These suckers can be used to capture and hold prey, or to hold onto rocks in a swift current or tide. Interestingly, although most mature echinoderms are benthic meaning that they live on the bottom , the larvae are usually planktonic with bilateral symmetry.
During the process of maturing, the echinoderm will change its body shape from bilaterally symmetrical to radially symmetrical, and in the process, settle down on the sea floor. Perhaps the most common echinoderm is the sea star. Although more well known as the star fish, sea stars are not actually fish. The scientific community prefers to reserve the term "fish" for vertebrates with fins. The subphylum Stelleroidea contains the two classes of sea stars.
The class Asteroidea contains the true sea stars and sun stars. The class Ophiuroidea contains the brittle stars and basket stars. The distinction between the two classes lies in the way the arms connect with the body. Ophiuroids have a distinct central body part called a central disk with arms radiating out from the body. Adjacent arms do not connect with each other. Asteroids, on the other hand, have arms which seem to connect together in such a way as to make it difficult to discern where the arms end and the central disk begins.
The sea star's aboral top surface is spiny looking when closely examined. The rumpled skin, when examined up close, contains several different types of formations. Some of the bumps on the surface are called Dermal Branchiae, which are used to absorb oxygen from the water.
Another type of adaptation of the skin are called pedicellaria. These pincher-like pairs of organs can be used to pluck things off of the skin of the sea star which might otherwise cause a problem.
For example, the larval form of a barnacle could settle onto a sea star and begin to grow if the sea star had no way of removing it. The single colored spot on the aboral surface of the sea star is called the madreporite mad-ruh-PORE-ite. This is a calcareous piece of the water vascular system filled with tiny holes, like a strainer. It is the interface between the water vascular system and the ocean, acting as a filter. The sea star has a light sensitive organ at the tip of each ray, called an eyespot.
When moving across the ocean floor, the sea star usually leads with one ray, probing the surface ahead. Although the star cannot see in the way we do, it can detect the presence and direction of light, and does seem to have some idea where it is headed.
Sea stars are capable of regenerating limbs in the event that one or more is severed or damaged. The wound first closes off, and in time, the new limb will begin to grow. In a few species, the severed limb can regenerate a new sea star, but in most species, the severed limb dies. Most sea urchins are herbivores and scrape algae from hard substrates with five tooth-like structures in the mouth on the lower surface of the body.
Small bits of food move through a long digestive tube to be digested and absorbed. Indigestible material passes out through the anus, opposite the mouth Figs. Sea stars belong to the class Asteroidea from the Greek word asteroid meaning like a star ; Fig.
Like sea urchins, sea stars inhabit the oceans worldwide, from nearshore tide pools to deep ocean seafloors. Sea stars come in a range of sizes, reaching up to one meter m in length, but most are much smaller. Sea stars may be red, blue, or many other colors. Most sea stars have a central disk with five radial arms; some species have 15 to 40 arms. A few species have arms so short that they barely protrude.
Their bodies look like pin cushions. These plates, called ossicles , are much smaller than those of sea urchins. Some sea stars have spines extending from the ossicles, to help defend them from predators. Sea stars have a water vascular system and tube feet much like those of the sea urchins. Ambulacral grooves from the Latin root ambul meaning walk are narrow channels in the oral surface of a sea star filled with tube feet.
The tube feet are used mainly for grabbing and locomotion. Sea stars have remarkable powers of regeneration. Many species can regenerate a whole arm that breaks off Fig. These regenerated pieces are called comets Fig. Sea stars are voracious predators, crawling over the ocean bottom in search of prey. They feed not only on sessile molluscs such as clams, oysters, and mussels, but also on dead organisms lying on the bottom.
The crown-of-thorns starfish Acanthaster planci consumes so much live coral that it is considered a significant threat to coral reefs in the tropical Indo-Pacific region. The mouth of a sea star opens into the stomach in the central disc. The anus is on the upper surface Fig. Most sea stars are carnivores.
Although a sea star has no teeth, it can eat coral polyps and molluscs by pushing its stomach out of the body, spreading it over its prey, and digesting it.
To eat a clam, the sea star grasps the bivalve in its arms, attaches its suction cups to both shells, pulls steadily until the shells open slightly, and extends its stomach into the clam. In this way it preys on clams whose shells are open as little as 0. After the sea star digests and absorbs the tissue of its prey, it sucks its stomach back into its own body.
Brittle stars are the most abundant echinoderms. About 2, species inhabit the ocean floor worldwide, from the shoreline to great depths. In some areas, clusters of millions of brittle stars thickly carpet the bottom. This group is active only at night, hiding under rocks and in crevices during the day. Brittle stars have long, flexible arms attached to a small central disc Figs. Skeletal ossicles form a series of scaly plates along the arms, and a series of large cylindrical ossicles runs through the center of each arm.
These ossicles look somewhat like the row of vertebrae in a fish skeleton. They are connected by muscles that contract, producing a snakelike action. This characteristic movement gives the class its name, Ophiuroidea from the Greek root words ophio - meaning snake and - uroid meaning tail-like.
It is also the basis for another common name, serpent star. Moving brittle stars can appear to be dangerous, but they are harmless to humans. A row of movable spines projecting from the sides of the arms helps the animal move along the bottom. Although the arms appear to be radial, one or two of them usually lead in pulling the animal along while the others trail Fig.
These animals got the name brittle star because an arm often breaks off if they are captured. The broken arm is left wiggling as the rest of the brittle star scoots away. The missing arm regenerates quickly.
Most brittle stars are small with a central disc diameter less than three centimeters, but the arms may be up to ten centimeters long. Brittle stars feed on detritus—small particles of food—on the bottom. Some brittle stars curve their arms up to collect food particles suspended in the water. The tube feet, shaped like pointed tentacles, are used mainly for collecting food. One tube foot passes particles to another toward the mouth.
They do not have a true heart and the blood often lacks any respiratory pigment pike haemoglobin. Echinoderms have a a poorly developed respiratory system.
They use simple gills and their tube feet to take in oxygen and pass out carbon dioxide. Echinoderms are either male or female and become sexually mature after about two to three years. Most release their eggs and sperm into the water where they are fertilized. A female can release one hundred million eggs at once.
Larvae devlop which eventually settle on the sea floor in their adult form. If an arm breaks off some echinoderms, a new arm or even a new echinoderm can regrow. Echinoderms have a simple excretory system with no kidneys and use diffusion to rid their bodies of nitrogenous waste which is mainly ammonia gas. Their lifestyles vary greatly depending on which group of Echinoderms a species belongs to.
Sea stars are generally predators or detritivores, eating decomposing animal and plant material. Crinoids and some brittle stars are passive filter-feeders, absorbing suspended particles from passing water; sea urchins are grazing herbivores and sea cucumbers deposit feeders removing food particles from sand or mud. Echinodermata are exclusively marine organisms. Sea stars Figure 1 , sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms.
To date, no freshwater or terrestrial echinoderms are known. Adult echinoderms exhibit pentaradial symmetry and have a calcareous endoskeleton made of ossicles, although the early larval stages of all echinoderms have bilateral symmetry. The endoskeleton is developed by epidermal cells and may possess pigment cells, giving vivid colors to these animals, as well as cells laden with toxins.
Gonads are present in each arm. In echinoderms like sea stars, every arm bears two rows of tube feet on the oral side. These tube feet help in attachment to the substratum. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system. An interesting feature of these animals is their power to regenerate, even when over 75 percent of their body mass is lost. Echinoderms possess a unique ambulacral or water vascular system, consisting of a central ring canal and radial canals that extend along each arm.
Water circulates through these structures and facilitates gaseous exchange as well as nutrition, predation, and locomotion. The water vascular system also projects from holes in the skeleton in the form of tube feet.
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