The flower hat jelly (Olindias formosus) is a species of hydromedusa in the hydrozoan family Olindiidae. Although they look like a jellyfish, they actually belong in the class Hydrozoa, while true jellyfish belong in class Scyphozoa. Flower hat jellies occur in the northwestern Pacific off central and southern Japan, and South Korea's Jeju Island. (close relatives live elsewhere, like O. sambaquiensis found off Argentina and Brazil). The adult form of the flower hat jelly only lives a few months and is typically seen from December to July, with peaks in April and May. During the day they rest on the bottom, often among rocks or algae, but at night they float up to hunt for their prey, typically small fish.
Stygiomedusa gigantea, commonly known as the giant phantom jelly is a part of the monotypic genus of deep sea jellyfish, Stygiomedusa. This is in the Ulmaridae family. It can't actually sting you as its—30-foot-long—”mouth arms” are just there to trap prey rather than sting them.
This ghostly giant is a rare sight.
MBARI's ROVs have logged thousands of dives, yet we have only seen this spectacular species nine times. The first specimen of the giant phantom jelly was collected in 1899, but it was not until 60 years later that scientists recognized this as a new species.