Stingrays, sawfish snouts & peeing sharks on the radio

This week I had another trip to London to catch up with the Home Planet team at the BBC. For the first time I appeared alongside the fantastic astronomer Carolin Crawford, which meant we were 50:50 girls:boys – usually there’s a distinct male bias to the proceedings. It also meant that Toby the producer had a bit of a task fitting together questions about space and the oceans.

On the aquatic side of the universe, we had one of those deceptively simple Home Planet questions that makes you think ‘Oh yeah, I hadn’t thought of that’:

Why are rivers fresh but the seas they run into are salty?

Philip Stott did a nice job of explaining the fresh/salty situation (it’s all about slightly acidic rainwater weathering rocks on land, dumping minerals and salts in the sea, where they get left behind when freshwater evaporates). Then I stepped in to talk about the challenges that aquatic life faces in fresh and salty water (or both)… time to dig out my GCSE biology and the whole spiel water diffusing across cell membranes from lower to higher concentrations in a grand attempt to even things out.

I decided elasmobranchs would be a neat example of a group of animals that figured out how to move from the sea into freshwater and back again.

And it gave me the perfect excuse to talk about sharks – they deal with life in the sea by loading their tissues with urea (hence shark meat smells and tastes terrible unless it’s prepared properly… or left to rot underground for months as is the case for Greenland sharks – yuk). Some sharks (including pajama sharks) sneak into less salty waters by peeing a whole lot more than they normally do in the sea.

Are they called that because it looks like they're wearing pajamas?

Then came part of my sharky story that didn’t make into the final cut of the show – I had a few things to say about river stingrays. These are the only group of elasmobranchs that are permanent, fully paid-up members of freshwater ecosystems. These guys permanently have less urea in their tissues which essentially makes them freshwater prisoners unable to return to the sea.

And I was slightly miffed they cut this bit out because I’d spent ages practicing how to say Potamotrygonids. Ah well, I’m sure that’ll come in handy some day…

We also had a nice quicky question:

What are sharks teeth made of? Answer: same stuff as human teeth.

Sharks are covered in tiny teeth (aka dermal denticles) which makes them slide silently through the seas – so you might not hear a shark sneaking up on you. But a very Helen Scales moment hit the cutting room floor, as I enthusiastically mimed (on the radio, yes) the sawfishes’ long, protruding nasal implement, adorned in modified teeth. *sigh*

And our quick chat about shark teeth also raised one question in my mind that I’ve not managed to answer yet: did teeth originally evolve from dermal denticles or the other way round? Or did they evolve on several separate occasions in different groups? Answers on a postcard please.

Home Planet wasn’t the only BBC Radio 4 show this week to mix together space and ocean science. Jonathan Freeland took The Long View of the end of the US Space Shuttle programme by comparing it with the decommissioning of the British oceanographic survey HMS Challenger. Well worth a listen.

And you listen to that episode of Home Planet here.

Share your thoughts