Is A Sea Cucumber An Animal, Fish, Or Vegetable? (Explained)

We can often find sea cucumbers on the bottom of the ocean as a non-moving, dark shape resembling cucumbers. We usually can’t notice them moving or swimming, so are sea urchins animals, vegetables, or perhaps fish? If you ever wonder about this, in this post, we’ll talk all about that. However, let’s begin with a quick answer:

Sea cucumbers are animals that belong to the phylum Echinodermata. These marine invertebrates are classified as Holothuroidea, and they have a digestive, nervous, respiratory, and circulatory system.

However, that certainly doesn’t tell the whole story. Below I’ll explain more about what kind of animal the sea cucumber is. Furthermore, I’ll explain why sea cucumbers aren’t fish or vegetables. Read on!

Is A Sea Cucumber An Animal?

Sea cucumbers are marine animals that belong to the phylum Echinodermata (Echinoderms), which is a collection of about 7,000 species. Echinodermata in Greek means “spiny skin,” which perfectly describes sea cucumbers’ relatives, including sea stars, sea urchins, sand dollars, brittle stars, and feather stars.

Echinoderms are marine invertebrates which means they don’t have a backbone. They’re characterized by their five-point radial symmetry, unique water vascular system, dermal skeleton, and mutable connective tissue. We can specify five classes in Echinoderms, where sea cucumbers belong to Echinoidea. There are about 1,250 different species of sea cucumbers.

Sea cucumbers’ anatomy

Sea cucumbers are typically 4 to 12 in (10 to 30 cm) in length, with the largest species reaching 10 ft (3m) and the smallest just 0.8 in (2 cm). The body of sea cucumbers is elongated and radially symmetrical along its longitudinal axis. Unlike some other echinoderms (ex. sea stars), they lack arms, but it doesn’t mean they can’t move or feed.

On the underside of sea cucumbers’ body (oral side), there are rows of tube feet which are small tubular projections. Echinoderms use them to move around the sea floor by filling them with the fluid and creating contractions to extend and attach them to the surface. As a result, sea cucumbers move forward steadily and can climb or attach to the rocks.

Another interesting fact about sea cucumbers’ anatomy is that they have a ring of modified tube feet surrounding their mouth, which is located at the anterior end. These feet are called feeding tentacles, and as the name suggests, sea cucumbers use them to feed themselves. They have a few ways of eating.

Filter feeders expand their feathery tentacles into the water and catch passing food particles. Next, they bring tentacles one by one into their mouth. Sediment feeders use their tentacles to sift through the sand, looking for food and capturing it directly.

Sea cucumbers also have a digestive system including the mouth, pharynx, esophagus, stomach, intestine, cloaca, and anus. In some species, food is digested and eliminated through the anus, but some use their mouth. Sea cucumbers also have a full respiratory and circulatory system. They also have a nervous system but without a true brain.

Sea cucumbers’ habitat

Sea cucumbers live in every ocean, in all marine benthic zones, from the splash zone of the intertidal pools to the depths of oceanic trenches. Many species adapted to different environments, such as coral reefs, sandy bottoms, rocky intertidal shores, seagrass meadows, mud, and even kelp forests.

You can read more about sea cucumbers’ habitat in my other post: “Where Do Sea Cucumbers Live?”.

Reproduction

Sea cucumbers’ reproduction methods are another factor that makes these creatures more fascinating animals than you have probably thought. Sea cucumbers can reproduce sexually and asexually. When reproducing sexually, females release their eggs, and males release their sperm into the water column, all at the same time. This behavior is called spawning.

Next, gametes (sperm and eggs) that were lucky and met in the water, fertilize, and form a zygote (fertilized egg). Furthermore, the egg transforms into a larva, larvae into juvenile, and to the adult stage. You can read more about every stage of sea cucumbers’ life cycle in my other post: “The Complete Life Cycle Of A Sea Cucumber”.

Sea cucumbers can also reproduce asexually by dividing their bodies into half or more pieces. After this remarkable event, these animals regenerate each part into a new and complete individual. It’s a very complex process that is accompanied by complex behaviors, such as softening the body, stretching, and twisting it.

As you can see, sea cucumbers are definitely living animals, and if the above reasons still don’t convince you, keep reading where I explain why sea cucumbers aren’t fish or vegetables.

Is A Sea Cucumber A Fish?

Sea cucumbers are not fish because of a very simple difference – fish are vertebrates, and sea cucumbers are invertebrates. Fish have a backbone, fins, gills, and scales. They use their gills for breathing, but some have lungs and may be able to spend some time out of the water.

Sea cucumbers don’t have lungs, but they use two respiratory trees inside their bodies. They “breathe” oxygen from the seawater by drawing water in through the anus and then expelling it. Therefore, their way of breathing differs from how the fish breathe.

Another critical difference is that fish can live in either salt or fresh waters, depending on the species. Sea cucumbers and other echinoderms such as sea stars are exclusively marine animals. They cannot survive in freshwater due to their state of isotonic stability with the surrounding saltwater.

This means that the same amount of water moves between body tissues and the saltwater. Sea cucumbers are simply animals, unable to push water out of cells actively. Vertebrates such as fish can do this because they have specialized organs like kidneys that help keep the body stable. This takes a lot of energy and a more complex organization of the body organs.

Is A Sea Cucumber A Vegetable?

Lastly, as you can already conclude, sea cucumbers are not vegetables. They are marine invertebrates with digestive, nervous, respiratory, and circulatory systems. They move around the sea floor, eat, breathe, and reproduce sexually and asexually.

The term vegetable usually refers to plants that are multicellular organisms with roots. Plants take in carbon dioxide and give out oxygen through their leaves. In contrast, animals breathe in that oxygen and breathe back out carbon dioxide.

Another critical difference is that plants create their own food using sunlight and water. Animals, including sea cucumbers, move from place to place to find food and feed on plants or other animals. Sea cucumbers mainly eat waste particles, algae, and small marine invertebrates.

The only similarity we can think of is that plans reproduce asexually, and sea cucumbers can do that as well. However, the process is completely different, and sea cucumbers can reproduce sexually as well.

In conclusion, sea cucumbers are called sea cucumbers only because their body shape resembles sea cucumbers. They’re not vegetables but marine invertebrates.