The Jellyfish is a planktonic animal, predominantly marine, belonging to the phylum of the Cnidarians. Generally represents a stage of the life cycle that ends after the sexual reproduction with the formation of a polyp.
Jellyfish are pelagic animals that are able to swim while contracting, but are generally transported by sea currents and often found on the beaches.
It is easy to understand why the plastic found in the sea: bags, nylon, polystyrene, etc. it is exchanged by animals with the jellyfish they eat.