Cnidaria: Aurelia | Zoology Optional Notes for UPSC PDF Download

Aurelia Aurita: Overview

• Habitat: Coastal waters of tropical and temperate oceans, cosmopolitan in distribution, floating or swimming freely.
• Size and Shape: Flattened, bowl or saucer-shaped medusa with tetramerous radial symmetry, typically 7.5-10 cm in diameter, can be larger.
• Color: Almost transparent with a bluish-white body and pinkish gonads.

External Structures

• Bell or Umbrella: Divided into exumbrellar and subumbrellar surfaces, exhibiting radial symmetry.
• Marginal Notches: Eight notches on the circular margin, each with a pair of marginal lappets enclosing tentaculocyst or rhopalium.
• Velarium: Thin ridge-like structure on the margin, lacking true velum.
• Oral Arms: Four long, frilled, tapering processes extending from the mouth, each with a ciliated groove and nematocysts.
• Manubrium: Short, inconspicuous structure at the center of the subumbrellar surface, bearing a large squarish mouth.
• Gonads and Sub-genital Pits: Four horse-shoe-shaped gonads, pinkish and visible externally; sub-genital pits present below gonads.
• Canal System: Per-radial, inter-radial, and ad-radial canals visible on the subumbrellar surface, joining to form a circular canal at the margin.

Histology

• Ectoderm (Epidermis): Columnar epitheliomuscular cells, sensory cells, nerve cells, and mucus-producing cells. Striated muscle fibers in epitheliomuscular cells.
• Endoderm (Gastrodermis): Columnar ciliated epithelial cells lining the enteron, covering gastric filaments and gonads. Nematocysts present.
• Mesogloea: Large quantity, contains amoebocytes and branching elastic fibers, making the bell rigid.

Nematocysts:

• Three types: Atrichous isorhizas, heterotrichous microbasic euryteles, and holotrichous isorhizas.
• Distributed on oral arms, ex- and subumbrellar surfaces, marginal tentacles, and gastric filaments.

Gastrovascular Canal System

• Gullet leads to a four-lobed stomach with gastric pouches.
• Eight canals (per-radial, inter-radial, and ad-radial) branch out and join to form a circular canal at the margin.
• Functions in digestion and circulation.

• Well-developed musculature in the subumbrellar surface.
• Striated and non-striated muscle processes in epitheliomuscular cells.
• Coronial muscle, radial muscles, and longitudinal muscles responsible for swimming movements.

• Ciliary beating in endodermal lining causes water circulation.
• Water drawn into mouth, through gullet, stomach, and canals, and expelled through oral arms.
• Facilitates feeding, removes waste, and aids respiration.

• Carnivorous, feeding on eggs, fish, larvae, and small animals.
• Captures food with oral arms and tentacles, utilizing ciliary feeding as well.
• Gastric filaments with nematocysts paralyze or kill prey.
• Extracellular and intracellular digestion occur.

Respiration and Excretion

• No specialized organs; presumed to occur through the body surface.
• Gastrovascular system aids in respiration, with water circulation carrying oxygen and removing carbon dioxide.
• Excretion managed by the whole body surface.

Nervous System and Sense Organs

• Two nerve nets: one concentrated in the ectoderm, controlling pulsations; the other diffuse with smaller cells in the endoderm, responsible for local reactions.
• Eight tentaculocysts, modified tentacles serving as sense organs.
• Olfactory pits, ocelli (photoreceptor cells), and statocysts present near tentaculocysts.
• Tentaculocysts function in equilibrium and sensitivity to light.

Reproductive System and Life History of Aurelia Aurita

• Dioecious: Aurelia Aurita has separate sexes. Testes or ovaries (four in number) lie inter-radially on the floor of gastric pouches.
• Gonad Characteristics: Horse-shoe-shaped, reddish in color, endodermal in origin. Sex cells originate in the endoderm of gonads.
• Gamete Release: Ripe gametes shed into the stomach and expelled through the mouth.

Development and Life History:

Fertilization:

• Separate Gametes: Male and female gametes formed in different individuals.
• Sperm Release: Spermatozoa discharged internally from a male medusa, exit through the mouth into seawater.
• Fertilization Site: Ova fertilized inside the gastric pouches of female medusa.

Development Stages:

Formation of Planula Larva:

1. Zygote Formation: Zygotes formed within gastric pouches, shed into the stomach.
2. Morula Stage: Zygote undergoes holoblastic cleavage, forming a solid ball of cells (morula).
3. Blastula Formation: Blastocoel appears, forming a hollow single-layered blastula.
4. Gastrula Formation: Invagination creates a double-layered gastrula with ectoderm and endoderm.
5. Planula Larva: Gastrula elongates, blastopore closes, forming a ciliated planula larva. Planula settles after a free-swimming period.

Development into Scyphistoma:

1. Metamorphosis: Planula metamorphoses into a small polyp or hydra tuba with oral cone and tentacles.
2. Hydratuba Stage: Hydratuba buds new hydratubae from a root-like stolon, feeding and storing food.

Strobilation:

1. Wintering Stage: Hydratuba may cease budding and winters over.
2. Transverse Fission: Undergoes transverse fission or strobilation, producing ephyra larva (saucer-like discs).
3. Ephyra Larvae: Released ephyra larvae swim away, with each having tentacles, gastric ridges, and gastric filaments.
4. Return to Hydratuba: After the ephyra larva stage, the basal part of the scyphistoma grows new tentacles, resuming its existence as a hydratuba.

Metamorphosis into Aurelia Aurita:

• Ephyra Growth: Ephyra grows, mesogloea increases, oral arms, marginal tentacles, and sub-genital pits appear.
• Maturation: Spaces between notched lobes fill up, septal funnels become sub-genital pits, and the ephyra transforms into an adult medusa.
• Completion of Life Cycle: Adult medusa represents the sexual generation, alternating with the asexual polypoid (scyphistoma) form.

Conclusion:

Aurelia Aurita exhibits a complex life history involving dioecious reproduction, intricate developmental stages, and alternation between sexual and asexual generations. The transformation from planula larva to scyphistoma, strobilation, and the subsequent metamorphosis into the adult medusa highlight the adaptability of Aurelia to different phases of its life cycle. While some debate exists on the strict categorization of alternation of generations, the extended metamorphosis and varied life cycle stages underscore the biological diversity within Aurelia Aurita.