It is found worldwide in tropical and warm temperate seas,[3][4] although it is suspected that records outside the North Atlantic region, which includes the Mediterranean and Gulf of Mexico,[10] represent closely related but currently unrecognized species.

[11] A fairly small and variably coloured species, both its tentacles and (unusual among jellyfish) bell are covered in stinging cells.

[3] When large numbers of this oceanic species are washed ashore, the local economy can be affected because tourists avoid the beaches and fishers are stung while trying to retrieve their nets, which can be clogged by the jellyfish.

[10][16] There are reports from most other tropical or warm temperate seas around the world, including both the Pacific and Indian Oceans, with its apparent southern limit being 42nd parallel south.

[21] The swimming ability of Pelagia noctiluca is limited and as a result large swarms (also known as blooms) of this oceanic species are occasionally carried by the wind or current to inshore areas, sometimes ending up stranded on beaches.

[3][10] This also means that the species sometimes appears in waters outside its normal temperature preference, with records as far north as the Shetland Islands and the Norwegian deep.

[22] Local populations fluctuate greatly and the species may go virtually unrecorded in a region for years, only to suddenly reappear in huge swarms.

[10][23][24] On occasion, a swarm may cover tens of square kilometers,[25] include millions of Pelagia noctiluca,[26] and reach densities of more than 500 individuals per m3 (14 per ft3).

This is a primitive gut or digestive cavity with only one opening that is used for ingestion and excretion; there are four long oral arms with crenulated margins that are the primary feeding surface.

However cnidaria have evolved cnidae, cells which serve for a variety of functions that include prey capture, defense, locomotion and attachment.

Four gonads arise as elongated endodermal proliferations, developing into ribbon-like folds in the interradial sectors of the stomach wall slightly distal to the rows of gastric filaments.

A very early description of bioluminescence was provided by Pliny the Elder in Historia Naturalis (77 AD), using the name "pulmo marinus", and now considered to refer to P.

[10][23] P. noctiluca rely on favorable trophic conditions to spawn, so when their criteria is not met, the medusae will stop reproducing immediately and lose weight when presented with an inadequate amount of food.

[32] P. noctiluca will eat small warty comb jellies (Mnemiopsis leidyi), potentially helping to control this invasive species.

[35] The sting causes pain that typically lasts 1–2 weeks, local redness, swelling and a rash, but it is generally not dangerous and there are no known fatalities.

Rarely, the sting can cause a serious allergic reaction and leave scars or hyperpigmented marks on the skin that can remain for years after the encounter.

[38] The sting of P. noctiluca can possibly be relieved with the use of Hydroxyacetophenone and Symsitive® since they are nematocyst inhibitor compounds, meaning they inhibit the discharge of cnidocysts.