It is the only species in the monotypic genus Periphylla and is one of the rare examples in Scyphozoa which life-cycle lacks a polyp stage.

This species is photophobic and inhabits deeper parts of the oceans to avoid light.

[3] They consist 90% of water, the rest being tissue and gelatinous mass, which give the animals their form.

[6] Not only have they become adapted and more abundant in darker environments, but they are also found in very opaque and cloudy waters.

Helmet jellyfish consistently have been found at higher depths, across different locations, if they are smaller in size or are less fit juveniles.

In this experiment it was found that each jellyfish had only an average of five different species of prey in their digestive system.

[16] The jellyfish release their eggs on the surface of the water, where they rapidly sink to a depth that limits visibility of predators.

This yolk supply is seen during the first stage of development and is found inside of a network of plasma strains.

As the jellyfish enters its third stage of development a smooth pit is visible on its anterior end.

The fourth stage of development shows their “umbrella” to have four indentations which creates the gastric septa on its inside.

The final eighth stage is known for when the purple pigmentation of the helmet jellyfish is now seen in their mouth and stomach.

The helmet jelly is found in nearly every ocean of the world, as well as in the Norwegian fjords and in the Mediterranean Sea.

Additionally, there has been an increase in their population throughout the northern Barents Sea in recent years.

The helmet jellyfish has also been found commonly in Arctic fjords that are located within Lurefjorden and western Spitsbergen, Norway.

[18][17] Their distribution throughout these different locations are greatly influenced by the depths of water, abundance of food sources, and preferred light intensities.

They have been observed to position themselves further from the surface of the water if there are harsh weather conditions[16] or to avoid too much sunlight.