Hi, I am very curious about jellyfish.
Yes they are considered animals but they lack a brain, they have no organs, no blood…
But still they manage to stay alive and survive.
Like… how?
In: Biology
They have only what is required for survival and reproduction.
Brains, nervous systems, and organs are just another way to achieve the same goal, but not a requirement to do so. Bacteria live and thrive while being a single cell. Viruses (viri?) manage it with even less.
Nature *usually* rewards biologies that achieve those goals with the minimum energy needed. Jelly fish are very low energy.
Survival of the fittest/evolution isn’t about making the “best”, it’s about getting to “just good enough”. Rarely does a trait with excessive energy consumption result in a benefit, the human brain being a good example.
They have organs, they’re just pretty simple. Blood evolved to carry oxygen to organs too far from the atmosphere. Jellyfish are thin enough (and they also don’t use as much oxygen as us!) that the oxygen gets where it needs to go.
As for brains well it depends what you call a brain. They have neurons and it’s enough. Think of very simple invertebrates as being more mechanical in nature. For example, ants count their steps to find their way home. In a jellyfish they have very simple eyes so that light = swim. Another example could be warm water + this much moonlight = release of sperm/eggs
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