User:Berkeleyb15/Periphylla periphylla

Reproductive cycle
Periphylla periphylla represents an exception, very rarely found in the phylum Cnidaria: the medusae do not go through a polyp stage, thus presenting a "holopelagic" life cycle. They also do not undergo an ephyra stage as well as a sessile stage. The helmet jellyfish is also unique in its growth and sexual reproduction in that they are the only known scyphozoan that undergoes sexual propagation but lacks the planula stage. During reproduction, the female helmet jellyfish contain thousands of eggs within their gonads. Their eggs are actually the largest sized eggs within all Cnidaria. Despite the eggs being very large, females will only produce a small number of eggs. The jellyfish release their eggs on the surface of the water, where they rapidly sink to a depth that limits visibility of predators.

Habitat and behavior
The jellyfish is found in depths up to 2700 meters and is adapted to its dark environment. Not only have they become adapted and more abundant in darker environments, but they are also found in very opaque and cloudy waters. They are found in these deep and dark waters due to the fact that sunlight can be very harmful to adult helmet jellyfish, and even deadlier to younger helmet jellyfish. The depth that they can be found at varies throughout the day. They're found in abundance at a depth of approximately 13.75 m during the nighttime, and as deep as 150 m during the daytime. The abundance of helmet jellyfish at the depth of 150 m during the day time is nearly three times the abundance of them found during the nighttime. The jellyfish were also noted as far down as 250 m. At this depth, one-third of the total population inhabited the lower water column during daylight while less than 10% were found at the same depths after the sun had gone down. This observation of migration shows that they are constantly migrating vertically throughout the depths of water based on the amount of available sunlight at the given time. However, they can also be seen commonly at different depths depending on their physical size and age. Helmet jellyfish consistently have been found at higher depths, across different locations, if they are smaller in size or are less fit juveniles. Through their vertical swimming, the jellyfish can swim several different speeds. The majority of their vertical swimming is seen at a speed of <2 cm/s. If they are swimming at full speed they can reach over 10 cm/s, but this is only for a short time span. If they do reach this speed, it is assumed that they follow this with a time of no vertical movement.

In 2017, many helmet jellyfish were caught to be studied to determine their main prey. In this experiment it was found that each jellyfish had only an average of five different species of prey in their digestive system. The prey in their digestive system was examined as well as the abundance of the prey. There was a 27% abundance of copepods, 23% abundance of pteropods, 20% abundance of amphipods, 17% abundance of euphausiids, and a 13% abundance of chaetognaths .With a full stomach it turns from the surface back to the depths. Other deep-sea inhabitants feed upon its faeces. They move by swimming with their tentacles being in an aboral position. Their tentacles also have various unique muscles. These include longitudinal, ring-, radial-, and diagonal musculatures. The two most unique are the longitudinal and the diagonal musculature. The longitudinal is used for consuming prey by moving very quickly to the jellyfishes mouth. The diagonal is used for the corkscrew reaction that is used in order to obtain and capture prey.

Distribution
The helmet jelly is found in nearly every ocean of the world, as well as in the Norwegian fjords and in the Mediterranean Sea. They can also be found in Iceland and Greenland Seas. Additionally, there has been an increase in their population throughout the northern Barents Sea in recent years. The helmet jellyfish has also been found in Arctic fjords that are located within western Spitsbergen and Lurefjorden, Norway. Their distribution throughout these different locations are greatly influenced on the location, abundance of food sources, and preferred light intensities. Their distribution can also be affected by physical conditions. They have been observed to not position themselves closer to the surface of the water if there are harsh weather conditions. Their distribution can also be affected by water temperature. The vast majority of helmet jellyfish live in temperatures ranging from four to 11°C. Despite this, they are able to survive in water temperatures reaching up to nearly 20°C.