lecture recording on 19 February 2025 at 09.46.22 AM

Overview of Zooplankton

• Definition: Functional group of organisms, not a phylogenetic group.

• Characteristics: Comprised of marine organisms that are not strong swimmers, unlike fish or sharks.

Key Features of Arrow Worms (Chaetognatha)

• Body Shape: Long, torpedo-shaped bodies; generally small (about 1 cm in size).

• Marine Presence: All species in the phylum discussed are exclusively marine; do not inhabit freshwater or terrestrial environments.

• Ecological Role: Key predators in the planktonic food web, mainly found in shallow, well-lit water bodies but some species exist in deeper waters.

Anatomy of Arrow Worms

Body Structure

• Regions: Divided into three body sections:

  • Head (anterior): Responsible for grasping prey.

  • Trunk (middle): Contains pairs of fins.

  • Tail (posterior): Features the caudal fin (tail fin).

• Septa: Thin lines of tissue that separate body regions internally.

• Locomotion: The caudal fin provides propulsion, with movement characterized by jerky, periodic strokes to remain concealed from predators.

Internal Systems

• Digestive System: Contains a simple, linear gut from mouth to anus; waste is expelled through the anus while metabolic waste diffuses through the body wall.

• Nitrogenous Waste Handling: Ammonia and other wastes diffuse directly across the body surface due to their thin morphology.

• Reproductive System: Hermaphroditic; internal fertilization occurs with adaptations to enhance genetic diversity and prevent self-fertilization:

  • Ovaries located in the trunk; testes in the tail, separated by septa.

  • Sperm delivery: Through seminal vesicles, sperm are transferred to the trunk of another individual during mating, promoting outcrossing.

  • Embryonic Development: Early-stage embryos are released into the water and are miniature versions of adults (direct development).

Feeding Mechanism

• Grasping Spines: Equipped with spines that can be extended to capture prey; they may secrete toxins (e.g., tetrodotoxin).

• Prey Capture: Utilize spines to seize copepods and other small organisms; these spines serve as external jaws to manipulate and consume prey.

Behavioral Adaptations

• Vertical Migration: They migrate to deeper waters during daylight hours to evade predators and stay near food sources.

  • Depth and Time Correlation: Populations are typically shallower during the night and migrate downwards as light increases.

Fossil Evidence and Evolution

• Fossil Analysis: Exploration of fossils from the Burgess Shale reveals anatomical similarities and differences:

  • Capitator praetermissus: Displays preserved ovaries and a possible neck region, indicating a more complex body plan than modern species.

  • Timor bestia caprei: Shows evidence of circular muscles not present in living species, suggesting a secondary simplification in body function over time.

• Size Comparison: Fossil species were significantly larger (up to 6 inches), indicating they were apex predators in their ecosystems.

Conclusion

• Significance in Ecosystem: Understanding the anatomy and behavior of arrow worms enhances comprehension of marine food webs and evolutionary trends within aquatic ecosystems.