Lecture Notes: Ascidiacea

Overview

• Ascidiacea: Paraphyletic class in the subphylum Tunicata, commonly known as ascidians or sea squirts.
• Characteristics: Have a tough outer test or "tunic" made of cellulose.
• Habitat: Found globally, mostly in shallow waters with salinities over 2.5%.
• Mobility: Sessile as adults, free-swimming as larvae.

Classification

• Domain: Eukaryota
• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Tunicata
• Class: Ascidiacea
• Included Groups: Aplousobranchia, Phlebobranchia, Stolidobranchia.

Anatomy

• Body Structure:

  • Rounded or cylindrical; size ranges from 0.5 to 10 cm.
  • Attached to substratum with root-like extensions.
  • Smooth, thick tunic with cellulose, proteins, calcium salts.

• Respiratory and Feeding Structures:

  • Two siphons: oral (buccal) and atrial.
  • Water drawn in through oral siphon, exits through atrial.
  • Pharynx with cilia and stigmata for feeding.

Digestive System

• Pharynx: Covered with mucus to trap food particles.
• Stomach: Digestion with enzyme secretion.
• Intestine: Leads to a cloaca.

Circulatory System

• Heart: Curved tube alternating blood flow direction.
• Blood Cells: 4 types including vanadocytes.

Nervous System

• Central Nervous System: Simple, consisting of a neural tube.
• Sensory Systems: No true brain, has ganglia and exocrine gland.

Reproduction and Development

• Hermaphrodites: Most are sessile as adults.

• Reproductive Strategies:

  • Solitary species release eggs and sperm into seawater.
  • Colonial species have mixed reproductive strategies.

• Larval Development: Free-swimming with notochord and nerve tube, undergoes retrogressive metamorphosis.

Ecology

• Impact on Environment: Filter feeders, sensitive to pollution, invasive potential.
• Predators: Include fish, birds, crabs, and sea otters.

Evolution and Fossil Record

• Fossil Evidence: Sparse due to soft bodies; Ediacaran species with possible affinities.

Phylogeny

• Molecular Evidence: Ascidians might be polyphyletic.

Uses

• Culinary: Consumed in various cultures, e.g., Sea pineapple in Japan and Korea.
• Research: Model organisms for studying chordate development.

Important Notes

• Sea squirts are key to understanding chordate evolution.
• Their simple body plan and transparency offer insights into embryological processes.