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| Sunday, 4 September 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Echinoderms : Amazing animals in the sea
A diverse group of animals numbering 6,000 species that are found only in the sea, usually on the ocean bed, are echinoderms. Sea urchins, starfishes and sea cucumbers belong to this group of animals with amazing adaptability. Even though the name echinoderms means spiny-skin in Greek, not all animals have spiny-skin. The spiny-skin has different types of growths on it, such as bumps which are used to absorb oxygen, and pincher-like organs that are used to clean the surface of the skin. Their skeletons are made of porous blocks of calcium carbonate and they have muscles to work their skeleton. They also have a hydraulic system, the water-vascular system with movable projections from the body called tube feet. Echinoderms usually have five appendages (arms or rays), however, there are exceptions. Many have suckers on the ends of their feet. They use them to latch onto rocks in a swift current. Sea stars which are also called starfishes are not real fish. The scientific community prefers to reserve the term 'fish' for vertebrates with fins, and starfishes lack both. There are two sub-types of sea stars, they are, Asteroids (the true sea stars and sun stars) and Ophiuroids (brittle stars and basket stars). The difference between the two sub-types is the manner in which their arms connect to the central disc. While Asteroids have arms that are connected to each other, Ophiuroids have arms that have a distinct boundary between the arms and the central disc. Most starfishes have five long arms or rays with the upper part covered in tough skin and short spines, and the underside with rows of tiny tube feet. These lie on either side of the groves which lead to the central mouth. These tiny tube feet which are also called podia can be filled with sea water. As the vascular system is filled with sea water, the starfishes travel along the sea bed by pumping water into their tube feet. Each ray of the sea star has a light sensitive organ called an eyespot at the tip. When moving across the sea bed it leads with one ray, probing the surface ahead. Starfishes may not be able to see their surroundings the way we do, but, they do have some idea of where they are going. Starfishes are perhaps the most common echinoderms, and have an amazing ability to regenerate. If a starfish is attacked by a predator such as a painted prawn, do you know what it does? It breaks away, leaving the predator with only the ray or arm it was holding onto. As long as the limb is not eaten by the predator, and still contains a section of the central body disc, the limb can grow into a complete starfish in some species. The starfish who shed the limb, also has the ability to grow back the part that was lost to its attacker. A sea star's or starfish's diet includes barnacles, snails, sea urchins, clams and mussels. A few species, such as the spiny star of the North Atlantic eat other sea stars! Sea urchins heart urchins, cake urchins and sand dollars which belong to a sub-group of echinoderms called enchinoidea, have many sharp spines pointing out in all directions. They provide protection from predators.
Like starfish, sea urchins too have their mouths underneath the body. The mouth contains five teeth that point towards the centre. Sea urchins are mostly ball-shaped but some are flat and are able to bury themselves in sand and mud. They are known as sand dollars in America. Sea urchins have five paired rows of feet with suckers. They are able to swivel towards a predator because their spines are connected to muscles. Sea cucumbers which belong to a sub-category of echinoderms called holothuroidea look very different from starfishes or sea urchins. In fact, they look very much like the cucumbers you find in the supermarket. Somewhat shaped like footballs, sea cucumbers have soft bodies and spines that are not very clearly visible. They have five rows of tube feet running lengthwise like the seams on a foot-ball. The mouth is at one end of the body and around the mouth are these long tube feet. Sea cucumbers feed on plankton and other organic matter. Some position themselves in a current that brings a steady supply of food, and spend hours waiting...then they collect food with their tentacles. The sea cucumbers have an interesting way of defending themselves. They can throw off some of their internal organs which would either scare off or satisfy their predators. They can then grow another set of internal organs. Sea cucumbers also secrete a sticky substance as a defence mechanism
which cannot be removed unless you shave! |
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