Give an illustrated account of life history of Branchiostoma. (IFS 2022/10 Marks)

Introduction:

Branchiostoma, commonly known as lancelets or amphioxus, are small, fish-like marine organisms that belong to the subphylum Cephalochordata. They are considered to be one of the closest living relatives of vertebrates and provide valuable insights into the evolution of vertebrates.

Life History of Branchiostoma (Amphioxus)

1. Adult Structure:

• Branchiostoma exhibits a body plan with distinct features:
  
  • Body Shape: Elongated, laterally compressed, and transparent, resembling a small fish.
  • Notochord: A flexible rod-like structure running along the body, providing support.
  • Pharyngeal Slits: Numerous gill slits used for filter feeding.
  • Tail: A muscular, post-anal tail used for locomotion.
  • Dorsal Nerve Cord: Runs above the notochord, characteristic of chordates.

2. Reproduction and Gametes:

• Branchiostoma exhibits dioecious reproduction (separate sexes).
  
  • Male: Produces sperm in the gonads located near the posterior end of the body.
  • Female: Produces eggs in similar gonads, which are released into the surrounding water.

3. Fertilization:

• External fertilization occurs in the water column.
  
  • Eggs and sperm are released simultaneously into the sea where fertilization takes place.
  • The fertilized egg forms a zygote that begins to develop.

4. Embryonic Development:

• The early stages of development follow typical chordate patterns:
  
  • Cleavage: The fertilized egg undergoes rapid cleavage to form a blastula.
  • Gastrulation: The blastula forms a gastrula, establishing the three germ layers: ectoderm, mesoderm, and endoderm.
  • Neurulation: The dorsal nerve cord and notochord are formed during this phase.

5. Larval Stage (Tadpole-like Stage):

• The zygote develops into a larva resembling a tadpole:
  
  • Shape: The larva is bilaterally symmetrical with a prominent tail and a small notochord.
  • Function: It swims actively and exhibits the same basic features as the adult, including a rudimentary notochord and nerve cord.
  • Development: The larva undergoes further differentiation and growth as it prepares for settlement.

6. Metamorphosis and Adult Stage:

• After a period of free-swimming life, the larva undergoes metamorphosis:
  
  • The notochord, pharyngeal slits, and dorsal nerve cord become more defined.
  • The adult morphology takes shape, and the amphioxus settles into a benthic lifestyle, burrowing into the sand.

7. Post-Metamorphosis:

• Once metamorphosis is complete, the animal adopts an adult form:
  
  • Adult Features: It now exhibits the characteristic features of Branchiostoma, including the fully developed notochord, pharyngeal slits, and a filter-feeding system.
  • Feeding: It continues filter feeding using its pharyngeal slits to capture plankton and small particles.

8. Longevity and Reproductive Cycle:

• The lifespan of Branchiostoma is generally around one year.
  
  • They reproduce seasonally, typically once a year, during favorable environmental conditions.

Conclusion:

Branchiostoma, with its simple body plan and close evolutionary relationship to vertebrates, provides valuable insights into the early evolution of chordates. By studying the life history of Branchiostoma, scientists can better understand the origins of vertebrates and the development of complex body structures.