16 Oct 2019 - Sea cucumbers are marine invertebrates that live on the seafloor. They're named for their unusual oblong shape that resembles a fat cucumber. Although people occasionally eat sea cucumbers, these chubby, worm-like sea creatures aren't related to their namesake fruit (and they wouldn't make a very appetizing salad topping if you were expecting a crunchy, refreshing bite).
There are about 1,250 species of sea cucumber, all of which belong to the taxonomic class Holothuroidea. This class falls under the Echinodermata phylum, which also includes many other well-known marine invertebrates, such as sea stars, sea urchins and sand dollars, according to National Geographic.
Sea cucumbers range in size from about three-quarters of an inch (1.9 centimeters) to more than 6 feet long (1.8 meters) and live throughout the world's oceans, from nearshore shallow waters to the ocean's deepest trenches, according to the National Wildlife Federation. No matter the depth, their main residence is on the ocean floor, often partially buried in sand.
Sea cucumbers, like all other echinoderms, exhibit radial symmetry, according to the University of California, Berkeley's Museum of Paleontology (UCMP). But instead of having five arms arranged in a circle like sea stars or sand dollars, sea cucumbers have five rows of tiny feet that run lengthwise down their bodies, from mouth to anus. Their tube-shaped feet serve mainly to anchor the limbless creatures to the seafloor, according to Woods Hole Oceanographic Institution (WHOI). Sea cucumbers move across the seafloor by changing the water pressure in their feet; they increase the amount of water in their feet to stretch them out and release the water to contract them.
What do sea cucumbers eat?
As the creatures slowly meander about, they use the extra 20 to 30 little tube feet around their mouths to shovel everything in, including sand. They feed primarily on tiny pieces of algae and marine creatures, which get broken down into smaller and smaller pieces, similar to how earthworms break down organic matter in gardens, according to the National Oceanic and Atmospheric Administration (NOAA).The sand that the sea cucumbers ingest passes straight through their system and comes out the other end in the form of a sandy poop log.
Along with the sand, sea cucumbers excrete byproducts that benefit ocean ecosystems, particularly coral reefs. A 2011 study published in the Journal of Geophysical Research found that sea cucumbers' natural digestion process gives their waste products a relatively high (or basic) pH, which means the water surrounding sea cucumber habitats is somewhat protected from ocean acidification. Sea cucumbers also excrete calcium carbonate, which is a primary ingredient in coral formation, and ammonia, which acts as a fertilizer and promotes coral growth.
Anatomy and reproduction
Sea cucumbers have a relatively simple internal anatomy, consisting of three main sections: digestive, respiratory and reproductive, according to the book "Marine Benthic Fauna of Chilean Patagonia" (Nature in Focus, 2010).
Although sea cucumbers don't have bones, many species of the animal have a rudimentary skeleton made of microscopic plates of calcium carbonate that lie loosely scattered underneath the skin, according to UCMP. Some species can align their skeletal plates when threatened so that their bodies become rigid, according to the University of Alaska Southeast.
The digestive tract consists of a long intestine coiled between the mouth and the anus that's approximately two to three times the length of the sea cucumber. Occasionally, if disturbed or stressed, sea cucumbers will expel their entire digestive system, but they can grow a replacement in just a few weeks, according to WHOI.
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