In this article we will discuss about Aurelia Aurita:-1. Habit, Habitat and Structure of Aurelia Aurita 2. Histology of Aurelia Aurita 3. Nematocysts 5. Musculature 6. Circulation 7. Nutrition 8. Respiration and Excretion 9. Nervous System 10. Sense Organs 11. Reproductive System 12. Development and Life History 13. Metamorphosis of Ephyra 14. Alternation of Generations.
Contents:
- Habit, Habitat and Structure of Aurelia Aurita
- Histology of Aurelia Aurita
- Nematocysts in Aurelia Aurita
- Musculature of Aurelia Aurita
- Circulation in Aurelia Aurita
- Nutrition in Aurelia Aurita
- Respiration and Excretion in Aurelia Aurita
- Nervous System of Aurelia Aurita
- Sense Organs of Aurelia Aurita
- Reproductive System of Aurelia Aurita
- Development and Life History of Aurelia Aurita
- Metamorphosis of Ephyra into Aurelia Aurita
- Alternation of Generations of Aurelia Aurita
1. Habit, Habitat and Structure of Aurelia Aurita:
Aurelia Aurita is a commonest jelly-fish or moon-jelly, occurs in coastal waters of tropical and temperate oceans of the world close to the surface of water. It is cosmopolitan in distribution. It lives either singly or in large groups found floating or swimming freely. It is carnivorous and reproduces both by asexual and sexual methods.
Structure of Aurelia Aurita:
(i) Shape, Size and Colour:
The medusa of Aurelia Aurita is flattened, bowl or saucer-shaped gelatinous structure called the bell or umbrella. It exhibits tetramerous radial symmetry. It usually measures about 7.5-10 cm in diameter, though much larger forms (up to 30 cm in diameter) have also been recorded from Atlantic coast. It is almost transparent having bluish-white body with pinkish gonads.
(ii) External Structures:
The bell or umbrella is distinctly divided into a slightly convex upper surface and a concave lower surface; the convex surface is known as exumbrellar surface and the concave surface is known as subumbrellar surface. The margin of umbrella is circular which is broken by eight notches. Each notch is provided with a pair of marginal lappets enclosing sense organ called tentaculocyst or rhopalium.
The margin of umbrella, in between notches, is provided with numerous, small, closely arranged hollow marginal tentacles. The margin of umbrella, bearing marginal lappets and marginal tentacles, is thin ridge-like called velarium or pseudo velum. A true velum like those of hydrozoan medusae is absent in Aurelia because it lacks muscles and nerve ring.
The subumbrellar surface bears the following:
Manubrium:
In the centre of subumbrellar surface, there is a well developed but short and inconspicuous manubrium bearing a large squarish mouth.
Oral arms:
All the four comers of the mouth are drawn out into four long, mobile, delicate, frilled and tapering processes called the oral arms. The oral arms are provided with a ciliated groove on its inner side and they are also provided with numerous nematocysts.
Gonads and sub-genital pits:
Four horse-shoe-shaped pinkish gonads, which later become circular, are situated between oral arms in the centre of the subumbrellar surface.
Gonads are, in fact, internal lying into the gastric pouches but they are visible externally. Just below each gonad in the subumbrellar surface, is a circular aperture called sub-genital pit of unknown function. The gonads, however, have no connection with sub-genital pits. The gonads are provided with delicate fine thread-like gastric filaments bearing nematocysts in the gastric pouches.
A system of fine branched or un-branched radiating canals are clearly visible on the surface of umbrella; these are per-radial, inter-radial and ad-radial in positions. The corners of mouth and oral arms are per-radial in position, while gonads and sub-genital pits are inter-radial in position. Out of eight marginal notches, four are per-radial and four are inter-radial in positions.
2. Histology of Aurelia Aurita:
Histologically, the general plan of the structure of Aurelia Aurita resembles to that of the medusa of Obelia.
However, its body consists of the following layers:
1. Ectoderm:
The ectoderm (epidermis) covers the bell all around, it has the columnar epitheliomuscular cells, sensory cells, nerve cells which produce mucus.
In the epitheliomuscular cells the epithelial part is much reduced, and on the sub-umbrella side it is entirely absent, the muscular part is converted into contractile striated muscle fibres. The sensory cells constitute a sensory epithelium between the epithelial cells. The manubrium and oral arms are also lined by epidermal cells.
2. Endoderm:
The endoderm (gastro dermis) has columnar ciliated epithelial cells, they have no muscle processes; the endoderm lines the enteron except the gullet. The gastric filaments are covered with endoderm and have a core of mesogloea. Gonads are endodermal and the cavities of tentaculocysts and marginal tentacles are lined by endodermal cells. The gland cells are present, while nerve cells and muscle processes are not found. The nematocysts are confined only to gastric filaments.
3. Mesogloea:
The mesogloea is large in quantity and it fills the bell between the ectoderm and endoderm. The mesogloea contains amoebocytes and is traversed by numerous branching elastic fibres which make the bell rigid. The amoebocytes are derived probably from the ectoderm, they wander freely and transport food and waste matter. Such a mesogloea is a collenchyma and it differs from the mesogloea of hydrozoan medusae.
3. Nematocysts in Aurelia Aurita:
The nematocysts are found distributed on the oral arms, ex- and subumbrellar surfaces, on the marginal tentacles and also on the gastric filaments.
Three types of nematocysts are found in Aurelia. These are as follows:
1. Atrichous isorhizas:
These have elongated capsule and there is no butt in them. The thread is without spines and it opens at the tip.
2. Heterotrichous microbasic euryteles:
These have small capsule and the butt is short in them. The butt is dilated distally and has unequal spines; it opens at the tip.
3. Holotrichous isorhizas:
These have oval capsule and their butt is narrow. The thread is long having spiral row of small spines and opens at the tip.
4. Gastro Vascular Canal System of Aurelia Aurita:
The mouth leads into a short gullet within the manubrium which finally opens into a four- lobed spacious stomach. The stomach occupies a greater part of the centre of the bell in the form of four extensions called gastric pouches situated inter-radially. At the four corners of the mouth are four per-radial canals situated at right angle to each other, four inter-radial canals are present in between the per-radial canals.
These eight canals are much branched and they end at the marginal lappets. Eight un-branched ad-radial canals arise, two from each gastric pouch, between the inter-radial and per-radial canals.
All these sixteen canals open in the circular or ring canal situated at the margin of the umbrella. Thus, the gullet, stomach with gastric pouches and all the canals constitute the gastro vascular canal system or enteron cavity. The enteron cavity except gullet is lined by ciliated endodermal cells.
5. Musculature of Aurelia Aurita:
The musculature of the body of Aurelia Aurita is well developed and confined to its subumbrellar surface only. It consists of the striated and non-striated muscle processes of the epitheliomuscular cells of epidermis only. The muscle processes are arranged longitudinally in tentacles, manubrium and oral arms.
The muscle processes constitute a strong, broad circular band called coronal muscle. Radial muscles extend from the manubrium to the coronal muscles. The rapid and rhythmic contraction of these muscle processes brings about swimming movements in Aurelia.
6. Circulation in Aurelia Aurita:
Beating of cilia of endodermal lining of the enteron causes a circulation of fluids:
Water is drawn into the mouth → gullet → stomach → gastric pouches → eight ad-radial canals → circular canal → inter-radial and per-radial canals → gullet → exhalant grooves of oral arms; water takes in food, removes waste substances and helps in respiration.
7. Nutrition of Aurelia Aurita:
Aurelia Aurita is carnivorous its food consists of eggs, fish, small larvae and pieces of animals.
The food is caught by oral arms and tentacles and transferred to the mouth. But ciliary feeding also takes place, as the animal moves downwards, plankton is trapped in mucus covering the sub- umbrella surface, flagella moves the food-laden mucus to the bell margin from where it is taken up by oral arms and transferred to the mouth, The gastric filaments kill or paralyze living food.
The endoderm of gastric filaments and enteron produces digestive enzymes. Food undergoes preparatory digestion in the stomach and its pouches, this is, extracellular digestion. Partly digested food which has been broken into small filaments is swept by the cilia into the canals. Food particles are ingested into food vacuoles by endoderm cells of gastric filaments and the canals where intracellular digestion occurs.
Enzymes digest proteins, carbohydrates, fats and even chitin. The digested food is taken up by amoebocytes and distributed by them. The endoderm of gastric pouches stores reserve food as fat droplets and glycogen. The gastro vascular system fulfils the functions of digestion and circulation.
8. Respiration and Excretion in Aurelia Aurita:
There are no specialised organs for respiration and excretion. These processes are presumably performed by the whole body surface.
The gastro vascular system brings about respiration since the current of water carries oxygen in its circulation and removes carbon dioxide. The oxygen requirement of the medusa is very small compared to its weight, because the jelly-fish contains about 96% of water. The gastro vascular system also removes excretory matter.
9. Nervous System of Aurelia Aurita:
The nerve cells and their fibres form two nerve nets joined to the sensory cells and muscle fibres. The nerve net in the sub-umbrella lies in the ectoderm. It is concentrated near the tentaculocysts and it extends into the oral arms and tentacles. This is the main nerve net, it causes pulsations of the bell by controlling the ectodermal coronal and radial muscles.
The second nerve net is more diffuse with smaller nerve cells in the endoderm of both sub-umbrella and ex-umbrella. It controls local reactions, such as feeding, but it can also inhibit bell pulsations because the two nerve nets are joined through the tentaculocysts.
10. Sense Organs of Aurelia Aurita:
Tentaculocysts or rhopalia are characteristic sense organs of Scyphozoa. In Aurelia, there are eight tentaculocysts, each lying between paired marginal lappets at the ends of inter-radial and per-radial canals. They are small modified tentacles. The edge of the bell projects over the tentaculocysts as a hood. Several sense organs are localised near the tentaculocyst.
There are two olfactory pits as depressions, one on the ex-umbrella and the other internal to the tentaculocyst, their sensory epithelium is olfactory.
The outer side of the tentaculocyst has an ectodermal pigment spot or ocellus containing pigment and photoreceptor cells, ocelli are sensitive to light. The tentaculocyst is made of two parts, a club- shaped projection called lithostyle or statocyst containing endodermal calcareous otolith (pigment cup) and a sensory pad on the inner side of the lithostyle.
When the animal is tilted, the lithostyle presses against the sensory pad, and the two together work as an organ of equilibrium, with the result that the bell automatically rights itself. The appearance of tentaculocysts marks the formation of an organ in coelenterata which have in general reached only a tissue grade of organisation.
11. Reproductive System of Aurelia Aurita:
Aurelia Aurita is dioecious, i.e., the sexes are separate. The four gonads (testes or ovaries) lie inter- radially on the floor of the gastric pouches.
The gonads are horse-shoe-shaped and reddish in colour. Gonads of Aurelia are endodermal in origin unlike those of Obelia medusae which are ectodermal in origin. Sex cells originate in the endoderm of gonads. The ripe gametes are shed into the stomach and passed out through the mouth.
12. Development and Life History of Aurelia Aurita:
Fertilisation:
The male and female gametes are formed separately in different medusae of Aurelia. The male gametes or spermatozoa from a male medusa are discharged internally and finally they leave the body through mouth with outgoing water current in the sea water. These are, then, drawn into the body of female medusa through its mouth along with the water current.
The ova are fertilised inside the gastric pouches of female medusa as soon as they are discharged from the ovaries. The zygotes, thus formed, are passed out of the mouth of female medusa along with outgoing water current and finally they get attached in the grooves of the oral arms. At this place, each zygote gets enclosed in a pouch where it undergoes development.
Development and formation of planula larva:
Inside the pouch, the zygote undergoes holoblastic cleavage but unequal and results in the formation of a solid ball of cells called morula. Soon a cavity appears in the centre of the morula called blastocoel which is filled with a fluid and the cells of morula get arranged in a single layer.
This single layered hollow morula is now called blastula. The cells of blastula, from one pole, invaginate to form a double layered gastrula.
The outer cell layer of gastrula represents ectoderm and inner cell layer represents endoderm which encloses a cavity, the coelenteron opening to the exterior by a wide aperture called blastopore. Then the gastrula elongates, its blastopore closes and ectoderm becomes ciliated to form the planula larva.
The planula differs from that of Hydrozoa in its method of formation and in having a coelenteron and blastopore. Planula larva may be seen in masses on the oral arms of female medusae. The planula is set free after a time from the oral arms, after a brief free-swimming existence it sinks, loses its cilia, the blastopore closes, and it gets fixed to some object by its aboral end.
Development and formation of scyphistoma from planula larva:
The planula will metamorphose into a small polyp or hydra tuba which has no perisarc. In this metamorphosis, an oral cone or manubrium is formed, the blastopore opens to become the mouth. Four hollow buds arise per-radially to become tentacles. Subsequently four inter-radial and eight ad-radial tentacles are formed.
The endoderm of coelenteron forms four inter-radial longitudinal ridges called gastric ridges or mesenteries. The mouth becomes square and the manubrium sinks down to form funnel-like depressions called septal funnels or infundibula.
These changes metamorphose a planula into a hydra tuba. A root-like stolon arises at the base of the hydra tuba, which feeds and buds off new hydratubae from its stolon throughout summer. These hydratubae may separate from the stolon of the parent, as in Hydra. After summer the hydratuba ceases to bud, it continues feeding and storing food.
The hydratuba generally winters over the first year and may bud other hydratubae, but next winter it undergoes a process of transverse fission and called strobilation, the dividing hydratuba is called a scyphistoma or strobila. The transverse discs of the scyphistoma which have been produced by strobilation look like a serial pile of saucers, and each disc is an ephyra larva.
One scyphistoma may strobilate a dozen or more ephyra in which growth begins at once in the margin. Ephyra larvae lie on each other, they are joined by muscular strands in the mesenteries which are continuous through all the ephyrae. The muscular strands contract and break, so that the ephyrae are constricted off one by one and they swim away.
The ephyrae break away one by one from the upper surface of the scyphistoma, and then they get inverted. Before the first ephyra is released, from the top of its 16 tentacles are absorbed and 8 notched lobes are formed with a tentaculocyst in each lobe. The successive ephyrae released from the strobila are devoid of tentacles.
The basal part of the scyphistoma which was not segmented grows new tentacles. After a period of rest it resumes its existence as a hydratuba, it starts strobilating again in the next winter. It may live for years feeding and budding in summer and producing ephyra in winter.
Ephyra larva:
The ephyra (Fig. 33.11) is a young medusoid form. It has eight notched lobes or arms which are per-radial and inter-radial. Each lobe has two exaggerated marginal lappets with a deep groove between them having a short tentacle which becomes a tentaculocyst.
An ephyra bears a short stomach or gastric cavity with a short manubrium and four-sided mouth on its subumbrellar surface. The gastric cavity grows into the lobes to form branched per-radial and inter-radial canals, later the ad-radial canals also appear. Four pairs of gastric filaments form in the inter-radial mesenteries.
13. Metamorphosis of Ephyra into Aurelia Aurita:
The ephyra feeds largely on protozoans which are caught by the lappets and transferred to the mouth.
The ephyra grows in size, the spaces between notched lobes fill up, mesogloea increases tremendously pushing the endoderm layers together to form an endoderm lamella except in the enteron, four oral arms and marginal tentacles appear, the septal funnels become subgenital pits, and by summer the ephyra becomes an adult medusa.
Thus, the stages in the life history of Aurelia are male and female medusae → gametes → zygotes → ciliated planula larvae → hydratubae → scyphistomae → ephyra → medusa.
In Aurelia Aurita, there are several phases that show variations which are found in the life-cycle of other Scyphozoa. Very large eggs of Aurelia become actinula larvae (an advanced type of planula which is like a polyp and has a short stem) which directly form ephyrae larvae. Smaller eggs form planula larvae which become hydratubae and then scyphistomae which strobilate ephyrae.
Strobilation is brought about by intensive feeding followed by a drop in the temperature. In Aurelia if the food is plentiful and the temperature is low, then many ephyrae are produced at one time (polydisc strobilation). If there is less food and the temperature is high then ephyrae are produced one at a time (monodisc strobilation).
14. Alternation of Generations in Aurelia Aurita:
In the life history of Aurelia Aurita, the sexual medusoid form always alternates with the asexual polypoid (scyphistoma) form.
Thus, in a sense, Aurelia Aurita exhibits the phenomenon of alternation of generations in its life history. The free swimming adult Aurelia, representing the medusoid form, is the sexual generation which reproduces by the formation of gametes. The zygote develops into fixed polypoid form called scyphistoma through a free swimming planula stage.
The polypoid form scyphistoma represents asexual generation, which reproduces asexualiy by budding or fission.
Some workers do not agree with this plan and deny the existence of alternation of generations or metagenesis in Aurelia.
However, alternation of generations shown by Aurelia does not truly correspond with that found in Obelia, because:
(i) The medusoid phase, in Aurelia Aurita, is formed by the metamorphosis of ephyra and not as a bud from the polypoid (scyphistoma) form
(ii) The polypoid phase— scyphistoma is greatly reduced in size and multiplies by fission in addition to budding.
Therefore, it will be better to regard the life history of Aurelia as a case of prolonged metamorphosis, which becomes complicated by the multiplication (budding or fission) in the larval (scyphistoma) condition, instead of alternation of generations or metagenesis.