Jellyfish are in the subphylum Medusozoa in the phylum Cnidaria (which comes from the Greek word cnidos, meaning stinging nettles). Most animals in this phylum have nematocyst cells which they use to either sting or inject a toxin into their prey. Moon jellies (Aurelia spp) are relatively plentiful off the Pacific Coast and worldwide. Discarded plastic bags that end up in the ocean are often mistaken for moon jellies and eaten by the various species that prey on jellies.
Recent trends of increasing jelly populations could indicate an unbalanced ecosystem. Scientists have discovered that jellies breed best in waters with an abundance of nutrients and therefore abundant plankton. In locations subject to heavy fishing pressure there is often an overabundance of plankton due to the absence of predatory fish species. This allows jellyfish populations to thrive well beyond normal sizes, thus consuming even more plankton and making it harder for fish to bounce back in subsequent years.