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Jellyfish also called sea jellies or pain-blobs (my favorite name) are unusual animals to say the least. Members of the subphylum Medusozoa, a major part of the phylum Cnidaria, jellyfish have no organs to speak of. No central nervous system, no pulmonary system instead they get their oxygen entirely through diffusion. About 95% water, these blobs are the last animal you’d expect to be fast. Although don’t get me wrong they are efficient.

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The bell shape scyphozoans are so famous for creates vortices that push them up or forward... these animals don’t really have directions. This is so good it has remained unchanged for over 500 million years. Though some species diverge from this shape.

My goal is to design a jellyfish capable of great bursts of speed.

The box jellyfish actively hunts its prey (small fish), rather than drifting as do true jellyfish. They are capable of achieving speeds of up to 1.5 to 2 metres per second. It’s also good to get away from hungry sea turtles. What I’m having trouble with is making this already perfect anatomy faster.

My first instinct was to stack jellyfish bells together to get more thrust. Another idea was to have tubes passing through the jellyfish to squirt water.

Creative solutions are welcome.

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  • $\begingroup$ While it's a bit like asking for a streamlined slug design, it is actually a fairly good intellectual exercise! $\endgroup$
    – DWKraus
    Sep 26, 2021 at 5:56

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I feel an unaccustomed humility, thinking of how to improve the jellyfish. The jellyfish! Oldest of us, how can my little schemings improve on 500 million years of your trials, your survival of the fittest?

Let us consider the squid.

squid vs jellyfish

https://www.researchgate.net/figure/Conceptual-diagram-of-underwater-jet-locomotion-for-both-a-squid-and-b-jellyfish_fig3_224329313

The squid is conceptually similar to the jellyfish but it has a difference. The output of its contracting mantle is funnelled through a small orifice, and so speeds up as it does. Squids are fast.

Why would a jellyfish not have evolved something similar? Are they structurally not equal to the pressures created? Squids are tough. Would jellyfish burst?


now getting hypothetical. Consider the human heart. The atrium provides a kick and then the ventricle even more. Could a chambered jellyfish do something similar? Let us consider a 10 chambered jellyfish in which each chamber passes to the next via a peristaltic wave. In each successive chamber the water is moving faster - and so by Bernoulli's principle exerts less force on the walls of the chamber - the faster the flow the lower the pressure.

Thus each chamber can add its pressure to the flowing fluid. The additive end result is a very fast fluid flow, but at no point did the jellyfish have to exert (and withstand) massive pressure to achieve that jet. It was accelerated incrementally.

Such a jellyish would be long to accomodate its 10 linear chambers. Maybe it would have an outer mantle to collect the water then an inner to focus and propel the jet.

Oh this is a squid again. Yeah, squids got jet propulsion pretty much figured out.

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  • $\begingroup$ Squid were the first things I thought of reading this question. $\endgroup$
    – DWKraus
    Sep 26, 2021 at 5:55
  • $\begingroup$ "Why would a jellyfish not have evolved something similar?" Jellyfish use the inside of their bell to feed. Streamlining it and providing a smaller jet hole impedes on the volume of water circulating/filtered inside the bell. One may need to explain how they get their food without vision to hunt their targets. $\endgroup$ Sep 27, 2021 at 7:36

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