Sea cucumbers extract oxygen from water using a pair of 'respiratory trees' near the anus. They 'breathe' by drawing water in through the anus and then expelling it.
Sea cucumbers are a very important part of marine food webs. Almost all sea cucumbers are detritivores eating tiny scraps of food that settle on the sea floor including invertebrates, algae, bacteria, and decaying organic matter. Some are "Direct deposit feeders” wiping their tentacles over the sediments to pick up tiny food particles that settle there from the water above. Others are "Suspension feeders" using special tentacles to pick particles out of the water.
Their predators include crabs, crustaceans, sea turtles, sea stars, fish and humans. Their tough skin is a protection against predators as is their burrowing behaviour and camouflage. They can also expel some of their internal organs to confuse predators. These organs can be regrown in a couple of months or less. Some also release a poison into the water to kill their predators.
In most sea cucumbers, the calcium carbonate plates are microscopic and are embedded beneath the skin. Their leathery skin is mainly made from collagen fibres, these fibres can be loosened and tightened to allow sea cucumbers to squeeze through small gaps as if it was liquefing its body.
Most sea cucumbers are either male or female and fertilization takes place outside of the body in the water. The fertilized eggs develop into free-living larvae that are dispersed with water currents and become part of the plankton. They eventually settle onto the sea floor and develop into adults.
Over 30 species of sea cucumber fertilise their eggs internally and a few species of cold-water sea cucumbers brood their larvae in special pouches on their body.
At depths below 8.8 km sea cucumbers are found in large herds that move across the ocean searching for food. They can also be found in great numbers in some shallow waters.