Jelly fishes are scyphozoans in which medusa is the dominant adult stage. The most common scyphozoan jelly fish is Aurelia aurita, which is also called moon-jelly that occurs in seas all over the world.
MORPHOLOGY & ANATOMY
Aurelia has inverted cup or umbrella-shaped body with a convex aboral or exumbrellar surface and a concave oral or subumbrellar surface. There are four horseshoe-shaped gonads on its upper surface and a short manubrium with a central mouth hanging downward from the lower surface. Generally body is semitransparent and light bluish in colour. Mouth is squarish and attached on its sides are four oral arms which have a ventral ciliated groove leading into mouth and its edges are armed with nematocysts.
The circular margin of umbrella or bell is divided into 8 lobes and notches, 4 of which are perradial and the other 4 interradial. In each notch, there are leaf-like marginal lappets or rhopalium, which carry sensory organs such as tentaculocyst and eye spot. The free edge of umbrella is beset with a row of numerous small marginal tentacles bearing stinging cells or nematocysts. Margin of the subumbrellar surface forms a thin and flexible fold called velarium or pseudovelum which carries a canal inside it.
Like all coelenterates, Aurelia is diploblastic having ectoderm and endoderm and mesogloea. All outer parts of umbrella are covered by epidermis but the gastrovascular canal system is lined by endoderm called gastrodermis. Gonads are also gastrodermal structures in Scyphozoa. Mesogloea is very thick in jelly fishes and forms the bulk of body and contains numerous branching elastic fibres and wandering amoebocytes. This type of mesogloea is known as collenchyma.
There are three types of nematocysts in jelly fishes, namely, Atrichous isorhiza, in which capsule is elongated and thread tube is open at the tip and is without a butt and spines. The second type is Holotrichousisorhizas, in which capsule is oval and butt is narrow and the whole thread tube is armed with spines. The third type of nematocysts are Heterotrichous microbasic euryteles in which capsule is small and thread tube is open at the tip and covered by minute spines. Butt is short and its distal dilated position bears unequal spines.
Musculature system is mainly confined to the subumbrellar surface where coronal, longitudinal and radial muscles are present. These muscles bring about swimming movements in Aurelia during which the exumbrellar surface is kept upwards. Contraction of circular muscles forces water out from the subumbrellar cavity in a jet, propelling the body forward and upward. Gelatinous mesogloea also helps in buoyancy because of its low density.
Mouth opens into a four-lobed stomach in the centre of umbrella. Inside the gastric pouches of stomach are located four C-shaped gonads and small gastric filaments or phacellae that bear nemotocysts. The diet is carnivorous containing planktons and small marine invertebrates. Prey is captured by tentacles and oral arms and carried into the stomach by the incoming water current. Prey is killed by the nematocysts of oral arms and gastric filaments. Digestion takes place in stomach and gastric pouches. Glands of endothelium secrete digestive enzymes for extracellular digestion and partially digested food particles are ingested by the gastrodermal cells for intracellular digestion. Undigested food is egested through mouth along with the outgoing water current.
Canal system is well developed in jelly fishes and helps in nutrition, respiration, excretion and reproduction. Inhalant water current enters the mouth and passes through the narrow gastrogenitalcanals, then into adradialcanals and finally entering the circular canal on the margin. From circular canal, the exhalent water current returns through per-radial and inter-radial canals to stomach from where it passes out through the basal grooves of oral arms.
Nervous system is made of diffused nerve net that thickens to form loose radial nerves that supply to sense organs in rhopalia. Eight rhopalial ganglia are formed by the aggregation of nerve cells.
Sensory organs include Tentaculocyst or statocyst, two ocelli and two olfactory pits.
LIFE CYCLE OF AURELIA
Aurelia is dioecious i.e. the male and female gonads are in separate individuals but there is no sexual dimorphism. Testes and Ovaries are similar in appearance. A medusa has 4 horseshoe-shaped gonads in each gastric pouch. Ova and sperms are released into the gastrovascular cavity and pass out of mouth along with the outgoing water current. The ova are lodged in the oral arms which serve as brood pouches and fertilization takes place there.
The zygote undergoes holoblastic cleavage to produce a solid ball-like morula. Soon it is transformed into blastula that encloses a central fluid-filled blastocoel. Gastrulation takes place by invagination producing an outer ectoderm and an inner endoderm lining the coelenteron. Eventually a ciliated planula larva is formed within the oral arms.
The planula larva escapes from the oral arms, swims about and fixes on substratum and metamorphosed into a 5 mm long hydra-like polyp that has tentacles around the mouth. Larva at this stage is referred to as hydratuba or young scyphistoma. It feeds and grows up to 12 mm in height and may survive and reproduce by budding for several months.
Scyphistoma after full growth undergoes transverse fission or strobilation and is called strobila and each of its segments is called an ephyra, which is only 1 mm in diameter. Ephyrae grow and then break off from the parent strobila and swim away as little medusae of jellyfish, feed and grow to become adults. When all the ephyrae get detached, the basal part of scyphistoma grows new tentacles and mouth and develops into a new hydratuba. It may live for several years, feeding, growing and multiplying by budding in summers but producing ephyrae by strobilation in winters. With the appearance of 4 oral arms and numerous marginal tentacles, ephyra is finally transformed into adult Aurelia. An ephyra formed in winter becomes a sexually reproducing adult medusa by spring or summer.