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Learn About Jellyfish Anatomy, Lifecycle, & Reproduction 

  Facts About Jellyfish: Jellyfish have Survived for 650 million Years

Jellyfish belong to the phylum Cnidaria. This phylum is divided into definitive classes which include all anemones, corals, fire corals and what is commonly referred to as true jellyfish. Anthozoa contains sea coral and anemones. Hydrozoa includes the Portuguese Man o`War which contrary to popular belief is not an actual jellyfish but a massive colony of hydrozoans. The class Cubozoa contains box jellies, the deadliest creatures on Earth. True jellyfish belong to the class Scyphozoa which includes over 200 species.

Flower jellyfish stings, W
Jellyfish exist in every ocean on the planet. They cover the entire spectrum of oceanic depths from shallow estuaries and lagoons to the deepest, largely unexplored regions of the aquatic domain. The most geographically diverse and easily recognizable of these creatures is the moon jellyfish (Aurelia) or common jellyfish which contains twenty separate species that are so identical morphologically that it takes DNA testing to distinguish one form another. From a non-scientific standpoint, moon jellyfish might as well be a single species.

See Moon Jellyfish (Aurelia)

Despite the fact that jellyfish are one of the most prolific life forms on the planet,  until recently it was thought impossible to keep jellyfish alive in captivity.  In fact, the world's first public jellyfish exhibit  opened at the Monterey Bay Aquarium just a little over twenty years ago.

The technological advancements needed to keep these delicate creatures alive and healthy in captivity paved the way for the newest segment of the aquarium  industry, the jellyfish fish tank aquarium.  This specialized branch of the home aquarium industry is still in its infancy.  A  decade ago there was no such thing as a home jellyfish tank.  It has just been in the past two years that this new trend in aquarium home ownership has produced an affordable desktop jellyfish  aquarium.  For more information go to Jellyfish Aquariums.
See this Desktop
Jellyfish Tank Aquarium
desktop jellyfish fish tank aquarium

Check Out the Newest Desktop Jellyfish Aquarium on the Market

 Facts About Jellyfish: Jellyfish Anatomy

Anatomically, jellyfish can best be described as a sac within a sac. They are composed of approximately 95% water, 3% salt and 2% protein. They have no eyes, no brains, and no supporting skeletal system but yet are one on the oldest multi-cellular creatures known to man. They existed long before the first dinosaurs roamed the Earth and will almost certainly still be here long after the human race has vanished. Without a brain, jellyfish have managed to survive three planetary wide extinctions: This alone bears testimony to their long term survivability as a life form.

moon jellyfish anatomy, W

Jellyfish are one of the simplest multi-cellular organisms in existence. They are most accurately described as gelatinous zooplankton. The actual term jellyfish is a universal misnomer. Jellyfish are, of course,  not fish. Jelly refers to the gelatinous substance that accounts for most of the mass in a jellyfish’s umbrella or bell. This jelly (mesoglea) is surrounded by two layers of epithelial cells. The top layer forms the upper portion of the umbrella. The bottom layer forms the subumbrella or underbelly of the bell.

Jellyfish do not have specialized digestive, respiratory or circulatory systems. In fact, they don’t even have blood cells. Oxygen is absorbed by simple diffusion through their thin outer membranes. A second membrane within the jellyfish contains a gastrodermal lining which forms a gastrovascular cavity. This primitive cavity functions in place of a digestive system. Nutrients are absorbed and distributed throughout the body. Jellyfish either have a single mouth or multiple mouth openings positioned on oral arms that function for both intake of nutrients and expulsion of waste products.

Many species in the order Rhizostomae have a symbiotic relationship with algae colonies living inside them. These jellyfish provide a host body for the algae. In exchange, the algae provide carbon rich nutrients for the jellyfish. In addition to essential nutrients, the algae’s photosynthetic process produces oxygen to help support metabolic respiratory functions in poorly oxygenated environments. 

See Symbiotic Rhizostomae Jellyfish in Action

Jellyfish do not have brains or a central nervous system. Instead they have a rudimentary neural network called a nerve net running throughout their epidermal membrane. They have no eyes, although many have ocelli. Ocelli are basic light sensory organs. They are not sophisticated enough to form visual images but can distinguish light from darkness. This gives them the ability to sense a foreign object so they can maneuver around it. Aside from distinguishing between light and darkness in some species, a jellyfish’s awareness of external environmental factors is limited to being able to determine between up and down, and tactile impulses. They can sense when they have come in contact with another object.

Jellyfish also lack any semblance of an advanced skeletal system. They have what is called a hydrostatic skeleton that provides structural integrity and allows for limited mobility. Hydroskeletons are common in many lower life forms, specifically cold blooded and soft bodied organisms. A hydroskeleton consists of fluid or gelatinous filled cavity called a coelom. The coelom is typically surrounded by muscular tissue or muscle-like membranes. As the muscle tissue contract or expand the pressure of the fluid in the coelom is changed. This change in fluid pressure is what allows jellyfish to change shape and achieve locomotion. Most jellyfish are poor swimmers. They spend the vast majority of their adult lives drifting haplessly on the ocean currents. Box jellies, however, are quite good swimmers. Sea nettles are such accomplished swimmers that they spend most of their time swimming, quite frequently against  prevailing currents. This is why they appear to be swimming upside down.

The Sea Nettle: Pure Poetry in Motion

 Facts About Jellyfish: LifeCycle & Reproduction

Jelly fish have two distinctive phases to their lifecycle. The first is the polypoid or polyp stage. The second is the medusa or adult phase. These are the free swimming jellyfish to which people are accustomed to.

Reproduction in jellyfish typically occurs both sexually and asexually. The initial stage of a jellyfish’s life cycle begins with sexual reproduction. A male jellyfish releases sperm from its mouth into the water. Some of these sperm make their way into the mouth of the female jellyfish where the eggs are stored. After initial fertilization, embryonic deployment occurs inside the female’s mouth or in brood pouches inside her oral arms depending on the specific species involved. Eventually, free-swimming larvae are released into the water. These larvae sink to the ocean floor and attached themselves to stationary objects. At this point they develop into sessile polyps. They will remain firmly anchored until nature signals that the asexual stage of their reproductive cycle is about to commence.

moon jellyfish lifecycle, W

Asexual reproduction involves strobilation of the existing polyps. Stobilation simply means that the polyps segment and re-segment to form a colony of polyps much like a single cell organism divides to form a second, unique organism.  Most jellyfish species strobilate in the winter months. They will remain attached together as a sessile colony of polyps until spring signals the next phase of their lifecycle. The chart above pictures the complete jellyfish life cycle. 

With the warming of the water, this colony of polyps breaks apart into individual, free-swimming medusa. It is this medusa, or adult phase of the jellyfish’s life that beach goers come into contact with. Many species of jellyfish instinctively seek shallow water to initiate the sexual stage of their reproductive cycle. It is in this stage of the reproduction process that human interaction is most likely to occur. Some species of adult jellyfish congregate in massive swarms called blooms. It is not uncommon for a bloom to contain 100,000 adult jellyfish.

Jellyfish are cyclic in nature. In the wild, most jellyfish’s lifecycle takes one calendar year to complete. Jellyfish in captivity are not bound by the mandates of nature. They have been documented to live for over 10 years in public aquariums.

In 2009 scientists discovered a previously unknown species of jellyfish, Turritopsis dohmii. This species of jellyfish is unique in that it has the ability to reverse its lifecycle and revert from an adult to a juvenile thus cheating the seasonal dictates of a jellyfish’s lifecycle. This is the only animal known in creation to have the capability immortality.

See What Immortal Jellyfish Look Like

Facts About Jellyfish: Global Jellyfish Invasion 

box jellyfish stings warning sign

In recent decades jellyfish bloom density and populations have increased dramatically. Scientists point to the rise of ocean temperatures (due to global warming) and over fishing as possible catalysts behind these population explosions. While warmer water temperatures are conducive to jellyfish polyps morphing into adult medusa, there is no empirical data to support these speculations. In truth, there is simply not enough documentation existing on long-term jellyfish population patterns to offer anything more than scientific speculation. Case in point: The sudden and devastating rise in the locust populations that plagued the Midwestern United States in 1874. We have no way of knowing whether this was a cyclic rise in population or was caused by man’s intervention with nature. Periodic locust infestation could have taken place for centuries in the Midwest. American settlers were just not around to document the devastation. 

One thing is certain: The rise in jellyfish populations has cost untold millions in revenue loss to the tourism industry. Australia, Indonesia, Japan, the U.S., Central America, and countries throughout the Mediterranean coastline have all experienced significant rises in jellyfish populations in recent decades. 2005 Newport Beach California: 1,400 people had to be treated for jellyfish stings in a single day. Costa Brava Spain 2006: Over 19,000 beach goers were treated for jellyfish stings. Mediterranean beaches have experienced jellyfish concentrations of over 10 jellyfish per square meter in recent years. The beach resorts in Mexico have had to be closed on more than one occasion due to jellyfish infestation.

Australia’s deadly box jellyfish populations have also risen dramatically. Box jellyfish have migrated all the way to the shores of Hawaii. Hawaiian beaches have had to start posting the same “Deadly Jellyfish” warning signs that Australians have learned to accept as a way of live in the height of jellyfish breeding season.

Box jellies and their smallest members the Irukandji jellyfish inundate the coasts of Northern Australia from November through May. An adult box jellyfish contains enough venom to kill 60 people. They can kill a full grown adult in as little as 2 minutes. Sting victims frequently go into anaphylactic shock if anti-venom is not promptly administered. Netted areas are provided for Australian beach goers to keep them out of harms way. Irukadji jellyfish present an entirely different problem in regards to human interaction. These jellyfish are no larger than your finger nail and are completely transparent. For all intents and purposes they jellyfish are invisible to beach goers. Nets are completely ineffective in isolating them from the public. While their venom is not as fast acting as that of their larger cousins, there is no anti-venom currently available for Irukandji Syndrome. Doctors can only treat the sting victim’s symptoms and hope the patient survives the ordeal. If their presence is detected, the beaches are closed.

Tourism is not the only industry to suffer in light of the global jellyfish explosion. On October 23, 2008 alarms began to sound at the Diablo Canyon Nuclear power plant in San Luis Obispo County. Water pressure readings from the plants cooling systems were skyrocketing without explanation. Underwater scuba teams surveyed the plant’s cooling intake system and determined that 100s of moon jellyfish had completely jammed the intake pipes. Officials had no choice but to power down the plant. A bloom of seemingly inauctious jelly fish had succeeded in doing what years of environmental protestors had failed to accomplish. Jellyfish have also clogged desalination plants and ship intakes.


Perhaps hardest hit industry in recent years is the commercial fishing industry. Jellyfish blooms have clogged and broken nets, poisoned fish catches and threatened to shut down fishing operations completely in more than one region of the world. The giant Nomura’s jellyfish (Nemopilema nomurai) has even capsized fishing boats off the coast of Japan.

 Japanese Jellyfish Invasion

  Jellyfish Attacks: The Truth about Jellyfish Stings

Contrary to popular belief, no jellyfish has ever actually attacked a human being. Jellyfish are, in fact, one of the most fragile creatures in creation. If not for their ability to sting, they would be defenseless against predators and left to starve from malnutrition. The jellyfish’s nematocysts (stinging cells) are nature’s way of compensating for their delicate constitution. A  jellyfish’s nematocysts are triggered by touch. This is a strictly autonomic response to physical contact.

Box jellyfish are the most lethal creatures on the face of the Earth. But not all jellyfish stings are deadly. There are almost 30,000 species of jellyfish worldwide. The sting of many of these species is imperceptible to human beings. Some jellyfish nematocysts do not even generate enough firing pressure to penetrate human skin. Still others are no more aggravating than a bee sting to the average person. They will produce no more than moderate irritation and a minor skin rash. This is not to say that a jellyfish sting should be taken lightly. Just like a bee sting, each individual reacts differently to the introduction of invasive toxins to their system. A bee sting for most people is no more than a minor annoyance. To others an adverse allergic reaction to bee venom means an emergency trip to the hospital.

But even the most deadly jellyfish venom may prove beneficial to mankind. Modern science has already found one aspect of a particular species of jellyfish extremely helpful in the field of biological research. Crystal jellyfish (Aequorea Victoria) contain a naturally occurring biofluorescent gene called Green Fluorescent Protein (GFP). Scientists have been able to isolate and synthesize this gene. GFP almost single-handedly revolutionized cellular biology. It has made it possible for researchers to see inside living cells and understand how they work for the first time. It is hoped that the paralyzing aspects of jellyfish venom may lead to unlocking the key to the cardiovascular system.

Collecting Box Jellyfish Venom


There is one place on the planet where tourist can swim worry free while completely surrounded by millions of jellyfish. It is Clear Lake on the island of Eil Malk in Palau. Eli Malk is one of a series of volcanic outcroppings that make up the Rock Islands. While permanently stratified marine lakes are exceedingly rare, the Rock Islands contain over 70 of them.  Clear Lake (also known as Jellyfish Lake or Lake Palau) has been isolated from predators for over 1,2000 years even though it is still connected to the ocean. It is connected through a series of tunnels and fissures in ancient limestone reefs that

date back to the Miocene Era. This lake is teaming with millions of golden jellyfish (Mastigias cf. papua salii), an algae dependent jellyfish that has evolved for thousand of years in nutrient rich waters. Between their symbiotic relationships, abundance of supplementary nutrients and total lack of predators, the golden jellyfish has transformed into the most benign jellyfish on the face of the planet. Golden jellyfish no longer have the ability to sting.  

Facts About Jellyfish: Jellyfish
Anatomy, Lifecycle, & Reproduction
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