Can Fish Swim Backwards?

[andywg]

Only "ram fed" sharks prank (i.e. those that only pass water over their gills by movement).

A notable exception is the nurse shark which has a habit of sitting on the bottom as it has an ability to pass water over its gills without needing movement. There are a number of other species whcih share this trait.

(ps: I assumed by "sharks" you meant those of the Elasmobranch Order, rather than the Cyprinids often refered to as sharks)

 

[Jules H-T]

Only "ram fed" sharks prank (i.e. those that only pass water over their gills by movement).

I accept this is the prevailing opinion, but I do seem to recall seeing a David Attenborough documentary some time ago that dispelled it as a myth that there were sharks that died if they stopped moving completely (or at least disproved it for some species traditionally accepted as being in this category). It showed footage of sharks previously though to never stop, sitting there doing nothing (not even holding station against a current) for prolonged periods. Whether they just conserved oxygen for a few hours I don't recall.

Perhaps someone else will remeber this as I have no details. Might have been part of the Blue Planet series, might not.

 

[Bignose]

@ Bignose, that's very interesting information!

On the topic of oscilliatory (sp?) vs pectoral swimming, and purely from my own observation, it seems to me that most fish in the aquarium trade are incapable of reversing the oscillations along their body (presumably due to muscle structure), and do resort to pectoral swimming (or at least a combination of the two methods) in order to reverse- the knife fish being an obvious exception, and they do spend as much time going in one direction as the other.

Yes, the math from the 1960's was a very simplified analysis. No consideration of muscle structure at all. In one of Prof. Lighthill's very next papers he introduces a better description of the wave. In the first paper, it is assumed that the entire fish oscillates with the same amplitude. Specifically, the amount the head goes back and forth is exactly the same as the tail. This is not true for most species, really only eels and maybe the knife fish. The next paper used an exponentially increasing sine wave... the head moves only a few percent of what the tail can move. This puts an implied direction in swimming movement now, though mathematically it is still possible to swim backwards. However, it is most likely just as easy to turn around, especially in case of emergency.

Here is where the different muscle groups come in, as well. Most fish have at least some white burst muscles. The white muscles can be activated very quickly for immediate bursts of speed, but cannot be used for very long. But, these white muscle groups are just for escape (or lunging at prey) so, if a fish can activate them and turn quickly and then swim head-first, that is probably preferred to awkwardly swimming backwards. Also, there is at least one practical consideration: the fish can see better what is in front of them rather than behind.

 

[andywg]

Can't find much on that, ecept for the fact that some sharks that are ram-ventilated seem to sit still in front of freshwater caves and the like where the flow of the freshwater is enough to provide respiration (as well as a possible benefit against parasites).

You have to remember that sharks are not bouyant, they are heavier than water, they have no developed gas bladder (such as teleosts, or "modern" fish) which is why the bones are made of cartilage (bone has an SG of around 2, cartilage around 1.2 IIRC). If a shark stops swimming it sinks to the bottom. If it has no way of getting water to pass over the gills it will suffocate.

It may well be that there are some that were thought of as ram-venitlaters are in fact not (perhaps through an ability to pump water across the gills in the short term) or have different ways of doing it (sitting in front of water currents).

 

[dgwebster]

i recall that show myself, and they were using cave and labrynth structures which even in the stillest had a small current which they used - it solved the question of why they all faced the same way inside it however it efectively allowed them to stop swimming and nature breath for them

 

[Jules H-T]

Fair enough, that makes sense. I couldn't recall properly what I'd seen. Cheers.

 

[csjasona]

I see most of my fish swim backwards. Certainly the male dwarf gourami does. And the clown loaches seem to defy all laws of water-bourne movement!