HERDMANIA

Contents hide

1) Introduction

2) Classification of Herdmania

3) Geographical distribution

4) Habit and habitat

5) Characteristics of Herdmania

6) Ecological Importance

7) Conservation Status

8) Special features

9) Identification

Introduction

Herdmania, a genus of sea squirts belonging to the subphylum Tunicata (Urochordata), is a marine invertebrate commonly found in shallow coastal waters. These organisms are sac-like, sessile filter feeders enclosed in a protective tunic composed of tunicin, a cellulose-like material. As tunicates, Herdmania exhibits fascinating evolutionary traits, including the presence of chordate characteristics like a notochord and dorsal nerve cord in its free-swimming larval stage.

Known for their simplicity in adult form yet complexity in larval stage, Herdmania serves as a crucial subject in evolutionary biology and developmental studies. Its life cycle offers insights into the transition from motile larval stages to sessile adulthood, shedding light on chordate ancestry.

Classification of Herdmania

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 :- Stolidobranchia (Neural gland ventral to nerve ganglion. Gonad one.)
Family :- Pyuridae
Genus :- Herdmania

Geographical distribution

Different species of Herdmania (=Rhabdocynthia) are widely distributed in the Pacific, Atlantic and Caribbean seas, besides Indian Ocean.
Found in tropical and temperate marine environments worldwide.
Common in shallow coastal waters, especially in intertidal and subtidal zones.
Often attached to rocks, ship hulls, coral reefs, and other submerged surfaces

Habit and habitat

Solitary, sedentary and sometimes living as commensal in association with gastropod shells, specially over Xaneus pyruns (Shankh) and Xaneus angulatus (Conch).

Habit:
- Sessile in adulthood, permanently attaching to solid substrates.
- Free-swimming, motile larvae that settle during metamorphosis.
Habitat:
- Found in shallow, nutrient-rich waters with ample plankton.
- Often inhabit estuaries, rocky shores, and artificial structures

Characteristics of Herdmania

It is commonly called as Monoacidian or Sea squirt. Das S.M. (1936) wrote a memoir on this animal.
It is more or less like a purse or large oval potato, measuring 6.5 to 11.8 cm in length and 5.2 to 6.9 cm in breadth.
Body dark brown, reddish brown or yellowish brown in colour and regionated into soft body proper and foot.
Foot large, dirty, rough, leathery and with a number of foreign objects. It forms one-third of the body.
Body enclosed in a thick, tough and supporting transparent test or tunic in the form of a thick translucent protective investment meant for respiration and reception of stimuli. It is composed of polysaccharide, called as tunicin, and protein.
Without cutting the animal open at the free end, body is drawn to form 1.0 cm long branchial or incurrent siphon and 1.5 cm long atrial or excurrent siphon. The incurrent opening of mouth is somewhat laterally placed, while the excurrent opening is directed upwards. Branchial and atrial apertures are found on branchial and atrial siphons respectively.
Body proper lies within test.
Herdmania is hermaphrodite and protogynous.
Fertilization external. Development includes a fully formed larva, called ascidian tadpole larva. It contains all the chordate characters i.e., notochord and nerve chord, etc.
Metamorphosis is retrogressive in which notochord, nerve cord, tail and tail fins are degenerated. By the time adult is formed, all chordate characters disappear.

Ecological Importance

Water Filtration: Act as natural biofilters, removing plankton and organic particles, contributing to water quality.
Nutrient Recycling: Aid in nutrient cycling by consuming organic matter and releasing simpler forms back into the ecosystem.
Habitat Formation: Serve as substrates for colonization by algae, bacteria, and other microorganisms.
Food Chain Contribution: Provide food for predators such as fish, crabs, and starfish

Conservation Status

Herdmania species are not globally threatened but face challenges such as:
- Pollution and habitat degradation in coastal waters.
- Threats from invasive species in certain regions.
- Biofouling control measures may lead to accidental removal.

Special features

Herdmania in adult form is devoid of any chordate characters which are exhibited only by its tadpole larva. Animal has also a peculiar symmetry; branchial aperture marks the anterior end and opposite end is the posterior end.
The atrial aperture indicates dorsal side and the area diagonally opposed to it represents ventral side. Such abnormal symmetry is brought about by metamorphosing larva into sedentary adult.
Chordate Larval Stage: Possess notochord, dorsal nerve cord, and a tail, making them an evolutionary link between invertebrates and vertebrates.
Tunic Composition: Unique tunic material (tunicin) provides protection and structural support.
Regenerative Abilities: Capable of regenerating damaged tissues, highlighting their resilience.
Filter Feeding Efficiency: Process significant volumes of water daily, aiding in plankton control and water clarity.
Metamorphosis: Undergo rapid transformation from a motile larva to a sessile adult, an evolutionary hallmark.