Jellies: Hypnotic and Fascinating cover

Jellies: Hypnotic and Fascinating

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The ocean is home to many species of gelatinous animals, from sea jellies to comb jellies, as well as some tunicates, gastropods, and worms, which can all have translucent bodies similar to sea jellies.
Sea jellies are members of the phylum Cnidaria (pronounced nigh-DARE-ee-uh). Within this phylum is the class Scyphozoa, which includes the most familiar types of sea jellies, with bell-shaped bodies and tentacles or oral arms. This includes moon jellies, purple-striped jellies, Pacific sea nettles, and many other species.





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Jellies: Hypnotic and Fascinating

What is a Sea Jelly?

The ocean is home to many species of gelatinous animals, from sea jellies to comb jellies, as well as some tunicates, gastropods, and worms, which can all have translucent bodies similar to sea jellies.

Sea jellies are members of the phylum Cnidaria (pronounced nigh-DARE-ee-uh). Within this phylum is the class Scyphozoa, which includes the most familiar types of sea jellies, with bell-shaped bodies and tentacles or oral arms. This includes moon jellies, purple-striped jellies, Pacific sea nettles, and many other species.

Indonesian Sea Nettle

Indonesian Sea Nettle

Photo Courtesy Aquarium of the Pacific

What is a Sea Jelly? (cont.)

Other classes of sea jellies in the phylum include:

Hydrozoa (small, usually transparent species like umbrella and crystal jellies; this class also includes the Portuguese Man-o-War, which is actually a colony of jellies in their medusa and polyp forms), Cubozoa (box jellies and sea wasps), and Staurozoa (stalked jellies that live attached to rocks and other surfaces).

Together, there are several thousand species of jellies in the Cnidarian branch of the animal family tree. At the Aquarium, jellyfish are referred to as sea jellies because technically they are not fish.

Comb Jelly

Comb Jelly

Photo Courtesy Aquarium of the Pacific

Comb Jellies

While they have similarly translucent bodies, live in the same waters, and eat the same kinds of food, the jellies described above are members of a different phylum from comb jellies, or ctenophores.

There are at least 100 species in this phylum, Ctenophora (Greek for combbearers and pronounced teen-o-for-uh). Scientists at one time categorized sea jellies and ctenophores in the same phylum, but the differences between these groups have now been well characterized.

The most notable is that instead of tentacles armed with stinging cells, comb jellies have sticky cells called colloblasts that do not sting and eight rows of cilia, or combs, that propel them through the water and produce a flickering rainbow-like appearance with their movements. Both jellies and comb jellies are on display at the Aquarium.

Natural History

Jellies and comb jellies have lived on Earth for at least 500 million years, making them three times as old as dinosaurs.

They are the earliest known animals to have organized tissues and to swim using muscles instead of drifting. These animals vary widely in size, from tiny medusae and comb jellies smaller than a pea to the lion’s mane jelly, which occasionally reaches 6 feet in width with tentacles up to 100 feet long.

Stinging Cells

Stinging Cells

As Cnidarians, jellies are related to corals and anemones. They all possess stinging cells called nematocysts, or cnidae. This word comes from the Greek word cnidos, meaning stinging nettle. In addition to nematocysts, Cnidarians have several other characteristics in common, including a similar method of capturing food and an oftentimes complex life cycle.

Indonesian Sea Nettle

Photo Courtesy Aquarium of the Pacific

Biology

Sea jellies survive without a heart, brain, or lungs. They are 95 percent water, and their movements are governed by the flow of the water they live in.

Although they contract their bells to propel themselves, jellies are planktonic animals, meaning that they drift with currents, being too weak to swim against them.

While they lack organs, Cnidarians have a net of sensory nerve cells, tentacles or oral arms, gonads that produce reproductive cells, and a gastrovascular cavity, where digestion takes place. The cavity opening is also used to expel waste and release reproductive cells.

Biology (Cont.)

Jellies have the simplest-known nervous system among multicellular animals. Nerves in the jelly’s external layer form a nerve net.

The nerves generate pulses that contract the jelly’s bell, allowing it to swim. The nerves also detect changes in water chemistry and can sense touch. Jellies in the class Cubozoa even have complex eyes and can respond by swimming toward or away from the light and dark they see. The jelly’s circulation system is made up of canals filled with fluid that transports food and distributes nutrients.

Diet

Diet

Jellies use their stinging cells to capture food, which mostly consists of zooplankton and other sea jellies.

These stinging cells are primarily located along feeding tentacles and sometimes on the bell itself. If the food gets stung by the nematocysts, those cells will hold onto and incapacitate the prey with venom. The tentacles then pull the food into the gastrovascular cavity where it is digested.

Egg Yolk Jelly

Photo Courtesy Aquarium of the Pacific

Importance of Jellies

Jellies are eaten by many kinds of fish and sea turtles, so they are important for a healthy and balanced ecosystem.

Leatherback turtles feed almost exclusively on jellies. Humans also have many uses for sea jellies:

Food

People from many countries, including Indonesia, China, Japan, Korea, and other Asian countries, harvest sea jellies for food.

Jelly fisheries in China date back 1,700 years, and worldwide more than 900 million pounds of jellies are caught each year, according to the Smithsonian Institution. Fishermen are beginning to harvest sea jellies off the coast of the United States for Asian markets. Jellies are often dried for storage and eaten either dried or rehydrated.

Importance of Jellies (cont.)

Medicine

The ocean is increasingly seen as a potential source of medicines.

While marine life found on coral reefs are the most researched, scientists are also studying biochemicals derived from sea jellies that show some promise in treating various human diseases.

Science

In 2008 scientists Osamu Shimomura, Martin Chalfie, and Roger Y. Tsien were awarded the Nobel Prize in Chemistry for their work with green fluorescent proteins found in crystal jellies.

When exposed to a certain kind of light, the proteins glow bright green, allowing scientists to use them as markers in cell and molecular biology research.

Conservation

Conservation

Humans have found many uses for sea jellies, but human activity is changing ocean ecosystems, affecting jelly reproduction and habitats and potentially reducing their populations in the wild. Jellies are important for a healthy and balanced ecosystem.

Moon Jelly

Photo Courtesy Aquarium of the Pacific

Jelly Populations

An abundance of jellies is seen by some scientists as a signal that an ocean ecosystem is out of balance.

As humans have removed fish from the food web by overfishing certain species, there is less competition for the zooplankton jellies feed on, so their numbers may have grown.

Another human activity that may lead to higher numbers of jellies is fertilizers entering the ocean through runoff. Fertilizers cause algae to bloom, which quickly depletes oxygen from the water, creating what are known as dead zones. Some jellies thrive in low-oxygen environments.

Jelly Populations (cont.)

Jelly Populations (cont.)

A changing climate and warming ocean are also likely to affect sea jellies. Those that thrive in warmer waters may increase in number, while those that live in cold-water habitats may diminish. Large jelly blooms can pose problems for ocean swimmers because of the danger of being stung by some species. They can also clog cooling water pumps at coastal power plants, causing regional power outages.

Moon Jelly

Photo Courtesy Aquarium of the Pacific

Ocean Pollution

Ocean Pollution

Pollution and trash in the ocean pose major threats to ocean life. For example, sea turtles and other animals sometimes mistake plastic bags for jellies. If these animals eat the plastic bags they find in the ocean, it can be extremely harmful to them, even deadly.

Pacific Nettle

Photo Courtesy Aquarium of the Pacific

Information Gap

Assessing human impacts on jelly populations is difficult because of a lack of available data.

Some species seem to have disappeared for several years at a time, but returned later. Some species of jellies were more abundant in San Francisco Bay at a time when waters there were more polluted.

Conversely, while the waters around the San Juan Islands in the Pacific Northwest appear to be relatively pristine, jellies populations there have decreased over recent decades. The causes for these changes have yet to be confirmed. In order to best understand sea jellies, the changes to their populations, and our impacts, more research needs to be done.

What Can You Do?

What Can You Do?

The public can help track jellies as citizen scientists. At jellywatch.org, people can learn about the jellies they see along the coast while providing important scientific data. Jelly blooms are not currently tracked in any permanent record. Citizens can help scientists by reporting their observations.

Pacific Nettle

Photo Courtesy Aquarium of the Pacific

Husbandry

Husbandry

The Aquarium’s aquarists have successfully cultured several species of jellies for many years. In protected environments such as aquariums, jellies can live longer than their lifespans in the wild because of the absence of predators and the availability of an adequate food supply.

Purple Stripe

Photo Courtesy Aquarium of the Pacific

What are they fed?

What are they fed?

The Aquarium’s jellies are fed brine shrimp twice a day. Some of the larger species’ diets are supplemented with additional types of food, like krill and tiny crustaceans called copepods. The sea nettles, lion’s mane jellies, and egg yolk jellies are also fed young moon jellies, which is a normal part of their diet in the wild.

Purple Striped

Photo Courtesy Aquarium of the Pacific

Do they sting?

All the jellies (except for the comb jellies) can sting to some extent.

At the Aquarium, the sea nettles are the most likely to produce a sting that is painful to humans. When handling this species, the best line of defense against stings is latex gloves because the stinging cells on the tentacles cannot penetrate the latex.

If an aquarist handling the jellies happens to get stung despite precautions, pouring vinegar on the sting will neutralize it and stop the pain. The aquarists also rinse all tools used to clean jelly exhibits with a freshwater hose to knock off any stray tentacles that might be stuck to the tool.

Kreisel tanks

Sea jellies are planktonic—in the ocean, they drift with the currents and have little control over their locomotion.

For this reason, at the Aquarium jellies are kept in special tanks called kreisels (German for “spinning top”), which are designed to keep them in suspension in a constantly moving flow of water. These tanks have rounded corners to prevent jellies from getting stuck, and water circulates around the perimeter to prevent jellies from settling at the bottom.

Reproduction

While sea jellies have the simplest anatomy of almost any animal, they have complex and varying lifecycles and reproduce both sexually and asexually. Different jelly species reproduce in different ways.

Most adult Scyphozoans release sperm, eggs, or both into the sea. Fertilized eggs develop into a planula, a flattened, free-swimming, larval-stage organism. The planula settles on a suitable surface, such as a rock, shell, dock, or piece of driftwood, and then develops into the polyp stage. A jelly in its polyp stage looks like a tiny sea anemone and feeds in the same way. With adequate food and space to develop, the polyp will divide asexually forming a stack.

Reproduction (cont.)

Suitable water conditions cause the polyps to pulsate and bud off the stacks.

They become free-swimming ephyrae, the juvenile version of sea jellies. The ephyrae drift and eat, soon metamorphosing into the small stages of what is later the sexually mature adult or medusa.

Cubozoans reproduce sexually, with the male passing sperm to the female, who either hosts or releases fertilized eggs as they become larvae. The settled polyps can then reproduce asexually by budding. Hydrozoans and Staurozoans also have both sexual and asexual methods of reproduction.

Jelly Blooms

Jelly Blooms

A jelly bloom occurs when an abundance of nutrients and other conditions allows many jellies to reproduce at once. This often occurs when temperate waters are warmed by sunlight in the spring, leading to an abundance of phytoplankton, or tiny plants in the surface waters. These plants feed tiny animals called zooplankton, triggering an abundance of food for jellies, which provides them the excess energy to grow and reproduce.

Umbrella Jelly

Photo Courtesy Aquarium of the Pacific

Life Span

Life Span

Different species reproduce at different times of the year. Small species generally have short lifecycles ranging from as little as an hour or two to perhaps three to four months. Jellies can die because they are eaten by predators and other jellies; because they are damaged over time by rough waves, boat propellors, or bacteria; or because of lack of food.

West Coast Jelly

Photo Courtesy Aquarium of the Pacific

Comb Jellies

Comb jellies have a much simpler reproductive process.

Most are hermaphrodites, with individuals carrying both male and female sexual organs. After they grow to a certain size, they release eggs and sperm daily, which drift in the water for minutes to hours until they are able to fertilize and grow into new comb jellies.

If conditions become unsuitable for reproduction, for example, due to a lack of food, comb jellies can refrain from releasing these reproductive cells and may shrink in body size until conditions improve.