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Jellyfish are mainly free-swimming marine animals with umbrella-shaped bells and trailing tentacles.
A few are not mobile, being anchored to the seabed by stalks.
The bell can pulsate to provide propulsion and highly efficient locomotion.
Its tentacles are armed with stinging cells and may be used to capture prey and defend against predators.
Its main feature is the umbrella-shaped bell.
The bell is a hollow structure consisting of a mass of transparent jelly-like matter known as mesoglea.
The bell forms the hydrostatic skeleton of the animal. and 95% or more of the mesogloea consists of water, but it also contains collagen and other fibrous proteins, as well as wandering amoebocytes which can engulf debris and bacteria.
Under the bell is the manubrium, a stalk-like structure hanging down from the center, with the mouth, which also functions as the anus, at its tip.
There are often four oral arms connected to the manubrium, streaming away into the water below.
Jellyfish have a complex life cycle, and are found all over the world, from surface waters to the deep sea.
They are common in coastal zones worldwide.
They are eaten by humans in certain cultures.
Thousands of swimmers are stung every year.
Stung patients experience effects ranging from mild discomfort to serious injury or even death;
Small box jellyfish are responsible for many of these deaths.
They can form vast swarms, and damage to fishing gear, and cooling systems of power and desalination plants.
The term jellyfish corresponds to medusae.
They have limited control over their movement.
Some can navigate with the pulsations of the bell-like body, while some species are active swimmers most of the time, while others largely drift.
Box jellyfish have more advanced vision, having 24 eyes, two of which are capable of seeing color, and one of the few kinds of animal to have a 360-degree view of its environment.
They range from about one millimeter in bell height and diameter, to nearly 2 meters in bell height and diameter.
The tentacles and mouth parts usually extend beyond this bell dimension.
Box jellyfish are small and venomous, and armed with nematocysts in their tentacles.
The contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject venom.
Only some species’ venom causes an adverse reaction in humans.
The venom effects of stings range from mild discomfort to extreme pain and death.
A jellyfish sting can cause pain, swelling, difficulty breathing, nausea, vomiting, muscle pain, numbness, headache, problems swallowing, and coma.
Most jellyfish stings are not deadly.
Some box jellyfish stings can be deadly.
Stings may cause anaphylaxis.
Jellyfish kill 20 to 40 people a year in the Philippines.
Vinegar may help with box jellyfish stings, but not the stings of the Portuguese man o’ war.
Salt water may help if vinegar is unavailable.
Rubbing wounds is not advised as it can encourage the release of more venom.
Clearing the area of jelly and tentacles can reduce nematocyst activity.
Scraping the affected area, may remove remaining nematocysts.
After the site has been cleaned of nematocysts, hydrocortisone cream applied locally reduces pain and inflammation, and antihistamines may help to control itching.
Antivenins are available and are used for serious box jellyfish stings.
A severe sting from the box jellyfish can cause unconsciousness and death in minutes.
The Portuguese man-of-war is the most common jellyfish sting treated in U.S. emergency rooms.