Echinoderms: Starfish, Sand dollars and Sea Urchins

Phylum That Includes Sea Stars, Sand Dollars and Feather Stars

Starfish
Starfish. Kerstin Meyer/Moment Open/Getty Images

Echinoderms, or members of the phylum Echinodermata, are some of the most easily-recognized marine invertebrates. This phylum includes sea stars (starfish), sand dollars, and urchins, and they are identified by their radial body structure, often featuring five arms.   You can often see echinoderm species in a tidal pool or in the touch tank at your local aquarium. Most echinoderms are small, with an adult size of about 4 inches, but some can grow to as much as 6.5 feet in length.

Different species may be found in a variety of bright colors, including purples, reds, and yellows. 

Classes of Echinoderms

The phylum Echinodermata contains five classes of marine life: Asteroidea (sea stars), Ophiuroidea (brittle stars and basket stars), Echinoidea (sea urchins and sand dollars), Holothuroidea (sea cucumbers) and Crinoidea (sea lilies and feather stars.) They are a diverse group of organisms, containing about 7,000 species. The phylum is regarded as one of the oldest of all animal groups, thought to have appeared at the start of the Cambrian era, about 500 million years ago.  

Etymology

The word echinoderm means comes from the Greek word ekhinos, meaning hedgehog or sea urchin, and the word derma, meaning skin. Thus, they are spiny-skinned animals. The spines on some echinoderms are more obvious than others.  They are very pronounced in sea urchins, for example. If you run your finger over a sea star, you'll likely feel small spines.

The spines on sand dollars, on the other hand, are less pronounced. 

Basic Body Plan

Echinoderms have a unique body design. Many echinoderms exhibit radial symmetry, which means that their components are arranged around a central axis in a symmetrical manner. This means that an echinoderm has no obvious "left" and "right" half, only a top side, and a bottom side.

Many echinoderms exhibit pentaradial symmetry—a type of radial symmetry in which the body can be divided into five equally-sized "slices" organized around a central disk.

Although echinoderms can be very diverse, they all have some similarities.These similarities can be found in their circulatory and reproductive systems.

Water Vascular System

Instead of blood, echinoderms have a water vascular system, which is used for movement and predation. The echinoderm pumps sea water into its body through a sieve plate or madreporite, and this water fills the echinoderm's tube feet. The echinoderm moves about the sea floor or across rocks or reefs by filling its tube feet with water to extend them and then using muscles within the tube feet to retract them.

The tube feet also allows echinoderms to hold on to rocks other substrates and to grip prey by suction. Sea stars have very strong suction in their tube feet that even allows them to pry open the two shells of a bivalve.

Echinoderm Reproduction

Most echinoderms reproduce sexually, although males and females are virtually indistinguishable from one another when viewed externally. During sexual reproduction, echinoderms release eggs or sperm into the water, which are fertilized in the water column by the male.

 The fertilized eggs hatch into a free-swimming larvae that eventually settles to the ocean bottom.

Echinoderms can also reproduce asexually by regenerating body parts, such as arms and spines. Sea stars are well-known for their ability to regenerate arms that are lost. In fact, even if the sea star has only a small part of its central disk left, it can grow an entirely new sea star. 

Feeding Behavior

Many echinoderms are omnivorous, feeding on a variety of living and dead plant and marine life. They serve an important function in digesting dead plant material on the ocean floor and thereby keeping waters clean. Plentiful echinoderm populations are essential to healthy coral reefs.

The digestive system of echinoderms is relatively simple and primitive compared to other marine life; some species ingest and expel waste through the same orifice.

Some species simply ingest sediments and filter out the organic material, while other species are capable of catching prey, usually plankton and small fish, with their arms. 

Impact on Humans

While not an important source of food for humans, some forms of sea urchin are regarded as a delicacy in some parts of the world, where they are used in soups. Some echinoderms produce a toxin which is fatal to fish, but which can be used to make a medicine used to treat human cancers. 

Echinoderms are generally beneficial to ocean ecology, with a few exceptions. Starfish, which prey on oysters and other mollusks have devastated some commercial enterprises. Off the coast of California, sea urchins have caused problems for commercial seaweed farms by eating young plants before they can become established.