What Is a Group of Jellyfish Called?

Group of pink jellyfish with long tentacles

Jellyfish are primitive species that have been on Earth for more than 500 million years. You might already have heard many fascinating facts about these species. But do you know what a group of jellyfish is called?

Stick with us as we discuss the various collective terms used to describe a group of jellies, from the groups’ descriptions to the reasons for the terminology. We also have fun facts about jellyfish you might not know!

What Is a Group of Jellyfish Called?

Group of golden jellyfish floating in clear blue water

A group of jellyfish can be called different names depending on the context in which they are observed. The most common names for a group of jellyfish are bloom, swarm, and smack. Other less common collective nouns for jellyfish include fluther, brood, smuth, shoal, or stuck.

These terms not only identify the group but also reflect the jellyfish’s behavior when they gather.

When the jellyfish population explodes in an area, it is called a bloom. These blooms can consist of over a thousand jellyfish and become quite a sight in the ocean.

The word bloom is also suggestive of how flowers increase in spring. Similarly, due to their astounding reproductive capabilities, jellyfish can dramatically multiply in favorable conditions, creating a large population.

Meanwhile, a swarm of jellyfish often describes a smaller collection of jellyfish and implies their movement as a unit.

It typically refers to jellyfish collectively moving in the water, which usually happens when the same current carries them or when they are attracted to a particular food source.

This term is similar to how we describe a group of insects moving together, emphasizing coordinated movement rather than social interaction.

A smack of jellyfish is a more traditional and informal term that may have originated from the sensation felt after a jellyfish sting, akin to a minor but sharp impact.

This term may also refer to how the bodies of jellyfish gently hit each other as they move through the water.

Fluther, brood, shoal, smuth, and stuck are other less common terms used to describe a group of jellyfish.

Although some of these collective names have no clear explanation, the term fluther could have originated from the word flutter, which means to fly unsteadily or hover by flapping [wings for birds].

Reasons Behind the Terminology

The collective names used to describe a group of jellyfish are usually attributed to their behavior and characteristics. The names are not just random choices; they have specific origins.

Terms such as swarm, bloom, and fluther are often associated with how a group of jellyfish moves and behaves. On the other hand, the collective name smack may refer to the biological characteristics of jellyfish.

For instance, a swarm of jellyfish usually directs to a large number of jellyfish moving together, much like insects do when seen in large groups.

Meanwhile, the term a bloom of jellyfish is used when there is a sudden rise in the jellyfish population, visually reminiscent of plants blossoming.

Fluther, a supposed derivation of the word flutter, may refer to how jellyfish hover and glide with the current by flapping or “fluttering” their tentacles like how birds flap their wings.

Meanwhile, the term a smack of jellyfish is believed to be associated with the sensation of a jellyfish sting, which can feel like a sharp slap. It is also often related to the sound of jellyfish gently hitting each other’s soft bodies.

These terms have developed from human interactions with jellyfish and observations of their behavior. The language used to describe these groupings reflects how we interpret their movements and biological traits.

Fun Fact: Although not all jellyfish sting, some jellyfish species have potent stingers that can be fatal even to humans, such as the Australian Box Jellyfish and Irukandji Jellyfish. This makes the term “smack” of jellyfish more appropriate when referring to these particular species. 

Behavior of Jellyfish in Groups

A bloom of white jellyfish near water surface

Jellyfish, when grouped, display fascinating behaviors in their simplicity and effectiveness.

Jellyfish do not gather in groups to socialize, unlike other animals. Often, their group behaviors occur due to environmental factors, such as converging currents, abundance of food, and breeding conditions.

Grouping serves protective functions against predators or aids in the reproductive process by increasing the likelihood of gamete encounters.

The reasons behind their grouping vary, involving both biological and ecological factors.

For instance, swarming could increase the likelihood of jellyfish encountering mates or could be a strategic response to the presence of abundant prey, allowing them to feed more effectively.

Moreover, moving as a collective may reduce individual vulnerability to predators, as there is safety in numbers. These behaviors are dynamic and context-dependent, varying with the conditions of their marine habitats.

It is vital to note that while jellyfish can coordinate group behaviors, they do not exhibit social interactions as seen in higher organisms.

As established, their groupings are largely influenced by instinctual and passive responses to the ocean’s currents and food availability.

I was once lucky enough to witness a swarm of Cassiopea Jellyfish on their feeding grounds behaving rather oddly.

This jellyfish species is also called the upside-down jellyfish due to their unusual swimming pattern, where they turn their bodies upside down.

This behavior is part of their unusual feeding process and symbiotic relationship with photosynthetic algae called zooxanthellae, which share nutrients produced through photosynthesis with their host jellyfish.

I was in awe while I watched a swarm of Cassiopea Jellies pulsing their tentacles almost synchronously near the surface of the water, but I was also amazed at how this movement benefits the zooxanthellae as well.

Watch this video of Cassiopea Jellyfish swarm drifting upside down:

Jellyfish Sunbathing | BBC Earth

How Many Jellyfish Are in a Group?

Smack of jellyfish near the surface

The size of a jellyfish group can significantly vary, often depending on environmental factors such as food availability and ocean currents.

While the term for a group of jellyfish — be it a swarm, bloom, smack, or fluther — can give us imagery of their collective behavior, it doesn’t specify the exact number of individuals within these groups.

A swarm might be seen as a relatively small number of jellyfish, and a bloom can consist of hundreds or thousands of jellyfish, particularly when conditions are right for reproduction​​.


Although they lack a complex nervous system to socialize, a drifting group of jellyfish can be really mesmerizing to watch. Do you have other cool facts about groups of jellyfish? Let us know in the comments!

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