The six thousand species of marine animals in the phylum Echinodermata ("spiny-skinned") are, like annelids, arthropods, chordates, and mollusks, characterized by a true coelom, or body cavity. However, echinoderms differ from all other coelomates (except for chordates) in their embryonic development. Very early in this development, a ball of cells called a blastula develops an infolding called a blastopore, which eventually reaches the other side of the embryo and forms the digestive tract. If the blastopore forms a mouth, the embryo is a called a protostome, meaning that the mouth (stoma ) forms first (proto ) after the anus. If the blastopore forms an anus, it is called a deuterostome, meaning that the mouth (stoma ) forms second (deutero ) after the anus. Echinoderm embryos are deuterostomes. This difference in development is so fundamental that protostomes and deuterostomes are thought to have diverged before any other branchings that led to the modern coelomate phyla. In other words, echinoderms and chordates are more closely related to each other than to any other organisms.
Although both are deuterostomes, echinoderms and chordates have significant differences. All echinoderms have a calcium carbonate skeleton just beneath the skin which typically bears projecting spines, hence the name of the phylum. Like cnidarians (jellyfish), echinoderms are radially symmetrical as adults, whereas chordates are bilaterally symmetrical. However, larval echinoderms are also bilaterally symmetrical. The late development of radial symmetry in echinoderms indicates that it is relatively recently evolved in the taxon. Modern echinoderms probably evolved from a mobile, bilaterally symmetrical ancestor by adding a sessile life stage, which then evolved radial symmetry. Many echinoderm species have since evolved mobility as radially symmetrical adults.
Echinoderms possess a unique water vascular system, which provides structural support for a set of tube feet used for locomotion. This system consists of internal canals lined with protruding tube feet and muscular sacs called ampullae. This system is also connected to the outside of the organism by an opening called a madreporite, through which water goes in and out of the system. When the ampullae contract, water is pushed into the tube feet, making them rigid. Most echinoderms have muscles in the ends of the tube feet that contract to create suction between the foot and a surface upon which locomotion occurs. The tube feet are also used for gas exchange.
Phylum Echinodermata is made up of five classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies and feather stars), and Holothuroidea (sea cucumbers). Sea stars have the typical echinoderm body plan —a central disk from which five or more arms radiate. They have no head or brain, and their sensory perception consists of eyespots at the end of the arms and neurosensory cells scattered throughout the epidermis. A ring of nerves around the mouth connects to nerve cords extending down the arms and coordinates movement. Sea stars feed on bivalve mollusks by prying them open with their arms and tube feet, then turning their stomachs inside out into the opening to digest the prey while it is still in its shell.
Brittle stars look like sea stars but have thinner arms. Like sea stars, they are mobile, but their tube feet lack suction and are not used for locomotion. Also like sea stars, brittle stars can regenerate limbs that have been lost. Incredibly, a leg can regenerate an entire body. Some species reproduce asexually by dividing and regenerating.
Sea urchins and sand dollars do not have arms, but retain radial symmetry in the rows of tube feet poking out of their hard skeleton. Whereas urchins are spherical, sand dollars are flattened along the axis of radial symmetry. They are armed with movable spines that can be poisonous. Many species have powerful jawlike structures called "Aristotle's lanterns," which they use for grazing on algae and other food attached to a surface, such as rock or coral. As with sea stars and brittle stars, the mouths of urchins and sand dollars are located on the bottom of the body.
Scientists believe that sea lilies and feather stars resemble the first echinoderms because they are sessile and their mouths and arms are oriented upward to gather food from the water. This was probably the intermediate evolutionary step through which the other echinoderms passed on their way to a more mobile adult stage. In fact, 500-million-year-old sea lily fossils are virtually indistinguishable from modern species.
Sea cucumbers are the most recently evolved echinoderms. They have lost most of the skeleton, which remains in the form of small bony particles in the skin. Although they retain five rows of tube feet, they are elongated from head to tail and display partial bilateral symmetry; some tropical species attain lengths of several meters. Sea cucumbers are the most mobile class of echinoderms, eating plankton from the water column or digging into the bottom sediments. They also have the ability to regenerate their guts after they expel them in response to predators, presumably as a deterrent.
Echinoderms play important roles in the ecological community of species. Ecologist Robert Payne conducted a famous experiment in Pacific-coast tide pools in which he removed Pisaster, a species of sea star. Because sea stars prey on mussels, removing them resulted in an explosion in the mussel population and disrupted the ecological balance of the entire community. Sea stars are so important to tide-pool communities that they are considered a " keystone species. "
Echinoderms reproduce sexually, with male and female individuals releasing gametes into the water. The larvae that result are small and lightweight, like many other floating organisms. They transform into the relatively immobile adult form in order to grow and produce more gametes.
Why did echinoderms evolve such a peculiar adult form of sexual reproduction ? Why do the larvae not grow bigger and reproduce themselves? Like insects, echinoderms have evolved a strategy of specializing in different activities at different stages in the life cycle. The larval stage specializes in dispersal, which is important for finding new habitats and avoiding competitors. The adult stage specializes in growth and reproduction by moving only enough to capture prey or graze. The ability to metamorphose from a dispersal stage to a growth stage allowed echinoderms to perform each function more effectively than their ancestors did. But other primitive deuterostomes failed to evolve this sophisticated adaptation; one of them gave rise to the chordates, and eventually humans. span>
see also Keystone Species; Phylogenetic Relationships of Major Groups.
Brian R. West
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Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishers, 1989.
Ridley, Mark. Evolution, 2nd ed. Cambridge, MA: Blackwell Science, Inc., 1996.