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Limits on the size/mass of sea creatures.

  1. Sep 26, 2009 #1
    A limit to the size of a land creature would be mass, and another one might be the ability to pump blood throughout such a huge body.

    For a sea creature, what such biomechanical limits exist (taking aside already availability of food that might limit size)?
  2. jcsd
  3. Sep 28, 2009 #2
    Not entirely sure on your definition of "creature", but I don't see how/why limitations of size/mass should differ on land or sea.
  4. Sep 28, 2009 #3
    Well, they do. Whales for example are much more massive than even the dinosaurs. This is because of the buoyancy of water. Land animals have problems with supporting their mass for one thing.

    What I'm saying though is that there still must be a biological limit even for sea creatures like whales. I've heard estimates for land animals that suggest an animal could get no more massive than 500 tonnes. In the sea this should be a lot higher, but I'm wondering what the limitations would be in that environment.

    Obviously, there's food resources, but I'm thinking more along the biomechanical lines of its sheer mass (and it has to maintain buoyancy in the sea), as well as its organs and circulation systems.
  5. Sep 28, 2009 #4
    The ONLY attribute that limits the size of an organism is the ratio of surface area to volume. This is true for all organisms and many to most biological phenomena (hint: in bio classes the answer is almost always surface area). As the volume of a cell, organ, organism increases, it becomes ever more difficult to transport necessary nutrients through the given volume, and remove metabolic wastes. So, as an animal increases in size, the delivery of vital nutrients becomes more difficult. At the cellular level, simple osmosis tends to be enough to get nutrients to and throughout the cell, while at the organismal level, modifications, such as blood and lymph vessels become necessary.

    Terrestrial Animals are even more limited by gravity since many animals must also carry around their entire mass, a task considerably more difficult than floating in the water. So technically, there is no upper limit for the size of an organism (there are mats of fungal hyphae that extend for acres in area), so long as said organism can effectively obtain, deliver and remove nutrients and metabolic waste. Someone brought up the point of buoyancy, but remember that buoyancy is mostly determined by an animal's average density, which doesn't necessarily change as your increase total mass, hell even saturn is technically buoyant enough to float in water.
  6. Sep 28, 2009 #5
    So, could you have a creature say 10 x the approximate proportionate size of the blue whale that could still live?
  7. Sep 28, 2009 #6
    You can find the answer on this website. :smile:
    http://www.acsonline.org/factpack/bluewhl.htm [Broken]

    I love whales. I love dolphins too.
    Last edited by a moderator: May 4, 2017
  8. Sep 29, 2009 #7
    It's somewhat important what you mean by creature. From the sounds of it, you're talking about mammals in which case as Biophreak pointed out, we should be concerned with the fractal capillary branching to deliver oxygen supply to the body. The reason I single out mammals is because of the way nutrients are delivered to the body - other organisms can deliver them differently. (such as diffusion of nutrients for cells, again as Biophreak pointed out)

    Everything Biophreak talks about concerns metabolisms (you should read about the metabolic theory of ecology - really interesting stuff), and they often talk about size limits, but in terms of lower bounds, not upper bounds.

    I think the answer you may be looking for may be more physiological. This is not my area of expertise, but... our bones support our mass, allowing us to move on land, and the rate at which the bones need to grow thicker outpaces the mass of mammals. That is, if there existed a 50ft giant with our proportions, there's no way it'd be able to stand, much less walk.
  9. Sep 29, 2009 #8


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    Can't be.

  10. Sep 29, 2009 #9
    Hahaha, it's amusing because that movie is exactly what I was refering to :rofl:
  11. Sep 29, 2009 #10
    Farful mentioned the "metabolic theory of ecology" so I found this to be interesting.

    Proc Natl Acad Science, USA
    2009 Aug 18;106(33):13860-4. Epub 2009 Jul 30.

    LinksLatitudinal variation in lifespan within species is explained by the metabolic theory of ecology by Munch SB, Salinas S., School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, USA.

    "Many ectotherms exhibit striking latitudinal gradients in lifespan. However, it is unclear whether lifespan gradients in distantly related taxa share a common mechanistic explanation. We compiled data on geographic variation in lifespan in ectotherms from around the globe to determine how much of this intraspecific variation in lifespan may be explained by temperature using the simple predictions of the metabolic theory of ecology. We found that the metabolic theory accurately predicts how lifespan varies with temperature within species in a wide range of ectotherms in both controlled laboratory experiments and free-living populations. After removing the effect of temperature, only a small fraction of species showed significant trends with latitude. There was, however, considerable residual intraspecific variation indicating that other, more local factors are likely to be important in determining lifespan within species. These findings suggest that, given predicted increases in global temperature, lifespan of ectotherms http://www.ncbi.nlm.nih.gov/sites/e...Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSummay be substantially shortened in the future."

    Borek, I've stood over a little :rofl:toy train set before.
  12. Oct 1, 2009 #11


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    This is not an area that I'm familiar with, but I have a thought about it. So far, everyone has quite capably dealt with metabolism. It seems to me, although I might well be mistaken, that the maximum size would also be inversely proportional to the 'sophistication' of the organism based upon nerve conduction velocities. It wouldn't be too practical to have muscles perform actions several seconds after the brain orders them to. Even the contraction of a huge muscle itself, after the motor neurons trigger it, might take an inconveniently long time.
  13. Oct 2, 2009 #12
    this isn't such a terribly difficult obstacle to overcome, though. When you're dealing with animals that large, relatively instantaneous reaction speeds are hardly important.
  14. Oct 2, 2009 #13


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    Um, what biophreak said implies exactly the opposite of that! An animal 10x the length/width would have 100 times the surface area, but 1000 times the volume and that creates problems.

    Geometry presents limitations on scaleability for all sorts of things. Consider that you can make a small plane out of balsa wood - imagine trying to scale up a balsa wood plane to support a jet engine the size of a van!

    With whales, the problems of structural support against gravity and blood pooling in the lower extremities goes away, but there are other issues that don't: A whale 10x the physical dimensions of another is 1000x the mass, so requires 1000x the energy to sustain life. But a whale 10x the physical dimensions only has a mouth 100x the size - so how can it eat enough to sustain itself?
    Last edited: Oct 2, 2009
  15. Oct 3, 2009 #14
    Well, I see it would die of being unable to fuel itself long before this becomes significant, but at what point would it be so large that the sheer mass itself is a problem just as it is on the land? Cause even with the buoyancy of water, would there be a point that the internal mass would crush itself? Like the whale having organs so massive that they can't support themselves? Structural support should still be relevant in some ways. A ship, for example, is supported by the water due to the air it contains, but if you scaled the ship up proportionately enough, the increasing weight of its walls and supporting structures could not be held up by the cross sectional area of the material they are made from. Isn't that right?

    Another thing I've just thought of about when thinking of extremely large animals is the efficiency of their immune systems. Is size an issue against bacterial/viral attack?
    Last edited: Oct 3, 2009
  16. Oct 3, 2009 #15


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    Don't forget about overheating.
  17. Oct 5, 2009 #16
    I don't see any limit to the size a lifeform unless and until it depletes it's environment of nutrients and oxygen. It's a design problem. Distribute the mass appropriately so as to have an adequate surface to mass ratio and no excessive concentrations of mass for the conditions. A spreading fungus with nodes linked by tubular connections is an example. In theory, I don't see why a weird animal, something like a spreading mobile fishnet-like creature couldn't exist in a marine environment. Good SF stuff.
  18. Oct 5, 2009 #17


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    I was thinking earlier about something in the form of long tube and able to eat and breathe at every point of its body (say annelid with each segment having its own mouth). But I think it wont work - unless we can find sea without waves and currents, as water motion will sooner or later break the animal.
  19. Oct 5, 2009 #18
    My fishnet architecture would be better. It's self reinforcing. It could extend over a large area of the sea surface. If some pieces break, it could repair itself. The net could be as strong as commercial fishnet but perhaps more flexible so it could stretch. This thing would be a real horror story.

    I think your annelid (or some kind of giant "worm") could work too if had the right physical properties. I don't think a 20 km or longer line rope (used to secure ships at docks) would break up.
    Last edited: Oct 5, 2009
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