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I have some species of aliens that have pseudopodia, less like the temporary protrusions of amoeba but more like the retractable eye stalks of snails. Their body is covered in areas where limbs can protrude from by forcing blood into that area.

Their organs have a degree of movement within their body. They are similar to blob creatures but with less freedom and liquidity in their form. They can shape their body to most simple shapes and add detail with temporary limb structures. When they need to move or interact with things temporary limbs can protrude and retract when the their job is done.

Would a creature that can change its shape and has temporary limbs have a default/resting shape and what would be the most efficient shape if they did?

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From what you describe, these creatures seem primarily composed of organs, muscle tissue, connective tissues, and vascular structures. The "default" resting shape can be determined by:

  • Skin thickness and flexibility.
  • Firmness of muscle tissue.
  • Connective tissues between muscles.
  • Blood pressure.
  • Gravity.
  • Shape of its immediate surroundings.

Invertebrates can hold their shape, even in a resting position. Relaxed muscles will probably make the creature conform more to its surroundings, but not become a puddle.

Generally, the resting position of an animal will balance energy conservation with positioning itself to react to changes in its surroundings.

Unless a scientific explanation for this shape is necessary for plot development, choose a shape that best fits the story and the plot elements this creature satisfies. For instance, if this creature needs to lay still in order to ambush prey or opponents, make the creature as flexible as you need to hide in whichever places and shapes that you want.

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    $\begingroup$ A silly picture popped into my head reading this answer. The least energy used would result in a squashed egg-shape blob with the lower pseudopodia extended proportionate to how far up the body they are, with the lowest being fully extended, and the highest perhaps even being inset. $\endgroup$
    – GOATNine
    Commented May 31, 2022 at 17:18
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    $\begingroup$ Draw it, @GOATNine! You have my upvote in advance! $\endgroup$
    – Willk
    Commented May 31, 2022 at 17:36
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    $\begingroup$ @Willk I would, but I don't do self portraits! $\endgroup$
    – GOATNine
    Commented May 31, 2022 at 17:38
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What's the weather like?

It occurs to me that the answer to this question depends quite a lot on the creatures environment. On a very basic level, if this is a cold environment, and the creature is concerned with maintaining body temperature, then it will seek to minimise its surface area by tucking itself into a ball shape, and therefore retracting its pseudopods. If, however, the creature lives in a hot environment, and is more concerned with venting out surplus body heat, then I can imagine it would want to keep its extremities elongated, specifically for the purpose of radiating out body heat.

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I would probably say that a convenient resting position would be like a flat circle. To stick out a limb would imply the use of energy, so a circular shape would ensure that essentially no energy is exerted.

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It will probably be Worm or Slug like.

enter image description here

Before you can exactly answer this question you first need to figure out the organism's pattern of symmetry. Every multicellular organism can be generally classified as Asymmetrical (Sponges and some Plants and Fungi), Radially Symmetrical (Jellyfish, Starfish, most plants, etc.), or Bilaterally Symmetrical (Most animals).

If we assume your alien is evolved enough to be able to use these temporary limbs to some effect, we should assume it has some kind of complex organ structure as most animals, meaning it will probably have some kind of Bilaterally Symmetry which is helpful for designing things like uni-directional digestive tracts, and minimizing organ redundancy.

As for the other aspects of its shape, its degree of flexibility means it has a generally fluid like internal structure. This suggests that it's skin will probably obey some kind of surface tension like rules when at rest.

The best example we have of this in the real world is the Amoeba. Even though it's able to form itself into distinctly asymmetric shapes, at rest it prefers a generally worm like shape.

enter image description here

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  • $\begingroup$ Worm or slug like depends on the position of the head, so when searching/moving the stretched form with the head at the front will occur. Some of the species do take this form but I am not sure if it is the most efficient to handle interactions from all angles. $\endgroup$
    – user96146
    Commented Jun 1, 2022 at 18:31
  • $\begingroup$ @RayHammond Amoebas are also bilaterally symmetrical at rest and they are basically the microscopic version of what you are looking for. An elongated resting form means the organism has the advantage of being able to stretch longer than a radially symmetrical organism of similar mass. Also, there will be organs in this alien that you don't want to be more redundant than they need to be. If your alien needs 1 and only 1 Xylobladder to survive, then having to replicate it over and over again radially could make you more full of extra organs and less full of stretchy stuff. $\endgroup$
    – Nosajimiki
    Commented Jun 1, 2022 at 21:53

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