Sea cucumbers are marine animals that resemble plants, found throughout the oceans: from tidal wetlands (brackish water) to depths of thousands of meters below the sea, their traces can be seen. Sea cucumbers attach themselves to the hulls of ships, affecting the speed of travel and increasing fuel consumption; they can also cling to underwater pipes, causing blockages. However, the tail of juvenile sea cucumbers
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(Photo: guiamarina) |
contains a spine, indicating that they are higher-level animals, and sea cucumbers are classified as vertebrates. They play a crucial role in studying the origins of vertebrates in the process of evolution.
The shape of a sea cucumber resembles an eggplant, sometimes looking like a flower, or even a teapot. If you poke it with your finger, it will shoot out a strong jet of water, and then it will gradually stand upright before drooping down softly like cotton.
Sea cucumbers are sessile animals, covered on the outside by a protective layer of cellulose that shapes their form. This is a unique phenomenon in the animal kingdom. For this reason, they are called sea cucumbers (where “cucumber” refers to the outer covering). They absorb oxygen through water conduits via their gills and take in nutrients and small organisms from the water through their digestive system.
Sea cucumbers reproduce by growing a bud from their body, which gradually enlarges and then detaches from the parent to develop into a new organism. This is known as the asexual reproductive bud of the sea cucumber. Some species also reproduce sexually, where the sea cucumbers are hermaphroditic, but their eggs and sperm do not mature simultaneously, so they must “mate” with another sea cucumber.
Juvenile sea cucumbers resemble tadpoles, with well-developed eyes, brains, and tails, and they have a spine in the middle. The dorsal side of the spine features a nerve cord extending straight to the tip of the body, with gills present in the throat, allowing juvenile sea cucumbers to swim freely. After several hours, the tip of the body gradually protrudes and attaches to another object. Subsequently, the tail shrinks and eventually disappears, leaving only a nerve segment. The transformation from juvenile to adult in sea cucumbers is the opposite of evolutionary progression, which zoologists refer to as reverse metamorphosis.
(Photo: seawater)
