ASCIDIA

Contents hide

1) Introduction

2) Classification of Ascidia

3) Geographical distribution

4) Habit and habitat

5) General Characteristics of Ascidia

6) Ecological Importance

7) Conservation Status

8) Special features

9) Identification

Introduction

Ascidia refers to a genus of sea squirts, which are sessile marine invertebrates belonging to the subphylum Tunicata (Urochordata) in the phylum Chordata. These intriguing organisms are often regarded as a bridge between invertebrates and vertebrates due to their chordate features, such as a notochord and a dorsal nerve cord in their larval stage. Commonly found in shallow coastal waters, Ascidia is known for its sac-like body structure encased in a tunic made of cellulose-like material.

Ascidians play a critical role in marine ecosystems by serving as filter feeders, removing plankton and organic particles from the water. They also contribute to the ecological balance by providing habitat for microorganisms and serving as prey for other marine species. Additionally, their ability to regenerate tissue and their production of bioactive compounds have made them subjects of significant scientific research.

Classification of Ascidia

Phylum :- Chordata (Dorsal tubular nerve cord, notochord and gill-slits present.)
Group :- Acraniata (No head, cranium or brain)
Sub phylum :- Urochordata (Marine. Body covered by a thick test. Notochord present only in larval tail.)
Class :- Ascidiacea (Test with scattered muscles. Many pharyngeal gill-slits. Bottom-dwelling.)
Order :- Phlebobranchia (Neural gland ventral to nerve ganglion. Gonad one.)
Family :- Ascidiidae
Genus :- Ascidia

Geographical distribution

Found worldwide in marine environments, from shallow intertidal zones to deep-sea habitats.
Prefer temperate and tropical waters, attaching to hard surfaces like rocks, ship hulls, and coral reefs.

Habit and habitat

Habit:
- Sessile in adulthood, attaching permanently to substrates.
- Larvae are free-swimming and use their chordate features to locate a suitable surface for settlement.
- It is solitary, marine and sedentary urochordate inhabiting shallow water but some occur deep upto 5500 metres.
Habitat:
- Inhabit shallow coastal waters, estuaries, and coral reefs.
- Found on artificial structures like piers and ship hulls.

General Characteristics of Ascidia

Commonly called as sea squirts.
Shape of body short and cylindrical with a broad base attached to rocky substratum.
Test translucent, wrinkled and brownish in colour. Below test is mantle.
Anteriorly body contains eight-lobed terminal branchial siphon and six-lobed sub-terminal atrial siphon having mouth and anus, respectively. Mouth is surrounded by 50-100 tentacles.
Branchial siphon leads into pharynx which is perforated by stigmata. Pharynx contains endostyle.
Dorsal tubercle separated from the nerve ganglion neural gland and membranous dorsal lamina.
Alimentary canal consists of mouth pharynx, stomach, intestine, rectum and anus.
Gonads, gonoduct and gonopore distinct.
Monoecious. Ovary, testes in the same animal. Reproduction sexual. Oviparous. Metamorphosis retrogressive.

Ecological Importance

Filter Feeders: Help maintain water quality by filtering plankton and organic matter.
Nutrient Cycling: Contribute to marine nutrient cycling by breaking down organic matter.
Habitat Formation: Provide surfaces for algae, bacteria, and other microorganisms to grow.
Food Source: Serve as prey for fish, starfish, and crustaceans.

Conservation Status

Ascidians, including Ascidia species, are not globally threatened but face localized risks such as:
- Pollution and habitat destruction.
- Biofouling on artificial structures, leading to removal and destruction.
- Invasive species competition in some regions

Special features

Regeneration Ability: Capable of regenerating lost tissues, making them a model for scientific studies.
Bioactive Compounds: Produce compounds with potential pharmaceutical applications, such as anti-cancer and anti-bacterial properties.
Tunic Structure: Outer tunic is unique among animals, containing a cellulose-like compound.
Chordate Features in Larvae: Larval stage exhibits a notochord and dorsal nerve cord, linking them evolutionarily to vertebrates.
Efficient Filter Feeding: Can process large volumes of water, contributing to water clarity and ecosystem health.
There are some compound ascidians in which individuals are buried in a common test. The development includes tailed larva exhibiting typical chordate characters having notochord, nerve cord, etc. Kowalewsky (1886) carefully studied the development of larval ascidians and brilliantly demonstrated true position of the group among chordates.
The tunicates are best understood by studying first free-living larva of an ascidian, than the adult. The larva shows chordate characters but same are absent in adult and others are modified to sessile mode of life. Compound ascidians reproduce asexually by gemmation and also produce eggs and sperms for sexual reproduction.