Unit 2: Cnidaria and Ctenophora

Table of Contents

1. Phylum Cnidaria

Cnidaria (from Greek: knide = nettle) are radially symmetrical, diploblastic animals. They are characterized by the presence of stinging cells called cnidocytes or nematocysts. They exhibit a tissue grade of organization.

1.1 General Characteristics and Classification

General Characteristics

Classification of Cnidaria (Up to Classes)

Classification is based on the dominance of the polyp or medusa stage in the life cycle.
Class Dominant Form Key Characteristics Examples
Hydrozoa Both Polyp and Medusa Polyp stage is often colonial. Medusae have a velum. Mesoglea is non-cellular. Hydra, Obelia, Physalia (Portuguese man-of-war)
Scyphozoa Medusa is dominant "True jellyfish." The polyp stage is reduced or absent. Medusae are large and lack a velum. Aurelia (moon jelly), Rhizostoma
Anthozoa Polyp only No medusa stage. The gastrovascular cavity is partitioned by septa or mesenteries. All marine. Metridium (sea anemone), Gorgonia (sea fan), Corals

1.2 Life Cycle of Obelia

Obelia is a colonial hydrozoan that exhibits a classic example of metagenesis (alternation of generations).

Metagenesis: The alternation between the asexual polyp form and the sexual medusa form in the life cycle of an organism.
  1. Colony (Asexual Stage): The life cycle begins with a mature, branching colony of polyps attached to a substrate. The colony has two types of polyps (zooids):
    • Hydranths (Gastrozooids): Feeding polyps with a mouth and tentacles.
    • Gonangia (Blastostyles): Reproductive polyps that produce medusa buds asexually.
  2. Medusa Formation: The gonangia produce tiny, free-swimming medusae by budding. These medusae are released into the water.
  3. Medusa (Sexual Stage): The medusae are dioecious (separate male and female). They mature and produce gametes (sperm and eggs).
  4. Fertilization: Fertilization is external, occurring in the water. The fusion of sperm and egg forms a diploid zygote.
  5. Larval Stage: The zygote develops into a free-swimming, ciliated planula larva.
  6. Settlement: The planula larva swims for a while, then settles on a suitable substrate, loses its cilia, and develops into a single polyp.
  7. Colony Growth: This initial polyp grows and buds asexually to form a new Obelia colony, completing the cycle.

1.3 Polymorphism in Siphonophora

Polymorphism is the occurrence of multiple structurally and functionally different types of individuals (zooids) within a single colony. Siphonophores (an order of Hydrozoa) show the highest degree of polymorphism.

In a Siphonophore colony like Physalia, different polyps and medusae are modified to perform specific functions, acting like organs of a single individual.

  • Pneumatophore (Float): A gas-filled modified medusa that keeps the colony afloat.
  • Nectophores (Swimming Bells): Modified medusae that propel the colony through water by jet propulsion.
  • Gastrozooids (Feeding Polyps): Responsible for digestion.
  • Dactylozooids (Defensive/Prey-capturing Polyps): Long tentacles armed with potent nematocysts.
  • Gonozooids (Reproductive Polyps): Produce gametes for sexual reproduction.
Exam Tip: Polymorphism is an excellent example of division of labor at the colonial level, allowing for increased efficiency and survival.

1.4 Corals and Coral Reefs Formation

Corals

Corals are marine invertebrates belonging to the Class Anthozoa. A coral "head" is a colony of numerous tiny, genetically identical polyps. Each polyp secretes an exoskeleton of calcium carbonate. Over generations, these exoskeletons build up to form the massive structures known as coral reefs.

Many corals have a symbiotic relationship with single-celled algae called zooxanthellae that live within their tissues. The algae provide the coral with food via photosynthesis, and in return, the coral provides a protected environment. This is why most reef-building corals are found in clear, shallow tropical waters.

Coral Reef Formation

Coral reefs are vast underwater ecosystems built by corals. They are often called "rainforests of the sea" due to their immense biodiversity.

  1. Fringing Reefs: Grow directly from the shore, forming a border along the coastline.
  2. Barrier Reefs: Located further offshore, separated from the land by a wide, deep lagoon. The Great Barrier Reef in Australia is the most famous example.
  3. Atolls: Ring-shaped reefs that enclose a central lagoon. They typically form on submerged volcanic islands.

Coral Bleaching: When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic zooxanthellae living in their tissues, causing them to turn completely white. This is a major threat to coral reefs worldwide.

2. Phylum Ctenophora

Ctenophora (from Greek: cten = comb, phora = bearers), commonly known as comb jellies or sea walnuts, are a phylum of marine invertebrates.

2.1 General Characteristics and Affinities

General Characteristics

  • Habitat: Exclusively marine and pelagic (live in open water).
  • Symmetry: Biradial symmetry.
  • Body Organization: Diploblastic, with a well-developed, jelly-like mesoglea.
  • Comb Plates: Their most distinctive feature is the presence of eight external rows of ciliated plates, called ctenes or comb plates, which are used for locomotion.
  • No Cnidocytes: They lack stinging nematocysts. Instead, many have specialized adhesive cells called colloblasts on their tentacles to capture prey.
  • Bioluminescence: Many species are capable of producing light.
  • Digestion: They have a gastrovascular cavity with a complex canal system.
  • Reproduction: They are hermaphroditic, and reproduction is sexual with external fertilization. Development includes a cydippid larval stage.

Affinities of Ctenophora

Ctenophores show relationships with some phyla but also have unique features, making their phylogenetic position debated.

  • Affinities with Cnidaria (Similarities):
    • Radial (biradial) symmetry.
    • Diploblastic body plan with a gelatinous mesoglea.
    • Presence of a gastrovascular cavity.
    • Absence of organ systems like excretory and respiratory systems.
  • Differences from Cnidaria:
    • Ctenophores have comb plates for locomotion; Cnidaria do not.
    • Ctenophores have colloblasts; Cnidaria have cnidocytes.
    • Ctenophores lack polymorphism and alternation of generations.
    • Ctenophores have a more complex digestive system with anal pores.
  • Affinities with Platyhelminthes (Advanced Features):
    • Some developmental similarities.
    • Presence of mesenchyme in some ctenophores, hinting towards a triploblastic condition.
Conclusion on Affinities: While Ctenophora and Cnidaria were once grouped together in Coelenterata, the significant differences (especially comb plates vs. cnidocytes) justify their placement in a separate phylum. They are considered a distinct evolutionary lineage that diverged early from other metazoans.