May 19, 2022

Jellyfish or sea jellyfish is a collective name for a group of sea-dwelling, free-swimming cnidarians (Cnidaria). Jellyfish are simply built marine creatures with a gelatinous, umbrella-shaped body and drooping tentacles. The body can be pulsed in such a way that efficient locomotion is created. The tentacles are armed with stinging cells and are used to capture prey and deter predators. Jellyfish are found worldwide, from surface waters to the deep sea. Disc jellyfish (the 'true jellyfish') live exclusively in the sea, but there are also some hydroid polyps with a similar appearance that live in fresh water. Jellyfish have a complex life cycle; the meduse can swim around and after sexual reproduction produces small larvae, the so-called planulae. From these larvae grows a clinging polyp that pinches off small jellyfish. Jellyfish are considered a delicacy in some Asian countries. They are first dried and pressed and then processed into a dish. Jellyfish also play a role in scientific research. The green fluorescent protein was originally isolated from a bioluminescent jellyfish and has become a very important marker in biotechnology. Every year thousands of people are stung by jellyfish while swimming. The effects range from mild discomfort to serious injury or even death; small box jellyfish in particular are responsible for many of the deaths described.


The word 'jellyfish' was mentioned in Dutch documents as early as 1619, at the time with the spelling qualle, and had the meaning 'wet, slimy or bulging creature'. The term is most commonly used today to refer to the meduse stage (jellyfish stage) of certain cnidarians. However, the name 'jellyfish' is an informal collective term that includes several (unrelated) animals. In addition to the disc jellyfish or 'true jellyfish', the name also refers to animals from the related classes hydroid polyps (Hydrozoa), box jellyfish (Cubozoa), and stalked jellyfish (Stauromedusae), and from the unrelated phylum comb jellyfish (Ctenophora).


The main characteristic of a jellyfish is the umbrella-shaped swimming bell. This is a soft structure made up of a transparent, jelly-like substance called the mesoglea. The mesoglea acts as a hydrostatic skeleton; it offers sturdiness and freedom of movement. About 95% of the mesogloea consists of water, the rest is collagen, fiber proteins and wandering amoebocytes that can clear up cell debris and bacteria. Tentacles hang on the edge of the swimming bell, as well as a number of rudimentary sense organs, the rhopalia. The mouth opening of a jellyfish is located at the bottom of the swimming bell. The mouth opening is part of a stem-like structure that hangs down from the center, the manubrium. Tentacles are also present around the mouth opening. The mouth provides access to the gastrovascular cavity, the body cavity where digestion takes place and nutrients are absorbed. Around the manubrium, on the inside of the swim bell, are four gonads. These make the sex cells. Cube jellyfish, a separate group of cnidarians, have a similar anatomy. They are characterized by their square, cube-shaped swimming bell with one or more slender tentacles at each corner. The edge of the swim bell is folded inwards allowing efficient jet propulsion; these jellyfish can swim faster than disc jellyfish. Hydroid polyps also have a jellyfish stage in their life cycle, usually with only four tentacles on the edge of the swim bell. However, many hydroid polyps are colonial and do not have a prominent meduse. Morphological diversity of jellyfish

Internal anatomy

Jellyfish have a simple internal build. In fact, an adult jellyfish (meduse) has only one internal organ: the gastrovascular cavity