How do Knidarians obtain food?


Knidarians are a diverse group of marine organisms that includes jellyfish, coral, and sea anemones. Despite their varied appearances, they all share a common method of obtaining food. In this article, we will explore the different feeding strategies employed by knidarians, focusing on their unique adaptations and behaviors. By the end, you’ll have a comprehensive understanding of how these fascinating creatures acquire their sustenance.

1. Feeding Mechanisms of Knidarians

Knidarians possess a few distinct feeding mechanisms that allow them to capture and consume their prey effectively. These mechanisms include:

1.1 Passive Suspension Feeding

Many knidarians, such as sea anemones and some types of jellyfish, employ passive suspension feeding. This method involves capturing food particles that are suspended in the water column. Knidarians possess specialized structures, such as tentacles or mucus-coated surfaces, which aid in trapping and manipulating their prey. These organisms rely on water currents to bring food particles within their reach, where they are then captured and consumed.

1.2 Active Predation

Other knidarians, particularly some species of jellyfish and certain types of coral, are active predators. These organisms actively search for prey and use their tentacles, armed with stinging cells called cnidocytes, to capture and immobilize their victims. Once captured, the prey is brought towards the mouth, located at the center of the organism, where it is ingested.

1.3 Symbiotic Relationships

While not a feeding mechanism per se, it is worth mentioning that some knidarians engage in symbiotic relationships with other organisms to obtain their food. For example, certain types of coral have a mutualistic relationship with photosynthetic algae called zooxanthellae. The coral provides shelter and nutrients to the algae, which, in turn, produce food through photosynthesis. This symbiotic partnership ensures a constant supply of nutrients for the coral.

2. Passive Suspension Feeding in Knidarians

Passive suspension feeding is a prevalent feeding strategy among knidarians. Let’s delve deeper into the specific adaptations and behaviors associated with this feeding mechanism.

2.1 Tentacles and Mucus

Knidarians that engage in passive suspension feeding possess tentacles adorned with specialized structures, such as nematocysts or cilia. These structures aid in capturing and manipulating food particles. Additionally, many knidarians secrete mucus, which covers their tentacles and enhances their ability to trap prey effectively.

2.2 Water Currents

Water currents play a crucial role in passive suspension feeding. Knidarians position themselves in areas with sufficient water movement, such as near coastal reefs or in ocean currents. The water carries suspended food particles towards the organism, allowing them to be captured by the tentacles or mucus-covered surfaces.

2.3 Filter Feeding

Some knidarians, particularly certain types of jellyfish, utilize a filtering mechanism to extract food from the water. These organisms possess structures called oral arms, which are lined with tiny, hair-like structures called cilia. The cilia create water currents that draw in food particles, which are then captured and transported to the mouth for ingestion.


Nematocyst Animation – Feeding Tentacles

3. Active Predation in Knidarians

Active predation is a feeding strategy employed by certain knidarians that actively hunt and capture their prey. Let’s explore the adaptations and behaviors associated with this method of obtaining food.

3.1 Cnidocytes and Stinging Cells

Knidarians involved in active predation possess specialized cells called cnidocytes, which are located on their tentacles. Cnidocytes contain tiny harpoon-like structures called nematocysts, which are capable of injecting venom into their prey. When a potential meal comes into contact with the tentacles, the nematocysts are triggered, releasing the venom and immobilizing the prey.

3.2 Tentacle Lassoing

Some jellyfish species employ a unique feeding behavior known as tentacle lassoing. These jellyfish extend their tentacles in a long thread-like manner, capturing prey by entangling them in a sticky mucus. Once ensnared, the prey is then drawn towards the mouth for consumption.

3.3 Mouth and Digestive System

After capturing their prey, knidarians involved in active predation bring it towards their mouth, which is typically located at the center of their body. From there, the prey is ingested and enters the digestive system, where it is broken down and nutrients are absorbed.

4. Symbiotic Relationships in Knidarians

While not a feeding mechanism in the traditional sense, symbiotic relationships play a significant role in the nutrition of certain knidarians. Let’s explore how these relationships contribute to their food acquisition.

4.1 Coral and Zooxanthellae

Many species of coral form a symbiotic relationship with zooxanthellae, a type of photosynthetic algae. The coral provides a protective environment and nutrients to the zooxanthellae, while the algae produce food through photosynthesis. The coral benefits from the sugars and other organic compounds produced by the zooxanthellae, ensuring a reliable food source.

4.2 Anemones and Anemonefish

Anemones and anemonefish, popularized by the movie “Finding Nemo,” also engage in a mutualistic relationship. The anemone offers protection to the fish, while the fish, in turn, defends the anemone against predators and provides it with nutrients through their waste and leftover food.

5. Frequently Asked Questions (FAQs)

FAQ 1: How do jellyfish capture their prey?

Jellyfish capture their prey using their tentacles, which are armed with stinging cells called cnidocytes. When a potential meal comes into contact with the tentacles, the cnidocytes release venom, immobilizing the prey for ingestion.

FAQ 2: Do all knidarians have the same feeding mechanisms?

No, knidarians employ a variety of feeding mechanisms. While passive suspension feeding and active predation are common, some knidarians rely on symbiotic relationships to acquire their food.

FAQ 3: How do coral obtain food if they are immobile?

Corals form a symbiotic relationship with photosynthetic algae called zooxanthellae. These algae produce food through photosynthesis, which is shared with the coral. Additionally, coral can also capture small planktonic organisms using their tentacles.

FAQ 4: Are all knidarians carnivorous?

While the majority of knidarians are carnivorous, there are exceptions. For example, some types of coral are primarily autotrophic and rely on photosynthetic algae for sustenance.

FAQ 5: Can knidarians consume large prey?

Knidarians, particularly those involved in active predation, can consume prey that is smaller than their own size. However, they typically feed on smaller organisms, such as zooplankton or small fish.

FAQ 6: How do knidarians digest their food?

Knidarians possess a simple digestive system. Once the prey is ingested, it enters a central cavity where it is broken down by enzymes. The nutrients are then absorbed into the organism’s body.

FAQ 7: Can knidarians survive without food for extended periods?

While some knidarians can survive for short periods without food, they require a steady supply of nutrients for their survival and growth. Without food, knidarians would eventually face starvation and perish.


Knidarians have evolved a range of feeding mechanisms to ensure their survival in diverse marine environments. From passive suspension feeding to active predation and symbiotic relationships, these organisms have adapted to acquire food in various ways. By understanding their feeding strategies, we gain insight into the intricate dynamics of marine ecosystems and the remarkable adaptations that enable knidarians to thrive in their habitats.

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