UKC

Went to the London Aquarium the other day (some beautiful tanks, but a huge chav magnet - go at quiet times if you don't want to be obstructed by proles taking pictures on their mobile telephones).

We were listening to a talk about sharks when the woman explained that sharks (most species) have to keep swimming all the time because otherwise no water comes through their gills and they can't breathe. This is a bore for the poor things because they therefore can't sleep very well. However, sand tiger sharks, she said, have learned to pop up to the surface, gulp down some air, and use that for five minutes so they can have a bit of sleep without using their gills.

How interesting, I thought vaguely at the time, but on reflection, WTF?? How does that work? Fish can't breathe air, so what on earth do they do with this air they've gulped down? I wish I'd asked now - any marine biologists out there? Iain R, maybe?

Also, I thought sharks had to keep swimming because they had no swim bladders and otherwise they sank, not so that they could breathe. No? How do other fish manage to breathe when they're not swimming, then?

jcm

In reply to Anonymous:

You shouldn't think so much; just believe what people tell you

Sharks have no operculum.
Sand Tigers gulp air into their stomachs to float a bit, but I don't think they breathe it in.

It all sounds a bit fishy to me.

In reply to Anonymous:
I expect you'll find a bone fide RT expert to give first hand info, but your post made me curious, so I googled:

"Many sharks lead extremely active lives, so they need an efficient supply  of oxygen to their muscles and organs. Sharks breathe oxygen from the water by taking it in through their mouth, allowing it to flow over their gills and then expelling it through their slits.
Sluggish, bottom-living species such as Catsharks and Wobbegongs use special muscles to pump water over their gills even while at rest. But fast, highly active species such as Mackerel sharks use their forward motion to force water through their gills; a  process known as ram-jet ventilation. These sharks must keep swimming in order to breathe. Between these extremes are species like the Grey Nurse which pumps water over its gills at rest, but switches to ram-jet ventilation when swimming to save energy."                   

LOL, just noticed it's a primary school website... they've obviously got the kids researching sharks. And very nice too...

Then there's this from a diving site:
"called a Reef Shark. While many species of open water sharks must keep swimming to survive, the Caribbean reef shark can remain motionless on the bottom by pumping water through its gills. Although rare, these sharks have been seen resting on the bottom, in caves or under ledges. The Caribbean reef shark usually travels alone or in groups of 3 to 6 sharks."

And this from the uni of delaware:
"Another difference from typical finfish is that sharks lack an air-filled swim bladder, and hence, most sharks must keep swimming to keep from sinking."

Another interesting one:

"NOVA: Do sharks have to keep swimming to breathe?

Gruber: Several species do, including hammerheads and mackerel sharks. Typically, pelagic sharks that never encounter the bottom are adapted to swim all their lives. But the vast majority of sharks have a buccal (mouth) pump and are not so-called "obligate ram ventilators." (Ram ventilators like hammerheads and great whites must swim to pass water over their gills.) So the answer is no, emphatically. There are even some sharks with spiracles, holes on the top of their head that allow water to enter their gill chambers when the mouth is on the bottom."

Ooh, that's from a site advertising an IMAX film, island of teh sharks. Sounds good.

" Great White sharks must swim constantly or they would sink to the bottom of the ocean. Like other sharks , they have no gas filled bladder to keep them afloat like bony fish do. Also like other sharks they have a large , oily liver that provides some floating ability. Great Whites cannot swim backwards or even come to an abrupt stop , because their fins are not flexible like other fish. Great Whites also must keep moving to breath , this is why if they get caught in nets the die very fast. Great Whites are also only one of a couple species of sharks that jump out of the water while catching prey. They sometimes go down deep in the water and rocket skyward to catch their prey."

ps That one was sharks.com

In reply to gingerkate:
Stop hogging threads Kate!

In reply to Mike C:
Nope, I'm obsessed now:
"Based on current knowledge, the sand tiger shark is the only shark species which swims to the water's surface and swallows air in order to regulate its buoyancy, enabling the animal to move around and remain at any desired depth. "

That's from a site called shark info...

In reply to gingerkate:
I can see that. You have brief mail. Dum.., dum, dum, dum. Dum.., dum, dum, dum.

Right, there are loads of other sites saying exactly the same. So sounds like she had it a bit wrong, John.

Reminds me of the guide who took us round White Scar caves and described one formation as 'the arum of lily' and also commented that whoever had named it had obviously never seen a lily as it looked nothing like a lily.

In reply to Anonymous:

Most of your points have already been answered John, but on the point about fish gulping down air, some fish can do this. Some simply absorb oxygen from air bubbles in the gut or the spaces around their gills (some catfish, for instance); some have specialised organs designed to help extract oxygen from air (anabantids, like the popular gouramis in your average tropical fish tank). And then there are the various sorts of lungfish, of course.

Many species of loach and catfish kept in aquaria can often be seen doing this. I don't think it applies to bottom living marine fish though. Apart from anything else, it has to be done quickly to avoid predators and, even in the absence of a swim bladder, dashing up and down the sorts of pressure gradients involved would be a problem, I think.

In reply to gingerkate:

Can I just point out that I said all this, with some brevity, before 07h00

In reply to Kipper:
No you didn't, you talked about operculums... and all over the country, climbers peered blearily through their hangovers wondering wtf an operculum is.

Then you said you THOUGHT they did it to float. But we didn't know how well-informed your THOUGHT was. It might've been a down-pub-7-pints-talking-about-something-I-know-nowt-about thought, or it might've been a professorial expert seven-year-study scratching tiger sharks' bellies thought

In reply to gingerkate:

Re: operculums - I left the discovery of these to the reader.

I have tickled Sand Tiger bellies, and I know they don't breathe through them.

In reply to Kipper:
There you are then! You should've said, "I have tickled Sand Tiger Bellies..." (the caps make it look even more impressive) and everyone would have known you to be the man.

But I thought that bit about ram-raiding (or whatever they called it) to get the air driven into them was rather good.