Comox Valley Naturalists Society
Knowing Nature . . .
Cool as a Sea Cucumber
June 16, 2006
June has some great low tides, so we recently explored Shelter Point, near Willow Point. Equipped with old running shoes, sun hats and dip nets, we walked out over the cobble. Each tide pool was crammed with life; barnacles and mussels vied for space, limpets and chitons clung to rocks. Crabs scuttled by and sculpins darted away. Purple and orange sea stars clambered over each other. We could hear the crackle and pop of all these organisms shifting under the sun.
“Look at this!” my husband called out with boyish enthusiasm. He had picked up a large, orange sausage-shaped creature that looked slightly repulsive at first. “A sea-cucumber!” I came over to examine the thing. It was blotchy orange-brown with 5 rows of tube feet that ran the length of the body. We found several more under the rocks. Underwater, brilliant feathery orange tentacles emerged, but retracted abruptly when touched. We also discovered some smaller, white cucumbers. I became more and more curious about these strange sea creatures.
Sea cucumbers, known as Holothurians, have been around for 400 million years. They are echinoderms, which is latin for “spiny skin”. Echinoderms include organisms like sea stars, urchins, and sea cucumbers. All echinoderms have three features in common: 1) an internal skeleton made up of calcite (crystallized chalk) 2) a water-vascular system that controls tube feet; and 3) five rows of tube feet that radiate from a central ring.
Unlike other echinoderms, cucumbers do not have a skeleton that is a solid plate.
Generally, sea cucumbers have five rows of tube feet (some species lack tube feet entirely). The tube feet, called podia, are small tubes with suckers at the tips. Tube feet can be easily observed along the sides of the cucumber. Tube feet assist with locomotion. The water-vascular system controls the movement of each foot.
At one end of the cucumber’s cylindrical body is the mouth; at the other end is the anus. Cucumbers feed by extending feeding tentacles. Small particles in the water stick to a coating of mucous that covers each tentacle. When a tentacle becomes saturated, the cucumber places it in its mouth and sucks off the food particles. Some cucumbers have feathery tentacles, while others are more mop-like.
Inside the cucumber’s body, there are respiratory “trees” that function as the creature’s lungs. Cucumbers breathe by drawing oxygenated water through the anus. There is a simple blood system, but no heart. Remarkably, sea cucumbers are the only echinoderms to have haemoglobin, a component of blood cells that pick up oxygen and release it to body cells. Cucumbers have a digestive system, and a simple nervous system. Waves of contraction controlled by longitudinal and circular muscles help cucumbers move, in addition to the tube feet. The only hard part of the animal is a calcareous ring, a collar made up of ten plates.
Sea cucumbers spawn every year. Each individual has a gender, but this is difficult to determine in the field. Many species broadcast egg and sperm into the water. Spawning occurs when there is a signal, such as water temperature, or a plankton bloom. Fertilized eggs develop into larvae that feed on plankton. Larvae eventually develop into adults. Females of some species brood fertilized eggs.
Sea stars and fish are the main predators of sea cucumbers. Cucumbers do not have too many defenses, so many hide under rocks and in crevices. Some contain toxic chemicals that help deter predators. When very disturbed, many cucumber species will expel all of their internal organs. This extreme behaviour, called evisceration, may distract a predator. Others theorize that it may help the cucumber rid itself of internal parasites. Just as sea stars can regenerate lost rays, sea cucumbers can regenerate a new set of organs.
Several species of large and small cucumbers frequent coastal BC, in a wide variety of habitats. The most likely to be encountered are the Orange Sea Cucumber (Cucumaria miniata), frequently found at low tide under rocks. The White Sea Cucumber (Eupentacta quinquesemita), smaller than the Orange Sea Cucumber, may be found in similarly rocky places.
The California Sea Cucumber (Parastichopus californicus) is the largest to be found in BC, and grows up to 50 cm long. It is brown to red in colour and has distinctive spiky outgrowths on the body. This species is occasionally found at low tide on mud, gravel or rock rubble. The California Sea Cucumber is the only species to be commercially harvested in BC.
Having learned a little about sea cucumbers, I realize that these are complex and fascinating creatures. After all, what can be more interesting than a creature that can turn out its guts and grow a new set! Sea cucumbers are easy to find on rocky beaches in the low tide zone. Take care to handle them gently, and return them to where they were found.
Click on a link below to view the CVNS newspaper column.