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I have a concept for a special kind of jellyfish that somewhat bears a resemblance to the generic slimes of videogames. This jellyfish was of the Cubozoa class and existed roughly 300 million years ago during the Pennsylvanian period. Sporting the same kind of advanced rhopalium as present day box jelly fish and complex nervous systems, these jellyfish evolved to expand their oxygen storage capabilities for longer than the average 48 minutes a normal jellyfish can survive on land before death. They evolved a reinforced mesoglea as well, expanding on their Cubozoan capabilities of active movement through muscular contractions to slowly drag themselves across land. (Not sure how fast.) These adaptions allowed them to traverse the swamps of the Carboniferous for one to two hours.

Early land jellyfish were detritivores, dragging themselves across swampland in order to eat. They looked like relatively flat pancakes, with tentacles splayed. Their rhopalium was incredibly advanced, able to see colour, light, etc, similar to modern day box jelly fish but even better so that they could move across land. Later on tentacles were used as collectors, dragging along detritus and passively moving it towards the jellyfish's mouth.

These jellyfish became more and more terrestrial in nature, eventually being able to store oxygen for many hours like turtles. A species of these jellyfish emerged, with even more reinforced mesoglea, enough to make it stand somewhat upright and was the size of a small baby. They had three rhopalium on their 'face', with two large ones acting as its main 'eyes' and a smaller one beneath mostly dedicated to light perception and contracting the mesoglea to become very hard. This species leaped at prey, contracting and hardening their mesoglea so that the force of impact would be enough to stun them. Eventually either body trauma or brain damage killed their prey, where they would drag themselves over and eat the prey over a long period.

Is any of this plausible and realistic? What are some prevalent issues?

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  • $\begingroup$ Anything is possible if you can imagine it. I think the jellyfish are a really cool concept. What role do they play in your story? $\endgroup$ Aug 14 '20 at 12:35
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    $\begingroup$ Planet of the Slimes, lol. I'm creating a speculative evolution book on the idea of what if slimes were the first terrestrial animals? Ecosystem, etc. $\endgroup$ Aug 15 '20 at 3:50
  • $\begingroup$ The hard-shelled conulariids might have been some sort of medusozoan cnidarians also known as jellyfish... (That's the best hypothesis. If they were not hard-shelled jellyfish then nobody has any better idea of what they were.) $\endgroup$
    – AlexP
    Aug 17 '20 at 15:52
  • $\begingroup$ Hard science tag should not be used with a reality-check tag. Since you ask "Is any of this plausible and realistic?" I believe this is meant to just be a reality-check question. I've taken off the Hard Science tag for you. $\endgroup$
    – Nosajimiki
    Aug 17 '20 at 19:34
  • $\begingroup$ @Nosajimiki, I have rolled back the tag edit. Let it be the OP to decide which of the two to keep. $\endgroup$
    – L.Dutch
    Aug 18 '20 at 3:40
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The most obvious, prevalent issues would be moisture-retention and structural support. Not sure how to solve these though while still keeping them being recognizable jellyfish since we so few (dare I say virtually zero) working examples. The Portugese man-of-war is the only one I know of that has parts that are persistently dry and self-supported in the absence of water.

Other more insidious issues are related to inherent limitations in their morpholgy as we know it. The behaviours you describe would require very drastic changes due to these limitations. So much that you almost have free-reign so long as it doesn't seem completely unbelievable. The most immediate example that comes to my mind is the efficiency (or lack thereof) of their respiration and the apparent speed and strength limitations of their actuators, both of which are closely tied together.

Since you do not see any running, walking, or leaping molluscs, it would seem that actions such as leaping are reserved for animals that have some sort of rigid support such as vertebrates and arthropods.

For these reasons, I think the most implausible part of your description is how they somehow have the strength to leap such that their mass is sufficient to kill prey through blunt trauma. This is pretty implausible even for animals that exist today. It's a Catch-22. You need the square-cubed law so that your mass is enough that the impact is effective, but the square-cubed simultaneously limits the strength which you need launch yourself high enough. Not to mention that the predator is exposed to the same amount of deadly blunt trauma as the prey. I know of no animal that is capable of doing this, let alone one that actually does it unless predator dwarfs prey, in which case there are safer, more effective, more efficient methods.

Compare this to ramming where you can build up momentum over time which does not require huge muscles capable of a very short, powerful impulse to launch your own body weight into the air at sufficient height to cause the damage you seek. Strength like that would allow for a very deadly toss of a much smaller object at much longer range or a powerful targeted strike. And I still know of no animal that hunts with fatal ramming.

You might want to take a look at are molluscs which, like Cnidarians, are moist soft-bodied creatures which are not tetrapods, except that some members are terrestrially adapted. They might be the only real-life example you have to work off of.

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  • $\begingroup$ This is not a hard science answer. $\endgroup$
    – L.Dutch
    Aug 17 '20 at 19:22
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    $\begingroup$ @L.Dutch-ReinstateMonica Oh, I guess it does add for hard science. Not sure a hard science answer is possible for this question though since very few working examples. $\endgroup$
    – DKNguyen
    Aug 17 '20 at 19:23
  • $\begingroup$ Consider killing prey by leaping and stinging with toxin instead of blunt force trauma. More consistent with both physics and biology of today’s jellyfish. The leap allows targeting face or unshielded areas that might not be possible from the ground, and toxin might couple with suffocation if airways are obstructed. $\endgroup$
    – SRM
    Aug 18 '20 at 14:16
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    $\begingroup$ @SRM Face hugger jellyfish $\endgroup$
    – DKNguyen
    Aug 18 '20 at 14:17
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No

I can not find any examples of jellyfish with the ability to walk on land much less with the level of mobility you've described. But there are a few details about jellyfish biology that would suggest that they could not exist, even in the hypothetical.

Jellyfish biology is poorly adapted to life on land, even by marine organism definitions. The reason for this is that the structure of the body comes from its mesoglea. Mesoglea is a mostly acellular sac of fluid that relies on buoyancy to give it structure. Without the buoyancy of water to support it, mesoglea is not stiff enough to support the weight of the body causing it to collapse when on land. Also, bigger jellyfish tend to have higher mesoglea ratios; so, while tiny jellyfish might survive for over half an hour on land, larger ones crush under thier own weight far more easily making death almost instant. This becomes a big problem for your body slam killing mechanism if your land jellies are restricted to the size of insects.

Furthermore, its epidermis is not designed to retain water when removed from its water habitat. When a Jellyfish is removed from water it rapidly deliquesces turning into little more than a puddle of water in a matter of hours. So, even if you were to expand its oxygen capacity on land, it would still die pretty quickly.

Is any of this plausible and realistic?

Speculative evolution is not really hard science, but ... I don't really want to leave you high and dry either (pun total intended). With enough time and evolution, it may be possible that a distant descendant of the jellyfish could become terrestrial, but to do this it would have to evolve a much less water permeable epidermis and a more rigid structure than mesoglea. The issue with this is that the properties that such adaptations would require would likely make the jellyfish far less slime like thereby taking away the qualities you are looking for. In general, the faster an animal moves, the more it requires a rigid body to prevent collapse under the strain of pushing itself off of things. Shy of giving the jellyfish some kind of bone like structures or lots of thick, dense muscles, its maximum mobility on land would more likely resemble that of a slug; so, if you want to make a plausible slime like creature, I would suggest nix the leaping hunter idea and go with more of an ambush hunter idea. Maybe the land-jelly stings and parallelizes passer-bys and then slowly crawls over and consumes it.

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  • $\begingroup$ Interestingly, the ambush hunter was one of my initial ideas. It would deliberately leave its tentacles spread over a wide surface area, stinging animals and consuming them. $\endgroup$ Aug 19 '20 at 2:44

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