Station 3: Eumetazoa: Radiata - Phylum Cnidaria

Most adult cnidarians appear as either free-swimming medusae or sessile polyps, and many hydrozoans species are known to alternate between the two forms. 

Both are radially symmetrical, like a wheel and a tube respectively. Since these animals have no heads, their ends are described as "oral" (nearest the mouth) and "aboral" (furthest from the mouth). Most adult cnidarians appear as either free-swimming medusae or sessile polyps, and many hydrozoans species are known to alternate between the two forms.

Most have fringes of tentacles equipped with cnidocytes around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies of zooids that serve different purposes, such as defense, reproduction and catching prey. The mesoglea of polyps is usually thin and often soft, but that of medusae is usually thick and springy, so that it returns to its original shape after muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a sort of jet propulsion.

Cnidaria are diploblastic animals; in other words, they have two main cell layers, while more complex animals are triploblasts having three main layers. The two main cell layers of cnidarians form epithelia that are mostly one cell thick, and are attached to a fibrous basement membrane, which they secrete. They also secrete the jelly-like mesoglea that separates the layers. The layer that faces outwards, known as the ectoderm ("outside skin"), generally contains the following types of cells:

• Epitheliomuscular cells whose bodies form part of the epithelium but whose bases extend to form muscle fibers in parallel rows. 

• Cnidocytes, the harpoon-like "nettle cells" that give the phylum Cnidaria its name. These appear between or sometimes on top of the muscle cells.

• Nerve cells. Sensory cells appear between or sometimes on top of the muscle cells, and communicate via synapses (gaps across which chemical signals flow) with motor nerve cells, which lie mostly between the bases of the muscle cells. Some form a simple nerve net.

• Interstitial cells, which are unspecialized and can replace lost or damaged cells by transforming into the appropriate types. 

In addition to epitheliomuscular, nerve and interstitial cells, the inward-facing gastroderm ("stomach skin") contains gland cells that secrete digestive enzymes. In some species it also contains low concentrations of cnidocytes, which are used to subdue prey that is still struggling.

Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles when capturing a brine shrimp. Used cnidocytes have to be replaced, which takes about 48 hours. To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes: nearby sensory cells detect chemicals in the water, and their cilia respond to contact. This combination prevents them from firing at distant or non-living objects. 

Anthozoa is a class of marine invertebrates which includes the sea anemones, stony corals and soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of the adult is the polyp; this consists of a cylindrical column topped by a disc with a central mouth surrounded by tentacles. Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual. Colonies are strengthened by calcium carbonate and other materials and take various massive, plate-like, bushy or leafy forms.

Anthozoans are carnivores, catching prey with their tentacles.  Many species supplement their energy needs by making use of photosynthetic single-celled algae that live within their tissues.  These species live in shallow water and many are reef-builders. 

Unlike other members of this phylum, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs into the water. After fertilization, the planula larvae form part of the plankton. When fully developed, the larvae settle on the seabed and attach to the substrate, undergoing metamorphosis into polyps. Some anthozoans can also reproduce asexually through budding or by breaking in pieces. More than 16,000 species have been described.

Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their cube-shaped medusae.  Some species of box jellyfish produce extremely potent venom.  Stings from these and a few other species in the class are extremely painful and can be fatal to humans.

The medusa form of a box jellyfish has a squarish, box-like bell, from which its name is derived. From each of the four lower corners of this hangs a short pedalium or stalk which bears one or more long, slender, hollow tentacles. The rim of the bell is folded inwards to form a shelf known as a velarium which restricts the bell's aperture and creates a powerful jet when the bell pulsates. As a result, box jellyfish can move more rapidly than other jellyfish; speeds of up to 6 metres (20 ft) per minute have been recorded.

In the center of the underside of the bell is a mobile appendage called the manubrium which somewhat resembles an elephant's trunk. At its tip is the mouth. 

A fully grown box jellyfish can measure up to 20 cm (7.9 in) along each box side (or 30 cm (12 in) in diameter), and the tentacles can grow up to 3 m (9.8 ft) in length. Its weight can reach 2 kg (4.4 lb).  Each tentacle has about 500,000 cnidocytes, containing nematocysts.

Hydrozoa are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in fresh water. 

Most hydrozoan species include both a polypoid and a medusoid stage in their lifecycles, although a number of them have only one or the other. For example, Hydra has no medusoid stage.

The hydroid (polyp) form is usually colonial, with multiple polyps connected by tubelike hydrocauli. The hollow cavity in the middle of the polyp extends into the associated hydrocaulus, so that all the individuals of the colony are intimately connected. Where the hydrocaulus runs along the substrate, it form a horizontal root-like stolon that anchors the colony to the bottom

The medusae of hydrozoans are smaller than those of typical jellyfish, ranging from 0.5 to 6 cm (0.20 to 2.36 in) in diameter. Although most hydrozoans have a medusoid stage, this is not always free-living, and in many species, exists solely as a sexually reproducing bud on the surface of the hydroid colony.

Anatomy

To see the tissues of the body wall, use your compound microscope to look at a prepared slide of a longitudinal section of Hydra.  How many well-defined cell layers do you see in the body wall?  The outer epidermal cells (derived from the ectoderm) are tightly joined to one another.  These columnar cells have a basal-lateral extension, giving an isolated cell somewhat the appearance of an inverted letter T.  The basal extension, called a myoneme, contains contractile proteins, that can shorten the body or reduce its width. Widely scattered nerve cells (that you will not be able to identify) underlie the myonemes and coordinate their contractions. Underlying the epidermis is the mesoglea layer composed of hydrated collagen and polysaccharides linked to other proteins, creating a jelly-like consistency.  The mesoglea of Hydra is quite thin, but in jellyfish it makes up most of the body.  The gastrodermis includes a number of types of cells (derived from the endoderm) that form leak-proof seals with adjacent cells. Some secrete digestive enzymes into the gastrovascular cavity when food is present. 

Study the epidermis of the tentacles closely to see cnidocytes.  The threads that you saw earlier are, when untriggered, coiled up as a coiled filament in unique organelles, called nematocysts, that are not found in other animals.  A small projection from the nematocyst, called a cnidocil, is a trigger. When stimulated it causes the nematocyst vesicle to take on water, raising the internal pressure to the point that the coiled filament is rapidly forced out, uncoiling in the process.  If available, look at the demonstration slide of discharged cnidocytes.  Compared to the cell, how many times longer is the extended thread?  Prey caught on the tentacles are moved to the mouth and stuffed into the gastrovascular cavity. 

Digestions

Hydra, as all cnidarians, has a sac-like digestive system. This is in contrast to a tubular digestive system found in most animals. Tubular systems allow continuous processing of food with residues passing out the anus.  In a sac-like system, any nondigestible material must be regurgitated.  Note how the gastrovascular cavity extends into the tentacles from the body column.  As digestion proceeds, food molecules can pass into these spaces and nourish the surrounding cells.  Realize that you observed no specialized organs other than the gastrovascular cavity. How do you think that Hydra performs the important physiological functions of gas exchange, circulation, and excretion?  Hydra has a hydraulic skeleton. When the myonemes in the body wall contract, the fluid in the gastrovascular cavity is pressurized, stiffening the body column. By controlling which myonemes contract and which relax, different regions of the body (column or tentacles) can be extended or retracted as you observed with your live Hydra. 

Gas Exchange

Gas exchange in Hydra primarily occurs through the body surface, as it lacks specialized respiratory organs such as gills or lungs. 

The outer layer of Hydra's body is called the epidermis, which is in direct contact with the surrounding water. The epidermal cells of Hydra are thin and permeable to gases, allowing for the exchange of oxygen and carbon dioxide between the organism and its environment. Oxygen from the water diffuses across the epidermis into the cells of Hydra, while carbon dioxide produced by cellular respiration is released back into the water.

In addition to the body surface, Hydra can also perform gas exchange through the cells of the gastrovascular cavity. The gastrovascular cavity serves both as a digestive chamber and a means of distributing nutrients and gases throughout the body. As water is taken in through the mouth, it fills the gastrovascular cavity and comes into contact with the cells lining the cavity. Gas exchange can occur between these cells and the water, facilitating the uptake of oxygen and release of carbon dioxide.

Overall, Hydra relies on the thin and permeable nature of its body surface, as well as the cells lining its gastrovascular cavity, to carry out gas exchange with its surrounding environment.

Reproduction

Hydra, like most solitary polyps, can reproduce asexually by budding.  Buds form as an outgrowth of the parent's body wall that becomes a small replica of the parent.  It eventually breaks off and lives independently.  In colonial species such as corals, new individuals resulting from budding remain attached to the adjacent parent, so that a spreading mat of individuals develops.  All polyps asexually produced are genetically identical and represent a clone.  Sexual reproduction is often triggered by harsh environmental conditions. In species having only a polyp stage in their life cycles, gonads develop as clusters of gamete producing cells in the body wall. These temporary gonads are seen as swellings. Those near the base are ovaries and will produce eggs.  Those near the mouth will produce flagellated sperm.  Sperm swim to the eggs and fertilize them in position in the body wall. The resulting zygote develops into a free-swimming larval stage that leaves the parent.  Cilia on its surface cells allow it to feebly swim at the mercy of water currents.  After a few hours to a few days, the larva settles to develop into a new polyp.  

Cnidarians in the genus Obelia are small marine colonial animals that attach to seaweeds and pilings in shallow waters along the Atlantic and Pacific coasts of North America.  Obelia is studied because it has a life cycle with both polyp and medusa stages that are easily observed.  The polyp stage is a colony of several individuals and reproduces asexually. The medusa stage is solitary and reproduces sexually with separate male and female individuals. The medusa stages are often short lived compared to the polyps.

Get a prepared microscope slide of a colonial polyp stage of Obelia from the supply area.

Below is a look at a very unusual species found in a lot of places including the Gulf of Mexico along the Texas coast.  The Portuguese Man-of-War has been the cause of many a bad day at the beach. 

The Scyphozoa are an exclusively marine class of cnidarians, referred to as the true jellyfish (or "true jellies"). 

Most species of Scyphozoa have two life history phases, including the planktonic medusa or jellyfish form, which is most evident in the warm summer months, and an inconspicuous, but longer-lived, bottom-dwelling polyp, which seasonally gives rise to new medusae. Most of the large, often colorful, and conspicuous jellyfish found in coastal waters throughout the world are Scyphozoa.  They typically range from 2 to 40 cm (0.79 to 15.75 in) in diameter, but the largest species, Cyanea capillata can reach 2 meters (6.6 ft) across.  Scyphomedusae are found throughout the world's oceans, from the surface to great depths; no Scyphozoa occur in freshwater (or on land).

As medusae, they eat a variety of crustaceans and fish, which they capture using stinging cells called nematocysts.  Some species, however, are instead filter feeders, using their tentacles to strain plankton from the water.

Behavior

If living Aurelia or other schyphozoan genera are available, observe their swimming movements otherwise review the video below and be able to answer the questions below.

• How is movement achieved?

• Are they strong swimmers?

Anatomy

Using a ladle (the medusae is too fragile to be handled with a forceps), transfer a preserved specimen of Aurelia to a finger bowl of water and spread out flat.  Note that Aurelia is more discoidal and less cup-shaped than Obelia.  When spread flat, the jelly shows a circular shape broken at eight regular intervals my marginal notches (see figure).  Each marginal notch contains a rhopalium, a sense organ consisting of a statocyst (a gravity sensing organ) and an ocellus (a light sensing structure).  

Snip out a rhopalium with scissors and examine under the higher power of a dissecting microscope. 

In the center of the oral side are four long, throughlike oral arms.  These are modifications of the manubrium.  Note that the oral arms converge toward the center of the animal where the square mouth is located.  The mouth opens into a short gullet, which leads to the stomach.  From the stomach, four gastric pouches extend.  They can be identified by the horseshoe-shaped gonads that lie within them.

A complicated system of radiating canals runs from the gastric pouches to the ring canal, which follows the outer margin.  This system of stomach and canals, resembling hub, spokes, and rim of a wheel, forms the medusoid gut, or gastrovascular cavity.  Contrast this with the simple, saclike gut of polyp individuals such as Hydra or Obelia polyps.