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Aquarium of the Pacific - Online Learning Center - Species Print Sheet

Conservation Status:  Safe for Now

Climate Change:  Uncertain

AquaticJapanese Sea Nettle

Chrysaora pacifica CnideriansSea jellies

Japanese Sea Nettle on blue background The appearnce of this sea nettle while similar to other sea jellies, is distinctive. | Aquarium of Pacific
Japanese Sea Nettle on blue background
The appearnce of this sea nettle while similar to other sea jellies, is distinctive. Aquarium of Pacific

Species In-Depth | Print full entry

At the Aquarium

The Japanese sea nettle’s Aquarium habitat is in the Northern Pacific gallery.

Geographic Distribution

Southwest Pacific Ocean around Japan.

Habitat

subtropical ocean waters at temperatures of 12 to 25 degrees C (54 to 77 degrees F)

Physical Characteristics

The color of rounded dome-shaped bell ranges from gold to red with a distinctive darker sunburst pattern of stripes on the exumbrella from the top to almost the rim of the bell. Adult medusae have eight clusters of tentacles each hanging from the bell margin. Juvenile medusae have three tentacles per cluster. There are 48 dark lappets above the bell margin. Extremely long oral arms hang down from the center of the bell.

Size

Bell diameter to 30.5 cm (12 in) Tentacles extended to 3 m (10 ft) Oral arms may be as long as the tentacles

Diet

Like other sea nettles, the Japanese sea nettle is a voracious carnivore. Its preferred prey is ctenophores (comb jellies). It also eats small fishes, copepods, larvae, anchovy eggs, and other zooplankton.

Reproduction

Medusae are either male or female. There are two phases to the reproduction cycle, sexual and asexual. An adult female produces eggs that are held in its oral arms around its mouth. The male jelly releases sperm into the water, and the female uses her oral arms and tentacles to bring in sperm to fertilize her eggs. The fertilized eggs remain on the oral arms until they develop into ciliated pear-shaped larvae, the planulae. Eventually they drop off the oral arms, swimming freely for a time while searching for a suitable substrate on which to attach, usually the bottom of a structure that is shaded and rough. They attach upside-down with their tentacles pointing upward to filter feed. They are now known as sessile planocysts. Depending on environmental conditions, the planocysts go through one of two reproductive phases, either asexual or sexual. They either bud off non-motile clones of themselves or begin a process called strobilation. Strobilation only occurs when the water quality, temperature, and salinity are favorable and food supply is adequate. Miniature medusa-like structures called strobilia are formed on a stalk, one on top of the other like a stack of dinner plates, with the most mature on top. The strobilia bud off as individual eyphyrae that develop marginal lobes, rhopila, tentacles, elaborate oral arms, and finally, the bell shape of the adult medusa, the form in which sexual reproduction occurs.

Behavior

Trailing tentacles sting and transport food up the central tentacle to the jelly’s gastrovascular cavity where digestion takes place. Nettles can and do feed constantly because their many tentacles can function independently of each other. As they trail they offer a large surface area with which to capture prey. Large prey are partially digested on the oral arms before being transported to the gastrovascular cavity.

Adaptation

Although the sting of this jelly is considered to be mild, the toxin can cause a rash and a burn mark for humans.

This sea nettle has both coronal (circular) and radial muscles. As it swims, the shape of its bell changes markedly from flat to bell-shaped during extension and contraction of the bell. The coronal muscles act first, drawing the bell inward and downward. The radial muscles then cause the bell to bend diameter. There is an initial backward thrust of the bell on the water followed by an outward jet of water with each contraction. There is sufficient drag on the bell to virtually stop the jelly in the water at the end of recovery. The jelly then accelerates again during the next beat. In the resting state at the end of the swimming stroke, the jelly’s bell is flattened and the lobes give it the appearance of an eight-pointed star.

Conservation

The Japanese sea nettle’s habit is subtropical ocean waters. The impacts of climate change on ocean water temperatures may affect its preferred habit unless it can adapt to a different temperature that that of the subtropical waters (12 to 25 degrees C or 54 to 77 degrees F) in which it is found currently.

Special Notes

It was not until 2009 that the polyps of the Japanese sea nettle were discovered in the wild and then the discovery was accidental. A Japanese scientist who was surveying an area off of Japanese waters was looking for a different jelly species when he accidentally discovered polyps and planocysts of the Japanese sea nettle that were attached to shells of dead crabs, bivalve shells, and stones. In a laboratory setting, the researchers were able to induce strobilation.