Aurelia (A Jellyfish)- An Overview

Aureliais commonly referred to asjellyfishbecause it is made of a jelly-like substance. A jellyfish is not true fish which is a vertebrate animal with a backbone. It belongs to the classScyphozoaof phylumCoelenterata. The most common scyphozoan jellyfish isAurelia aurita(popularly known asmoon jelly). It represents the dominant medusoid stage.

Table of Contents

Systematic Position

Phylum: Coelenterata

Class: Scyphozoa

Order: Semaeostomae

Family: Ulmaridae

Genus:Aurelia

Species:aurita

Image Source:Alexander Vasenin.

Habit and Habitat ofAurelia(Jellyfish)

It iscosmopolitanjellyfish occurring in warm and temperate seas ranging in temperature from 6-19ºc all over the world.
It lives in coastal waters singly or in large shoals.
It can live in waters with a salt content as low as 0.6%. Salt content and water temperature affect the shape, size, and reproductive strategies.
They thrive best in waters with temperatures are above 17.5°C and salinity greater than 38.0%, largest.
It ranges in depth between the epipelagic zone and the mesopelagic zone (200 to 1,000 m).
It is found either floating with water currents or waves or swimming feebly by the contraction movements of its bell.
It iscarnivorous,feeds on small organisms with the help of its long oral arms.
It responds to various stimuli and is most active in diffuse light.

External morphologyofAurelia(Jellyfish)

1. Shape and size

It is a large version of the medusa ofObelia.
It looks like a soft bell or umbrella-shaped body with 4 red or purple horseshoe-shaped gonads on its upper surface and 4 long and narrow oral lobes hanging downwards from the lower surface.
Its circular body measures about 90mm in diameter and presents a convex aboral orexumbrellar surfaceand a concave oral orsubumbrellar surface.

2.Colouration

Its body is perfectly transparent and bluish-white in color.
The reddish or pinkish horseshoe-shaped gonads are clearly visible from the body surface.

3. Manubrium, mouth, and oral arms

A very short and inconspicuousmanubriumis present in the center of the subumbrellar surface.
At its free distal end is a squaremouth, from each corner of which hangs down a long, tapering much-frilled and delicate process, theoral arm.
Each of the four oral arms has a ventral ciliated groove leading into the mouth and its edges are armed with nematocysts.
The radii, along which angles of the mouth and oral arms lie, are referred to asperradii.
Midway between two adjacent perradii is aninterradius, and between each perradius and its adjacent interradius, on either side, isadradius.

Image Source:Study and Score.

4. Subgenital pits

On each interradius, a little distance from the mouth, the sub-umbrellar surface bears a circular aperture. It leads into a small shallow cavity, thesubgenital pit, lying immediately between a gonad and of uncertain function.

5. Gonads

A horseshoe-shaped and frilledgonadare present just above each subgenital pit within the umbrella.
It is red or purple in color.
Free arms of all the four gonads are directed towards the center of the umbrella. There is no connection between gonads and subgenital pits.

6. Lappets and tentaculocysts

The circular margin of the umbrella or bell is broken into 8 lobes by 8 indentations or notches, 4 of which are perradial and the other 4 interradial.
In each notch, there are two delicate leaf-like processes, called themarginal lappets.
Between lappets lies small sensory organs, thetentaculocystorrhopalium.

7. Marginal tentacles

Between notches or rhopalia, the free edge of the umbrella is beset closely with a row of numerous small, delicate and hollow threads or marginal tentacles.
The tentacles bear a number of stinging cells or nematocysts.

8. Velarium

The margin of the subumbrellar surface, bearing lappets and tentacles, forms a thin and flexible flap, calledvelariumorpseudovelum.
It differs from the true velum ofObeliain having gastrodermal canals running into it. Such medusa with a pseudovelum(e.g.,Aurelia) is calledacraspedote medusa, while a medusa with atrue velum (e.g.,Obelia) is calledcraspedote medusa.

HistologyofAurelia(Jellyfish)

The basic histological plan ofAureliamedusa is more or less the same as that of theObeliamedusa. It isdiploblasticand derived from two embryonic layers, ectoderm and endoderm.

a. Epidermis

All the exposed parts of an umbrella, that is, exumbrellar surface, velarium, tentacles, subumbrellar surface including subgenital pits, oral arms, and manubrium, are covered byepidermis.
The gullet is formed by the evagination of the epidermis.
It consists of epithelial cells (on the exumbrellar surface), epithelio-muscle cells (confined to subumbrellar surface) beside sensory cells, nerve cells, gland cells, and cnidoblasts. Sensory cells form a sensory epithelium between epithelial cells.

b. Gastrodermis

Gastrodermis has columnar ciliated epithelial cells, they have no muscle processes; all the parts of the gastrovascular canal system, except the gullet, are lined bythe gastrodermis.
The gastric filaments are formed by the thin core of the mesogloea and a double layer of gastrodermis.
Gonads are also gastrodermal structures.
The cavities of tentaculocysts and marginal tentacles, being an extension of the gastrovascular canal system, are also lined by the gastrodermis.
Gastrodermis mainly consists of flagellated columnar endothelial cells.
The gland cells are present but nerve cells and muscle processes are wanting. Cindoblasts are confined to gastric filaments.
In the interspace between gastrovascular canals, the gastrodermis of ex-and sub-umbrellar surface fuse to form a thin sheet, of thegastrovascular lamella.

c. Mesogloea

It constitutes the main bulk of the body, forming a thick gelatinous layer between the epidermis and gastrodermis.
The mesoglea ofAureliais not structureless, contains numerous branchingelastic fibersand wanderingamoeboid cellsderived from the epidermis.
This type of mesogloea is more or less like connective tissue snd is known ascollenchyma.
It is ectodermal in an origin and endo-mesodermal as in higher metazoans.
The mesogloeal fluid resembles seawater except that it has more potassium and less sulfate.

Nematocysts

They occur on oral arms, ex- and subumbrellar surface, marginal tentacles, as well as gastric filaments. They are of 3 types:

1. Atrichous isorhizas: Capsule is elongated. The thread tube is open at the tip and is without a butt and spines.

2.Holotrichous isorhizas: Capsule is oval and butt is narrow. The thread tube is long, open at the tip, and armed with a spiral row of small spines.

3. Heterotrichous microbasic euryteles:Capsule is small. The thread tube is open at the tip and covered by minute spines. The butt is short nad its distal dilated portion bears unequal spines.

Musculature and locomotionofAurelia(Jellyfish)

a. Musculature

Aureliahas a well-developed musculature mainly confined to the subumbrellar surface.
The musculature is formed by muscle processes of epithelio-muscle cells of the epidermis.
A broad, circular, and peripheral muscle band, known ascoronal muscle, extends along the periphery of the sub-umbrella.
The conspicuous纵肌are present in the tentacles, manubrium, and oral arms.
Theradial muscleextends from manubrium to coronal muscle along the main radii of the umbrella.
Coronal and longitudinal muscles of tentacles are striated, while others are unstriated

b. Locomotion

The highly developed musculature brings about swimming movements of Aurelia, during which the ex-umbrellar surface is kept upwards.
Rhythmic contractions of circular muscles force water out from the sub-umbrellar cavity, like a jet. As a result, the body is propelled forward or upward. This type of jet-propulsion is known ashydropropulsion.
When the contractions stop, the body gradually sinks to the bottom.
Horizontal movements depend on wave action and help in buoyance because of their low density.
If the body is tilted, equilibrium is maintained with the help of 8 tentaculocysts.

Digestive systemofAurelia(Jellyfish)

1.Gastrovascular canal system

The rectangularmouth通向一个肖tgullet, within manubrium, which opens into a spacious four-lobedstomachoccupying the center of the umbrella.
Extending laterally from the stomach are four wide interradialgastric pouches.Within each gastric pouches is a C-shapedgonadand arrow of smallgastric filamentsorphacellae, bearing nematocysts.
The gullet communicates with each gastric pouch through a groove-likegastro-genital canalthat runs between the two free ends of the gonad.
Around each gonad runs anexhalent channelthat also communicates with the gullet through thegastro-oral canal.
Radial canalsbranch out from pouches to join thecircularorring canalin the bell margin.
From the periphery of the stomach as well as from the gastric pouches, sixteen radial canals originate, four areper-radials, fourinter-radials, andeight adradials.
Both per radials and inter-radials are branched, while the adradials are un-branched.
所有的径向运河被纤毛排列胃dermis, which ultimately open into a ring or circular canal situated at the margin of the body.

2.营养

a. Food

Aureliais exclusivelycarnivorous.It is mainly a suspension or ciliary feeder. It feeds upon the planktonic organisms and small marine invertebrates, such as crustaceans, worms, their eggs, and larvae.

b. Ingestion

The small planktonic organisms are carried directly into the stomach with the entering water current.
Some small organisms that may get entangled in the mucus of the sub-umbrellar surface are collected by oral arms.
Sometimes, as medusa sinks slowly or swims gently downwards, the prey is captured in contact with tentacles and oral arms.
Nematocysts of oral arms paralyze and entangle these organisms, which are then swept up along the lateral tracts of oral arms and passed into the mouth.
Undesirable particles are rejected and dropped on the way.
If the prey, still in living condition, reaches the gastric pouches, it is paralyzed and killed by the nematocysts of gastric filaments.

c. Digestion

The digestion takes place in the stomach and gastric pouches, where the gland cells of these regions secrete enzymes for theextracellulardigestionof protein, fats, carbohydrates, and even chitin.
Partly digested food particles, circulate through the canal system and are ingested by gastrodermal cells forintracellular digestionin food vacuoles.
The undigested material passes out of the body with the outgoing current of water.

d. Distribution

水流通交易的运河系统nsports digested food to the gastrodermis of all parts. Further, wandering amoeboid cells in mesogloea transports food from the gastrodermis to the epidermis.

e. Reserve food

The food is reserved in the gastrodermal cells of gastric filament in the form of glycogen and fat droplets.

Circulation of water inAurelia(Jellyfish)

The circulation of water is due to the beating of cilia of the gastrodermal cell lining of the gastrovascular canal. It circulates through the gastrovascular canal system along a fixed route.
The inhaled water current enters the mouth and passes through the narrow gastrogenital canal into gastric pouches, and finally through the unbranched adraial canal enters the circular canal.
From the circular canal, exhalent water current returns through perradail and interradial canals.
The Preradial canal conveys it directly into the stomach from where it passes into the gullet and finally exists along the basal grooves of oral arms.
Interradial canals convey it to the basal grooves of the oral arms via exhalent and gastro-oral canals. This arrangement greatly prevents the mixing of water currents entering and leaving the gastrovascular canal system.
The contraction of the bell during locomotion and movements of oral arms helps in circulation.
One complete circulation takes about 20 minutes.
The circulation of water helps in nutrition, respiration, excretion, and reproduction.

Respiration and excretionofAurelia(Jellyfish)

Aurelia has no special organs for respiration and excretion. Oxygen dissolved into water diffuses directly into the epidermis as well s the gastrodermis, both of which are constantly bathed by water.
Carbondioxide and nitrogenous wastes diffuse out into surrounding water.
Some workers are of the opinion that the subgenital pits facilitate gaseous exchange. This is based on the observation that, at the time of swimming, foul water constantly leaves the pits and freshwater with dissolved oxygen) enters into them.
99% weight of jellyfish is formed by water.
Nothing is known about the nature of excretory products.

Nervous systemofAurelia(Jellyfish)

It consists of the a.main nerve net, b.adiffuse nerve net, and c. 8 rhopalial ganglia

a. Main nerve net

Each nerve net or plexus consists of nerve cells and fibers.
The main nerve net is more developed and lies in the subumbrellar surface and extends into tentacles, rhopalia, manubrium, and oral arms.
The presence of the main nerve net on the subumbrellar side is correlated with the presence of well-developed musculature on that side; the former co-ordinates muscular movements during locomotion.
Its nerve elements form a sort of nerve ring along the margin of the umbrella near-circular canal.
主要的神经网络是沿着每增厚和interradii due to the concentration of its nerve element along these radii.
Each radial thickening, near the margin of the umbrella, is connected with the rhopalial ganglion, situated near the rhopalium on that radius.

b. Diffuse nerve net

It lies in the epidermis of the subumbrella as well as the exumbrella.
Its nerve elements consist of smaller cell bodies.
It is also connected with rhopalial ganglia.’It controls the local responses, like feeding, and can inhibit contractions of the umbrella because the 2 nerve nets are joined through tentaculocysts.

c. Rhopalial ganglia

These are formed by the aggregation of nerve cells.
There are 8 such rhopalial ganglia, one near each sense organ or rhopalium.
Nerve impulses received by the sense organs are conducted through nerve nets to the muscle fibers which react accordingly.

Sense organsofAurelia(Jellyfish)

The sense organs of Aurelia are 8rhopaliawhich are situated one in each of the per-and interradial marginal notches. each rhopalium comprisestentaculocystorstatocyst, 2ocelli, and 2olfactory pits.

1. Tentaculocyst or statocyst

It is a hollow club-shaped structure, situated in the marginal notch between twomarginal lappets.
It is covered on the outer side by the process of bell margin, termedhood, which also connects the bases of 2 marginal lappets.
Just below the club is a pad of tall ciliatedsensory epithelial cellswhich are connected with the subumbrellar nerve net lying below the epidermis.
Tentaculocyst is hollow tentacles. Projecting into tentaculocyst is an extension of a circular canal lined by gastrodermis. Lying in the distal part of the tentaculocyst is a mass of polygonalstatolith cellsof gastrodermal origin.
Each statolith cell contains a self-secreted particle, thestatolith(Gr.,statos, standing +lithos, stone), composed of calcium sulfate and calcium phosphate.
Statoliths act as a weight, causing the club of tentaculocyst to bend up and down at its base, whenever the animal tilts to one side or other, during swimming.
Tentaculocysts control theequilibriumof the umbrella during swimming. If the umbrella is tilted, the clubs of tentaculocysts press against their sensory pad beneath, the sensory cells which become stimulated. Higher the tilt greater the stimulation. The impulse is conducted through the subumbrellar nerve net to muscle fibers which react accordingly.
In response, the upper half of the umbrella drives less water than the lower half at each beat, so that the umbrella automatically rights itself.

2.Ocelli

There are two ocelli, one of ectodermal and the other of endodermal origin.
The former, known aspigment spot ocellus, consists of a patch of pigmented and sensory epidermal cells on the outer side of the club of tentaculocyst.
The latter, known aspigment cup ocellus, consists of a cup-shaped cavity lined by pigmented and sensory gastrodermal cells and is situated on the inner side of tentaculocyst, in association with statoliths.
The sensory cells of both the ocelli are connected with their respective underlying nerve nets.
Ocelli arephotoreceptors.

3. Olfactory pits

These are in the form of depression of thickened epidermis containing sensory cells.
One such depression lies at the base of the hood. It is termed theouteroraboral olfactory pit.
The other, known as theinneroradoral olfactory pit, is situated on the inner side of the tentaculocyst at the base of the pad of ciliated sensory epithelial cells
These olfactory pits are probablychemoreceptors.

Reproduction and life historyofAurelia(Jellyfish)

Aureliaisdioecious也就是说,男性和女性的性别separate but there is no sexual dimorphism.

1.Sex organs

Testes and ovaries are similar in appearance. A medusa has 4 horseshoe-shaped gonads radially lying on the floor of the stomach periphery, that is, one in each gastric pouch.
The gonads areendodermalin origin.
They are reddish violet in color.
他们通过半透明果冻是可见的of the umbrella as frilled organs with their concavities facing inwards.
On maturity, ova nad sperms break into the gastrovascular cavity and pass out of the mouth with the outgoing water current. The ova and eggs are lodged in the frill of oral arms.

Figure: Life-cycle stages of Aurelia. (A) Life-cycle scheme depicting sexual reproduction of mature medusa and asexual proliferation of the polyp. (B − G) Photographs of the six analyzed stages: planula (B), polyp (C), early strobila (D), advanced strobila (E), ephyra (F), and mature medusa (G). The dashed line in G represents the excised part of the medusa that was used for RNA-seq. Bar in B, 50 μm, and in C − F, 500 μm. Image Source:Vera Brekhman et al. 2015.

2.Fertilization

Spermatozoa, swimming about in the water, reach the ova and fertilize them either in the stomach of females or in the frills of oral arms. Thus, fertilization is eitherinternalorexternal.

3. Formation of planula larva

The frills of oral arms serve as temporary brooding members where fertilized egg or zygote undergoes early development into a ciliated larval stage calledplanula.
The zygote undergoes holoblastic and equal segmentation to produce a solid ball-likemorula.
Morula becomes a single-layeredblastulasoon by the accumulation of fluid in its interior.
The two-layered gastrula develops by invagination, having outer ectoderm and inner endoderm lining an enteron cavity, with itsblastoporeorgastral mouthnot completely closed. Thus, it differs from the gastrula of Hydrozoa (e.g., Obelia) which develops by the process of delimitation and multipolar regression of cells into blastocoel having no blastopore.
The embryo, now elongates, its outer cell becomes ciliated, blastopore closes and the typicalplanulalarva is formed. At this stage, masses of planulae are visible as minute opaque patches on the oral arms of female individuals.

4. Formation of scyphistoma

The ciliated planula eventually escapes and after a short free-swimming existence attaches itself to a stone or seaweed by its aboral end.
The cilia are lost and a mouth opens at its free distal end where the blastopore had closed.
The larva now becomes elongated and metamorphosed into a small trumpet-shaped orHydra-likepolyp orhydratuba, about 5mm high.
The larva proximal part is narrowed into a stalk-like organ, attached to the substratum by an adhesive basal disc.
Its tentacles bud out around the mouth where 4 tentacles are perradial, subsequent 4 interradial, and then 8 adradial in position. Thus, 16 long and slender tentacles are formed.
Its mouth becomes square in outline and its edges become elongated to form a short manubrium.
The larva now looks like a trumpet-shaped polyp orHydraand is called hydratuba or young scyphistoma (Gr.,skyphos, cup+stoma, mouth).
The endoderm of its cavity is raised into 4 interradial longitudinalgastric ridgesortaenioles, characteristic of Scyphozoa, which divides the enteric cavity into four perradial diverticula or pouches.
Simultaneously, the ectoderm between mouth and tentacles also becomes invaginated as four interradial funnel-like depressions, known asseptal funnelsorinfundibula, which sink into four gastric ridges.
Scyphistoma feed and grows up to 12mm in height and may survive in this stage for several months. Sometimes, it multiplies either by lateral budding or by growing horizontal creeping stolons, which bud off fresh hydratubae. These buds eventually separate from the parent, as inHydra.

5. Formation of ephyrae( strobilation)

Scyphistoma undergoes a remarkable process of budding or transverse fission of oral end in autumn and winter, calledstrobilation.
Distally, the body develops a series of ring-like transverse constrictions or furrows which gradually deepens so that the organism resembles a pile of minute saucers or disc, placed one above the other. At this stage, scyphistoma with a segmented body is calledstrobilaand each of the segments is calledephyralarva.
Theephyraeorephyrulaeare connected together by muscular strands. As they grow older, they constricted off one by one and swim away as little medusae or jellyfish.
About a dozen ephyrae are formed in single strobilation.
When the food is plentiful and the temperature is low, several ephyrae are produced at one time (polydisc strobilation). When food is scarce and the temperature is high, a single ephyra is produced at a time (monodisc strobilation).
When all the ephyrae get detached, the basal unsegmented part of the scyphistoma grows new tentacles and continues to live as a polyp or hydratuba. It may live for several years, feeding, growing, and multiplying by budding in summers, but producing ephyrae by strobilations in winter.
The scyphistoma may be metamorphosed into a single adultAurelia, without forming ephyrae under exceptional circumstances.

6. Ephyra larva

A newly released ephyra is a microscopic gelatinous creature, about 1mm in diameter and with a well-developed tetramerous symmetry.
The edge of its umbrella is greatly fluted, being produced into 8bifid lobesorarms( 4 per and 4 interradial), separated by 8 deep adradial indentations orclefts.
Each lobe has 2 exaggerated marginal lappets with a groove or notch between them is a sensory recess or niche bearing short tentacles, which becomestentaculocystorrhopalium.
Ephyra contains a small segment of the stomach of scyphistoma with gastric ridges.
It remains opens on the subumbrellar side and grows out to form a short manubrium baring a four-sided mouth at its apex.
Spacious enteric cavity projects into 8 arms as branched perradial and interradial canals. Adradial canals appear later on.
The gastric ridges or mesenteries are replaced bygastric filaments. 4 interradial septal invaginations probably persist as 4subgenital pits.
The first ephyra differs from the rest in bearing the original tentacles of scyphistoma.
Ephyrae swim actively in sea-water feeding on minute organisms, such as protozoans, which are caught by lappets and transferred to the mouth.

7. Metamorphosis

As the ephyra grows in size, the space between the notched lobe fills up, mesogloea increases enormously so that two layers of endoderm(gastrodermis)f fuse to form a solidgastrodermal lamella, except in regions of gastrovascular canals.
The adradial region grows more rapidly, gradually filling up their wide clefts, so that the umbrella of 8 -8-rayed ephyra becomes circular and saucer-shaped ], as in an adult medusa.
With the appearance of 4 oral arms and numerous tentacles, ephyra finally transferred into adultAurelia.
An ephyra formed in winter becomes a sexually reproducing adult medusa by spring or summer.

Alternation of generation (metagenesis) inAurelia(Jellyfish)

There are 2 contrasting views regarding the life history ofAurelia.
According to the older view, the development ofAureliaillustrates in a sense, the phenomenon ofalternation of generations.
The free-swimming adultAureliarepresents thesexual generations, producing ova or spermatozoa, like the medusoid phase ofObelia. Here, the fertilized egg gives rise to a small fixed polyploid form, or scyphistoma, after passing through a free-swimming ciliated planula larval stage.
Scyphistoma represents theasexual generation, equivalent to the polypoid Obelia colony. It reproduces asexually by strobilation, budding of ephyrae, each of which metamorphoses into an adultAurelia.
According to the modern view, the development of Aurelia does not show true alternation of generation or metagenesis.
However, the alternation of generation shown byAureliadoes not truly correspond with that ofObeliabecause, the medusoid stage inObelia发展作为芽分支多倍体的殖民地,whereas theAureliamedusa is formed by the metamorphosis of an ephyra larva, produced as one of several transverse segments of the polyploid scyphistoma.
Thus, the life cycle ofAureliamay be considered as a simple continuous process involving a prolonged metamorphosis further complicated by the multiplication of scyphistoma larva into several ephyrae.

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References and Sources

Brekhman, V., Malik, A., Haas, B.et al.Transcriptome profiling of the dynamic life cycle of the scypohozoan jellyfishAurelia aurita.BMC Genomics16,74 (2015). https://doi.org/10.1186/s12864-015-1320-z.
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