Interesting one @amavi
They may have poisoned hundreds of the humans and can be considered as a threat but still, they also have given us several enhancement in the field of biomedicines. You may have heard of Aequorea victoria a known jellyfish for the production of Green Fluorescent Protein (GFP).
Osamu Shimomura, Martin Chalfie and Roger Y. Tsien shared the Nobel prize in chemistry for the discovery and development of Green Fluorescent Protein by this you can just guess the importance of it.
And yeah, they are among the oldest of all creatures and still there is so much more to study.....
Cheer