|Periphylla periphylla jellyfish. Image credit: Christian Skauge.|
Periphylla periphylla are mesopelagic jellyfish typically found up to depths of 4000 m in the open ocean. However, in various Norwegian fjords, these jellies are found at much shallower depths due to enhanced light attenuation from the dark, nutrient-rich waters coming in from the Norwegian Trench. In Lurefjorden in particular, Periphylla have reached vast abundances, due to the shallow sill of the fjord (approx. 20 m), resulting in the jellies becoming trapped. Here, the jellies rule, and Periphylla are the top predator, as visual predators such as fish cannot hunt in the abnormally dark waters.