[Pia Asa]

Pia Asa asked the Naked Scientists:

Hi there,

I read recently that some sharks breathe through their mouths so they need to keep swimming all the time so that they can pass water through their lungs.  But some sharks don't need to do this.  So which ones evolved first?  Are the ones which need to keep swimming and breathe through their mouths more primitive or more advanced?  If they are more advanced then why would they want to do this?  Doesn't it take more energy to keep swimming all the time?

Thanks, Pia.

What do you think?

[dentstudent]

I don't know the answer to this, but an interesting thing about sharks is that they urinate through their skin!

[DoctorBeaver]

Pia - sharks do not have lungs. Like all fish, they have gills where oxygen from the water permeates into their bodies.

As far as I am aware, the question of them having to keep swimming is as follows. Unlike most fish, sharks do not have gill covers. Consequently, it used to be thought that all sharks had to keep moving forwards to prevent water getting into their gills the wrong way. This, however, has been proven false.

from http://www.google.co.uk/imgres?imgurl=http://www.delportdupreez.co.za/diving/sharks/assets/images/sharkgills.jpg&imgrefurl=http://www.delportdupreez.co.za/diving/sharks/html/anatomy.html&h=272&w=445&sz=29&tbnid=EZ1-icPss7gJ::&tbnh=78&tbnw=127&prev=/images%3Fq%3Dshark%2Bgills&hl=en&usg=__IkwYSiXEZv97Hfu2eI50HHUVwPU=&sa=X&oi=image_result&resnum=3&ct=image&cd=1

Water enters the gill chambers through the mouth or spiricles in order for the shark to breathe. Previously it was thought that sharks had to move to keep breathing, but know we know that they can pump water over their gills by opening and closing their mouth. Due to low blood pressure the sharks rely on muscular movement to circulate their blood. Blood in the gill filaments absorb oxygen from the water and water then exits through the gill slits.

I think that probably answers your question about "mouth breathing sharks" - they all do it.

[atrox]

How Sharks breath depends on their way of living.
palagic sharks (means, sharks living in the deep oceans) have to move to get enough oxigen ... the great white shark for example has to swim at least 3 km/h so that the water can flow trouhgt their open mouth over the gills.
Some of these sharks rest in underwater hives, where the flow is strong enough. But mostly they are far aways from any ground...so their is no question if they really"want" to rest or if they want to rest during swimming. They just don´t have the choice, so it´s no question of energy-wastage at all.
benthic sharks (means sharks living at reefs or at the shores) are able to breath while resting. They use a completely different method for respiration. The open an close their mouth alternately. When they open their mouth, water gets sucked in...the gills are closed by that moment. Then they close their mouth, open their gills and the water gets pushed out through the gills.
Some sharkes are able to use both techniques.

I don´t know, which method is the primary one..therefore you would have to know where sharks first did evolve... maybe some specialists know ... I don´t know why, but I would bet, that they did evolve at open seas...

bye
aj

[DoctorBeaver]

atrox - that article I quoted from made no distinction between palagic & benthic sharks. What I have seen, though, is that the main reason some sharks have to keep moving is not to do with breathing but because their blood circulatory system is weak. If a large shark is transported in a box of water it may die because of inadequate blood circulation.

It is true, though, that the larger species of whaler shark have to keep moving to ventilate their gills.

[atrox]

Hi Doc...

...hmmm.. I started to investigate a bit more, and I also read the article you linked to.

Previously it was thought that sharks had to move to keep breathing, but know we know that they can pump water over their gills by opening and closing their mouth.

Thats not true. Not all sharks are able to do active respiration! It´s as wrong as the general statement, that sharks have to move to keep breathing. Some sharks have to move to keep breathing because they are just not able to do active breathing... they need to use the flow of the water... for example many of the lamnidae/ mako sharks like the great white shark. They use the difference of pressure between the area in front of their mouth and the area behind their gills... this process gets its drive by the movement of the shark (it´s called "ram ventilation")
Then there are sharks, which use active respiration like the nurse sharks for example.
And there are few shark-species which are able to switch between ram ventilation and active respiration like the spiny dogfish or sandtiger shark


I may have found an interessting link, but I have to admit, that I didn´t read much more than the Title yet  []
http://jeb.biologists.org/cgi/reprint/103/1/185.pdf
...its about energetic costs of active branchial ventilation in sharksuckers, which are also able to switch between both types of respiration... I think there would be no great difference to energetic costs for sharks as well.

ok, thats what I "know" about how sharks breathe (but of course, I only "know" what I read about it in this case..never saw a living wild shark yet)

bye
aj

[DoctorBeaver]

atrox - thanks for that. I too have been more diggng and found the same as you. That first article was a bit misleading.