PHYLUM: ECHINODERMATA - THE SPINY-SKINNED ANIMALS
Introduction:
Echinoderm includes about 6000 living species, such as starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers.
They usually show a superficial five-part radial symmetry, and generally are equipped with peculiar tube feet (discussed below).
The phylum name is derived from the spiny skinBechinos = spiny and derma = skin..
Some characteristics of the Phylum:
Calcareous endoskeleton composed of separate plates or ossicles
Five-part symmetry
Water vascular system ("tube feet")
Digestive tract is complete with a mouth and anus.
Decentralized nervous system
Marine only; echinoderms are common on the ocean bottom at all depths; in the deep sea they often make up the bulk of living material.
General Structure of echinoderms
An echinoderm such as the starfish typically has a mouth surrounded by five arms that bear minute, fleshy tube feet with which the animal clings and crawls.
The tube feet are supported by a complex internal hydraulic system that is inflated with seawater.
Some echinoderms, especially brittle stars, crawl or swim by moving their arms.
Sometimes, as in sea urchins, rows of tube feet extend along the body surface, and arms are absent.
The spines are particularly well developed in sea urchins.
The skeleton, made up of calcium carbonate, may form a large proportion of the body, or, as in some sea cucumbers, it may be quite small.
Deviations from the five-rayed pattern are common as shown in the fossil record and still found today, with some having up to 50 rays..
The animals have a well-developed gut, but the nervous and circulatory systems are simple.
Echinoderms are slow-moving and without complex behavior patterns.
Classification:
In traditional taxonomy, there are five classes of living echinoderms:
Asteroidea (starfish),
Crinoidea (sea lilies),
Ophiuroidea (brittle stars or snake stars),
Echinoidea (sea urchins and sand dollars), and
Holothuroidea (sea cucumbers).
Class Asteroidea - the starfish
Starfish may well be the most unusual well-known creature.
They have no front or back: they can move in any direction without turning.
Often brightly colored & range in size from a centimeter to a meter in diameter.
They do have an oral and aboral surface with very different features.
Both surfaces are covered with a ciliated, pigmented epidermis.
A starfish is a marine animal with thick arm-like extensions called rays protruding from its body.
A starfish can have up to 50 rays, but never fewer than five.
Each of the arms has an eyespot. This eyespot doesn't see images, but senses light instead.
Pedicellaria are small, snapper-like skeletal elements that are used by the echinoderm to keep small organisms from settling on its body & aid in food capture. The tiny jaws are manipulated by muscles.
Dermal branchiae are Agills@ which enable the organism to take in oxygen from the water.
Are soft, delicate projections of the coelomic cavity.
Covered with epidermis & lined with peritoneum from the coelomic cavity.
Extend out through spaces between the ossicles.
Starfish has a nerve net:
The starfish has no central nervous system (no brain or spinal cord).
The starfish's movements and behaviors are controlled by a nerve net: a system of functionally connected nerve cells and fibers spread throughout most of the animal.
More than one nerve net can be present- each responding to different kinds of stimuli and causing different forms of response.
For example, a large radial nerve runs down the center of each ambulacral groove.
Nerve nets, having no centralized system, are efficient in terms of resistance to injuries and the production of local response.
The behavior of starfish is such that a nerve net is sufficient to control movements (i.e., it doesn't take a big brain to move a starfish).
All starfish bodies include a round, disk-like body called the central disk.
Ambulacral grooves run down the undersurface of the rays & contain the tube feet.
The mouth is located on the underside of the body, and leads to a bag-like stomach made of two parts, the lower cardiac and upper pylorus.
The water-vascular system:
Rather than using muscles to move their hundreds of tiny tube feet, starfish use a complex hydraulic system to move around or cling to rocks.
The intake valve (a sieve) for this system is generally located on the top of the starfish, just off center.
Parts: Madreporite, stone canal, ring canal, radial canals, lateral canals (valves prevent backflow of water), tube foot with its associated ampulla (has circular muscles).
This hydraulic system extends or contracts the tube feet.
Tube feet have muscles which shorten to pull the starfish along.
Starfish also have a spacious coelom.
It is an open, fluid-filled body cavity lined with tissue) which contains amoebocytes.
Amoebocytes pick up wastes & migrate to the outside thru the dermal branchiae or tube feet.
Tips of dermal branchiae may be filled with them and be pinched off.
Starfish sexual reproduction:
Male & female have large gonads, but are indistinguishable externally and internally.
Starfish are dioecious and shed their eggs and sperm freely into the water, so fertilization is externally.
The very small chance of fertilization is compensated by the enormous amounts of eggs and sperm cells.
A female starfish sheds in two hours several millions of eggs into the water, with a mean diameter of 0.16-0.19 mm .
After fertilization, a hollow ball develops, called the blastula. The cells of the blastula possess cilia on the outside for swimming.
After one day a deep groove develops, leading to the gastrula.
Then differentiation starts.
The common starfish develops a so-called bipinnaria larva, with ciliated bands running about the periphery, a beautiful sight.
After several weeks the bipinnaria larva takes on a more elaborate form, with longer projecting arms and after some more weeks, a brachiolaria larva is formed.
The larvae have their own gut, with inside cilia to inhale and transport food particles.
They feed themselves with diatoms and other organisms in the plankton.
The stomach is large and round and situated at the back side.
After this phase a large part of the larva degenerates and at the rear side a rudimentary formed juvenile starfish develops. The organs of the young starfish are formed anew.
Habitat
Starfish are found in shallow waters in the Indian, Pacific, and Atlantic Oceans.
They live in kelp forests, sandy ocean bottoms, rocky coast lines, tide pools, and coral reefs.
Diet
Starfish eat shelled animals, like mussels, oysters, and sea urchins.
Once the starfish locates its bivalve prey it will attach itself to the shell and pry it open.
Exerts a steady pull, using its tube feet in relays.
In a half hour or so, the muscles of the bivalve tire and the shell gapes open slightly.
Once the shell is partly open, the starfish will turn its stomach inside out and put it inside the shell.
It then injects the animal with a digestive juice that kills it.
Digestive juice comes from the pyloric cecae in each ray.
The stomach will then proceed in digesting the animal inside the shell.
When the animal is digested, the stomach returns to the inside of the body.
A short intestine leads from the stomach to the anus.
Starfish are pests, as when found in oyster beds.
Other starfish food:
Will eat other echinoderms such as brittle stars, sea urchins or sand dollars.
Swallow them whole and later regurgitate indigestible parts.
May eat other starfish; if they are larger will start eating at one end of a ray.
Predators
Starfish are preyed on by sharks, large bony fish and rays.
Large starfish will occasionally eat smaller starfish, too.
To escape predators, they can drop arms off.
The removed arm will soon be replaced by a new one (called regeneration).
Even if the starfish is cut in half, another half replaces the missing one, creating two identical starfish as long as there is a portion (at least 1/5) of the central disk present. (How does this contrast with regeneration in sponges?)
Oyster fishermen used to cut starfish into half with a hatchet. What did this accomplish?
Autotomy in starfish:
Ability to cast off an injured ray near the base.
May take months to regenerate a new one.
Class: Crinoidea (sea lilies)
Crinoids are neither abundant or familiar organisms today.
However, they dominated the Paleozoic fossil record of echinoderms and shallow marine habitats until they suffered a near complete extinction.
Many Paleozoic limestones are made up largely of crinoids.
Living sea lilies inhabit the deep sea and are so seldom seen.
Class: Ophiuroidea (brittle stars)
The more familiar brittle stars, usually have five arms and superficially resemble true starfish (Asteroidea).
However, brittle stars have long, flexible arms (hence the other common name for ophiuroids, "snake stars" and a central, armored, disk-shaped body that is clearly demarcated from the arms.
Instead of crawling on hundreds of tube feet like starfish, brittle stars move fairly rapidly by wriggling their arms.
These agile arms are supported by an internal skeleton of calcium carbonate plates that superficially look like vertebrae, and that are in fact called vertebral ossicles.
These are moved by a system of muscles and linked together by ball-and-socket joints.
The body and arms are also protected by calcium carbonate plates, and the arms generally bear delicate spines.
Class: Echinoidea (sea urchins and sand dollars)
Echinoids are one of the more diverse and successful echinoderm groups today, including familiar echinoderms such as the sea urchins and sand dollars.
The roe (egg mass) of some species, notably certain sea urchins, is eaten in some cultures, notably in Japanese sushi; as a result, certain echinoid species are commercially fished.
Sea cucumbers are also eaten by some humans.
In echinoids, the skeleton is almost always made up of tightly interlocking plates that form a rigid structure or test -- in contrast with the more flexible skeletal arrangements of starfish, brittle stars, and sea cucumbers.
Test shapes range from nearly globular, as in some sea urchins, to highly flattened, as in sand dollars.
Living echinoids are covered with spines, which are movable and anchored in sockets in the test.
These spines may be long and prominent and used for defense against potential predators, as in typical sea urchins.
In sand dollars, however, the spines are very short and form an almost felt-like covering.
The mouth of most echinoids is provided with five hard teeth arranged in a circlet, forming an apparatus known as Aristotle's lantern.
Because most echinoids have rigid tests, their ability to fossilize is greater than that of more delicate echinoderms such as starfish, and they are common fossils in many deposits.
Sea urchins:
You could call sea urchins the porcupines of the sea.
Like a porcupine's quills, sea urchins count on their long spines to deter hungry predators from making them a snack.
In fact, the sea urchin gets its name from an Old English word for the spiny hedgehog, a land animal similar to the American porcupine.
You can pick up most sea urchins without getting harmed--except for the long-spined sea urchin found in south Florida, whose poisonous sharp spines can penetrate human skin and break off.
In cooler waters up north, more harmless varieties like the purple and green sea urchins predominate.
Although difficult to see it through all the spines, sea urchins also have a hard outer body like that of its relatives.
Their outer skeleton--called a test--is made up of ten fused plates that encircle the sea urchin like the slices of an orange.
Every other section has holes through which the sea urchin can extend its tube feet.
These feet are controlled by a water vascular system. By changing the amount of water inside, the animal can extend or contract the feet to move about.
Mainly sea urchins use their feet to hang on to the bottom while feeding, but they can move fast, walking on their feet, their spines, or even their teeth.
When a sea urchin dies, all its spines fall off, leaving only the test.
If you look carefully at a test, you can see tiny bumps covering it where the spines were once connected.
The base of the spines once fit over the bump like a snug-fitting cap.
The spines can rotate extensively around this bump.
In a live sea urchin, skin and muscle cover the test and can be pulled on to move the spines.
Sea urchins eat using a structure called Aristotle's lantern. It is made up of five hard plates that come together like a beak.
They use their beak-like mouth to scrape rocks clean of algae.
This scraping can wear down the plates--so sea urchin teeth grow to replace worn-down ones.
Their mouth is located on the underside of their body, while any wastes are excreted through the anus at the top of the animal.
Sea urchins mainly congregate in colder, offshore waters, but sometimes travel into shallower waters looking for food.
Sand dollar is the common name for a marine animal in the same phylum as the starfish .
The sand dollar has a rigid, flattened, disk-shaped test, or shell, made of firmly united plates lying just beneath the thin skin.
Small spines that densely cover the test enable the animal to burrow in sand just below the surface.
Like other members of its class, the sand dollar is radially symmetrical.
Tube feet are similar to those in other echinoderms and are used for locomotion and to convey small food particles, mostly organic matter found in sand, to the mouth.
Gills on the upper surface are used for respiration.
Sand dollars differ from the closely related sea urchins by their shorter spines and more flattened shape.
More convex, short-spined sand dollars are called sea biscuits.
The sand dollar lives slightly buried in the sand in shallow coastal waters.
Its thin, circular body is about 2 to 4 inches wide.
Its dried skeleton looks like a large, white coin while a living sand dollar resembles a fuzzy brown cookie.
The animal eats tiny aquatic organisms that it finds among the sand grains or traps from the surrounding waters.
A sand dollar releases eggs from small openings near the center of the top of the body.
The eggs develop into free-swimming larvae which eventually sink to the ocean floor and grow into the adult form.
Sand dollars are abundant on the sandy bottom of deeper waters on both the Atlantic and Pacific coasts.
They live in large numbers in many parts of the world and are frequently tossed ashore after storms.
Class: Holothuroidea (sea cucumbers).
A casual passerby might not realize that a holothurian, or sea cucumber, is an echinoderm.
Sea cucumbers are generally long and wormlike; they don't look much like starfish or sea lilies.
However, sea cucumbers have five-rayed symmetry, with five rows of tube feet running from the mouth along the body
Sea cucumbers have the skeleton of echinoderms, but in most species the skeletal plates are mere microscopic spicules, often shaped like wheels, bars, or anchors.
Sea cucumbers are common in the oceans of today; in fact, in certain parts of the deep sea, the ocean floor swarms with vast herds of sea cucumbers.
Sea cucumbers are also common in shallow-water habitats such as tidepools.
The spicules of sea cucumbers are found as fossils, but they have not received much study compared to other types of microfossils.
Body fossils of whole sea cucumbers are rare.