Sea cucumbers may look like the fruit in your garden, but they’re actually animals – related to sea stars, sea urchins, sea lilies and sand dollars. Most sea cucumber bodies are covered with tube feet, but only the ones located on the bottom of their bodies are developed enough for use. (However, the hairy cucumber is covered with slender tube feet.) Sea cucumbers use these feet mainly to attach themselves to the bottom, rather than for motion. The tube feet are controlled by changing the water pressure of the animal. By increasing the amount of water, the feet can be extended; and by lessening the amount of water, the tube feet contract -much like how a sea star moves.
Surrounding its mouth are ten to thirty modified tube feet. These sweep the surrounding water, capturing bits of food, and are then transferred one by one to its mouth to wipe off the food. Other sea cucumbers, such as Leptosynapta, burrow in the sediment, digesting what is edible, and excreting the rest.
Sea cucumbers breathe as water is pumped through two respiratory trees located on each side of their digestive tract. Some sea cucumbers can eject their digestive system and associated organs when disturbed (or due to overcrowding or foul water) and grow a new set within a few weeks.
They are sometimes host to the pearl fish, which slides backwards into the respiratory openings of sea cucumbers, often limiting itself to an entire life spent within a single individual. Together, they form a relationship, with one member as an uninvited guest, causing no harm and gaining a little from the association.
Sea cucumbers use their tentacles to capture tiny animals (zooplankton). Some sea cucumbers, such as the worm-like Leptosynapta, burrow and ingest detritus present in the sediment. In Asia, dried sea cucumber bodies (called trepang) and their sex organs are considered a delicacy by humans.
Sea cucumbers are the champions of organ regrowth because they direct their wound healing abilities towards restoring their organs, according to research published in the online open access journal, BMC Developmental Biology. The discovery that Holothuria glaberrima uses similar cellular mechanisms during wound healing and organ regeneration gives us the opportunity to discover how to repair our own wounds and, perhaps eventually, how to regenerate body parts.
The research was carried out by the investigators José San Miguel-Ruiz and José García-Arrarás, at the University of Puerto Rico. “Sea cucumbers should be viewed as the tissue regeneration equivalent of the squid for our knowledge of nerves and Drosophila for genes and the genome. They can help us learn to fix ourselves,” commented Professor Garcia-Arraras. “Many people, including scientists, regard sea cucumbers and other echinoderms like star fish and brittle stars as bizarre, exceptional outcasts because of their regenerative abilities. But we’ve shown that they use the same ‘ordinary’ mechanisms and processes to both regenerate and heal wounds.”
All animals possess some kind of tissue repair mechanism. The sea cucumber, H. glaberrima, belongs to a group of marine animals that are well known for their ability to regenerate, along with the axolotl salamander, which is also famous for regrowing lost limbs. The scientists made observations over a four-week healing period and found that sea cucumbers healed up rapidly after receiving a 3 to 5 millimeter cut along the body wall. The repair process involved special cells called morula cells moving to the injury site and full repair was achieved after just a couple of weeks. The cellular events observed during the healing of sea cucumber muscular, nervous and dermal tissues that correspond to those observed during intestinal regeneration include extracellular matrix remodeling and the differentiation of muscle cells.
Although all animals have wound repair processes, not all regenerate injured or lost body parts. There must be some unusual properties of the healing processes found in animals capable of organ regeneration. So it remains to be seen at a molecular level what limits healing processes being used for regeneration by all animals in all tissue.
“Many of these regenerative mechanisms are the same as those being used by other animals to heal and repair – this includes us humans, “concluded Professor Garcia-Arraras. “Sea cucumbers will probably provide us with the key to deciphering how to regenerate our tissues, or at least find out what is needed to do this.”