Tag Archives: Jellyfish

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.

In addition to this blog, I write a monthly Mystery Newsletter about true crime in Alaska. If you are intrigued by Alaska and/or true crime, sign up for my newsletter. Each month I profile a crime that is in some way unique to Alaska.




There is nothing simple about jellyfish, except the creatures themselves. From their taxonomy to their lifecycles, jellyfish are complicated. When I am out on the boat with our summer guests, I answer many questions about jellyfish, and since I am never 100% certain of my answers, I decided to tackle jellyfish for a blog post. Once and for all, I planned to conquer these gelatinous creatures. The more I read, though, the more confused I became. There are many species of jellyfish, so, of course, lifecycles vary between species and may also vary depending on water temperature and nutrient availability. In this post, I will try to keep things as simple as possible and will describe a generalized jellyfish lifecycle. Keep in mind, though, when it comes to jellyfish lifecycles, there are many exceptions and exceptions to those exceptions.

Our guests ask us: How long can a jellyfish live? What do jellyfish eat? What eats a jellyfish? Why do jellyfish form large groups? How do jellyfish reproduce? Will these jellyfish sting me? Are these jellyfish dangerous?

First of all, most biologists now refer to jellyfish as jellies, because they aren’t fish. This week I will cover basic facts about jellies and tell you about some of the poisonous species. Over the next two weeks, I will discuss two of the most prevalent jelly species in the North Pacific near Kodiak Island.

Let me begin by attempting to explain one of the most complex lifecycles in the animal kingdom. Most species of jellies reproduce by a combination of sexual and asexual reproduction, and their lifecycles include several stages. The jellies you see floating in the ocean are in the adult or medusa stage of their lifecycle. Adults are usually either male or female (not even this is a given with jellies, though), and they release eggs and sperm at incredible rates. When an egg and sperm unite, a larva is produced. Each larva attaches to a hard surface, such as a rock on the ocean bottom. At this point in the lifecycle, the organism is called a polyp.

Polyps have only rarely been seen in the wild, but biologists believe polyps may blanket large expanses of the ocean bottom in some areas. Scientists also think that a jelly may remain in the polyp stage of its lifecycle from days to years or even decades until temperature and food availability are favorable for it to survive as an adult. When conditions are favorable, a polyp elongates and reproduces asexually by budding. These buds develop into young jellies that grow into adults, completing the life cycle. A single polyp may produce a large number of jellies, and a large field of polyps can produce tens of thousands of jellies at a time.

The medusa phase of the jelly may last from a few hours to several months, depending on the species. Most jellies live 2 to 6 months in the medusa stage. The medusa stage is usually the end of a jelly’s lifecycle, but one unusual species, Turritopsis dohrnii, has the ability under certain conditions, to transform from the medusa stage back to the polyp stage, making this species theoretically immortal.


Because polyps bud when conditions are favorable, a large number of medusae may be formed at one time in a particular area. Waves and tidal currents often congregate the medusae in groups of thousands. These groups are called a bloom, a swarm, or a smack. We often see blooms of moon jellies, and when viewed from the deck of our boat, they make a large, white patch in the water that can be seen from quite a distance. When we get closer, we can make out individual medusae in the bloom.

Jellies range in size from a few millimeters in bell height and diameter to nearly two meters in bell height and diameter. The lion’s mane jellyfish (Cyanea capillata) is considered one of the longest animals in the world; its fine, thread-like tentacles may extend to 120 ft. (36.5 meters) in length, although most are much smaller than that.

Medusae are carnivorous and eat plankton, crustaceans, fish eggs, small fish, and other jellies. They use the venom-filled nematocysts on their tentacles to sting and stun their prey, and then they trap the prey in their mucous. Jellies ingest their food and eliminate their waste through the same hole in the center of the bell. Some fish and invertebrate species are immune to the stings of certain jellies and may form symbiotic relationships with them.

One of the questions we are most often asked is what eats a jellyfish? Other jellies are some of the most common predators, but jellies are also food for sea anemones, tuna, shark, swordfish, sea turtles, shore birds, and possibly even salmon. Nevertheless, jellies are not eaten in large numbers, and since many jelly populations have expanded in recent decades, biologists worry that jellies are becoming more dominant in some ecosystems, replacing fish that once thrived in these areas. Jellies can live in areas with low-oxygen levels. In water that has been polluted by agricultural runoff, nutrient levels are high, but oxygen levels are low. These conditions favor jellies over fish that cannot tolerate such low levels of oxygen.

The nematocysts or stinging cells of most jellies are so small they can’t penetrate human skin. Others may cause a slight sting or irritation, but the sting of a few jellies can cause severe pain and in some cases, even death.


The sea wasp box jelly, found in Australia, is considered the deadliest jelly in the world. Since 1954, 5,568 people have died from the sting of this jelly. A sea wasp has 15 tentacles, extending up to ten feet in length. On each tentacle, there are approximately half a million microscopic darts, and each dart is full of venom. One of these darts holds enough venom to kill 60 people. The venom acts very quickly and may cause cardiac arrest in a few minutes. In addition to the venom, the pain of the sting is so intense; it can lead to shock. Some other species of box jellies are also poisonous. One member of the box jelly family, the Irukandji jelly is only 0.2 inches in length, and is nearly transparent, making it almost invisible. Its toxin is 100 times more deadly than that of a cobra’s. The Portuguese Man o’ War is not a jelly but is an organism called a bluebottle. Its sting leaves a welt like a whip mark and may remain painful for days. The venom can cause fever, shock, and may lead to cardiac or pulmonary arrest.

Most jelly stings cause only mild pain similar to a bee sting, so what is the treatment for a jelly sting? Barrier clothing, even something as thin as pantyhose can protect a swimmer or diver. If stung, use a credit card to scrape the affected area to remove remaining nematocysts. A 10% solution of aqueous acetic acid or vinegar may be used to soothe the skin. You can also wash the affected skin with salt water, but do not wash the skin with fresh water, alcohol, ammonia, or urine. These solutions may cause the nematocysts to release more venom.

Over the next two weeks, I’ll write about the two species of jellies we most often see around Kodiak Island. I also want to invite any of my readers who haven’t already done so, to sign up for my monthly Mystery Newsletter. Each month I write about a true crime in Alaska.