“Sea urchin excitement again”
Which diver doesn’t know them, the hundreds of sea cucumbers, sea stars and sea urchins encountered during a diver’s lifetime. Often a first time diver is surprised and amazed by the appearance of a sea cucumber or sea urchin and pointing enthusiastically on one of those animals urging the divemaster or instructor to make a photo of those fascinating animals. And how many times we must have thought: “There we go again, sea cucumber excitement once again, don’t you know that we find those guys everywhere?” But what do we really know about those animals we often consider as “not so special”? Reading deeper into the topic, those animals are in fact pretty amazing. In the next few articles I write, I’ll try to enlighten the readers a bit more about those quite bizarre and interesting animals. But before we go deeper into the details of the different classes (sea urchins, sea cucumbers, sea stars etc.), first a few general facts about the “echinoderms”.
Ancient all-rounders of the oceans
Sea cucumbers, sea stars, sea urchins and sea lilies belong all to the same group of animals, the so called echinoderms. Echinoderms are animals belonging to the phylum Echinodermata, which is made up of approximately 7’000 species. Echinoderms are very abundant and can be found pretty everywhere in the oceans; from the shallow intertidal zones of just a few meters below surface down to the deepest areas of our oceans at several thousands of meters, from tropical warm waters to the coldest zones near the poles. There are no echinoderms living in fresh water or outside the seas though, they are indeed purely marine animals. The echinoderms are incredible old and emerged first around 540 million years ago, long time before the first fish organisms evolved. They are extremely resilient, able to live under high pressure, ice cold water and even regenerate their bodies once they have been cut into pieces.
The “Spiny skins”
The word echinoderma is derived from ancient Greek and means spiny skin. Like their name indicated, most echinoderms do possess spines. Those spines are made of calcite (a form of calcium carbonate) and so are their internal skeletons (also called “endoskeleton”). Those endoskeleton are made up of plates hold together by more or less elastic connective tissue, which allows them to take a softer from when they are moving, or make them quite stiff when they stay still. Besides the outer spines, echinoderms possess small pincers which are used to keep their body free from parasites and help to capture prey (in some species like the flower urchin, they are also loaded with toxins).
Beautiful and poisonous
Echinoderms appear in many forms, colors and sizes. The form depends of course on species and animal class, but is also determined by their age. All echinoderms start their lives as little larvae with a bilateral symmetry, resembling more larvae from gastropods (snails and slugs) and other animals than the looks of their parents. Only after passing through a number of grow stages do they reach their common radial looking (except the sea cucumbers, which partially retain their bilateral symmetry). Their often stunningly colorful looks are produced by pigment cells living in their skins. Those pigment factories can produce blue, green and violet hues and are often light-sensitive. Some echinoderms can change their appearance completely as night falls. Many echinoderms also possess toxin producing glands and can cause very painful stings (flower urchin or the crown-of-thorns starfish for instance).
Echinoderms’ bodies are all made up of equal sections surrounding a central axis. In many species each of those radial sections contains a duplicated set of the animal’s life-containing organs. This radial symmetry provides the echinoderms with incredible regeneration abilities. If an animal is cut into several pieces, each of the equal sections is able to regrow a full body, a thing also called “asexual reproduction”. Some species can even do that on purpose and deliberately break themselves into two, thus creating two identical clones. Sea cucumbers are able to spit out their guts and internal organs (more to that later) and regrow them later.
No blood, no heart, but a water vascular system
Echinoderms don’t possess a cardiovascular system like more complex animals. Instead of a heart and blood vessels, they possess a so called “water vascular system” which is unique to echinoderms. Echinoderms have tubular feet through which they are able to suck water inside their vascular system. Through muscle contractions they are able to pump and circulate water through different parts of their body and provide their cells with oxygen. The water vascular system also allows them to move. Through contradictions of their body cavities they are able to pump water into their tiny tube feet. The expansion and contradiction of their tube feet allows them to move, grasp surfaces and prey for items.
Brainless but not (that) stupid
Echinoderms don’t have a brain in the common sense. But how are they able to detect and hunt prey, sense dangers and move to different locations? The answer to that question is: Echinoderms do have a “distributed brain”. Instead of a large mass of interconnected neurons in one section of the body, echinoderms possess a radial nervous system connected by neurons which are distributed throughout their body (rather than concentrated at a certain place which we call “brain”). Through this “distributed brain”, they are able to gather information delivered by sensory cells in their skin and tube feet, such as temperature, light, touch and pain. Their best sense is probably smell, sensory cells in their tube feet are able to detect chemical changes in the water surrounding them. Although they don’t possess a brain in the common sense, they are able to interact with their environment and each other (starfish wrestle with each other while competing for food for instance).
Echinoderms love life
Expect for a few species, all echinoderms are gonochoric, which means they are either male or female. Sexual maturity is reached at age of about 2-3 years (estimated life span of most echinoderms are between 5-10 years). To reproduce, eggs and sperm cells are released into the open water where fertilization takes place. The release of eggs and sperms is often synchronized by the lunar cycle and some species gather during breeding times. There is usually no parental care of the egg. However a few species of starfish, brittle stars, sea urchins and sea cucumber carry their eggs around till they hatch.