Ascidiacea: An Overview

Introduction

• Ascidiacea, also known as sea squirts, is a class of marine invertebrates in the subphylum Tunicata.
• Characterized by a tough outer test or "tunic" made of cellulose.
• Ascidians are filter feeders.
• Temporal range: Cambrian Stage 3 to Present.

Classification

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

Habitat and Distribution

• Found worldwide, typically in shallow waters.
• Prefer salinities over 2.5%.

Types of Ascidians

• Solitary: Single animals attach to substrates.
• Social: Form clumped communities.
• Compound: Many zooids form large colonies.

Anatomy

• Rounded or cylindrical shape, size ranges from 0.5 to 10 cm.
• Firmly attached to solid surfaces.
• Covered by a tunic composed of living tissue.
• Two siphons for water intake and expulsion.

Feeding Mechanism

• Water enters through oral siphon, passes through pharynx, and exits through atrial siphon.
• Mucus traps food particles in pharynx.

Circulatory System

• Heart pumps blood alternating directions every few minutes.
• Four types of blood cells.

Nervous System

• Simple central nervous system.
• Lack special sense organs.

Reproduction

• Most are hermaphrodites.
• Solitary ascidians use external fertilization.
• Colonial ascidians exhibit mixed reproduction.
• Asexual reproduction through budding.

Ecology

• Important indicators of marine pollution.
• Some species are invasive.

Evolution and Phylogeny

• Fossil record is sparse.
• Recent studies suggest ascidians might be polyphyletic.

Human Uses

• Some species are consumed as delicacies globally.

Research Importance

• Used as model organisms due to their simple structure and chordate linkage.

Notable Species

• Ciona intestinalis: Commonly studied species.

Environmental Impact

• Accumulate pollutants due to filter-feeding.
• Contribute to marine biodiversity but can disrupt ecosystems if invasive.

Conservation

• Monitoring of invasive species is crucial to maintain marine ecosystems.