Right now they just scale the damage, with upgrades for specific levels (if you upgrade jellyfish and octopus souls 10 times i.e: 1 jellyfish, 9 octopus you get an upgrade)
but perhaps I should make the upgrades more visual instead of just upgrading their damage like increasing their size, range or adding more like you said, I agree it would be more fun :)
thank you for the valuable feedback!