Model Answer
0 min readIntroduction
*Branchiostoma*, commonly known as the lancelet, is a primitive chordate often considered a living fossil due to its retention of ancestral chordate characteristics. It provides valuable insights into the evolution of vertebrates. This small, fish-like marine animal inhabits shallow coastal waters and is of significant interest to zoologists studying chordate origins. Understanding its habitat, habits, and life history is crucial for appreciating its evolutionary significance and ecological role. This answer will comprehensively describe these aspects of *Branchiostoma*.
Habitat
Branchiostoma is exclusively marine, inhabiting shallow, sandy or muddy bottoms of coastal waters, typically at depths of 5-30 meters. Several factors define its preferred habitat:
- Substrate: It burrows into fine-grained sediments like sand or mud, leaving only the anterior part exposed for feeding.
- Salinity: Prefers normal marine salinity (around 35 parts per thousand).
- Temperature: Thrives in temperate and subtropical waters, with optimal temperatures ranging from 15°C to 25°C.
- Water Quality: Requires clear, well-oxygenated water. Pollution and turbidity are detrimental to its survival.
- Geographical Distribution: Found in coastal regions of Europe, North America, Asia (including India), and Australia.
Specific species exhibit slight variations in habitat preference. For example, Branchiostoma lanceolatum is common in European waters, while Branchiostoma belcheri is found in Japanese waters.
Habits
Feeding
Branchiostoma is a filter feeder. Water enters the body through the oral siphon, passes over the gill slits, where food particles are trapped by mucus secreted by the endostyle. Cilia then transport the food-laden mucus to the pharynx, and finally to the digestive tract. It feeds on phytoplankton, zooplankton, and other microscopic organic matter.
Locomotion
Locomotion is primarily achieved through undulatory movements of the body, facilitated by the myotomes (muscle segments). It can swim backwards rapidly by forcefully expelling water through the atrial siphon. However, it spends most of its time buried in the sediment.
Social Behavior
Branchiostoma is generally a solitary animal, although aggregations can occur in areas with favorable conditions. There is no evidence of complex social behavior.
Life History
Reproduction
Branchiostoma exhibits sexual reproduction with distinct sexes. Gametes (eggs and sperm) are released into the water column for external fertilization. Spawning typically occurs during spring and summer.
Development
The fertilized egg undergoes cleavage to form a blastula, which then develops into a gastrula exhibiting all the characteristic features of chordate development – notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail. The larval stage is free-swimming and resembles the adult in many respects, but is smaller and lacks fully developed reproductive organs.
- Egg: Microscopic, translucent eggs are released into the water.
- Larva: The larva is planktonic and feeds on microscopic organisms.
- Metamorphosis: The larva settles on the seabed and undergoes metamorphosis into the adult form, burying itself in the sediment.
- Adult: The adult remains buried for most of its life, feeding and reproducing.
Lifespan
The lifespan of Branchiostoma varies depending on the species and environmental conditions, but typically ranges from 1 to 2 years.
| Stage | Description |
|---|---|
| Egg | Microscopic, fertilized egg |
| Larva | Free-swimming, planktonic stage |
| Juvenile | Settles on seabed, begins burrowing |
| Adult | Buried in sediment, filter feeder, reproduces |
Conclusion
In conclusion, *Branchiostoma* is a fascinating chordate whose habitat, habits, and life history reflect its primitive yet highly adapted nature. Its burrowing lifestyle, filter-feeding mechanism, and characteristic chordate features make it a valuable subject for evolutionary studies. Understanding its ecological requirements is also crucial for conservation efforts, particularly in the face of increasing coastal pollution and habitat degradation. Further research into its genomic and developmental biology will undoubtedly reveal more about the origins of vertebrates.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.