Blue jellyfish are very rare. They are known to be very agile and stealthy, and they create blue jelly instead of purple or pink like the normal jellyfish. In "Jellyfish Hunter," SpongeBob captures the only blue jellyfish in all of Jellyfish Fields and names him "Friend."
SpongeBob tries to capture one at the start of the episode but fails. Later, the same blue jellyfish follows SpongeBob to his house, captures him, and takes him to Mr. Krabs' factory. The blue jellyfish and SpongeBob stop Mr. Krabs from using the jellyfish SpongeBob captured for his own good and sets all the jellyfish free. The blue jellyfish then enters SpongeBob's net, and SpongeBob calls the blue jellyfish "Friend."