Cnidaria: Obelia | Zoology Optional Notes for UPSC PDF Download

Introduction

Genus: Obelia

Class: Hydrozoa

Phylum: Cnidaria

Habitat: Mainly attached to surfaces like seaweeds, rocks, wooden piles, and molluscan shells in shallow waters (up to 80 meters).

Life Cycle and Reproduction

• Life Cycle: Two distinct stages - polyp and medusa.
• Generations: Two generations involved in the reproductive process.
• Polyps: Small, immobile animals resembling sea anemones; collectively known as hydroid polyps.
• Reproductive Strategy: Takes two generations to complete.

1. Diploblastic Structure:
   • Polyp Form: Two true tissue layers - epidermis (ectodermis).
   • Medusa Form: Gastrodermis (endodermis) with a jelly-like mesoglia between the tissue layers.

1. Colony Structure:
   • Branched: Obelia forms a branched, fixed colony.
   • Horizontal Branches: Called Hydrorhiza, anchor the colony.
   • Vertical Branches: Known as Hydrocaulus.
   • Granular Coenosarc: Each branch consists of a granular coenosarc with two cell layers enclosing the coelenteron.
   • Perisarc: Transparent horny perisarc surrounds the coenosarc.
2. Zooids:
   • Branch Endings: All branches end in zooids.
   • Structure: Zooids are part of the vertical branches toward the base.
   • Function: Responsible for various functions within the colony.

Conclusion: Obelia, belonging to the class Hydrozoa in Phylum Cnidaria, is a marine invertebrate found in shallow waters. Its unique life cycle involves two generations, the polyp and medusa forms, each characterized by distinct tissue layers. The branched colony structure, with horizontal and vertical branches, showcases its sedentary nature, and zooids play a vital role in the overall functioning of the colony.

Obelia Zooids and Medusa: Detailed Description

1. Polyps or Gastrozooids (Nutritive Zooids):
   • Barrel-shaped zooids responsible for colony nutrition.
   • Enclosed in a cup-shaped hydrotheca, an extension of the perisarc.
   • Distal end features a conical projection, the hypostome or manubrium, bearing a mouth.
   • About twenty-four solid tentacles surround the hypostome, equipped with cnidocytes.
   • Proximal end connects with the coenosarc.

2. Blastostyles or Gonozooids (Reproductive Zooids):

   • Located at the basal end of the hydrocaulus.
   • Long, cylindrical structure without a mouth and tentacles.
   • Enclosed in a transparent gonotheca, a modified perisarc.
   • Develops small buds known as medusae buds on its walls.

Medusa:

1. Maturity and Release:

   • Umbrella-like medusa buds detach from the blastostyle and enter the seawater through the gonotheca aperture.
   • Hydro-medusa: Solitary, free-swimming modified zooid.

2. Medusa Structure:

   • Bowl-shaped gelatinous disc with a subumbrella (concave) and exumbrella (convex) surface.
   • Manubrium: Cylindrical projection hanging from the middle of the subumbrellar surface.
   • Mouth: Square mouth at the apex of the manubrium leading to the gastric cavity or stomach.
   • Four gastrodermal canals radiate from the gastric cavity to the bell margins, forming radial canals.
   • Radial canals open into a ring or circular canal along the bell margin.

3. Gonads and Maturation:

   • Four gonads project from the middle of the radial canal.
   • Gonads mature after medusae escape from the gonotheca.
   • Gonads can be either testes or ovaries, and they are sex-specific.

4. Additional Structures:

   • Velum: Thin fold produced inwards at the edge of the bell.
   • Tentacles: Numerous solid tentacles with swollen bases (vesicles or bulbs) containing cnidocytes.
   • Sense Organs: Ocelli (pigment granules and nerve cells) near the bulb, marginal sense organs (statocysts or lithocysts) attached to tentacle bulbs.
   • Statocyst: Circular, closed vesicle with otoliths, providing stimuli for swimming coordination.

5. Swimming Movements:

   • Muscles coordinate snake-like swimming movements of the medusa.
   • Marginal sense organs (statocysts) contribute to swimming coordination.

Histology of Hydromedusa

1. Surface Covering:

   • Both exumbrellar and subumbrellar surfaces covered by ectoderm cells.
   • Whole canal system lined by endoderm cells, extending to the surface ectoderm at the lip of the mouth.
   • Thin sheet of endodermal lamella, formed by fused upper and lower layers of endoderm, lies between radial canals and ex- and subumbrellar layers of ectoderm.

2. Velum:

   • Composed of a double layer of ectoderm enclosing a ring canal with a strip of narrow mesoglea between the canal and ectoderm.

3. Tentacles:

   • Solid tentacles with a core of vacuolated endodermal cells covered by ectoderm.
   • Tentacles contain cnidocytes.

4. Mesoglea:

   • Gelatinous mesoglea forms the main bulk of the body and contains non-cellular fibers.

5. Musculature:

   • Well-developed musculature with regularly arranged circular, longitudinal, and radial tracts.

6. Interstitial Cells:

   • Accumulated mainly at the bulbs or vesicles of tentacles.
   • Interstitial cells give rise to cnidocytes of only one type.

Gonads:

1. Medusae are dioecious—testes and ovaries are borne by separate individuals.
2. Four gonads situated on the subumbrellar surface, one on the middle of each radial canal.
3. Gonads mature after medusae escape from the gonotheca.
4. Gonad structure: Outer covering of ectoderm continuous with subumbrellar ectoderm, inner lining of endoderm continuous with radial canal endoderm.
5. Space between layers filled with interstitial cells that differentiate into ova or sperms.
6. Germ cells originate in the ectoderm of the manubrium and migrate to gonads for maturation.
7. Ripe gonads rupture ectodermal covering, shedding gametes (ova or sperms) in water.
8. Ova are large rounded cells; sperms are minute, actively swimming flagellated cells.
9. Medusae die soon after liberating gametes.

Fertilization and Development in Obelia:

1. Fertilization occurs in seawater where germ cells are set free or by spermatozoa carried by water currents to female medusae.
2. Zygote undergoes holoblastic cleavage to form a blastula.
3. Blastula transforms into a ciliated planula larva by invagination.
4. Planula settles on submerged substratum, developing a disc for attachment.
5. Distal end expands, developing a manubrium and a circlet of tentacles, turning into a hydrula or simple polyp.
6. Hydrula sends out lateral buds, repeating the process to form a complex Obelia colony.

Alternation of Generations:

• Distinct alternation of generations (metagenesis) present in Obelia's life history.
• Asexual generation (Obelia colony) reproduces by repeated budding of hydrula.
• Sexual generation involves medusae buds, some developing gonads, leading to the formation of a new Obelia colony from fertilized eggs.
• Asexual generation is dependent on and alternated by the sexual generation.

Significance of Planula Larvae:

• Non-locomotory Obelia produces free-swimming planula larvae.
• Larvae can swim with cilia and water currents, preventing overcrowding and facilitating species continuity.