The Cleaners of the Ocean

The ocean is a like large purifying device protecting the earth’s environment. The polluted substances on land continuously flow into the ocean, and the numerous creatures living in the ocean continuously discharge excrement. Another problem is the remains of creatures decaying on the ocean floor. The ocean, which contains 1,300,007,000km³ of water, is silently purifying polluted materials from the past to this day. There are creatures participating in this noble effort. These creatures are crabs, sea cucumbers, starfish, and sea lice, which are called the cleaners of the ocean.

Crabs, Eating Polluted Materials

 Among the 4,500 different crab species in the world, about 183 species live in Korea. As can be assumed by this diversity, the living environment and food consumption differs for various species. Crabs usually feed on smaller crabs, squids, octopi, or lugworms. However, a common food for crabs are the corpses of sea animals falling to the ocean floor or organic matters mixed in the mud flats.

Crabs significantly contribute to purifying mud flats, as well as the ocean itself. It is easy to see little sand balls piled up near places where globular ghost crabs live, such as mud flats near ocean coasts. These sand balls, which are extremely clean, are leftovers of globular ghost crabs after consuming plankton and organic matters in the sand. The method of consuming the organic matters is extremely scientific. The sand and water swirls inside a crab’s mouth, which causes the heavy sand to sink and the light organic matter to float on the water, which subsequently is swallowed by the crab.

The filtered sand is spit out, making a sand ball. The quantity of sand purified by a crab is several hundred times the weight of a crab. Since several hundred globular ghost crabs are living in a mud flat area of 1m², the quantity of sand purified by these crabs is huge when considering the entire area of mud flats.

Sea Lice, the Cleaner of Seashore Rocks


It is easy to see a black group scattering in all directions when setting foot on a seashore rock or tetrapod. These are sea lice, which are arthropods of the Crustacean class. They normally crawl all over the rocks and quickly escape by using seven pairs of legs when an abnormality is sensed by its long pair of antenna.

 Because it looks similar to a cockroach, a sea louse is also called ‘the cockroach of the sea’ in Korea. However, a sea louse would feel that it is unfair to be compared to a cockroach considering its role in contributing to the environment. Whereas cockroaches are public enemies for polluting food and environment by carrying all sorts of germs on their bodies, sea lice are diligent cleaners consuming various organic matters or food wastes between tetrapods or rocks on the seashore. Without sea lice, the seashore would be overloaded with waste radiating bad smells.

Thankful Starfish

People usually think of starfish as pirates of the seas, indiscriminately preying on ocean resources such as clams. However, this is a huge misunderstanding. For example, due to structural limitations such as short arms or slow movements, an astrina pectinifera, which is a local breed, cannot catch up with abalones or clams, which are faster than an astrina pectrinifera, and cannot effectively prey on the creatures by using its arms to wrap the target. As a result, the main food of an astrina pectrinifera are fish corpses or sick and rotten clams.

The ocean would be polluted if fish were left decaying on the ocean floor. However, if the astrina pectinifera consumes fish corpses, the pollution can be reduced. Moreover, it is said that sea spiders, which live along all coastlines of Korea, significantly contribute to improving the ocean environment. Sea spiders have thin and long arms like land-based spiders and do not attack live animals at all, consuming only decomposed fish and organic matters.

Sea Cucumbers, the Worms of the Ocean

Sea cucumbers can be distinguished by color. The reason they have different colors is because they consume different food. Because sea cucumbers mainly consume organic matters in mud or corpses of fish and shellfish, they are called the cleaners of the ocean.

Sea cucumbers swallow organic matters with the sand or mud and then filter out the organic matters and discharge the sand or mud through the anus. Small goby minnows can be seen in the middle of long crawling traces of sea cucumbers on the ocean floor. These goby minnows are the traces of organic matters filtered and excreted after sea cucumbers consume the sand on the ocean floor. This consumption habit of sea cucumbers is worth comparing to worms, which purify land polluted with heavy metals.