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There have been questions about sizing giant animals, giant humanoids, giant flying creatures (also discussed particularly for dragons and giant floating mammals), and giant spiders, one type of invertebrate. The first link has some general applicability here, but I am particularly interested in the mechanics and feasibility of a different type of invertebrate, giant worms, similar to what this question mentions:

60 feet in length and 20 feet in girth burrow up from under the mountain

That question has the worms carnivorous, but I am more interested in a generally docile worm, much like a large earthworm (though obviously at that type of size, could incidentally seriously injure or kill a person).

Parameters for Feasibility:

  • Environment: Lives on Earth-like world (gravity, atmosphere, etc.); essentially, if in question, pretend we are talking about Earth.
  • Ecosystem: Land based (burrows like an earthworm, though [preferably] may be considered to in some way penetrate harder rock than just loose earth); may be able to swim as well; must dwell exclusively subterranean with possible deep ocean excursions (whether swimming or just poking itself out of the earth at deep ocean depths).
  • Size: 10-15 feet in diameter at the most, but may be however long it is deemed necessary within reason; proportions of earthworms do not directly scale width/length according to the previous Wikipedia link that states:

    10 mm (0.39 in) long and 1 mm (0.039 in) wide to 3 m (9.8 ft) long and over 25 mm (0.98 in) wide [the larger size being the Giant Gippsland earthworm]

    I would not desire my 10' diameter worm to be 1000' long! So they will be a wider, stouter variety (more like a Goliath Beetle's larva proportions, which "are capable of growing up to 250 millimetres (9.8 in) in length" with a much thicker shape). Something in the 50'-150' range seems reasonable to consider.

  • Shape: May have some type of small "leg-like" appendages either in the front or full length, but generally still a worm more than a centipede, so either very few, short stout legs (like the grub example above) or smaller, almost "hairlike" (in proportion to its size, anyway) legs the run the length.
  • Diet: Not naturally carnivorous, though if they are "capable" of eating flesh, that is okay; ideally, while earthworms subsist not off earth itself, but plant matter in the earth, some consideration of how they might actually digest and subsist off earth/rock would be nice (acidic juices that "dissolve" the rock before ingestion, etc.), since they are purely subterranean.
  • Intelligence: Nothing more than animal (or even that of an earthworm) needed.

What things need to be considered to have such a worm be "naturally" feasible? A small suspension of belief is acceptable, but generally what kind of systems (physiological, ecological, etc.) would it need to have in place to grow to that size and possibly live off "earth" (or if not, what would it need to survive)?

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    $\begingroup$ I'm assuming you have read Frank Herbert's Dune, which is pretty much the de facto literature on the development of large worms. $\endgroup$ – Cort Ammon Apr 16 '16 at 0:18
  • $\begingroup$ Pier Anthony's "Magic of Xanth" has a series of friendly worms including a large one called a diggle. Removing its magical ability, you can have it replace all earth it eats (like a dwarf in the Artemis Fowl books) to prevent cave-ins from their paths and more solid rocks in the earth to help stabilize it and mitigate some earthquakes caused by the worms moving. $\endgroup$ – Marion Apr 16 '16 at 0:39
  • $\begingroup$ @CortAmmon: I am familiar with but have not read Dune (though I probably should make it a point to); however, the sandworms are far larger than what I envision (or would expect could conceivably dwell on an Earth-like planet; the desert sands of Arrakis makes sense), since they are 10+ times larger in diameter. $\endgroup$ – ScottS Apr 16 '16 at 4:58
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First thing I would consider is gravity on the worm, you have, essentially, a giant sack of water. You need to consider how it holds itself up, things like whales tend to die out of water because they are unsupported.

Potentially it could use the walls of its tunnels to support itself, that could be an interesting limitation on the worm, if it exposes too much of itself it can die.

Next you have heat, living things need to dissipate heat, depending on how fast your worm is going the heat needs to:

a) get removed from the worm (conductive slime to transfer heat to walls? excrement?)

b) get moved far away from the worm so it can continue to produce and eject heat

Maybe your worm makes holes in the surface to shoot out piles of high-temperature fecal matter, maybe it needs certain chemicals to run endothermic reactions internally, maybe its just very very heat resistant (and has a high conductivity meaning the heat transfer through the worm is fast).

Food, urgh, this thing is going to be using a huge amount of energy to move. I think biological machines are about 30% efficient, so all that heat it made, it needs all that and more (i.e. only 30% of the energy it eats is food, the rest is heat). This is probably going to need some suspension of disbelief. It needs to either consume dirt that gives it more energy than it takes to move through the dirt, or eat something else.

Maybe you could get away with it eating coal or something, a high energy-density material like that. Otherwise its going to be biomass of some kind. Probably not flesh because theres probably no way to get enough flesh efficiently enough to sustain it (unless its a stationary trap worm). Meat tends to run away from things that try to eat it. Also it would need a very large prey population. This would have to maybe eat trees? Maybe it eats a tree and then just sits there and digests it. So you dont have the heat and the efficiency problem, since its only the minimum energy it needs. Then it goes on to the next tree, since this is a forest. I guess then it kind of rejuvenates the forest behind it. Maybe it serves as the forest fire of its planet?

Hope this helps.

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  • $\begingroup$ +1 for giving me some stuff to think about (holding itself up, heat distribution, energy), but I forgot to add a parameter into my specs that does affect your thoughts; namely, these need to essentially spend their time subterranean (with possible deep ocean forays). $\endgroup$ – ScottS Apr 16 '16 at 4:45
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This started as a comment on Taha's answer, but grew in length.

The problem with giant animals is going to be how to fuel them and what sort of lifestyle they will need in oder to get enough food and extract enough energy from it to survive.

Monster earthworms will have to extract a lot of energy from the environment somehow, and will also be using a lot of energy to burrow through the ground. Their sheer size will also make significant impact on the local ecosystem, so the worms will be co evolving in some sort of environment where they "make sense".

Forest worms actually seem to fit the bill, since tree trunks represent a concentrated form of energy (as anyone with a fireplace or wood stove knows). A forest worm that rises up, eats through a root and then knocks over the tree to consume the trunk and foliage will be able to extract a fair bit of energy, but their biology will be very different from any conventional worm.

In order to consume a tree, the forest worm will need a mouth full of cutting teeth resembling a lamprey in order to cut through roots and the trunk of the tree. This wood chipper like mouth will not be enough to actually digest the tree, so internally there might be a gizzard full of rocks to grind the wood into a pulp, or a series of stomachs and intestines full of bacteria that digest the wood chips (somewhat like a termite), with the worm actually living off the byproducts of its microbiome.

Churning slowly through the earth will use up a lot of energy, so the worm's lifestyle will be to slowly burrow up to a tree, cut through the roots, then rise up and slowly consume the fallen trunk. Since the worm will be partially on the surface and exposed at that point, it will have an armoured skin to resist predation, and presumably retreat into the burrow after cutting through a part of the trunk, rising to take another "bite" later on. The worm's sense organs will have to be attuned to the presence of trees and roots, as well as sensing vibrations, electrical fields or other means of locating potential threats and being able to analyze the nature of the soil it is burrowing through to bypass large rocks and other underground hazards. Being able to locate other worms underground is also a bonus for finding a mate or defending it territory from other worms.

The forest will benefit through the deeply tilled soil filled with organic matter in the forest worm's castings, as well as the ability of the tunnels to absorb and hold water during rainstorms (this might be especially important if the climate is extreme like a rain forest or a monsoon environment.

The worms will have grown large not only to consume trees (presumably there are smaller sub species which live on the edge of forested areas which consume scrub and shrubs), but in response to predation. We already have an armoured skin, and a wood chipper like mouth, so a question you might ask is what sort of creature would feast on a massive forest worm? (presumably there is also a subset of parasites and symbionts which live in or off the worm). Working out the rest of the ecology of the giant forest worm should be interesting...

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    $\begingroup$ +1 for the thoughtful answer, though I forgot to mention in my parameters that these need to essentially spend their time subterranean (with possible deep ocean forays). Still, this answer could be useful to others and has some well thought out points for giant tree-eating worms. $\endgroup$ – ScottS Apr 16 '16 at 4:47

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