Despite their different shapes, starfish and sea urchins, including 1 100 species, are closely related. Their bodies are radially symmetrical, and like them, they crawl on their legs driven by turbulent seawater. However, sea cucumbers are called "cucumbers" by marine biologists because they have other skills. They breathe through the back of the body, suck seawater into the body from the anus, fill a system called "breathing tree", and then excrete it together with all other potential digestive wastes. In other words, although there is only one opening, it has two functions. In fact, small pearl fish like eels take advantage of this process. Pearl fish wait until the sea cucumber opens its anus every morning, then dive into the sea cucumber, swim around in its internal organs, spend a leisurely day in it, and then come out from the sea cucumber for food at night. Some pearl fish even beat sea cucumbers to open their anus and let them in. Young pearl fish are not very popular with sea cucumbers, because they have a bad habit of biting the gonads of sea cucumbers.
Sea cucumbers are nocturnal animals. In order to fill their stomachs, they should eat at least twice a night. So the life of sea cucumber is basically alternating between looking for food and resting in the sand like a vacuum cleaner. If they feel nervous or threatened, they will have a series of impressive escape strategies. The body of sea cucumber is mainly composed of a kind of connective tissue called "capture collagen", which gives them an unimaginable ability to change from solid to liquid. They can flow into the smallest crack in the rock and then harden again, so they won't be pulled out. Some species can make their bodies swell to the size of football. There are other kinds that can remove water from their bodies and make them look like pebbles. However, for all kinds of sea cucumbers, one of the most basic skills is to expel their internal organs from their anus, so that the surrounding waters are filled with a poisonous thick soup. This method is called "sea cucumber nuclear weapon". If you are in a small fish tank, this venom can kill all the fish in it, even the sea cucumber itself.
Some species also have more complex and advanced skills, and the sticky thin thread produced in the "giant Ye Wei tubule" in the body is ejected from the back. These thin threads can form an amazing sticky net, which can bind a hungry crab for hours without moving. In Palau in the Pacific Ocean, islanders squeezed mucus from the tubules of sea cucumbers and smeared it on their feet to make temporary canvas shoes. This mucus will also be used as a disinfectant dressing for dressing wounds. Strangely, a sea cucumber with its internal organs, gonads or tubules removed will grow back in less than a few months.
Dry sea cucumber is called edible sea cucumber, which is regarded as delicious throughout Asia. It is also called aphrodisiac and painkiller. The global sea cucumber market has grown to 2.3 billion pounds, which has led to the survival crisis of some kinds of sea cucumbers, thus promoting the establishment of some sea cucumber farms and "marine pastures".