In recent years, the number of jellyfish “blooms” (large groups of jellyfish congregating in a relatively small area) appears to be increasing. These blooms not only impact the local marine ecosystems, but have great effects on human life as well.

This Saturday, November 20th, there will be a public outreach event to inform the general public about the importance of jellyfish locally and globally. This free event, called Jellyfish ROCK: Reaching Out to the Community & Kids, will be held at the Santa Barbara Natural History Museum from 6:00 to 8:15PM. There will be talks on these fascinating creatures by experts from around the world.

The Cnidarians: As jellyfish “blooms” are reported more and more frequently in the news, it’s important to understand what exactly these animals are to figure out what the occurrence of these large groups of jellyfish means for us. The image of the jellyfish that we’re so familiar with – a gelatinous, bell-shaped dome with long, dangling tentacles underneath – is just a small glimpse of what these creatures are and the larger group of seemingly bizarre marine animals they belong to. Jellyfish, or “jellies,” are part of the phylum Cnidaria (despite the name, jellyfish aren’t actually fish at all). The Cnidarian group, which has been around some 580 million years (about 100 million years before any animal developed a spine), also includes sea anemones and

corals. Jellyfish themselves joined the scene around 500 million years ago, and have evolved lifecycles very distinct from the sea anemones and corals.

The Curious Life of a Jellyfish: The jellyfish lifecycle has two key stages (although biology tends to be more complex than expected, and many jellyfish have only one of these stages): a polyp stage and a medusa stage. Most jellyfish start out life as a small, mouthless, flat larva which swims or crawls around searching for a good location on the ocean floor to call home. Once the larva finds an ideal spot, it settles down and becomes a coral-like polyp. The polyp is roughly cylindrical, with one end attaching itself to a surface (a rock or other polyps) and the other end enclosing its mouth. Its mouth is surrounded by several finger-like tentacles that trap prey that swim by, which get broken down in its digestive cavity. Many polyp tentacles contain nematocysts, which can shoot into the skin of their prey and release a poison. These nematocysts are retained throughout the lifecycle, and can be painful or even deadly to humans. (For information on treating jellyfish stings, visit WebMD.) Polyps are solitary or live in interconnected colonies, depending on the species. While sea anemones and corals remain polyps their entire lives, most jellyfish have another, subsequent stage: the medusa stage.

The brown jellyfish (or Japanese sea nettle, <em>Chrysaora melanaster</em>) resides in Pacific coastal waters, where its yellow-brown body can reach about a foot in diameter and have tentacles dangling down six to eight feet.
Courtesy Photo

How does this stationary polyp turn into the jellyfish we’re so familiar with? Most often, the polyp amazingly splits itself horizontally into disks (in a process called “strobilation”). Each disk wanders off and turns into a medusa, which may take years to mature. (And as if that were not enough, some polyps can actually re-grow themselves and divide into more medusae later.) The medusa is the form that we usually think of when thinking of jellyfish: often transparent, bell- or umbrella-shaped, with a mouth tucked away underneath this gelatinous dome, surrounded by long, dangling tentacles and oral arms. The tentacles and arms are covered with nematocysts for defense and trapping prey, which is brought to the jellyfish’s mouth by the oral arms. These delicate creatures have no heart, bones, brain, or gills, but get by just fine with a relatively simple nervous system and diffusing oxygen through their skin.

Jellyfish are found in an array of shapes, sizes, and life styles. Medusae can range from the size of a contact-lens to the diameter of a truck tire, with tentacles over 100 feet long. While jellyfish primarily drift with the current, smaller jellyfish can collect water in their bell and contract it to “jet” across the water, although larger jellyfish can only contract the rims of their bells. These movements also create localized currents to draw in prey, which mostly consist of plankton-sized animals.

A Diverse Lineage: There are four primary groups of jellyfish (Scyphozoa, Staurozoa, Cubozoa, and Hydrozoa), which are not only distinguished by their body shapes and sizes, but also by many variations on the basic lifecycle described above.

The Scyphozoans: Several different types of Scyphozoans, or “true jellyfish,” inhabit waters of the Pacific coast (stings from these jellies are usually not deadly, but can still be dangerous). The grandest of these is probably the lion’s mane jellyfish (Cyanea capillata), which is usually considered the world’s longest animal; the longest specimen found had tentacles 120 feet long. These jellyfish have a milky yellowish bell that can reach six feet in diameter. Although they’re found along the Pacific coast, these giants rarely travel as far south as California.

The purple jellyfish (Pelagia panopyra), however, can be found in the coastal waters of Southern California. It’s only about one foot in diameter and sports flashy dark purple stripes down the sides of its otherwise white bell, which is surrounded by 20-foot-long tentacles. Also in Pacific coastal waters, the brown jellyfish (Chrysaora melanaster) is of a similar size (but with shorter tentacles) and, as its name implies, is yellow-brown in coloring.

The moon jellyfish (or “common jellyfish,” Aurelia aurita) can be found around the world, including along the Pacific coast. It’s a nearly colorless jellyfish about one foot in diameter with four horseshoe-shaped markings on the center of its umbrella (these are actually its reproductive organs) and many tiny, short tentacles. Dr. Cathy Lucas, who is a lecturer at the National Oceanography Center in Southamptom, studies moon jellyfish extensively and will be talking at the Jellyfish ROCK event this Saturday.

The Cubozoans: The Cubozoans, or “box jellies” (named for their cube-shaped bell), reside in warm, tropical waters and include some of the most poisonous creatures in the world. It’s estimated that box jellyfish stings cause the death of some 20 to 40 people a year in the Phillipines alone. The sea wasp (Chironex fleckeri), a basketball-sized jellyfish that resides in the Indo-Pacific, can kill an individual in three minutes from an untreated sting. It’s considered by many to be the world’s most venomous animal, and kills about one person a year in Australia. The sea wasp is the largest member of this group, though size doesn’t seem to matter much to them; other often fatal jellyfish in this group include the thumbnail sized Carukia barnesi and Malo kingi (which also reside in waters near Australia).

The Staurozoans: Probably the most bizarre jellyfish in appearance are the Staurozoans, or “stalked jellyfishes,” which go straight from a crawling, larva form to an attached, upside-down medusa form (skipping the polyp stage). They attach to a substrate (rock or algae) by a stalk on the top of the “umbrella,” while their tentacles project outwards. Members of this class are commonly found in relatively cold waters, close to the shoreline. For a long time they were grouped with the Scyphozoans, and only recently did they become their own group as a result of genetic studies (the taxonomical position of many jellyfish, and even Cnidarians in general, has long been debated).

The Hydrozoans: The hydrozoans have the strangest biology of the jellyfish world. Some remain polyps their entire lives, sometimes forming colonies intricately interconnected by tube-like structures. There can even be specialized polyps in these colonies that are responsible for certain key duties, such as reproduction, defense, or acting as a central surface for other polyps to attach to.

A hydrozoan’s medusa stage (if present) is fairly small compared to other jellies; hydrozoan medusae are usually no larger than two and a half inches in diameter. The freshwater jellyfish Craspedacusta sowerbyi is a hydrozoan less than an inch in diameter that has astonishingly found its way into freshwater rivers and lakes on nearly every continent. Also a hydrozoan, the famous Portuguese Man o’ War is not actually a jellyfish but only superficially resembles one; it is in fact made up of a colony of specialized polyps and medusae. The amazingly “immortal” jellyfish in the genus Turritopsis is also a hydrozoan.

Jellyfish Blooms: While “blooms” are often a normal part of being a jellyfish, and are thought to have occurred naturally for millions of years, it’s becoming more and more clear that climate change and other factors are positively affecting many jellyfish species around the world, possibly resulting in an increase in jellyfish blooms (congregations of jellyfish en masse in small areas of water). Normally blooms occur if conditions are favorable, such as during certain seasons in an area that has the right water currents, temperature, water quality, and amounts of prey and predators.

But lately people have been noticing higher occurrences of jellyfish blooms. It is difficult to tell which, if any one, factor is most responsible, although it is most likely a combination of several. The list of suspects includes climate change, overfishing, increasing nutrient levels in waters, and increasing amounts of artificial structures in marine areas. Overfishing can eliminate predators and competition, while higher nutrient levels in the water (due to farm fertilizer run-off) can increase the numbers of prey for the jellyfish and kill more competitors (this is also part of what’s driving coral reefs to extinction). Marine structures provide more areas for the polyp stage of the jellyfish to thrive in. Jellyfish additionally have been hitching rides on ships to areas where they didn’t previously live and can flourish in the absence of an established predator. People have helped in multiple ways to make the world more jellyfish-friendly.

It is difficult to tell exactly how much more common these blooms, and jellyfish numbers in general, have become because there haven’t been good records of their numbers in the past. But that said, there are some clear examples of species that are more abundant. Probably the most reported is Nomura’s jellyfish (Nemopilema nomurai, an enormous jellyfish similar in size to the lion’s mane jellyfish, possibly reaching nine feet in diameter and over 400 pounds. Over the last decade, these giants have been blooming in the waters between China and Japan and causing immense disruption of the fishing industry there; they have destroyed fishing nets and capsized fishing boats by getting their colossal bodies caught in nets in incredible numbers.

While this is one of the most publicized disruptions caused by jellyfish blooms, increases in jellyfish numbers have also caused decreases in tourism to tropical waters, disruption of fishing globally (not just by destroying nets, but also by poisoning and crushing fish and clogging boat engines), and damage to desalination and nuclear power plants (by clogging pipes carrying water).

Jellyfish ROCK! To talk to experts about these fascinating creatures, and learn more about our impact on them and the causes of jellyfish blooms, as well as the impacts of these blooms on us, this Saturday attend the free event, called Jellyfish ROCK: Reaching Out to the Community & Kids, at the Santa Barbara Natural History Museum from 6:00 to 8:15PM.

For more on jellyfish, see Kylie A. Pitt and Jennifer E. Purcell’s “Jellyfish Blooms: Causes, Consequences, and Recent Advances” (Kylie A. Pitt will also be speaking at the Jellyfish ROCK event), the National Science Foundation’s report “Jellyfish Gone Wild,” the University of California, Merced’s Wiki on Scyphozoa, a previous “Biology Bytes” article on “The Immortal Jellyfish: Turritopsis Can Change the Direction of its Life Cycle,” Christina Parson’s book “Dangerous Marine Animals of the Pacific Coast,” or Wikipedia’s article on Jellyfish.

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at


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