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Invertebrate Anatomy OnLine

Aurelia aurita ^©

Moon Jelly

30jun2006

Richard Fox

Lander University

Preface

This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises can be accessed by clicking on the links to the left. A glossary and chapters on supplies and laboratory techniques are also available. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.

Systematics

Cnidaria ^P, Medusozoa, Scyphozoa ^C, Semaeostomeae ^O, Ulmariidae ^F (Fig 7-56, 7-75)

Cnidaria ^P

The cnidarian body consists of a central blind sac, the coelenteron (= gastrovascular cavity), enclosed by a body wall comprising two epithelia, the outer epidermis and the inner gastrodermis (Fig 7-1, 7-2). A gelatinous connective tissue layer, the mesoglea, lies between the two epithelia. The mouth opens at one end of the coelenteron and marks the oral end. The mouth is at the tip of a process, the manubrium that elevates it above the oral surface. The opposite pole is the aboral end. The imaginary line connecting the oral and aboral poles is the axis of symmetry around which the radial symmetry of the body is organized. The mouth is usually surrounded by one or more circles of tentacles.

The defining cnidarian feature is, of course, their possession of stinging cells, or cnidocytes (Fig 7-8). Characteristic of the epidermis, they are also sometimes found in the gastrodermis. Cnidocytes contain an explosive organelle, the cnida, which, upon proper stimulation, inverts and ejects a slender, often barbed and toxic thread in the direction of prey or predator (Fig 7-9). Three types of cnidae are found in cnidarians (Fig 7-10). Nematocysts (in nematocytes), spirocysts (in spirocytes), and ptychocysts (in ptychocytes). All toxic cnidae are nematocysts whereas spirocysts are sticky, and the everted tubules of ptychocysts are used for constructing feltlike tubes. Most cnidae are nematocysts and these are present in all three higher cnidarian taxa. Spirocysts and ptychocysts are found only in Anthozoa.

The basic body plan described above can be manifest as a swimming medusa or attached polyp. In some taxa only one generation is present whereas in others both are found. A life cycle featuring alternation of sexual, swimming medusae with benthic asexual polyps is typical of many cnidarians.

All cnidarians are carnivores feeding on live prey which they usually capture using tentacles armed with cnidocytes. Digestion occurs in the coelenteron which is typically equipped with ciliated canals for distribution of partly digested food. Cnidarians are ammonotelic and diffusion across the body and tentacle surface eliminated the ammonia from the body. Gas exchange is across the general body surface. The nervous system is a plexus of basiepithelial neurons serving sensory and motor systems (Fig 7-6). Most cnidarians are gonochoric. The life cycle typically includes a planula larva. Cnidarians are chiefly marine but the well-known Hydra is an exception.

Medusozoa

Medusozoa comprises those cnidarians whose life cycle includes a medusa generation that alternates with a polyp generation (Fig 7-75B). Symmetry is radial and tetramerous. Nematocysts are the only type of cnidocyte present. Included taxa are Scyphozoa (jellyfishes) and Hydrozoa (hydroids, Hydra, Portuguese men of war, etc).

Scyphozoa ^C

Scyphozoans, the jellyfishes, are cnidarians in which the medusoid generation is large and noticeable and the polypoid is small and inconspicuous. The medusae, known as scyphomedusae, tend to be large, mobile, pelagic, drifting, solitary carnivores and all are marine. The mesoglea of the medusa is thick, gelatinous, and accounts for most of the mass of the organism, although it is mostly water. Cnidocytes are present, concentrated on tentacles, and are used for prey capture and defense. Locomotion is by muscular contraction antagonized by elastic recoil of the mesoglea. Sense organs are arrayed around the periphery of the organism. The sexes are separate. Polyps are small benthic scyphistomae. Scyphozoa includes about 200 species in five taxa; Semaeostomae, Rhizostomeae, Cubomedusae, Coronatae, and Stauromedusae.

Semaeostomae ^O

Semaeostomes are the “flag-mouthed” jellies, so named because of the four large pennant-like oral arms surrounding the mouth (Fig 7-46A). Semaeostomes, of which there are about 50 species, are mostly coastal and the taxon includes most of the species known as hazards to bathers. Aurelia aurita, the moon jelly, although common in coastal waters, is not dangerous.Cyanea capillata, the lion’s mane jelly, and Chrysaora quinquecirrha, the sea nettle, are a different matter and are a hazard to swimmers and fishermen.

Laboratory Exercise

1. Aurelia aurita , the moon jelly, is a common, cosmopolitan, largely coastal jellyfish. Place a living or preserved specimen in a glass culture dish or pan of water. Use seawater if the animal is alive, tapwater if preserved. If possible, select a small specimen that can be placed in a transparent culture dish and placed on the stage of the dissecting microscope when desired.

External Anatomy

Examine the specimen, using the dissecting microscope as needed. The body consists of a large, relatively flat disc, called the bell due to its bell-like shape (in life), or umbrella due to its umbrella-like shape, with the mouthparts hanging below it (Fig 1, 7-47A). The lower, or oral, surface of the bell bears the mouth and is the subumbrella. The opposite, aboral, surface is theexumbrella. The oral-aboral axis extends from the center of the aboral surface to the center of the oral surface and is the axis of symmetry. The radial symmetry of cnidarians is organized around this axis. The margin of the bell is shallowly indented at eight positions to impart a slightly scalloped appearance to its outline. Sensory complices, the rhopalia, are located in the indentations between scallops.

The outer surface is covered by a thin epithelium, the epidermis, which you cannot distinguish from the underlying mesoglea. Deep within the animal, the coelenteron is lined with a second epithelium, the gastrodermis. Between these two thin epithelial layers is the thick, jellylike mesoglea, which makes up almost all of the mass of the animal. The mesoglea is a connective tissue composed of protein fibers and a watery ground substance and a few amoeboid cells.

Figure 1. The moon jelly, Aurelia aurita, sectioned on an interradial axis. Scyphozoa4L.gif

Orient the specimen in the dish with its oral surface uppermost, facing you. In the center of this surface are four long, lacy oral arms of the manubrium (Fig 1, 2, 7-46A). The oral arms are oriented at right angles to each other and form a conspicuous "X" across the oral surface. The two axes that pass through the oral arms, perpendicular to the oral-aboral axis and to each other, are the perradial axes (Fig 2). The axes bisecting the angles between adjacent oral arms are the interradial axes.

The surface of each oral arm bears two frilly, ribbonlike ridges that run its length and enclose a deep, ciliated brachial groove (Fig 2). The grooves of the four oral arms meet at themouth in the center of the oral surface. The margins of the grooves bear small brachial tentacles. The four oral arms radiate outward from the short, inconspicuous vertical column called themanubrium attached to the center of the oral surface (Fig 1).

Inspect the margin of the bell and find the numerous, short marginal tentacles which bear cnidocytes. In preservative the tentacles are greatly contracted but in life they are longer. There is an inconspicuous circular veil of tissue, known as the velarium, central to the fringe of tentacles. Eight sensory rhopalia are distributed evenly around the margin of the bell.

1a. >Make a wetmount of a tentacle and examine its nematocysts with high power of the compound microscope. You will not be able to distinguish the cnidocytes themselves in these preparations but the oval nematocysts within them are distinct. Look for the coiled thread within the nematocyst. If you have a living specimen, draw a little 1% acetic acid beneath the coverslip and watch nematocysts discharge in response. Compare discharged and undischarged nematocysts. (Acetic acid does not always cause the nematocysts to fire.) <

Coelenteron

Aurelia feeds on zooplankters which become entangled in the mucus on the aboral surface. The mucus is moved by monociliated epidermal cells to the margin of the bell where it accumulates until removed by the tips of the oral arms. Cilia in the brachial grooves then transport the mucus and food to the mouth.

The gut, or coelenteron (= gastrovascular cavity), of semaeostome scyphomedusae is an elaborate arrangement of chambers and channels, lined by the ciliated gastrodermis and extending through the mesoglea and opening to the exterior via the mouth. It is both the digestive system and the fluid transport system and is physically divided into two regions to serve these two functions. Extracellular digestion occurs in a central set of chambers into which digestive enzymes are secreted. From these chambers extend ciliated canals which distribute partly digested food to the body where phagocytosis and intracellular digestion takes place.

The digestive region of the coelenteron consists of the stomach and gastric pouches (Fig 7-46C). With your blunt probe, relocate the mouth. It opens into a central stomach whose presence can be demonstrated with the probe gently inserted through the mouth. Four semicircular gastric pouches open from the sides of the stomach and are the sites of extracellular digestion (Figs 1, 2, 3). Each pouch contains a conspicuous horseshoe-shaped gonad on its floor. The opaque gonads are much easier to see than the transparent gastric pouches but the edge of the gastric pouch is visible around the outside margins of the gonad. Insert the blunt probe in the mouth once more and demonstrate the continuity of the pouches with the stomach. Use a probe to show that neighboring pouches are separated from each other by solid partitions. The gastric pouches of semaeostomes are interradial in position so they are not homologous to the perradial gastric pockets of other scyphozoans.

The passageway between the stomach and each gastric pouch is partitioned to form three short, side-by-side, ciliated channels which move food between the stomach and the gastric pouch (Fig 3).

A fringe of short, threadlike gastric filaments is located on the floor of each gastric pouch along the inner margin of each gonad (Figs 1, 2, 7-47A). These secrete enzymes into the gastric pouches. With the probe, show that the filaments extend into the pouches.

The distributive portion of the coelenteron consists of a set of narrow, ciliated canals that transport partially digested food to and from the periphery (Fig 2, 7-47B). The system comprises a ring canal, in the margin of the bell, and a set of 16 radial canals that extend from stomach to the ring canal. Numerous short blind canals arise from the ring canal and extend into the marginal tentacles and sense organs.

Figure 2. Oral view of the moon jelly, Aurelia aurita. Scyphozoa5L.gif

Look carefully at the radial canals and note that some are branched and some are not (Fig. 2). The canals lying on the perradial and interradial axes are the four perradial and fourinterradial canals respectively. These canals are branched and move materials from the ring canal back toward the stomach and gastric pouches. The perradial canals arise directly from the stomach. The interradials and adradials arise from the gastric pouches. Between each perradial axis and its neighboring interradial axis is an adradial canal. These eight canals are not branched and flow in them is away from the gastric pouches.

1b. >If you have a living specimen, use a hypodermic syringe to inject toluidine blue/seawater or other dye into a gastric pouch and watch it enter the radial canals. The outgoing ciliary current in the adradial canals will carry it to the ring canal from which the incoming currents of the perradial and interradial canals will bring it back toward the pouch. <

The septal funnels (= subgenital pits) are four invaginations in the oral surface of the bell, one below each gastric pouch (Fig 1, Fig 7-47A). The pits extend up into the mesoglea almost to the gonads and presumably provide the developing gametes with oxygen. On cursory examination they seem to open into the gastric pouches but they do not. With a probe, (gently) demonstrate that the pits are blind pouches and are not continuous with the gastric pouches.

Figure 3. Detail of a gastric pouch of Aurelia aurita. Scyphozoa6L.gif

Sensory Complex

The sense complices of Aurelia are the eight rhopalia evenly spaced around the margin of the bell (Fig 2, 7-48). Each rhopalium contains a statocyst for gravity detection, mechanoreceptors, chemoreceptors, and an ocellus for photoreception. Two small, triangular, side by side rhopalial lappets are located beside the rhopalium. The complex is supplied by a tiny branch of the ring canal.

1c. >Place the jellyfish in its dish with the oral side facing up on the stage of the dissecting microscope and find one of the eight rhopalia on the margin of the bell (Fig 2). If your jellyfish is too large for the stage of the dissecting microscope, remove a single rhopalium and the surrounding tissue and make a wetmount for study with the compound microscope. In the center of the rhopalium is an elongate, ovoid, called the lithostyle (Fig 4). This structure is protected on the aboral side by a semicircular flap of tissue called the rhopalial hood, which overhangs it (Fig 4, 7-48C). Below it are two triangular, side by side lobes called rhopalial lappets. The lappets and hood extend slightly from the margin of the bell.

The lithostyle bears several sensory receptors, some more apparent than others. These include a pair of tiny dark pigment cup ocelli, two obscure patches of chemosensory epithelium, and a conspicuous spherical statolith containing tiny, opaque granules of calcium sulfate. The statolith is a device for the detection of gravity. A short branch of the ring canal, the rhopalial canal, extends into the rhopalium to supply it with food. The canal branches into two lappet canals and a central lithostylar canal. In living specimens injection of dye into the canal system may render these canals more visible. <

Figure 4. Oral view of a rhopalium of Aurelia aurita. Scyphozoa45L.gif

Nervous System and Musculature

The nervous system and musculature are not visible in gross examination. Aurelia has a subepithelial nerve plexus but no nerve ring. A concentration of neurons, called the rhopalial ganglion, is associated with each rhopalium.

The musculature includes a ring of circular epidermal muscles near the margin of the bell. Contractions result in pulsations of the bell margin. This ejects water from the oral surface and propels the animal in the aboral direction. Recovery is effected by elastic recoil of the mesoglea.

Symmetry

Aurelia has a distinct tetramerous (four part) radial symmetry arranged around the oral-aboral axis. There are four oral arms, four gonads, four subgenital pits, four gastric pouches, and four or eight of each type of radial canal. The mouth has four corners and there are eight sense organs.

Life History

1d. Examine a series of prepared slides of the life history stages of Aurelia. Specimens on commercial slides are often oriented in postures that make study difficult and you may need to look at several slides. The life cycle of Aurelia includes alternating generations of medusae and polyps (Fig 7-49). The jellyfish you have been studying represents the large, sexual, freeliving, pelagic, motile medusa generation. From it develops the asexual, sessile, benthic, and inconspicuous polyp generation.

Aurelia is gonochoric. In medusae, gametes are released from the gastrodermal gonads into the gastric pouches. Ova remain in the coelenteron but spermatozoa exit via the mouth into the sea. They enter the mouth of a female and make their way to the gonad where they fertilize the eggs. Cleavage begins in the gastric pouches. Embryos are released from the mouth and brooded on the oral arms. A hollow, ciliated blastula develops and invaginates to become a gastrula. The gastrula, known as the planula larva, is capable of independent existence and leaves the mother to swim in the plankton. The planula locates a suitable hard substratum, attaches, and grows into a polyp.

Study a commercially prepared slide of a planula larva (Fig 5). The planula is an ovoid gastrula with a blastopore at one end. You should be able to distinguish between the outer ciliated ectoderm and the inner endoderm.

Figure 5. An Aurelia planula larva. Scyphozoa7L.gif

Upon settling on an appropriate substratum, the planula develops into the characteristic scyphozoan polyp, the scyphistoma. Examine a slide of a scyphistoma (Fig 6). It is a tiny, asexual, sessile animal which feeds and produces more scyphistomae asexually from stolons. It is trumpet-shaped and attached to the substratum by its narrow, aboral end. The free oral end is wider and bears a central mouth surrounded by a ring of 16 long tentacles. The mouth sits atop a low mound, the manubrium.

Figure 6. An Aurelia scyphistoma. Scyphozoa8L.gif

Figure 7. An Aurelia strobila. Scyphozoa9L.gif

Look at a slide of a strobila (Fig 7). The scyphistoma undergoes an asexual process of transverse fission called strobilation to produce a stack of tiny disks, which will mature to become the next generation of medusae. A scyphistoma undergoing strobilation is a strobila. The young medusae are ephyrae which accumulate in a stack at the oral end of the scyphistoma. The ephyra farthest from the scyphistoma is the oldest and will be released first.

Look at a slide of an ephyra (Fig 8). The tiny ephyrae swim away from the scyphistoma and grow into sexually mature medusae in less than a year. Most of the adult features are present in ephyrae but in different proportions. Their most obvious feature is eight pairs of lappets, which are disproportionately large in comparison with the rest of the body. The lappets are inconspicuous on mature medusae but they dominate the ephyra. Most of the features of the adult, except for the gonads, are present and can be seen if you look carefully. Find themanubrium, mouth, gastric filaments, and rhopalia. The outlines of the developing coelenteron, including the stomach and three types of radial canals can be seen.

Figure 8. An Aurelia ephyra. Scyphozoa10L.gif

1e. > If your specimen is a living adult, look in the brachial groove for early life history stages. Zygotes, cleavage stages, blastulae, and planulae may be present. If you find any stages, make a wetmount and examine them with the compound microscope. The planula is a small ciliated ovoid organism. <

References

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Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 ^th ed. Brooks Cole Thomson, Belmont CA. 963 pp.