All the reference books I’ve seen and the links in Google state that sharks must continually swim to prevent sinking. However, I recall (vaguely) seeing a TV program that showed that this is not necessarily so, as the wave action of the waters will prevent them from sinking. So, what’s the straight dope?
I had always believed that they had to swim in order to force water through their gills. I’ve seen film of stationary sharks, but only in places where the current does the job for them.
Well sharks can could certainly be kept at a constant height in the water column without swimming if there was a very fast flow of water over their bodies, much as a pane can ‘hover’ in a very fast wind without actually moving. But that situation is almost never going to arise. For most sharks most of the time there will be no ‘wave action’ or any other appreciable water current to keep them up, and sharks lack a swim bladder. So the answer is that for almost any shark at almost any time it will need to keep moving or it will sink. For all practical purposes sharks need to keep swimming or they will sink.
As for sharks needing to swim to breathe, there isn’t any solid evidence for such a belief. Sharks use quite a bit more energy trying to breathe if they don’t keep moving, but it’s nowhere near enough to suggest they would ever suffocate as a result of lying on the bottom.
There are a lot of different sharks. Some have to swim, some don’t. Great whites, for example, have to keep swimming. A nurse shark on the other hand is quite happy sitting motionless on the bottom.
Actually depending on the species some sharks do need to either keep moving or have water flowing through the gills from the current. Ram ventillation is used by many large fast moving pelagic fish. Tuna use it, so do some sharks.
The sharks don’t have operculums to pump water through the gills, and thus need another way to get water flowing through the gills efficiently. Ram ventilation is simply swimming with the mouth open. Not all sharks need to do it however. Many species can lay on the bottom for hours on end gently huffing enough water to breath. For the more active species that ain’t enough.
Sharks compensate for no swim bladder by having huge oily livers for bouyancy, but that’s not usually enough either. Many species have fins and body shapes that help keep them from sinking as they swim such as the orientation of the pectorals, shape of tail, and having those odd noses that will deflect the water down (& push them up) as they move forwards. Of course there are also species that are bottom dwellers without such features.
Just swimming forward helps some active sharks both prevent sinking and aids in supplying enough oxygen to keep it all going.
Mike can you provide a reference for your claims that some sharks need to swim to get enough oxygen? All the studies I’ve seen show that sharks use a lot of energy to breath actively but I’ve never seen any that show that suffocation would result.
There was a Jacques Cousteau special called The Sleeping Sharks of [Someplace] that mentioned the belief that sharks had to keep swimming to breathe, and that these sharks didn’t. Might have been some other than Cousteau, but there was definitely a documentary.
On a related note see “40 Winks Under the Sea” by the ReefQuest Centre for Shark Research - fascinating article. Goes into a lot of detail about the issues discussed in this thread.
You don’t need to go too far to do the Jacques Costeau number. In places as close as the Florida Keys, one can find many sharks apparently sleeping underneath coral bulbs, wrecks, outcroppings, etc. Usually (as has been reported) these are nurse sharks, some quite large. They will stay there for quite a while, not moving, seemingly dead to the world. You can wake them up and scoot them along, so as to prove that they are, indeed, sleeping and not dead. And you don’t need intense SCUBA gear, either. You can observe this just skin diving.
Could you please provide a cite for this, particularly the relative energy demands?
Here are some cites:
http://homepages.tig.com.au/~idal/sharkfaq.htm
http://www.tnaqua.org/Animals/Fish%20ID/Bonnethead_shark.htm
http://www.biosci.ohio-state.edu/~eeob/eeob405/labs/sharkswimmingQ.html
(page is too short to quote)
Well here is one online reference. There are many more print references readily available if you want to take the time to do a search on Ovid or similar.
http://www.elasmo-research.org/education/topics/b_40_winks.htm
Frankly Colibri I’n surprised that a fellow biologist would find it all surprising that sharks that normally rely on ramjet ventilation would use more energy when stationary.
However that is beside the point. I would be very interested to see any actual evidence that any species of shark needs to keep moving in order to avoid suffocation. All the research I have seen says that there is a greater energy use for many sharks while stationary but that it is nowhere near enough to case any problems.
engineer_comp_geek those pages just repeat the commonly accepted assertion that some sharks need to swim to breathe. I would very much like to see some evidence that supports such assertions. A 9% energy use increase above ‘resting’ levels is hardly enough to warrant fears of immediate death. Humans experience greater energy increases going from sitting to walking and we are in no danger of incurring any oxygen debt when we walk around.
It’s obviously going to be hard to get a large pelagic shark and strap it down to test the theory one way por the other, but I’d be interested to know if there is any hard data at all to support the belief that any shark species dies when immobilised.
Here are a few pages talking about obligate ram ventilation in some shark species:
Many shark species are facultative ram ventialtors, and some are obligate ram ventilators. Obigates cannot meet absorb enough oxygen from the water to meet requirements without swimming (or having water flowing into the mouth). The meaning is kinda built right into the term… “obligate” ram ventilators.
As mmmiikkkee indicates, many sites on the web, including reputable ones like the American Elasmobranch Society and the British Museum of Natural History, indicate that some species of pelagic sharks are obligate ram ventilators, and state that they must swim in order to breathe. One specific species in which this is stated to be true is the Porbeagle Lamna nasus.
Another cite
Other sites mention that pelagic sharks will suffocate if prevented from swimming if confined in nets or prevented from swimming. One example is here
Of course I said nothing of the kind. It is of course reasonable that a shark will require more energy to breathe when stationary. What I asked you for a cite of was evidence for your assertion that:
The cite you offer does not provide such evidence. For one thing, the 9% increase in metabolic demand for Lemon Sharks is while they are “facing into the current and propped up on their fins;” in still water their metabolic demand will be greater. Also, the Lemon Shark is not a particularly specialized ram-ventilator. The Mako Shark or the Porbeagle are more typical of this group. As indicated in the article I cited, these kinds of species have a reduced brachystegial apparatus and hence less capacity to breathe when stationary.
Blake, please cite the titles of the “many more print references readily available” that you mention. I have on-line access to many scientific journals and can also obtain them from my library at work.
Unfortunately you have a history here of making statements that contradict standard textbook information (as in this case), and when asked to provide cites either posting not particularly relevant ones (like the one on the Lemon Shark), or else disappearing entirely from the thread. If you are going to dispute information that appears in a large number of journal articles as well as on authoritative websites (specifically that at least some pelagic sharks are obligate ram-ventilators), the onus is on you to provide support for your position.
Unfortunatlely Colibri oyu have a history here of speaking as though you were Cecil himself. Furthermore you frequently ask for references and then simply ignore them. I have provided a reference which supports exactly what I said. I’m sure you have access to Ovid or Blackwell or similar to find further articles yourself. What I have said has been fully supported, as usual.
Unfortunately you have not provided any cites for 2/3 of this, which seems to be your main point; that there is no such thing as an obligate ram ventilating shark species anywhere in the world and that all shark species’ physiological adpatations for breathing can be accurately generalized as being the same; ie a grey reef shark ventilates the same as a lemon shark. We await your cites disproving every (or any) scientific reference to obligate ram ventilators and of differences in ventilation strategies existing between species.
:D:D:D You said three things in the top quote, but the only one that was referenced was the irrelevent one… as usual.
What can be characterized as spirited debate and challenge is slipping dangerously close to a pissing contest. I’m waiting for “nyah nyah - did too,” “did not,” “did too…” Mods - stay tuned. xo C.
LOL, we’re simply asking for references showing established science and definitions to be invalid. Until then, (or more preferably instead)the answer to the OP is:
Most sharks do need to keep moving in order not to sink, although many rest on the bottom anyways so it’s no big deal. A side note is that most sharks don’t need to swim to breath, but there are a few specialized species that do.
Well, I may not be Cecil, but I do regularly help him out on questions of this kind. He has cited me by name in four of his columns (and I have assisted with several others). Cecil has also appointed me Straight Dope Curator of Critters. If The Master were answering the OP, there’s a good chance he would ask my opinion on it.
I think mmmiiikkkee has pretty well summed up the quality of your response. And you don’t do anything for your credibility by claiming that you know of “many more print references readily available”on the subject, and then not being able to produce one single example when asked for them - especially when you pulled exactly the same stunt in another thread last week. (I’ll be happy to provide the link if you want it.)
As far as me ignoring your references, you’ve got to be kidding. I responded directly to the only one you have cited in this thread. Meanwhile, you have ignored mmmiikkkee’s and mine. In other threads I have often responded to your points with cites of peer-reviewed literature, which you have rarely if ever responded to substantively. In fact, on several occasions you have simply abandoned the thread.
You don’t seem to understand either GQ or how to evaluate evidence. Blake made a statement that contradicts a large number of authoritative sources on the subject, and then has been unable to back it up with any references. I wouldn’t characterize what he is doing as “debating.”