ECHINODERMS OF BRITISH COLUMBIA

(Sea Stars, Brittle Stars, Sea Cucumbers and Sea Urchins)

The Northern Sunstar (Solaster dawsoni), photo by Phil Lambert

Introduction

by

Phil Lambert

Visit the atlas pages for the Sea Cucumbers of BC
Visit the atlas pages for the Sea Stars of BC
Visit the atlas pages for the Sea Urchins of BC
Visit the atlas pages for the Feather Stars of BC

Members of the phylum Echinodermata are strictly marine and occur in all the oceans from the intertidal zone to the abyssal depths. Ask any child to draw a picture of the seashore and chances are it will include a sea star, one of the five classes of echinoderms. Sea stars are almost synonymous with life in the ocean. The sea stars (Asteroidea), and four other classes, brittle stars (Ophiuroidea), sea urchins (Echinoidea), sea cucumbers (Holothuroidea) and feather stars (Crinoidea) make up the phylum.  The word echinoderm is derived from the Greek for spiny, ekhinos, and skin, derma.

Even though the classes of echinoderms look quite different, they have characteristics that link them and indicate their common ancestry. Adult forms have pentaradial symmetry, being built like a wheel with five spokes. In some groups like sea stars this symmetry is obvious but in sea cucumbers for example, pentaradial symmetry is obscured by the wormlike or cucumber shape. To see the symmetry of a sea cucumber look at a cross-section of the animal with its five longitudinal muscles, five rows of tube feet, or feeding tentacles around the mouth in multiples of five. Most of the classes start out as a bilaterally symmetrical larva which undergoes a radical transformation with a reorganization of the internal organs, or a tiny radially symmetrical bud grows into the adult and the larval body is discarded.

 

Giant Pink Star (Pisaster brevispinus).  Photo by Mike Edley

 

 The skeletons of all the classes consist of calcareous plates or pieces usually joined by connective tissue. The plates often bear external spines or bumps that help protect the animal. This internal skeleton varies from the solid sphere of a sea urchin made up of many interlocking plates forming a solid test, to the tiny crystals of calcium carbonate found in the skin of a sea cucumber and forming characteristic shapes. In sea stars the skeleton is a porous meshwork of ossicles creating the general body shape. The gaps in the skeleton allow the gills (dermal branchii) to protrude and absorb oxygen from the surrounding water. The connective tissue found in echinoderms is also a special type which has not been discovered in any other group besides echinoderms. Scientists have called it Mutable Connective Tissue (MCT) because of its special ability to be converted from stiff to soft in an instant, by nervous control. In some crinoids, MCT in the arms can contract like muscle but the mechanism of how this happens without muscle cells is yet to be determined.

Of all the characteristics that separate echinoderms from other groups, the water-vascular system is perhaps the most unusual and distinctive. This term describes the system of tubes and reservoirs that operate the tube-feet (podia) that we see on the underside of a sea star arm. Each podium bears a sucker at the tip that can attach to a surface and allow the animal to do things like climb vertical walls or to pull the shells of a clam apart. In other classes the system has been modified. For example, in feather stars and brittle stars the podia are more like tentacles that taper to a point and help to transfer food particles down the arm to the central mouth. Some sea cucumbers have suckered tube-feet; others have lost them except for some at the anterior end modified into feeding tentacles. In sea urchins, tube feet protrude through tiny pores in the solid calcareous plates, allowing the urchin to grasp and eat pieces of seaweed, attach to rocks, or to rapidly move out of the way when a predatory sea star cruises by.

The water-vascular system usually has a ring canal around the esophagus with five branches (radial canals) leading to the arms or rows of tube feet. This closed system operates on a hydraulic principle so that a pressure applied at one point will be transmitted throughout the system equally. Each tube foot has a bulb and a valve at its base to control the pressure and how far the tube foot extends. A short branch (stone canal) from the ring canal to the outside in sea stars and brittle stars, allows the animal to replenish the fluid in the system. The sieve plate (madreporite) in these two groups contains amoeboid cells that attack any unwanted foreign invaders. In sea cucumbers the stone canal is internal and can be found near the calcareous ring around the esophagus.

 With the exception of feather stars and many sea cucumbers, echinoderms have their mouth facing down onto the substrate. Feather stars have evolved from the more ancient sea lilies that are attached to the sea bottom with a stalk. In both these forms the mouth faces up and the animal uses its feathery arms to sieve small food particles from the passing currents. The oral surface refers to the side where the mouth is, the aboral side is the opposite.  Sea cucumbers usually have their mouth at the anterior end of the body, sometimes oriented downward but in armoured sea cucumbers the feeding tentacles stick up. Ecologically, echinoderms occur in all trophic levels from top predators that attack other sea stars down to detritus and mud feeders.

Echinoderms do not have a brain and yet they are capable of co-coordinated movements such as crinoids swimming, sea stars excavating clams, or sea urchins fleeing from a predatory sea star. All these behaviours are accomplished by not much more than a simple nerve ring around the mouth region and radial nerves along each ambulacral groove. Sense organs are also fairly simple, such as an eyespot at the tip of a sea star arm or a modified spine (sphaeridium) in sea urchins for detecting the animal's orientation. Skin cells may also be sensitive to certain chemicals given off by prey or predators.

 The body cavity or coelom contains a fluid that acts like a blood system to transport nutrients and oxygen to other parts of the body. A haemal or blood system does exist but the vessels are thin-walled and generally not capable of pumping or driving fluids through them. In some sea cucumbers a network of vessels surrounds the intestine and connects to the haemal system, presumably playing a role in transferring nutrients. Wandering amoeboid cells serve to defend against invaders, heal wounds and remove wastes from the coelomic fluid.

 Most echinoderms have a digestive tract with a mouth at one end and anus at the other but there are many variations. The gut may have an enlarged area or stomach near the beginning, such as seastars with two stomachs, or a series of coils before reaching the anus, like sea urchins and sea cucumbers. Only the asteroids have digestive glands in each arm that store nutrients from the digested food. The intestine and anus are reduced in most sea stars, and in brittle stars the anus is absent. Indigestible particles are regurgitated. The mouths of sea stars and brittle stars are usually surrounded by tooth-like spines, but he mouth of a sea cucumber has a circle of feeding tentacles to catch food and push it into the mouth. Sea urchins possess a complex arrangement of five teeth operated by levers and muscles to ingest their diet of sea weeds.

 In most echinoderms the gonads are situated radially and associated with each arm. In sea cucumbers the gonad is a single or double tuft of tubules attached near the anterior dorsal part of the gut. With a few exceptions echinoderms have separate sexes but the sexes are not distinguishable externally except by subtle signs like the shape of the gonopore. Most animals broadcast their gametes into the water and fertilization takes place by chance. But most families of echinoderms have a small percentage of brooders, often in higher latitudes, that protect a small number of large yolky eggs. When the embryos reach the crawling stage they move away from the parent and settle nearby, often creating large aggregations covering the substrate, such as some species of sea cucumbers.

Read Phil's books on Sea Stars and Sea Cucumbers of British Columbia for more information on this exciting groups of animals

 

Key References

The Sunflower Star--Royal BC Museum (PDF)

Introduction to Sea Cucumbers--Royal BC Museum

Marine Medicines--originally reprinted from Discovery 22 (3)

Psolidium bidiscum, a new shallow water, psolid sea cucumber from the northeast Pacific

Links

Sea Stars of Alaska and the Aleutian Islands

Echinoderm Morphology 

About Sea Urchins--the Echinoid Directory

About Feather Stars--Charles Messing's Crinoid pages

About Sea Cucumbers - Holothuroidea

About Sea Cucumber Fisheries

Sidney BC Coastal Stewardship Program