If you have been to the beach and seen starfish or sand dollars, brittle stars, sea cucumbers, or sea urchins you have seen some of the organisms that make up the phylum Echinodermata, the largest phylum that does not have any fresh water or land members.
Echinoderms are not microscopic, except for their larvae; they range from a few millimeters to a few decimeters in size, although the stalks of some crinoids could reach a length of over a meter. Echinoderms typically live on the bottom of the ocean. This pattern of behavior is known as benthic. When studying their history however, fossil records show that Paleozoic echinoderms were sessile, meaning that they were attached to a fixed place and cannot move. Today most living echinoderms can creep from place to place and a few echinoderms can float or are free swimmers.
Echinoderms have a number of life “modalities” or lifestyles including starfish that are predators, crinoids that are filter feeders, while holothurians, sand dollars, and ophiuroids often feed on detritus and sea urchins scrape algae from rocks. Starfish and sea urchins may be common in very shallow water, while the floor of the deep sea may be covered with ophiuroids or holothurians.
The phylum Echinodermata has approximately 6,000 species and its name comes from Greek and means “spiny skin.” While many echinoderms do have “spiny” skin others do not. All echinoderms have radial symmetry meaning that the creatures have appendages (or body construction) that point outward from the center of the body like the spokes on a bicycle wheel. Most living echinoderms have five-fold symmetry or pentameral, with rays or arms in fives or multiples of five. There are a number of fossil echinoderms however, which were not pentameral. Echinoderm larvae are typically planktonic with bilateral symmetry. As they mature, the echinoderm’s body changes shape from bilaterally symmetrical to radially symmetrical and settle down to live on the sea floor.
Another trait that is not as visually obvious is the water vascular system common to all echinoderms. When examining the oral (ventral) side (underside) of a sea star, you can see hundreds of tiny feet arranged into rows on each ray or appendage of the star. These tube feet, or podia, are filled with seawater. By expanding and contracting these chambers (by forcing water into the tube feet to extend them) and retracting these chambers (using muscles in the tube feet) the creature can walk. Many echinoderms can also form suckers on the ends of their tube feet that can be used to capture and hold prey, or to hold onto rocks in a swift current or tide.
To reproduce echinoderms typically use external fertilization (discharging both eggs and sperm into the water). However, a few sea urchins brood their eggs in special pouches, but most provide no parental care. After hatching, most echinoderms go through several planktonic larval stages before settling down.
There are five known classes of living echinoderms: Crinoidea (sea lilies), Asteroidea (starfish), Ophiuroidea (brittle stars or snake stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers).
Crinoids are no longer abundant nor are they very familiar organisms today. However, these creatures dominated the Paleozoic fossil record of echinoderms and shallow marine habitats until the Permo-Triassic extinction, when they suffered a near complete extinction: many Paleozoic limestones are made up largely of crinoid skeletal fragments. Stalked crinoids, or “sea lilies” lived attached to the bottom and filter food particles from the currents flowing past them. The extant Crinoids are the only attached suspension-feeding echinoderms alive today. This makes them an important group for Paleontologists to study to better understand the numerous extinct attached suspension-feeding echinoderms. These living crinoids are the only way to examine in detail this ancient mode of life.
The living stalked crinoids mostly inhabit deep water and are therefore difficult for the average underwater enthusiast to observe. The unstalked crinoid, or “feather star” superficially resembles a starfish, but the mouth faces up, and the comatulid crawls by “walking” on specialized structures called cirri.
The Stelleroids: Asteroidea and Ophiuroidea
The most common echinoderm or at least the best-known echinoderm is the sea star. Although better known as the “starfish”, sea stars are not actually fish. The scientific community reserves the term “fish” for vertebrates with fins.
The subphylum Stelleroidea contains two classes of sea stars. The class Asteroidea which is made up of the true sea stars and sun stars. The class Ophiuroidea contains brittle stars and basket stars. The distinction between the two classes is the way in which the arms are connected with the body. Ophiuroids have a distinct central body (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 that seem to connect together and 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 pincher-like pairs of organs called pedicellaria that can be used to pluck things off of the skin of the sea star such as the larval form of a barnacle that could otherwise begin to grow if the sea star had no way to remove 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 also 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 does not “see” like fish or mammals, it has the ability to detect the presence and direction of light, and appears to have a general idea of 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. Sea stars eat a variety of different things, including barnacles, clams, mussels, snails, sea urchins, and in some cases, other sea stars. Many sea stars, such as the Northern Sea Star, eat mussels and clams in a fascinating way. The sea star first surrounds its intended victim. Then it applies outward force (with its suction cup equipped tube feet) on the two mussel shells (called valves), to pull them apart. Contrary to popular belief, the sea star does not need to apply force for a long time in order to tire out the mussel. The sea star can apply so much force to the mussel valves (7 or more pounds!) that it will bend the shell. Seizing the moment, the sea star then everts its stomach out through its mouth, and into the mussel (only a 1/100th of an inch opening is required). Once the sea star begins to digest its victim the mussel dies. The sea star then finishes the meal and when it is finished nothing remains except a shell.
Echinoids include sea urchins, heart urchins, cake urchins and familiar sand dollars. The sea urchins are well known to most people, having a large number of sharp spines pointing out in all directions. These spines offer protection from many would-be predators. The spines are joined to the skeleton of the animal, called the “test,” in a form of ball-joint. This coupled with the fact that there are muscles attached to each spine enables the urchin to swivel its spines in the direction of a predator. The test is an egg-like spherical structure constructed of rows of radially arranged plates fused together.
The creature has 5 paired rows of tube feet which, when extended, are long enough to reach past the length of the spines. They contain suckers. The anus is on the top of the creature, while the mouth is on the underside. The mouth contains five teeth that are arranged pointing towards the center of the mouth. This structure looks and works much like the jaws of a drill chuck and is called “Aristotle’s Lantern”. It gets the name due to the fact that it was first described in detail, in a book by Aristotle, as “looking like the top of an oil lamp”. The creature uses its tube feet to pull itself against the substrate so it can gnaw away at algae with its mouth.
Sea cucumbers are usually oblong and somewhat football shaped and lay on their side on the bottom. They have 5 rows of tube feet running lengthwise similar to the seams on a football. Three of these rows of tube feet are well developed and are in contact with the substrate while the remaining two rows are usually either underdeveloped and not used or missing. Tentacles surround the mouth of the sea cucumber and are usually branched. These tentacles are actually special tube feet and are part of the water vascular system. The water vascular system, unlike sea stars and sea urchins, is not filled with seawater but with a special body fluid. Furthermore the sea cucumber has no direct interface (madreporite) with the ocean and its internal organs.
The sea cucumber feeds in an interesting way. It will either position itself in a spot on the ocean floor where a current will bring a steady supply of food (plankton and other organic particles) its way or it will sift through the sand for particles of food. The tentacles are opened to collect the food and then the cucumber sticks each tentacle in its mouth, one at a time, and licks them off. As soon as a tentacle has been licked, the cucumber takes it out of its mouth and collects more food while it licks the next tentacle. The sea cucumber
The sea cucumber has another interesting habit, known as self-evisceration. When a sea cucumber is attacked, it may expel some of its internal organs for a predator to eat. This could either satisfy a predator, or scare it off. The cucumber will then proceed to grow another set of organs.will do this for hours at a time.
Many sea cucumbers are quite poisonous. However, the poison of some sea cucumbers has shown promise as an inhibitor in the growth of cancer cells.
- https://www.britannica.com/animal/echinoderm/Form-and-function-of-external-features – Encyclopedia Britannica
- https://scripps.ucsd.edu/centers/echinotol/about-echinoderms/project – Assembling the Echinoderm Tree of Life
- https://invertebrates.si.edu/ – Smithsonian Department of Invertebrate Zoology
- Echinoderms – Echinoderms of Hawaii
- Dr. Maria Byrne Publications https://www.ibr.usyd.edu.au/pubs/byrne.html
- Dr. Bruno David Phylogeny of the Echinodermata and Spatangoid Urchins https://rnb.snv.jussieu.fr/Themes/membres/info/mem51.html