CHARACTERISTICS AND CLASSIFICATION OF THE PHYLUM ECHINODERMATA
1. Five fold symmetry: arms or rays occurs in multiple
2. Advanced Biological systems:
--Digestive system including mouth, alimentary canal (intestine), and anus
--Water vascular system: hydralic system which driven by water and muscular contraction serve to motion
tube feet. These tube feet, small structures with sucker-like endings, provide for locomotion
and or feeding.
-- Reproductive system
-- Nervous system
-- Skeleton made of CaCO3 which is developed as discrete plates or segments, each comprising a
single crystal of calcite. This monocrystalline structure is particularly illustrated in the structure of
spines of the urchins. Here, spines easily cleave along crystal faces after penetrating the soft fleshy
A. Class Crinoidea -- Sea Lillies
B. Class Asteroidea -- Starfish
C. Class Holothuroidea -- Sea Cucumbers
D. Class Ophuroidea -- Brittle Stars
E. Class Echinoidea -- Sea Urchins
Diagram of the basic anatomy of a "regular" sea urchin. Note the tube feet that are operated by the water vascular system that refills through the madreporite (sieve plate), the digestive system (mouth...intestine....anus), and complex feeding structure "lantern" comprising five regenerative teeth.
Role of Echinoderms on Reef:
Amongst this group of animals, the starfish and urchins are generally the most destructive. For example the feeding system of the common "regular urchin" (these are lantern shaped with the mouth on the base and anus positioned on the upper part of the structure. The mouth of the urchins comprises a structure termed Aristole's lantern. This structure house a set of five regenerative teeth (they are progressively replaced as they wear during normal use) which are manipulated by a set of strong muscles. Urchins use these teeth to scrape and nibble at the reef (generally dead coral skeleton) in search of minute particles of organic sustenance. The effectiveness of this feeding structure is evidenced by the volume of material that passes through their digestive systems in a single day. Scientists have measured quantities of reef particles gouged from the reef skeleton on the order of 2 tablespoons per day (approx. 5 grams). When on considers the number of urchins that typically occupy the reef, the volume of reef that is destroyed (or recycled) by urchins is a phenomenal quantity of material. Despite such destructive effects, most urchins only feed upon dead areas of the reef unless their population size expands abnormally above the sustainable limits of the reef. A common reflection of the presence of urchin is a cavernous, sculpted surface to the reef as these often create small depression or cavities through their incessant bioerosion of the coral skeleton.
Figure on right shows the five teeth of a "regular" urchin. The
development of spins on these urchins provides for defense, motion,
and the ability to wedge themselves into crevices of the reef.
A typical black urchin "Diadema" grazing the surface of the reef for particles of algae commonly produces deep cavities and depression on the reef surface.
Crinoids : Feather Stars
Perhaps one of the most fascinating of the Echinoderms is the Feather
Stars, or the Crinoids. These are filter feeding animals that use
their multiple arms to trap suspended food particles and direct them toward
the mouth. In this case, the mouth is centered at the base of the
arms. Note also, that in these examples, the crinoid is able to attach
itself to the surface of the reef with small grasping appendages called
Cirri, which allow the crinoid to temporarily hold fast on the surface
In contrast to urchins, the starfish can active
prey upon living coral. For example, Acanthaster....." the crown
of thorns" has nearly devastated large areas of the Pacific reefs.
These starfish feed by extending their stomachs over the living coral tissues.
Secretion of gastral juices (and in the absence of TUMS), progressively
digests the living tissue which is then absorbed into the stomach walls
of the starfish. When the feast is completed, the starfish moves
on to the next area of living coral polyps. In the case of the Great
Barrier Reef of Australia, the density of these starfish predator was so
great that they covered as much as 50 percent of the surface of the reef.
Fortunately, a viral infection ultimately lead to a sharp decline in the
numbers of starfish preying upon the reefal systems. Though saved
for the short term, the imbalances in the ecosystem created by mankind
will likely manifest in yet another opportunistic expansion of this or
another species which may devaste the living reef community.
Examples of typical starfish illustrating the five-fold symmetry of
the arms. Also note the Sieve plate which serves to recharge the
water vascular system of this group. In the figure on the left, the
anal plate (with anal opening) is also visible.
Acanthaster or "Crown of Thorns" Starfish actively preying upon a staghorn coral. Such intense predation, as shown below, can lead to the total devastation of the living reef.
Dense Population of Acanthaser on the Great Barrier Reef, Australia
(From Reader's Digest, 1990)
Other Animals Comprising the Reef System: Annelids and Mollusca
These photos illustrate the "Christmas Tree Worm", passive member of
the reef community. In the photo to the left, these worms actively
extend their feeding structures to filter particulate matter from the passing
currents of water. If disturbed, they rapidly retreat into their
"worm tubes" which are passively constructed alongside the living coral
In general, Mollusca (snails and clams) are relatively passive components of the reef system. In the case of Snails (Class Gastropoda), feeding is effected through a rasping or scraping of the surface of coral reef. Yes, snails have teeth.... small scrapers made of CaPO4 (same material of which your teeth are made), and as in the case of the urchins, these are generally passive in their feeding on the reef. They graze upon the algae which colonize areas of dead coral or of loose debris derived from the the breakdown of the reef skeleton.
In general, most of the clams are passive filter feeders and are
passive members of the reef. However, on group of clams is perhaps
on of the more interesting organisms that live with the reef. These
are the boring "rock eating" clams. These are termed lithophaga
(rock eaters). Boring deep within the discarded skeletons of the coral
reef, these clams secrete a mild acid (in addition to mechanical abrasion)
to bore through the solid rock structure of the reef in search of minor
quanities of organic foodstuffs. Spending their entire life moving
progressively deeper and farther into the reef, their burrows or tunnels
are distinctive as they expand into conical structures reflecting the increasing
growth and size of the clam with age.