Protochordata: General Features and Life History of Herdmania | Zoology Optional Notes for UPSC PDF Download

Habit and Habitat of Herdmania

• Herdmania's Nature: Herdmania, a fascinating marine organism, belongs to the category of solitary ascidians. These unique creatures are exclusively found in marine environments.
• Depth Range: Herdmania exhibits a remarkable range in terms of depth. It has been recorded to inhabit marine environments at depths varying from 9 meters to as deep as 21.6 meters.
• Lifestyle and Feeding Habits: Herdmania leads a sessile lifestyle, meaning that it remains attached to substrates, like rocks or the shells of mollusks, through its base or foot. As a microphagous animal, Herdmania feeds on microscopic animals and plants. This creature's tunic, or outer covering, provides a protective shelter for a variety of organisms, creating a unique ecological microcosm within its vicinity. A common occurrence in Herdmania's environment is the growth of green algae on its tunic, occasionally even concealing the entire animal. Other organisms that inhabit the tunic include hydroids, anemones, minute lamellibranches, gastropods, and various other marine life forms.

External Features of Herdmania

• Body Shape and Size Variability: The external appearance of Herdmania can exhibit a considerable degree of variability. It possesses an oblong bag-like body with variable sizes, and the average size of an adult Herdmania is approximately 9.5 centimeters in length. When present, the foot of Herdmania measures about 3 to 4 centimeters in length.
• Attachment Mechanisms: Herdmania's attachment to its surroundings is dependent on its location. When dwelling on sandy beds, it employs a narrow foot for attachment. In contrast, when attached to rocks or molluscan shells, it lacks a foot and relies on a flat base for stability.
• Oral and Atrial Funnels: Herdmania's body has distinct oral and atrial funnels located at the free end. These funnels house the oral aperture and atriopore, respectively. Notably, both the oral and atrial funnels exhibit four distinct lobes. The oral funnel is smaller in comparison to the atrial funnel.
• Tunic Characteristics: The tunic covering Herdmania's body is soft and leathery. It is transparent in the juvenile stage but becomes opaque in the adult stage. During the juvenile stage, a network of blood capillaries is visible through the transparent tunic. An intriguing feature of Herdmania is the presence of reddish patches on its body, created by the ampullae located at the terminal ends of blood vessels within the tunic. The tunic also serves as an accessory respiratory structure.
• Tentacles and Sieving Apparatus: Surrounding the oral and atrial funnels, Herdmania has tentacles, which serve distinct purposes. The oral tentacles, which are comparatively longer, function as a sieving apparatus, preventing the entry of larger particles into the pharyngeal cavity. These tentacles are called oral or branchial tentacles. Atrial tentacles, on the other hand, are smaller and vestigial in comparison to the oral tentacles.
• Blood Capillaries and Ampullae: Herdmania's body contains blood capillaries, and these folds exhibit red pigmented patches, which are separated by white streaks. Typically, there are eight red patches alternated with eight white streaks. These patches and streaks extend from the tips of the funnels down to the basal end. While in a living condition, Herdmania experiences a strong water current that enters through the oral funnel and exits through the atrial funnel.

Body Wall of Herdmania

Cellular Components: In the body wall of Herdmania, several distinct cell types are encountered, each with its own unique characteristics and functions. These cell types include:

• Amoeboid Cells: These cells are abundant in the test, contributing to its structure and function.
• Eosinophilous Cells: Eosinophilous cells are spherical with homogeneous cytoplasm containing fine granules that stain red with eosin. They exist in two varieties, one with a large vesicular nucleus and the other with a smaller eccentric nucleus.
• Granular Cells: These oblong-shaped cells have large nuclei and are surrounded by nerve fibers, playing a role as receptor cells.
• Spherical Vacuolated Cells: These cells contain numerous small vacuoles, and their nuclei are not visible.
• Nerve Cells: Nerve cells are small with conspicuous nuclei and typically have 2 to 6 dendrites.

Interlocking Fibers: The test is traversed by interlocking fibers that resemble the muscle fibers of the mantle. These fibers contribute to the structural integrity of Herdmania's body.

Spicules: Spicules are small, mineralized structures found within the test, serving various functions. They come in two varieties:

• Microscleres: These minute bodies consist of a spherical head and a long body, adorned with 5 to 25 equidistant rings of small spines. The head is typically smooth in most cases, and the average size of microscleres is around 50 micrometers.
• Megascleres: Larger than microscleres, megascleres are categorized into two types: spindle-shaped spicules and pipette-shaped spicules. Spindle-shaped spicules have 20 to 60 equally spaced rings of spines, and their average size is about 1.5 millimeters. Pipette-shaped spicules feature a large, round median swelling and multiple rings of spines, with lengths extending up to 3.5 millimeters.

These diverse cellular components and structural elements within Herdmania's body wall contribute to its overall resilience and function in the marine environment.

Coelom and Atrium in Herdmania

• Coelom Absence: Herdmania, in its mature state, lacks a true coelom, except in certain doubtful derivatives like the pericardial cavity and gonad. In the absence of a coelom, Herdmania's body is primarily occupied by the atrium.
• Atrium and Pharynx Attachment: The atrium, which is the central cavity in Herdmania's body, is divided into right and left halves due to the attachment of the pharynx with the mantle on the ventral and dorsal sides. However, both halves of the atrium are continuous dorsally and open to the exterior through the atriopore. This arrangement facilitates water circulation and various physiological processes within Herdmania.

Life Cycle of Herdmania

• Development and Hermaphroditism: Herdmania exhibits a hermaphroditic reproductive system. This means that it possesses both male and female reproductive organs. The hermaphroditic gonads in Herdmania consist of two large lobulated bodies, with the right gonad situated on the right side, just dorsal to the heart, and the left gonad lodged within the loop of the intestine. Each gonad comprises 10 to 25 lobes, which are arranged in two rows on either side of a central axis.
• Structure of Gonads: These lobes have distinct zones within them. The inner zone, known as the ovarian region, contains different stages of egg formation. The outer, or testicular region, contains spermatogonia, spermatocytes, and spermatozoa. Each gonad functions as an ovotestis, housing both male and female reproductive structures.
• Fertilization and Larval Development: The fertilization process in Herdmania is external. When the gametes (sperm and eggs) mature, they are released into the sea water. The subsequent developmental sequences and metamorphosis are similar to those observed in other ascidians, such as Ascidia.
• These reproductive and developmental characteristics provide insights into the life cycle of Herdmania and its role in the marine ecosystem.

Affinities and Systematic Position of Herdmania

• Historical Uncertainty: The systematic classification of urochordates, which include ascidians like Herdmania, has been historically challenging. Early attempts to place them within the animal kingdom were met with uncertainty.
• Chordate Affinities: The inclusion of ascidians under the Phylum Chordata is now widely accepted due to the presence of tadpole larvae that possess all the fundamental characteristics of chordates, including gill slits, a notochord, and a dorsal tubular nerve cord.
• Similarities with Cephalochordates: The striking similarities between the ascidian tadpole and adult cephalochordates provide compelling evidence of their evolution from a common chordate ancestor. Early developmental stages closely resemble each other, suggesting a shared ancestry.
• Debates about Hemichordate Inclusion: While there have been suggestions that urochordates share some similarities with hemichordates, the debate continues regarding whether hemichordates should be classified within the Phylum Chordata.