Sea Slug vs Sea Cucumber

Written by Janet F. Murray
Published: September 28, 2022
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Although they may look much alike at first glance, sea slugs and cucumbers differ. Unlike what some may assume, they are not in the same taxonomic group. Sea slugs are marine gastropods. Gastropods are more closely related to other animals in the Phylum Mollusca group, such as sea stars and sea urchins. On the other hand, sea cucumbers are part of the Phylum Echinodermata group with animals such as sand dollars and starfish. So, many soft-bodied invertebrates like the sea slug vs sea cucumber are hard to tell apart. Still, they have significant differences regarding their specific location in the ecosystem.

Comparing Sea Slug vs. Sea Cucumber

Sea slugs grow to 1/8 – 12 inches and live up to 4 years. Cucumbers grow to 6 feet and live up to 10 years.
Sea SlugSea Cucumber
Scientific Name/Class/FamilyNudibranchia/Gastropoda/OpistobranchesEchinoderms/Holothuroidea/Echinodermata
Length1/8  – 12 inches6 feet
LegsNone20 – 30
DietPlankton, algae, jellyfish (omnivore)Algae, plankton, invertebrates, waste (omnivore)
Lifespan1 – 4 years5 – 10 years
Eggs produced1,000,000130,000
Number of species2,0001,200
PredatorsFish, crabs, lobsters, humansGiant mollusk, tiger fish, putter fish, crabs, lobsters, sea turtles, some shark

The Primary Difference Between Sea Cucumbers and Sea Slugs

The primary difference between sea cucumbers and sea slugs is that sea slugs are generally much smaller than sea cucumbers. In addition, they have significant physical differences.

Sea slugs also don’t have legs while sea cucumbers have many, live for much shorter time periods, and have different predators. Let’s dive into each of these differences in more detail.

Sea Cucumber vs. Sea Slug: Size and Appearance

When it comes to appearance, sea slugs are usually smaller than sea cucumbers and have a more compact body. However, sea slugs have enormous body shapes, sizes, and color variations. Depending on the species, they can be anything from 1/8 inch to 12 inches long. On the other hand, sea cucumbers can grow up to 6 feet or more. Sea slugs have a mantle, which is a thin layer of tissue that covers their internal organs. Interestingly the sea cucumber’s inner organs are in the back half of its body.

Most sea slugs have a defined “head” with tentacles to sense their surroundings. In contrast, sea cucumbers have no recognizable head end. Still, they have a single tentacle that comes out of one end that they use to feed, but they can retract these back into their bodies. They also have tube feet extending from one end for moving and to anchor themselves. On the other hand, sea slugs lack tube feet. Instead, they move along the sea floor in a manner that is similar to their land cousins.

Sea Cucumber vs. Sea Slug: Protective Mechanisms

What Do Sea Cucumbers Eat - Sea Cucumber Defending Itself

Sea cucumber vs sea slug: Sea cucumbers can regenerate their internal organs and sea slugs secrete toxins for protection.

©Ethan Daniels/

Unlike their land-dwelling cousins, most sea slugs cannot withdraw into their shells for protection. Therefore, they have evolved various creative ways to defend themselves from predators. For example, most sea slugs’ bodies are brightly colored. This characteristic helps them to warn predators that they are toxic and not fit for consumption. In addition, some sea slugs secrete slime or ink to escape predators. Although this is an effective defense mechanism, it is also corrosive. It can damage the slug’s own tissue, so they only use it as a last resort.

In comparison, the sea cucumber’s primary method of defense is to spew its internal organs out of its anus. This seemingly strange behavior is quite effective in warning predators. Furthermore, as opposed to other animals, which depend on their organs for survival, sea cucumbers can simply regenerate any organs they lose. As a result, they can escape from many dangerous situations.

Habitat and Location

Sea slugs live in all oceans of the world in various environments. Most have very special diets, so they typically live near their food sources. Some inhabit estuaries, rocky shores, coral reefs, intertidal zones, and deep water.

Sea cucumbers live just about anywhere, although most prefer to stay in the shallows. However, many sea cucumbers prefer tropical waters, making their habitats in the Indian Ocean and Pacific. You may typically spot sea cucumbers on coral reefs and near sandbars, where they like to feed.

Diet in Relation to Ocean Ecosystem

Frontal Close-up of a Cristate Neon Slug, Padang Bai, Bali, Indonesia

Sea slugs vs sea cucumbers: Sea slugs like this Cristate Neon Slug use their radula to eat, which is a bit like a grater, Sea cucumbers spew out water jets to capture food and then suck it back in,

©Daniel Lamborn/

Sea cucumbers are important to the ocean. They benefit their environment by eating plankton and other bits of organic matter that settle on the seafloor. In addition, sea cucumbers play an essential role in keeping reefs healthy because of their capacity to consume large quantities of sediment. This ability helps to improve water quality and clarity around reefs. As a result, sea cucumbers are often called “the vacuum cleaners of the sea.”

Unlike the sea cucumber, the sea slug is known for eating mixed food items. Their diet includes sponges, corals, anemones, hydroids, bryozoans, tunicates, algae, and sometimes other sea slugs. Sea slugs use a radula to eat, which acts like a cheese grater. The radula moves back and forth to grasp and shred food. This feeding method allows them to break down their food to digest it properly. In contrast, sea cucumbers expel a jet of water that encircles them and then suck it back in with particles from the surrounding area.

Sea Cucumber vs. Sea Slug: Reproduction

Another interesting fact about sea cucumbers is that they are primarily genderless and can reproduce both sexually and asexually. Male and female gametes connect to form a larva, eventually growing into an adult sea cucumber.

Similarly, most sea slugs are hermaphroditic, possessing both male and female reproductive organs. Soon after mating, they lay their eggs in a cluster. The fertilized eggs then develop into either trochophores or veligers. Veligers eventually settle at the bottom and undergo metamorphosis into juveniles.


Sea slugs have a few natural hunters, including fish, lobsters, crabs, and humans. Sea cucumbers have slightly more hunters, including giant mollusks, tiger fish, putter fish, crabs, lobsters, sea turtles, and sharks. Of these hunters, humans pose the most significant threat as we harvest sea slugs and cucumbers for food. Both are considered food treats.

The photo featured at the top of this post is ©

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About the Author

I'm a freelance writer with more than eight years of content creation experience. My content writing covers diverse genres, and I have a business degree. I am also the proud author of my memoir, My Sub-Lyme Life. This work details the effects of living with undiagnosed infections like rickettsia (like Lyme). By sharing this story, I wish to give others hope and courage in overcoming their life challenges. In my downtime, I value spending time with friends and family.

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