the passages. So, you can see the structure and then you make a model so that you're going to do experiments on the model?

Jonathan - That's right, yeah and we made, as I said, a plastic model, but most of it was opaque and a little section of it which comprise the nasal region was transparent so we could see into it.

Chris - And this means that then you can begin to ask questions very consistently like, how does water flow around this when it's moving along, how does water flow along the nasal passages and all that kind of thing I presume?

Jonathan - That's right, yeah. So, in order to do that, we used tiny filaments of red dye and fired them at the transparent nasal region, and looked to see what happened.

Chris - So, you didn't make the head move. You made the water move because obviously, it's the same thing. The water is going to flow the same way whether the head is moving or the water is moving, and then you're tracing where those filaments of dye go.

Jonathan - That's right. It's much easier to make the water move. Getting a head to move is pretty difficult. Although it was a hammerhead shark and the head of a hammerhead shark wobbles from side to side, so hammerhead sweeps its head from side to side. So, it would be nice actually to mimic that motion and that's a sort of a future experiment.

Chris - So, hammerheads have their eyes on the ends of the hammers. Do they also have their nostrils there then too?

Jonathan - Yeah, they're right on that front edge, sort of in the face of the action as it were. So, the eyes are on the sides and the nasal region is right on the front edge of the head.

Chris - So, what happens then when in your case, the dye, but say this were a shark out in the open ocean and there was some chemical in the water that it was sniffing down, a blood of a prey for example. What's actually the process?

Jonathan - Several things happen. First of all, water enters nasal passages and gets distributed over the large sensory surface area that's got packed into its nasal chambers. And as a result of getting past over the sensory surface get sensed. But also weirdly, quite a lot of the water or flow actually passes over the head and we think that  this actually helps paradoxically flow to go through the nasal chamber, so it helps the flow through the nasal chamber.

Chris - What about the fact that those nostrils are so far apart? How does the shark exploit that? I mean, it must have evolved that way for a very good reason and this presumably does give it an enormous advantage of sniffing down concentration differences.

Jonathan - Yes. Well, it's still that the further apart your two noses are or two nostrils are, then the easier it is to locate the source of a scent. So yeah, that's what it seems to be, but it has not been proven.

Chris - So, going back to where we started which was that people say, sharks have this extraordinary ability to track down prey over enormous distances. Do your experiments bear that out?

Jonathan - That's a physiological experiment which we were not able to do, working with a plastic model, but what we were able to do is to show how the flow - in other words, the water carrying a scent is distributed over the sensory surface area within the nose.

Chris - So, once it actually goes up the nose, what happens then because there is some quite interesting studies done on humans showing that we close off one nostril at one time and have a relatively open nostril, and this changes the rate at which the fluid air in our case (it will be water in the shark obviously) flows over the surface that detects things because some things are more quickly detected by the respiratory - the ciliated surface that does the detecting. So, the more water or the more fluid that can go up there and the more odorant, the more you're going to smell while other chemicals take longer to dock on to the things that smell them. So, slowing down the flow a bit is beneficial.  Do you see the same thing in these sharks?

Jonathan - Possibly with a hammerhead because it sweeps its head from side to side. One swivel of the head, it's going to encounter odiferous water. In other words, that's got the odorant molecules in and the other side, the other nose as it were is going to be kind of blocked to use your terminology because it's not going to be in the scent, but then it all comes back again. And also, by shaking the head from side to side, it's possible that there's a little bit of agitation of the water inside the nasal cavities and a kind of asymmetric mixing of that water. None of that is proven, but that's something to be looked at.