Tag Archives: what eats a jellyfish



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.