Study of larval forms In Protochordates

Larval Forms in Protochordates

The larval forms of Protochordates, along with the subphylum Cephalochordata (lancelets) and Urochordata (tunicates), are central to the clarification of the phylogeny of Chordata. The larval stages of Protochordates exhibit very typical characteristics provided within all chordates, such as the presence of a notochord.

• Notochord – This is one characteristic of chordates. The notochord appears in the larval stages of Protochordates. The notochord is a rod-like structure which is unjointed, rigid, and flexible, lying dorsally in the body of such animals. The notochord provides support to the body of such animals and it acts as an ancestor of the vertebral column found in the vertebrates.
• Larval Characters– The larval forms of Protochordates possess some important characters of chordates except the notochord. These characters are a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.
• Dorsal Hollow Nerve Cord – The young forms have a nerve cord that goes along the back of the body. This nerve cord is hollow and acts as a main pathway for sending nerve signals. In chordates, this nerve cord grows into the central nervous system, which includes the brain and spinal cord.
• Pharyngeal Slits- Another characteristic that the young forms of Protochordates exhibit is the presence of pharyngeal slits. These are slit-like structures found in the throat area, and depending on the species, they play a different role. In some of the young forms, it aids in filter feeding, whereas in others, it is an aid for breathing and gas exchange.
• Post-anal Tail-This is the second feature observed in the young forms of Protochordates. It is the post-anal tail which protrudes from the anus to provide assistance with movement and balance. The importance of swimming depends on it; thus, larvae are moved in water by the tail.

The larval forms of Protochordates are essential in the study of chordate evolution and connections. They depict the fundamental features that all chordates share, including the notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail. Studying these larval forms informs scientists about them.

Larval forms of Protochordata

• Tornaria larva of Balanoglossus ( Hemichordates)
• Ascidian tadpole larva of Urochordata
• Larval tunicates of Lancet ( Cephalochordates)

1. Tornaria larva of Balanoglossus ( Hemichordates)

Tornaria larva is a peculiar larval form of the hemichordate Balanoglossus. J. Müller first described it in 1805 and thought that it was an echinoderm larva. In 1869, Metchnikoff recognized it as Balanoglossus, and named it Tornaria due to its circular rotation behavior.

• Larval Characteristics- The Tornaria larva has a transparent, elongated body with a clear front and back. The front features a prototroch for movement and feeding, while the back has a telotroch. These ciliary bands assist in swimming and feeding.
• Rotational Behaviour:-The Tornaria larva rotates in a circle, allowing it to feed and swim within the water. Its ciliary bands assist it in this motion.
• Identification of Balanoglossus Larva- The discovery of Tornaria larva was a part of the life cycle of Balanoglossus, which highly improved the taxonomy of hemichordates. Metchnikoff’s description defined the connection between the larva and adult Balanoglossus.
• Life Cycle – Tornaria is the planktonic larval stage in the life cycle of Balanoglossus. These larvae swim freely in the water until they change to become the adult form of Balanoglossus, arising from eggs laid by the adults.
• Ecological role – Tornaria larvae are important in the life cycle of Balanoglossus for the dispersion of population and feeding. This helps in achieving wider distributions of the populations as well as access to planktonic food.

It indicates the Tornaria larva is well recognized as the larval form of Balanoglossus, contributing to the scientific knowledge of the development and classification of hemichordates. Its features and behavior gave insight into the life cycle of Balanoglossus and its ecological role in Hemichordata.

Structure of Balanoglossus

• Balanoglossus belongs to the phylum Hemichordata and possesses some exceptional features. Of all the different stages in the life cycle of Balanoglossus, tornaria larva is one such stage. This one drifts around in water and strikingly resembles the bipinnaria larva that is also found in starfish and in other echinoderms. Tornaria larva of Balanoglossus shows some unique features.
• The tornaria larva of Balanoglossus is ovate and transparent, about 3mm in diameter. It is small in size and can easily be observed with a microscope. An apical plate is usually provided at the anterior end of the larva. This region is thickened around a tuft of cilia and by a pair of eyespots. Cilia and eyespots have important roles in the sensory perception and movement of the larva.
• One of the amazing features of the tornaria larva is its full digestive system. This means it has a way to eat and digest food that goes from the mouth to the anus, helping it take in and use nutrients while it is in the water. Having a full digestive system shows how important eating and growing are for the larva’s development.
• The larva has an entire band of very small hair-like structures all around its body. This band runs from the front to the back of the larva and forms a ring encircling the area behind the mouth. In various ways, the band helps in movement as well as creates water flow for the purpose of eating and breathing. The movement of the cilia pushes the larva through the water and also helps it to catch food particles.
• The tornaria larva of Balanoglossus is a structure that is adapted to its survival and development in the marine environment. The oval shape and transparency of the larva enable it to move efficiently and allow light to penetrate through it. The apical plate with cilia and eyespots enhances sensory perception, while the complete alimentary canal supports efficient feeding and growth. Finally, the continuous ciliary band aids in locomotion and the acquisition of nutrients.
• In general, the structure of Balanoglossus, especially the tornaria larva, exemplifies the diversity and unique adaptations seen in the phylum Hemichordata. These adaptations enable the organisms to thrive in their marine habitats and contribute to our understanding of the complexity and diversity of life forms in the ocean.

Reproduction of Balanoglossus

• Balanoglossus refers to an invertebrate marine animal from the class Hemichordata. In its life cycle, it is a hermaphrodite species but with separate sexes with external fertilization. Its method of reproduction involves the presence of gonads developed in longitudinal rows along the alimentary canal.
• In Balanoglossus, the gonads arise from the coelomic wall. Sperms and eggs are produced from germinal epithelium lying in the gonads by gametogenesis.
• The beginning of sperm and egg production through cell proliferation in the germinal epithelium is followed by differentiation and maturation into mature gametes. The fully developed sperm and eggs are then released from the gonads into the coelom.
• Balanoglossus has a genital pore that releases mature sperm and eggs into the outside environment. This pore provides an opportunity for these gametes to encounter one another in the surrounding water.
• The fusion of gametes involves the sperm and eggs meeting in water; water provides the medium where fertilization takes place through contact between the swimming sperm and the eggs. This method suits Balanoglossus, which is a marine creature dependent on water for the fusion of gametes.
• After fertilization, the zygote develops into a new Balanoglossus. It first becomes a free-swimming tornaria larva, then metamorphoses and settles on the sea floor as an adult.
• In summary, Balanoglossus reproduces with separate sexes, having gonads in rows beside the alimentary canal. The gonadal germinal epithelium produces sperm and eggs, which are released through a genital pore for external fertilization in water. This strategy aids Balanoglossus’s life cycle and reproduction in marine environments.

Development of Balanoglossus

• The development of Balanoglossus, a marine hemichordate, can be divided into two modes: indirect development, which includes a larval stage known as the tornaria larva, and direct development, which occurs in larger eggs without the presence of a larval form.
• In the development of larva, the development starts with the initiation of the protocoel, which eventually forms the proboscis coelom. Simultaneously, the proboscis pore, an opening in the young developing animal, is also formed. Further development leads to the invagination of the hind gut, resulting in the formation of the collar and trunk regions in the Balanoglossus larva.
• The gut undergoes further differentiation, forming distinct structures such as the esophagus, stomach, and intestine. Importantly, the intestine opens to the outside through the anus, which is located near the blastopore. This anatomical arrangement allows for the elimination of waste materials from the developing organism.
• As the Balanoglossus embryo matures, it forms cilia. These are very tiny hair-like structures that make it possible for the larva to lead a free-swimming life. The cilia help in locomotion, making way for the larva to move in the surrounding water column.
• During the larval stage, the tornaria larva utilizes its cilia to propel itself through the water, facilitating dispersion and increasing the chances of survival. The larva explores its environment, feeding on microscopic organisms and growing in size.
• It is to be noted that not all Balanoglossus species go through larval development. Direct development occurs in some cases. Here, the eggs of Balanoglossus are larger in size and lack a distinct larval stage. Instead, the embryo develops directly into a miniature version of the adult organism.
• In conclusion, the development of Balanoglossus can occur through indirect or direct modes. During larval development, the protocoel develops into the proboscis coelom, and the collar and trunk regions arise from the invagination of the hind gut. The gut differentiates into the esophagus, stomach, and intestine, with the anus opening near the blastopore. The development of cilia enables the larva to lead a free-swimming life, promoting dispersion and exploration of the environment. In cases of direct development, the absence of a larval stage leads to the embryo directly forming a miniature version of the adult Balanoglossus.

2. Ascidian tadpole larva of Urochordata.

Structure

• Urochordata’s ascidian tadpole larva has various distinctive traits. Its tiny oval body and extended tail distinguish this larval stage. Ectodermal cells release a thin test that protects the entire body.
• A caudal fin distinguishes ascidian tadpole larva. This fin is an extension of the test that propels and helps movement. Ectodermal cells form three sticky papillae at the trunk’s anterior end. These papillae let the larva cling to surfaces and explore.
• The sensory vesicle or brain of the ascidian tadpole larva is expanded. Primary sensory processing occurs in this hollow structure. A hollow nerve cord runs from the brain throughout the body. The larva’s coordinated movement and sensory integration are provided via the mid-dorsal nerve cord.
• The sense vesicle has a pigmented statocyst and two ocelli. The statocyst helps the larva balance and orient, while the ocelli detect light intensity and direction.
• The flexible rod-like notochord runs from tail anterior to posterior. The notochord supports and rigidifies the larva, giving it structure.
• Ascidian tadpole larvae have a full alimentary canal with an endostyle pharynx. The endostyle secretes mucus into the throat to promote filter-feeding and food particle collection. Stigmata allow water to enter and exit the pharynx, aiding breathing.
• The dorsal atrial opening opens the atrial cavity around the throat. This hollow stores water and circulates respiratory gases. The ascidian tadpole larva has a heart and pericardium for circulation.
• The Urochordata ascidian tadpole larva has a distinct body shape, a tail with a caudal fin, adhesive papillae, a hollow nervous system with a sense vesicle and nerve cord, pigmented statocyst and ocelli, notochord, complete alimentary canal, pharynx with endostyle and stigmata, atrial cavity, and a well-developed circulatory system. These physical traits let larvae survive, move, sense, and eat.

Development

• Ascidian tadpole larvae of Urochordata undergo intriguing retrogressive transformation. A free-swimming larvae becomes a stationary adult in this remarkable evolution.
• A fertilised egg becomes an ascidian tadpole larva. The egg divides into a blastula, a hollow cell ball. During gastrulation, blastula cells generate ectoderm, endoderm, and mesoderm.
• The ascidian tadpole larva develops. It has a long tail and short oval body. One thin test generated by ectodermal cells covers the body. This test shields and supports larvae.
• With the sensory vesicle or brain, the larva’s nervous system develops. Pigmented statocysts and ocelli are found in this hollow anterior nerve system. These organs allow the larva to detect light and orientation changes.
• From the anterior to the posterior of the tail, the larval notochord is a flexible rod. The notochord controls larval movement and provides structure.
• The larva undergoes retrogressive transformation through development. This involves larval structural regression and adult development. In retrogressive transformation, the tail, caudal fin, and adhesive papillae disappear. These structures are unnecessary for inactive adults.
• The pharynx changes significantly. Endostyle and stigmata, which were critical for larval filter-feeding, change or vanish. The larval atrial cavity gets more pronounced and functions as a water reserve in the adult.
• Metamorphosis alters the larva’s nervous system. The larval nerve cord shrinks and reorganises in adulthood.
• The adult ascidian retains the larval heart and pericardium. The tail muscles and other larval locomotor mechanisms decline and are replaced by stationary adult components.
• The ascidian tadpole larva matures into a sedentary adult. Although sessile, this adult form may connect to surfaces utilising adhesive papillae or other features.
• The Urochordata ascidian tadpole larva develops by retrogressive metamorphosis. This transition creates larval body, tail, test, nervous system, and notochord. At maturity, larval structures degenerate and are replaced by adult traits, resulting in a sedentary adult ascidian. Retrogressive metamorphosis lets the ascidian adapt to its ecological niche and become a filter-feeding adult from a free-swimming larva.

3. Larval tunicates of Lancet (Cephalochordates)

Structure

• Lancets, or cephalochordates, have a distinct larval anatomy. These fish-like creatures are 3.5 to 6.0 cm long and have various unique anatomical traits.
• Slim, transparent, and laterally compressed, the larval lancet looks elegant. Its ends are tapered and pointy. A snout or rostrum protrudes from the front. An oral hood with 20 or more tentacles is below the rostrum. These tentacles help the larva filter-feed by capturing small food particles from the water.
• Oral hoods are encased in cup-shaped buccal cavities or vestibules that lead to the digestive system. The body is nearly oval except for the front two-thirds, which are triangular. A mid-dorsal caudal fin promotes locomotion and swimming stability throughout the body. A ventral fin mid-ventrally from the caudal fin to the atriopore helps the larva move. The fin rays support these fins, giving them structure and flexibility.
• The lancet larva has a hollow dorsal nerve cord that runs down its body. This dorsal nerve cord sends brain messages and controls physiological activities. The ventral gut digests and absorbs nutrients.
• The larval lancet’s gill-slitted pharynx aids respiration and filter-feeding. Unlike other chordates, adult lancet gill clefts link to the atrium, a bodily chamber, rather than the outside. This configuration sets them apart from other chordates.
• Protonephric larval lancet excretion uses primitive nephridia to eliminate waste. Lancet larvae are male or female. External water fertilisation happens.
• For food, larval lancets use their oral cirri or tentacles to catch tiny particles and plankton from the water column. Their effective filter-feeding helps them to get nutrition as larvae.
• Recent studies suggest lancets, specifically cephalochordates, are the sister group of vertebrates. Mitochondrial and ribosomal evidence shows these creatures’ genetic and evolutionary links.
• The larval tunicates of lancets, or cephalochordates, have a slender, compressed body with a pointed rostrum, oral hood with tentacles, buccal cavity, triangular and oval body regions, mid-dorsal and ventral fins supported by fin rays, hollow dorsal nerve cord, ventral gut, perforated pharynx with gill slits, protonephric excretory system, separate sexes, external fertilisation, and suspension-feeding. These larvae play a crucial role in the life cycle of lancets and give vital insights into the evolutionary relationships between chordates and vertebrates.

Larval Development

Larval development encompasses a series of significant metamorphic changes that shape the transition from larva to the adult form. In the context of the given information, the following key transformations occur:

1. Ectodermal cilia and the club-shaped gland disappear during larval development. These larval structures are no longer needed and are lost.
2. The mouth position changes significantly. It goes from left to antero-ventral. The mouth edges expand inward to produce the velum during this change. Additionally, skin folds form an oral hood that encloses the mouth. Oral cirri and ciliated wheel organs develop in the oral hood.
3. The larva learns to filter-feed. The oral cirri and ciliated wheel organ filter and gather nutrients from the water while the larva feeds.
4. Larval gill-slits vary during development. Tongue bars divide the gill-slits, revealing more behind the initial sets. These changes aid respiration and filter-feeding.
5. The coelomic cavities shrink as the atrium develops. Pharyngeal, myocoel, and gonocoel reductions occur.
6. Gonads, reproductive organs, develop from myotomes’ anterior lower angles. These organs protrude into gonocoels. Gonocoels help gonads grow and function.
7. Finally, the larva’s tail elongates and changes. It becomes more bilaterally symmetrical as it matures. This transition completes larval development and establishes the adult body plan, which is vital to organismal growth.

Significance

Amphioxus oma is important in several ways:

• Evolutionary insight- Embryology, which studies development, gives significant insights into species’ evolutionary past. By studying Branchiostoma embryos, researchers can better comprehend chordate evolution and development. This information explains how evolution has shaped life’s diversity.
• Phylogeny and Classification – Animals are classified into Protostomia and Deuterostomia based on blastopore formation during embryonic development. In protostomia, the blastopore becomes the mouth, while in deuterostomia, it becomes the anus. The Deuterostomia category includes Branchiostoma and other chordates and echinoderms. Researchers can identify and track animal phylum evolution by analysing Branchiostoma development.
• Coelom Formation- Branchiostoma’s coelom formation reveals its evolutionary link with echinoderms. Coeloms, which support and organise internal organs, originate similarly to Branchiostoma and echinoderms. Researchers can connect chordates and echinoderms by researching Branchiostoma’s coelom development, revealing vertebrates’ evolutionary history.

Researchers have argued that two major invertebrate lineages evolved from coelenterates by studying the blastopore, mesoderm, muscle chemistry, and sera protein similarities. As a primordial chordate, Branchiostoma helps bridge chordates and invertebrates. It reveals vertebrate evolution and the common ancestry and evolutionary shifts that formed Earth’s diversity.

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