Echinoderms
Echinoderms are a distinctive group of marine animals belonging to the phylum Echinodermata, which includes well-known species such as starfish, sea urchins, brittle stars, sea cucumbers, and sea lilies. Characterized by their unique radial symmetry, which contrasts with the more common bilateral symmetry seen in many other moving animals, echinoderms exhibit both free movement and a body structure that typically consists of five or more radiating areas. They possess a calcareous skeleton made of ossicles and a specialized water vascular system that allows for movement using tube feet.
Echinoderms inhabit various marine environments, from shallow shorelines to the depths of the ocean, and their reproduction methods can vary significantly across species, including both sexual and asexual reproduction. Many members of this phylum also have fascinating feeding mechanisms, such as the ability of some starfish to evert their stomachs to digest prey externally. The diversity within this group is vast, with over 7,500 classified species, showcasing a range of anatomical adaptations and ecological roles. Overall, echinoderms are an essential component of marine ecosystems, contributing to both biodiversity and the health of ocean environments.
Echinoderms
Echinoderm Facts
Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Superphylum: Deuterostomia
Phylum: Echinodermata
Subphylum: Homalozoa, Blastozoa, Crinozoa, Asterozoa, Echinozoa,
Classes: Various (see below)
Orders: Various (see below)
Geographical location: Found worldwide
Habitat: Marine, from the shoreline to the ocean depths
Gestational period: Varies among species, but most species lay eggs a few days after fertilization; eggs usually hatch within a few days to a few weeks after being deposited
Life span: Varies among species; the average is four years, but some live eight to ten years
Special anatomy: Body unsegmented with radial, pentamerous (five or more radiating areas) symmetry; no head or brain and few specialized sensory organs; endoskeleton of calcareous ossicles with spines; a unique water vascular system that extends from the body surface as a series of tube feet; excretory system is absent; development through free-swimming bilateral larval stages that metamorphose into radial adult
The echinoderms represent a biological puzzle for zoologists. Biologists generally agree that bilateral symmetry is adaptive for free-moving animals and radial symmetry is adaptive for sedentary animals, but the echinoderms are free-moving yet exhibit radial symmetry. Since the echinoderm larval stages are bilaterally symmetrical, they obviously evolved from bilateral ancestry; however, the larvae metamorphose into radially symmetrical adults. The echinoderms live in marine environments and include a varied group that is considerably different from all other members of the animal kingdom. Echinoderms include starfishes, sea urchins, brittle stars, sea cucumbers, and sea lilies. A calcareous skeleton is present in all members of the phylum, and their name is derived from the presence of external spines or protuberances.
General Characteristics of the Echinoderms
Because of the tremendous diversity in this group of animals, a complete description of all echinoderm characteristics is beyond the scope of this article. Externally, the arms or rays are joined at the center to form a disc that bears the madreporite. Many echinoderms are protected from predators by a spiny skeleton made up of calcareous plates just below the epidermis. Many of the plates bear tubercles (bumps) and spines. In addition, predators are often discouraged by the presence of pedicellariae (tiny pincers) scattered over the body. When stimulated, these snap vigorously and remain shut for several days on anything they catch.
The mouth is in the center of the ventral surface and is surrounded by a membranous area, the peristome. The mouth opens directly into a large cardiac stomach, which in turn opens into a smaller pyloric stomach. A small intestine extends upward from the pyloric stomach and ends at an anus in the middle of the upper surface of the disc. Five gastric glands extend out of the rays and open into the pyloric stomach. When feeding, the cardiac stomach everts through the mouth and spreads over the food. A fluid containing digestive enzymes is secreted and rapidly breaks down the food materials. The nervous system is composed of a nerve ring around the mouth and five radial nerves to the lower epidermis. Other nerve fibers are located in the walls of the digestive tract and inside the upper body wall. The nervous system exhibits reciprocal inhibition. The nerve centers on one side of the body inhibit those on the other side, permitting the animal to move in a coordinated manner in one direction.
The circulatory system is composed of circular and radial vessels filled with a fluid similar to that of the body cavity, which is very different from seawater. The vessels lie above the nervous system and are enclosed in their own body cavity. Some of these vessels in some species can contract. A pair of gonads are in the base of each ray, with one on each of the gastric glands. These gonads hang free in the body cavity except where a short duct attaches them to a reproductive pore opening externally between the bases of adjacent rays. Notably, some sea cucumber, starfish, and brittle star species can reproduce asexually.
Representative Echinoderms
Most starfish (class Asteroidea) have five rays and a relatively small disc, but in some species, the body is pentagonal rather than star-shaped, and the disc is large relative to the rays. The animals in one genus, Leptasterias, have six rays, while the number of rays in other starfishes may be as high as twenty-five or fifty. In general, if the number of rays is greater than seven, the number will be variable within a species. When the number exceeds five, the embryo develops five rays first and the others later. Most starfishes eat only small bivalves and other organisms that are swallowed whole. Many of the larger species, ranging from one to three feet in diameter, feed primarily on other echinoderms.
Sea lilies (class Crinoidea) are echinoderms that attach to the ocean floor by a stalk. The mouth is directed upward, and the anus is located to one side. The rays generally branch to form graceful patterns. Ciliated grooves flanked on both sides by tube feet without suckers extend out of the mouth along the upper surfaces of all the rays and branches. Food is trapped and swallowed when tiny organisms and food particles are pushed against the ciliated groove by the action of the tube feet. Movement in sea lilies is restricted to spreading and folding together of the branches and postural changes of the body. In the feather stars, the larvae attach and grow a short stalk like that of the sea lilies, but later break loose. The general anatomy and method of feeding are unchanged, but the feather stars differ from the sea lilies in their locomotion. Feather stars can crawl through vegetation using the rays as prehensile organs, and they can swim by raising and lowering the ten arms more than one hundred times per minute.
Sea cucumbers (class Holothuroidea) creep or burrow in the mud or sand. Since the calcareous plates are small, the body is soft and flexible. The body is elongated between the mouth and anus, but one side usually becomes the permanent lower side, causing the radial symmetry to be imperfect. While five rows of tube feet extend from mouth to anus, often only three of the rows have suckers and are used for locomotion. A circle of branched tentacles form from the tube feet near the mouth. Sea cucumbers are notable for their ability to throw away their viscera. When environmental conditions are unfavorable, the sea cucumber contracts violently and ejects the entire digestive system, which can later be regenerated.
The skeletons of the echinoids (class Echinoidea) form rigid boxes. Five grooves with tube feet radiate from the mouth up around the sides and end near the anus. The tube feet on the upper surface are often long and filamentous and apparently used for respiration, while the lower tube feet usually have suckers and are utilized in locomotion. Sea urchins possess numerous long spines, some of which are used to aid the tube feet in walking. The urchins move about slowly, using their five sharp teeth to scrape and chew whatever they encounter. The sand dollars are a group of much-flattened echinoids. They usually creep about slowly on their short lower spines under the surface of the sand.
Brittle stars (class Ophiuroidea) possess slender rays attached to a circular disc. Each ray is composed of a row of large cylindrical skeletal pieces joined together by short, powerful muscles. Each ray is very supple, and the tube feet are poorly developed. Brittle stars move by pulling or pushing on surrounding objects, and thus slither like a snake. Most brittle stars feed on debris and mud, but some capture prey with their prehensile rays and bring it to the mouth, which opens into a simple saclike stomach. Undigested remains are eliminated through the mouth because no other digestive organs are present.
The group commonly called sea daisies (infraclass Concentricycloidea, order Peripodida) is a class of echinoderms discovered in 1986 off the coast of New Zealand. They have no arms, and the tube feet are located around the periphery of the disc rather than in grooved areas, as in other echinoderms. Three species have been identified. Xyloplax medusiformis and Xyloplax turnerae have a small, saclike stomach but no intestine or anus. Another species, Xyloplax janetae was discovered in the early 2000s, clinging to wood, but much remains to be learned about this species. These sea daisies absorb nutrients from the environment.
Over 7,500 echinoderms are classified into five subphyla containing many classes, many of which comprise extinct species. Classes with extant species include Asteroidea (starfishes), Ophiuroidea (brittle stars), Holothuroidea (sea cucumbers), Echinoidea (sea urchins, sand dollars), Concentricycloidea (sea daisies), Stelleroidea (true sea stars), and Crinoidea (feather stars). The orders, families, and species classifications of echinoderms are numerous.
Principal Terms
Bilateral Symmetry: An arrangement of body parts of an organism down a central axis which, when divided down the midline, produces right and left mirror images
Calcareous: A material composed primarily of calcium compounds
Madreporite: A fine-meshed sieve that opens from the seawater into the water vascular system of the echinoderms
Metamorphosis: The transformation of a larval form into an adult form
Radial Symmetry: An arrangement of body parts of an organism like the pieces of a pie around an imaginary central axis
Bibliography
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