The crown jellies, Coronatae, belong to the Scyphozoans, a class of jellyfish counting some 200 species, many of them stunningly beautiful. Mainly known by its very descriptive Latin name, the Periphylla (peri = around, about; and phylla = leaved) has a bell that may reach a height of 35cm, although they are rarely seen larger than 20cm in the north Atlantic. Their 12 tentacles, which are often held in an upward position may be up to half a meter long. With their tentacles spread out, this strange and menacing looking animal reaches over a meter in diameter.

Life in the abyss
The Periphylla is a true deep-water creature. It is found in all the world’s oceans, except the Black Sea, and typically live below 900 meters and as deep as 7,000 meters in certain areas. In the Antarctic, they grow bigger than anywhere else, and the Periphylla may be the most abundant and widely distributed deep-water jellyfish in the world.
 
Life in the abyss is one of eternal darkness, scarce food supplies, trouble finding mates—and voracious predators. Many of the deep-water animals have developed bioluminescence in order to overcome some of these challenges, and so too with the Periphylla. Sometimes it can be observed changing colour and almost pulsating with light. It is not known exactly why the Periphylla does this, but many scientists believe the jellies use light to communicate and signal such things as readiness to mate—after all, they have four eyes called rhopalia at the rim of the bell, surely capable of detecting the light. The dark red colour of the bell is said to be masking the light of ingested bioluminescent pray, which might otherwise attract predators.
 
Like vampires and trolls
So, how deep do we need to venture to catch a glimpse of this stunning deep-water creature? Luckily, the Periphylla has a trait that enables scuba divers to observe and photograph them at dive-able depths—even close to the surface. After the hatching of the eggs, which floats motionless for months in mid-water at a depth determined by a combination of temperature, salinity and water density, the Periphylla develops 12 powerful tentacles and starts a life-long cycle of swimming several hundred meters to shallower water to feed at night, then sinking back into the abyss as soon as day breaks. In that respect, they are like the vampires and trolls of ancient tales—if they see the sun rise, they will die. But this time, it’s true.
 
Despite being able to produce its own light, the Periphylla is photo-sensitive and will not survive being exposed to sunlight. Even strong moonlight makes them turn around and head for deeper waters, and so do powerful dive torches. Light will break down the brilliant red pigment in the bell and the tentacles, and the jellyfish will perish. The destruction spreads along the two nervous systems the jellyfish possess, also destroying its capability to move properly. In order to observe them during a dive, absolute darkness (apart from dive lights) is a necessity. And you have to be at the right place at the right time.
 
A rare treat
Although the Periphylla is abundant in the depths of most oceans, there are some special places where they congregate in the millions because the conditions are particularly favourable. One of very few such places on the planet is a fjord called Lurefjorden, just north of Bergen, and this is where we were heading at full throttle through the night. In the winter, the feeding cycle of the Periphylla coincides with mating season, which occurs from November until April. February, being right in the middle, usually offers the highest density of jellyfish—if you dive on a moonless night such as we were.
 
Lurefjorden is a 400-meter-deep, ...