Tag Archives: jellyfish in the North Pacific

Moon Jelly

You probably have seen moon jellies at an aquarium. They are the small, round, nearly- transparent jellies most of us picture when we think of jellyfish. The common name moon jelly refers to all species in the genus Aurelia. Species in this genus are so similar to each other they can only be differentiated by DNA analysis. Scientists do not know how many species belong to the genus Aurelia because more are being discovered all the time.

Moon jellies are found worldwide but are most prevalent in the Atlantic, Pacific, and Indian Oceans. They tolerate a wide range of temperatures and salinities and may even inhabit brackish water. In brackish water, the bell of a moon jelly is much flatter due to the decreased salinity. Moon jellies usually stay near the surface and are capable of moving upward on their own but are dependent on wind, tides, and currents for horizontal movement. Currents sometimes bring together thousands of moon jellies in a small area, and these groupings are called a bloom, a swarm, or a smack.

A moon jelly is typically 4 to 5 inches (10 to 15 cm) in diameter. It has a shallow bell that is colorless, except for the horseshoe-shaped gonads that may be tinted violet, pink or yellow. Since the gonads are near the bottom of the stomach, they often take on the color of the prey the jelly has just eaten. The margin of a moon jelly is divided into eight lobes that are fringed by numerous, thin tentacles. Each lobe is divided by a shallow cleft housing a sensory organ that aids the jelly in maintaining its equilibrium. The radial canals of the digestive system are clearly visible and repeatedly branch as they move toward the margin of the jelly. The oral lobes or arms are short and thick.

Moon jellies are carnivorous and feed mainly on zooplankton. They are eaten by lion’s mane jellies, sea urchins, crab, some sea anemones, sea turtles, and shorebirds. Like other jellies, moon jellies have tentacles with stinging nematocysts, but their stings are relatively harmless to humans.

Moon jellies have a lifecycle similar to other jellies. In the medusa or free-floating phase, the male releases a strand of sperm which the female takes through her mouth, fertilizing her eggs. The fertilized eggs develop into larvae in pockets in the oral arms that surround the mouth. The female then releases the larvae, and they settle and develop into polyps. A moon jelly may remain in the polyp stage of its lifecycle for 25 years until conditions are right for the polyp to reproduce asexually by budding. The buds float free and develop into medusae. In the wild, moon jellies only survive about 6-months in the medusa stage, but they may live up to a year as medusae in an aquarium.

Moon jellies are the most commonly kept species in both public and private aquariums. Their wide tolerance of both salinity and temperature make them a good choice as an aquarium species.



Lion’s Mane Jellyfish

The lion’s mane jelly (Cyanea capillata) is the largest known species of jelly. The largest specimen ever recorded was longer than a blue whale; it had a bell diameter of 7 ft. 6 in. (2.3 meters) and tentacles that were 121.4 ft. (37 meters) long. Lion’s mane jellies near Kodiak Island are not nearly that large; a large specimen here would have a bell diameter of 20 inches (50.8 centimeters) and tentacles 29.5 ft. (9 meters) long. Lion’s mane jellies are gorgeous animals that range from red-brown to yellow to white in color. Larger individuals are often red to dark purple while smaller jellies are a lighter orange or tan.

The lion’s mane jelly has a bell that is flattened and thick in the center and thin at the margins. The margin of a lion’s mane jelly is divided into eight, deep lobes, and each lobe is divided by a shallow cleft. A sensory organ called a rhopalium is located in each cleft, and these organs aid the jelly in determining its orientation. Although a jelly does not have a brain, it does have a central nervous system that receives input from sensory organs. Large, sticky tentacles emanate from the margin of the jelly. These tentacles are grouped into eight clusters with each cluster containing over 100 tentacles. Colorful oral arms that are much shorter than the tentacles extend from the center of the bell.

A lion’s mane jelly uses its long tentacles to capture and pull in prey to its mouth in the center of the bell. As with other jellies, the tentacles contain stinging nematocysts. When the tentacles touch a human’s skin, they cause temporary pain, itching, and localized redness. The pain is more intense if these nematocysts get into a cut or sore, or if the contact is around the eyes and nose. I frequently touch the tentacles when cleaning a fouled fish hook, and the tingling, burning sensation usually lasts only a few minutes. The pain is more severe if a person comes into contact with a large number of tentacles, but the stings are not fatal to humans who are in good health.

Lion’s mane jellies eat zooplankton, small fish, and moon jellies. They are preyed upon by anemones, some crab, shrimp, and nudibranchs. Leatherback sea turtles feed almost exclusively on lion’s mane jellies in the summer around Eastern Canada.

Some fish species, such as juvenile Pollack, and some amphipod species are immune to the sting of the lion’s mane jelly and form a symbiotic relationship with the jelly by swimming in the protection of its tentacles.

Our summer guests are often surprised to see an abundance of jellies in the frigid waters of the North Pacific, but lion’s mane jellies are a cold-water species that cannot tolerate warmer water.

Lion’s mane jellies inhabit the open ocean and stay near the surface, usually no more than 65 ft. (20 meters) deep. They can move forward with slow, weak pulses, but they are mostly dependent on wind and currents to move great distances.

Lion’s mane jellies have a one-year life span. The female jelly in the medusa stage carries its fertilized eggs in its tentacles until they grow into larvae. She then deposits the larvae on a hard surface, such as a rock, where they grow into polyps. The polyps reproduce asexually, creating a stack of individuals called ephyrae. Each individual ephyra then buds and breaks away from the stack, developing into the medusa stage.

I never grow tired of watching a beautiful lion’s mane jelly pulsing near the ocean’s surface, often with one or two small fish swimming in its tentacles. This gorgeous creature with its complicated lifecycle is one of nature’s most amazing creations.

My post next week will be about the moon jelly, the most common jelly seen near Kodiak Island. While not as showy as a lion’s mane jelly, it is a fascinating animal that often forms blooms of thousands of individuals.

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