Invertebrates without brains

Question
I had a small sea spider in my aquarium. I read that they have a nervous system, but no brain, or eyes or ears. When I came near the aquarium, it would always come up to the glass where my face was at and touch the glass with it's two front "feet".

Then I read about an aquarium (I think it was Norway) that had a really huge jelly fish in an aquarium. Again, nervous system, no eyes, no ears. I watched a video where when a lady from the aquarium came near the tank; it swam up to her. She would place her hand in the water with a dead fish and one tenacle would come up and take the fish from her hand.

If they cannot see us, think, hear us, etc. How do you think they even know we are near the tank? Especially take food from our hands without stinging us. She could touch it all over without any injury. Amazing!

Answer
Hi David,

Thanks for the question. Sea life is amazing!

I believe the best answer to give you is that marine life including invertebrates and fish can detect electricity given off by other animals, including humans; they can detect a human in the room, through their elaborate innervated sensors.

I wish you the best in all of your saltwater endeavors!

Regards,
David