Model Answer
0 min readIntroduction
*Nereis*, commonly known as sandworms, are marine polychaete annelids found in benthic environments. They exhibit a complex life cycle involving both direct and indirect development, characterized by distinct larval stages. Understanding this life cycle is crucial for comprehending their ecological role and evolutionary adaptations. The life cycle of *Nereis* is a classic example of metamorphosis in invertebrates, showcasing significant morphological and physiological changes as the organism develops from egg to adult. This answer will detail each stage of the *Nereis* life cycle, supported by illustrative diagrams.
Stages of the Life Cycle of *Nereis*
The life cycle of *Nereis* involves several distinct stages: egg, trochophore larva, nectochaete larva, and adult. Each stage is adapted to specific environmental conditions and plays a crucial role in the overall survival and propagation of the species.
1. Egg Stage
The eggs of *Nereis* are typically small, spherical, and translucent. Fertilization is external, occurring in the water column. Eggs are released along with sperm during synchronized spawning events, often triggered by lunar cycles. The eggs contain a yolk reserve that provides nourishment for the developing embryo.
Figure 1: Eggs of Nereis (Source: Wikimedia Commons)
2. Trochophore Larva Stage
Upon hatching, the egg develops into a trochophore larva. This is a characteristic larval form found in many annelids and mollusks. The trochophore is bilaterally symmetrical, oval-shaped, and possesses a prominent ciliated band (trochophore band) used for locomotion and feeding. It also has a tuft of cilia at the anterior end and a digestive tract. The trochophore is non-feeding initially, relying on yolk reserves, but later becomes capable of filter-feeding.
Figure 2: Trochophore Larva (Source: Wikimedia Commons)
3. Nectochaete Larva Stage
The trochophore larva undergoes metamorphosis into the nectochaete larva. This stage is unique to polychaete annelids. The nectochaete larva develops paired, lateral appendages called notopodia, each bearing chitinous chaetae (bristles). These chaetae and notopodia are used for swimming and attachment to substrates. The nectochaete larva is also capable of filter-feeding and possesses a more developed digestive system. It is positively phototactic, meaning it swims towards light, aiding in dispersal.
Figure 3: Nectochaete Larva (Source: ResearchGate)
4. Adult Stage
The nectochaete larva eventually settles on the seabed and undergoes metamorphosis into the adult *Nereis*. This involves significant changes in body plan, including the development of a segmented body, a well-defined head with sensory organs, and numerous parapodia (similar to notopodia but more developed) bearing chaetae. Adult *Nereis* are typically carnivorous or omnivorous, feeding on small invertebrates and organic matter. They are burrowing animals, creating tunnels in the sediment. Reproduction involves external fertilization, completing the life cycle.
Figure 4: Adult Nereis (Source: Wikimedia Commons)
Summary of Stages
| Stage | Key Features | Feeding | Locomotion |
|---|---|---|---|
| Egg | Spherical, translucent, yolk-rich | Yolk reserves | None |
| Trochophore Larva | Ciliated band, bilateral symmetry | Initially yolk, then filter-feeding | Ciliated band |
| Nectochaete Larva | Notopodia with chaetae, paired appendages | Filter-feeding | Notopodia and chaetae |
| Adult | Segmented body, parapodia, chaetae | Carnivorous/Omnivorous | Parapodia and body undulations |
Conclusion
The life cycle of *Nereis* exemplifies the complex developmental strategies employed by marine invertebrates. The distinct larval stages – trochophore and nectochaete – are crucial for dispersal and colonization of new habitats. Understanding these stages is vital for assessing the ecological health of marine ecosystems and predicting the impacts of environmental changes on these important benthic organisms. Further research into the hormonal and genetic control of metamorphosis in *Nereis* could provide valuable insights into developmental biology.
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.