Phylum:  Coelenterata (Also called Cnidaria) Introduction:

 

Coelenterates are the animal phylum that includes the coral, hydra, jellyfish, Portuguese man-of-war, and sea anemone.

 

Coelenterates rank in complexity above the sponges among the Metazoa, or many-celled animals.

 

They comprise more than 9000 species, distributed in all oceans. Only a few species (e..g. Hydra) are known to inhabit fresh water.

 

Two forms of coelenterates exist, the polyp and the medusa, both of which may develop alternately during the life cycle.

 

The phylum is divided into three classes:

 

Hydrozoa  in which the polyp form predominates; examples are Hydra and Obelia.

 

Anthozoa consisting solely of polyp forms; example all sea anemones

 

Scyphozoa,  composed primarily of medusa forms; example jelly fish

 

The polyp form, such as the reef-building coral and Obelia, may occur in colonies.

 

It has a plantlike appearance and attaches itself to rocks or debris on the sea bottom.

 

The polyp form is cylindrical, with the mouth and surrounding tentacles at one end.

 

By contrast, with rare exceptions, the medusae swim freely.

The medusa or jellyfish form has an umbrellalike surface, from which the tentacles project, with the mouth at the center of the body.

 

All coelenterates are more or less radially symmetrical, divisible into four or six vertically similar segments.

 

The cells of coelenterates are organized into tissues, some of which differentiate into organs.

 

A layer of indistinct tissue (mesoglea) lies between the internal lining and the outer cell layers.

 

This tissue may be thin and firm or thick and gelatinous, and it contains nerve, muscle, skeletal, and pigment cells.

 

Coelenterates do not have an anus or  a separate circulatory system, but they do have a gastrovascular cavity.

 

The mouth (sometimes called vent)  leads into coelenteron (which, despite the name is not confined to this phylum).

 

Networks of differentiated nervous tissue are present, as are muscle fibrils and simple sensory organs.

 

These organisms have stinging cells, called cnidocytes (the stingers are called nematocysts and the two terms are used interchangeably although there is a technical difference), that are characteristic of the phylum and from which its alternate name, Cnidaria, is derived.

 

Prey captured by the tentacles is killed by venom ejected by the stinging organs.

 

It is digested in the coelenteron by secretions of the cells lining that canal, which, through branches, conveys the nutrients to all parts of the body.

 

Because of the lack of an anus, waste matter from the digestive system is discharged through the mouth (vent) opening.

 

Coelenterates absorb the oxygen required for metabolic processes from their liquid environment.

 

They move by contracting their muscle fibrils.

 

Their sensory organs respond to light, heat, and mechanical, chemical, and gravitational stimuli.

 

They reproduce by both fission and sexual reproduction.

Class Hydrozoa: 

 

Hydra is a common example.            Hydra are small, aquatic, free-living animals characterized by a simple, two-layered, cylindrical body surrounded at one end by tentacles.

 

Most hydra are about 0.1 to 1 in.  long.

 

They are among the simplest in structure of all multicellular animals and are often studied in freshman biology courses.

 

A hydra is a hollow animal, closed at one end, which is known as the foot or base, and opening at the other into the mouth.

 

The six to ten tentacles that surround the mouth are used for capturing food.

 

Locomotion - It moves either by gliding on the foot, by somersaulting, or by secreting a             gas bubble at the base and floating off..

 

 

The body of a hydra is a simple polyp, made up of two layers of cells: an outside layer, or ectoderm, and an inside layer, or endoderm, separated by a thin layer of secreted jelly (mesoglea).

 

Both layers of the hydra's cells contain contractile fibers, somewhat comparable to the muscles in higher animals, which relax to allow the animal to expand and are used in locomotion.

 

Interstitial cells, scattered cells found among the ectodermal and endodermal cells, give rise to a network of nerve threads running over the entire body and to several testes and ovaries on each animal.

 

Each ovary contains a single egg, and each testis contains several sperm.

 

The eggs develop within the body wall of the parent hydra.

 

The embryos then rupture through the body wall and grow, like buds on a plant, into full-            sized adults still attached to the parent.

 

Several such buds may be growing from a hydra at one time.

 

Eventually the buds leave the parent and become independent, attaching themselves by their sticky feet to a floating leaf or to a twig.

 

To capture minute forms of life and to defend itself from larger animals, the hydra is equipped with poison-containing structures in the ectodermal layer.

 

These are called stinging cells, or nematocysts.

 

Small animals paralyzed by the stinging cells are brought into the mouth by the tentacles and then transported to the body cavity.

 

There they are either engulfed by the pseudopodia of amoebalike cells and digested inside the cells or decomposed in a funguslike manner by secretions from all the endodermal cells.

 

Hydras are remarkable for their powers of regeneration.

 

When a hydra is cut into fairly large pieces, each piece develops into a complete individual.

 

Small pieces of hydra, when placed in contact with each other, grow together to form a complete individual.

Two species of hydra are common in ponds of the United States: a brownish-gray species and a green species.

 

The green color of the latter is due to an alga that lives in symbiosis within the body cells of the hydra. 

 

The alga is passed on to the hydra's young after entering the sex cells.

 

 

Obelia is a colonial hydrozoan:

 

One of the most interesting aspects about Obelia is the fact that they have two life phases.

 

There is a medusan and a hydroid stage.

 

The medusa is free swimming and reproduces sexually. Their eggs grow to form ciliated larvae that attach themself to a surface.

 

Then the hydroid stage starts to grow into a colony of polyps. These polyps multiply asexually, forming buds.The medusa of Obelia:  

 

At the center is the manubrium, it's feeding opening.

 

Around it lie four gonads, male or female reproductive organs.

 

The tentacles are based on a muscular band called the velum. This is the swimming organ.

 

At the base of the tentacles little vesicles can be found, the statoliths ,which serve as organs for equilibration

Obelia differs from true jellyfish in size. Obelia medusa grows not much larger than a few millimeters.

 

The tentacles of Obelia appear only on the feeding heads (hydranths) and are housed in transparent cups (called hydrotheca) for protection. When touched the tentacles will withdraw and hide in the cups.After the colony of polyps is fully grown, reproductive polyps are formed. These bud off little medusae which swim away.The two kinds of polyps, feeding (hydranth) and reproductive (gonangium), bud but fail to separate from an upright, anchored stalk, remaining instead as branchlike structures.  

 

The individual polyps are all connected by the branching stalk.

 

You can see under the microscope how they share their food via a fluid running through the stalk.

 

The living inside of the stalk contains cilia that make a current so that the fluid can move in two directions

 

Each one, called a subindividual because its actions are governed by the entire colony's, has a transparent, horny covering into which it can withdraw.

 

The two types differ otherwise in structure.

 

Only feeding polyps have tentacles with which to draw food towards them, but because both are hollow, partially digested food can travel through the colony's continuous digestive cavity to nourish the reproductive polyps.

 

The medusa, or sexual stage of the organism, emerges from the opening of the reproductive polyp and swims away as saucer-shaped mass of jelly.

 

Portuguese Man-of-War            Portuguese Man-of-War is the common name for a species of free-living coelenterate with a brightly colored inflated bladder and very long stinging tentacles.

 

It is found floating in tropical seas and is assisted in its passive wanderings by a crest or sail on top of the bladder.

 

Actually a colony of specialized cells, the Portuguese man-of-war may reach a length of 6 in. and trail tentacles that may be more than about 100 ft. long.

 

These tentacles are armed with thousands of explosive poison-secreting cells called nematocysts.

 

When discharged, the nematocysts disable the small fish and other animals that are the Portuguese man-of-war's prey.

 

The toxin secreted by the cells is similar to the venoms of other animals in that it appears to be a mixture of enzymes rather than a single substance.

 

The mixture is a neurotoxin about 75 percent as powerful as cobra venom.

 

Nematocysts remain active and may sting even when the Portuguese man-of-war is dying or dead.

 

Large groups of these colonial animals drift together and are commonly stranded on beaches.

 

They are not to be confused with jellyfishes, which belong to a different class of this same phylum.

 

Class Scyphozoa - the true jellyfish (May be 99% water)

 

True jellyfish are graceful, and sometimes deadly creatures.

 

Their stings may cause skin rashes, muscle cramps, or even death.

 

Jellyfish range in size from a mere twelve millimeters to more than two yards across.

 

The largest is Cyanea arctica, which may have tentacles over 40 yards long!

 

Despite their often enormous size, jellyfish have no head, no skeleton, and no special organs for respiration or excretion.

 

They are umbrella-shaped with horseshoe shaped gonads and tentacles with stinging cells. 

 

The oral arms extend from around the mouth.

 

Their life cycle involves an alternation between a free-swimming medusa stage and a sessile polyp phase. 

 

Jellyfish reproduction typically is by alternation of generations.

 

The medusa is the sexual stage. 

 

There are separate sexes, male and female, though these are not easily distinguished by sight.

 

Reproduction begins when the male releases sperm through its mouth into the surrounding water.

These swim to the female where they enter her central oral cavity to reach the eggs.

 

Once fertilized, the zygotes emerge onto the oral arms to develop for a time, becoming planula larvae which swim about for a time and then settle on the bottom of the ocean.

 

The resulting polyp, called a scyphistoma, superficially resembles a Hydra. 

 

After feeding for a time, it develops into a strobilus and begins to bud asexually, releasing free-swimming medusae which go on to develop into adults which reproduce sexually.

 

The life cycle then repeats, alternating between the sexual and asexual stages.

 

 Jellyfish are not very important as a food source, though they are eaten in some countries.

 

While many species live solitary lives, some like Aurelia may travel in shoals of hundreds to thousands of individuals stretching for dozens of miles.

 

These mass accumulations can cause fishing problems, clogging nets and making them difficult to clean.

 

At times, large shoals are washed ashore by storms; these beached jellyfish make the shore unsafe for humans, and their decomposition creates a bad odor.

 

Some stinging jellyfish may injure swimmers -- causing fever and cramping, or even death.

 

Even the broken tentacles or the bodies of beached jellyfish can be dangerous, though children in some middle eastern countries toss handfuls of less dangerous species at each other until they become numb all over.

 

Some juvenile fish, which are immune to the stings, travel with the jellyfish for easy food or for protection.

 

Class Anthozoa - corals and sea anemones

 

Coral   Coral is the common name for members of a large class of marine invertebrates characterized by a protective calcium carbonate or horny skeleton.

 

This protective skeleton is also called coral.

 

True corals secrete calcium carbonate from the individual animal, or polyp, forming skeletal cups to which the polyps are anchored and into which they withdraw for protection.

 

In the flattened oral disc at the top of the stalk is an opening, edged with feathery tentacles and cilia, that is both mouth and anus.

 

At night the tentacles extend from the cup, seize animal plankton that wash against them, and carry the food to the mouth.

 

Stinging cells, or nematocysts, on the tentacles can also paralyze prey.

The majority are colonial.

 

Colonial polyps average from 1 to 3 mm (0.04 to 0.12 in) in diameter.

 

They are connected laterally by tubes that are an extension of the polyps' gastrovascular cavities, and the colony grows by asexual budding from the base or the oral disc of the polyps.

 

Living polyps build on the deposits of their predecessors. 

 

The wide range of branched or massive forms that result depends on the species

involved.

Colonial corals can grow in deep water, but reef-building corals are found only in warm, shallow seas.

 

They live no deeper than light can penetrate because the symbiotic algae that live in their tissues require light for photosynthesis, and the corals cannot exist without the algae.

 

Carbon is passed by the algae to the coral, increasing its energy. 

 

The food caught by the coral may supply nitrogen and phosphorus for both                                     organisms.

 

The dependence of the corals on the algae probably varies according to species and locality.

Coral Reefs:            A coral reef is a ridge or elevated part of a relatively shallow area of the sea floor, approaching the sea's surface.

 

It is formed by a rocklike accumulation of calcareous (calcium-containing) exoskeletons of coral animals, calcareous red algae, and mollusks.

 

Built up layer by layer by living corals growing on top of the skeletons of past generations, coral reefs grow upward at rates of 1 to 20 cm (0.4 to 7.8 in) per year.

 

Coral reefs are tropical, extending to about 30E° north and south of the equator and forming only where surface waters are never cooler than 68E° F.

Coral reefs are ecosystems with well-defined structures that involve both photosynthetic algae and consumers).

 

The outer layer of a reef consists of living polyps of coral.

 

Within the coral animals live single-celled, round algae called zooxanthellae.

 

Below and surrounding the polyps is a calcareous skeleton, both living and dead, that contains filamentous green algae.

 

Other species of algae, both fleshy and calcareous, grow in the surface of old skeletal deposits.

 

These algae make up most of the primary producers.

The algae transfer some food energy directly to the coral polyps.

 

Coral animals also feed at night on zooplankton, which they capture with their tentacles.

 

Coral animals prey on zooplankton not so much for the calories but for scarce nutrients, especially phosphorus.

 

Through digestion, coral animals release these nutrients to the algae.

 

Coral and algae then apparently cycle these nutrients between them, reducing nutrient loss to the water.

Herbivorous fish, such as the colorful butterfly fish, as well as sea urchins, sea cucumbers, brittle stars, and numerous species of mollusks, feed on algae.

 

Hiding in the numerous caves and crevices of a reef are predatory animals such as small crabs, wrasses (long, spiny-finned fishes), moray eels, and sharks.

 

The numerous microhabitats and the productivity of the reefs support a great diversity of marine life.

KINDS OF REEFS Coral reefs are of three types: fringing reef, barrier reef, and atoll.

 

Fringing reefs extend outward from the shore of an island or mainland, with no body of water between reef and land.

 

Barrier reefs occur farther offshore, with a channel or lagoon between reef and shore.

 

Atolls are coral islands, typically consisting of a narrow, horseshoe-shaped reef with a shallow lagoon.

CONSERVATION ISSUES:  

 

In many parts of the world, the health of coral reefs has been declining over the past several decades.

 

Pollution and destructive fishing practices damage the delicate corals.

 

Nutrients in runoff from agricultural areas cause large algae blooms that smother coral.

 

Scientists are also finding heavy damage from more than a dozen different coral diseases, some of which were unknown until recently.

Coral reefs have also been affected by bleaching, that is, the discoloration or loss of symbiotic algae.

 

In 1979 and 1980, several incidents of coral bleaching occurred at reefs around Okinawa, Easter Island, northeast Australia, and the Caribbean Sea.

 

Outbreaks of bleaching also occurred in 1982 and 1983, including reefs off east Africa, Indonesia, and the west coast of Central and South America, and from 1986 to 1988 in areas such as Taiwan, Hawaii, Fiji, Mayotte Island, and the entire length of the Great Barrier Reef.

 

The most extensive bleaching episode ever documented occurred in 1998 and affected reefs in the Indian Ocean, Pacific Ocean, Red Sea, Persian Gulf, and Caribbean Sea.

 

In some areas 100 percent of the corals were bleached and more than 70 percent of the corals died.

The cause of these widespread bleaching incidents is unknown; pollution, global warming, and ultraviolet radiation have been suggested as suspects.

 

Although it has not been conclusively shown that any or all of these are the cause of these coral bleaching episodes, recent research indicates that the cause may be unusually warm waters.

 

The optimum temperature for coral growth is between 78.8E° F and 80.6E°).

 

Temperatures above 84.2E° F have been shown to cause stress in corals, and may boost the rate of photosynthesis by the symbiotic algae, creating high concentrations of free-radical toxins in the coral tissue.

 

These stressed coral polyps may actively expel their algae, causing the coral to appear bleached.

Bleached corals have difficulty recovering; a reef can take years to recover, and subsequent bleaching incidents may make it impossible.

 

Without their symbiotic algae, corals are unable to deposit the calcium carbonate skeleton that makes up the foundation of a coral reef.

 

Not only corals, but all reef organisms could potentially lose their habitat because of bleaching incidents, as the calcium carbonate structure of the reef erodes away.

Sea Anemones -            Sea Anemone is the common name for marine, flowerlike polyp having a cylindrical, or vaselike, body.

 

Many species are colored; large specimens may attain a diameter of 3 ft.

 

The body is closed and attached to rocks or coral at one end and, at the other end,                         has a central mouth surrounded by tentacles armed with nematocysts (stinging cells and thread cells that paralyze and entangle the small fish and marine animals that constitute its prey).

 

Locomotion: they can creep on their basal disc or when frightened can swim with surprising speed for short distances. 

 

The slitlike mouth opens into a short esophagus opening into the body cavity.

 

At each end of the mouth, a permanent pore opens into a ciliated groove, called a siphonoglyph, in the side of the gullet, through which a continuous current of water flows, carrying oxygen to the tissues and removing waste matter.

 

The body cavity is divided into a number of sacs by septa extending from the body wall.

 

These septa increase the surface available for the secretion of digestive juices and the absorption of nourishment, and they contain the gonads that produce the sperm and eggs.

Most sea anemones reproduce sexually; budding and fission occur, but are comparatively rare.

 

The eggs are usually fertilized in the gastrovascular cavity. 

 

The young are discharged from the mouth as free-swimming larvae, which soon attach themselves to surfaces.

 

The larvae develop into adult anemones. 

 

Hermit crabs sometimes attach sea anemones to their shells.

 

Some anemones become completely parasitic on certain species of jellyfish.