Echinoderms : Sea Stars, Sea Urchins, Sea Cucumbers and More

Echinoderms are a group of marine invertebrates that includes organisms like sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, crinoids, and sea daisies. Together, they make up the phylum Echinodermida. There are about 7,000 living species. Echinoderms live in every ocean from warm tropical waters to polar seas, from shallow waters to some of the deepest depths of the ocean. As adults, they usually have a five-point radially symmetrical body plan with their circulatory, nervous, and skeletal systems radiating in five directions from a central axis. This makes Echinoderms likely to appear star-shaped, disc-shaped, or spherical. They come in a variety of colors and sizes. Most echinoderms grow around 4 to 5 inches. Most species are found traveling along the seafloor or other surfaces, while a smaller portion can swim in the open ocean. All species have an internal calcareous skeleton. In some species, like sea urchins, the skeleton consists of plates that are fused, forming a hard shell. In sea stars and brittle stars, the skeleton is made up of many small plates that move with one another, forming flexible joints. Sea cucumbers often have a very reduced skeleton, consisting of a series of small, isolated ossicles. Echinoderms lack well-developed nervous systems. They possess no brain or heart, only a nerve ring with radial nerves extending outward. A wide variety of organisms prey on echinoderms, from fish to birds, mammals, and marine invertebrates. Larger echinoderms often prey on smaller echinoderms. Some echinoderms are consumed by humans for food or medicine. In many parts of the world, sea urchins are harvested for their gonads, called roe. In some Asian cultures, sea cucumber is a delicacy. Echinoderms protect themselves from predators in many different ways. Their skeleton usually features spiny or knob like extensions that project from the body. Many sea urchins have sharp, movable spines. Depending on the species, the spines may also be venomous. Crown of thorns starfish also have sharp, venomous spines. Urchins and sea stars typically possess tiny jaw-like structures or pincers, often at the end of stalks, that serve a variety of functions, ranging from defense to assisting in food capture. The pincers can serve to remove debris and planktonic larvae looking for a place to settle, or the pincers can act to fend off predators. Some species, like flower urchins, bear venomous pincers. Some sea cucumbers eject sticky white threads that entangle predators. Sometimes sea cucumbers eviscerate their gut or other internal organs as a decoy to distract predators. The innards are later regenerated. Echinoderms like starfish and brittle stars can break free from an attack by discarding an arm. Discarded arms can also be regenerated. Many sea stars and sea urchins show evidence of being able to perceive light and dark. Their ability to see varies by species, with some detecting only light and shadows, while others can likely make out basic images. Echinoderms have a unique system for breathing, nutrient circulation, and locomotion called the water vascular system. It is a complicated system of tubes that uses seawater to convey nutrients and gases throughout an organism's body. The seawater circulation also allows for waste exchange. The system uses water pressure to help give echinoderms shape and to help them move. Most echinoderms have a structure on top of the body called the matroporite that regulates the amount of water in the water vascular system. Sea lilies, on the other hand, obtain water through many pores that cover their body. As part of the water vascular system, most echinoderms have water-filled tube feet that may be expanded or contracted using the pressure in the system. The tube feet are used for locomotion, feeding, and sometimes respiration. Inside the body, a muscular and flexible bulb is attached to each tube foot. When the bulb is filled with water, the tube foot is relaxed. To extend the foot, an echinoderm forces water from the bulb into the foot. Tube feet secrete sticky substances to adhere to surfaces. Some starfish use their tube feet to latch onto bivalves and pull them apart, exposing the organism within for consumption. In most echinoderms, there is a relatively straightforward digestive process with food entering the mouth, going through the digestive tract, and exiting via the anus, typically on the opposite end of the body. Brittle stars in certain groups of sea stars lack an anus, however, and vent food by the mouth once their digestion is complete. Echinoderms may be grazers, filter feeders, scavengers, or voracious predators. Many feed on stationary organisms such as algae, corals, sponges, clams, and oysters. Others filter food particles from sand, mud, or water. Echinoderms are typically separated into five groups. Crinoids include feather stars and sea lilies. They have a saucer shaped body extending into repeatedly branching, feathery arms that are used as filter feeding appendages to filter plankton and particles from water currents. The food is coated in mucus and passed down the arms and into the mouth. Sea lilies attach to the seabed by a jointed stalk, but feather stars break free when they are young to become swimming adults. Sea stars, or starfish, are characterized by their star-shaped body with five or more arms merging into a central body disc. Their tube feet and mouth are located on their underside. They may feed on a wide range of organisms from algae to moving prey. In many species of starfish, the stomach can be everted to digest food outside the body. Brittle stars and basket stars have a disc-shaped body with five flexible arms. Basket star arms are branched and finely divided. Sea urchins and sand dollars have a body shape ranging from a disc in sand dollars to a spear in urchins with five double rows of tube feet. Most sea urchins are grazers, scraping algae off rock surfaces using their teeth. Some sand dollars have visible slits on their body that radiate from their center. These slits are called lunules and they help keep the sand dollars from being lifted off the seafloor by currents. Sea cucumbers have a sausage-shaped body and most have five rows of tube feet. The tube feet encircling the mouth are modified into feeding tentacles. They are primarily detritivores, feeding on decaying particles and sediment or water currents. Sand and mud typically accompany their food through their simple gut. In most echinoderms, the sexes are separate. Typically, when spawning, males and females release their gametes into the water where fertilization occurs. Individuals often gather to spawn at the same time. The fertilized eggs hatch into larvae that drift with the current before settling in an appropriate habitat. The early larval stages of all echinoderms have bilateral symmetry. Some echinoderms brood their eggs, especially cold water species. In brooding species, the young fertilized eggs usually develop directly into young adults without a planktonic larval stage. Echinoderms may also reproduce asexually through regeneration from body parts. For example, some sea stars can reproduce by splitting off a piece of their central disc and then regenerating the lost body parts. For more marine facts, click the SUBSCRIBE button!!