Amphioxus (Lancelet)

Amphioxus (Lancelet)

The Amphioxus also known as lancelet, Consisting of 30 to 35 species of “fish-like” chordates.

Branchiostoma is one of the few living genera of lancelets. It is the type genus of the family Branchiostomidae. The lancelets are believed to be related to Pikaia (530-million-year-old) fossils.

The first representative organism of the group to be described was Branchiostoma lanceolatum, the European lancelet or Mediterranean amphioxus, lancelet in the subphylum Cephalochordata. It is a marine invertebrate with a notochord but no backbone and is used as a model organism to study the evolutionary development of vertebrates. It was described by Peter Simon Pallas

Scientific classification of Amphioxus:

• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Cephalochordata
• Class: Leptocardii
• Order: Amphoxiformes

Habitat of Amphioxus:

Lancelets are distributed in shallow subtidal sands flats in temperate, subtropical, and tropical seas around the world. The only exception is Asymmetron inferum, a species found in the depth of 225m. They are able to swim; adult amphioxi are mostly benthic. They burrow in the sand through rapid movement of the body, covered by a sheath. They swim by contracting the muscles block or myotomes.

Feeding habit of Amphioxus:

The habitat reflects the feeding habit of Amphioxus.

• They only expose the front end to the water and filter feed on plankton by means of a branchial ciliary current that passes water through a mucous sheet.
• Branchiostoma floridae is capable of trapping particles from microbial to small phytoplankton size.
• while lanceolatum preferentially traps bigger particles.
• The “gill” slits are solely devoted to feeding.

The amphioxus is inactive filter feeders i.e. the Filter feeders are a sub-group of suspension-feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Their half body is buried in sand most of the time.

In the lancelet, Hatschek’s pit is found which produce mucus. By the ciliary action, with the help of mucus, they trap food particles.

Respiration in Amphioxus:

Amphioxux do not possess a respiratory system. They breathe through the skin (made up of simple epithelium)

Excretion in Amphioxus:

The excretory system consists of segmented “kidneys” containing protonephridia. There is absence of nephrons. In lancelets segmented gonads are found.

Reproduction in Amphioxus:

• These are gonochoric means consisting of two sexes. They reproduce through external fertilization.
• numerous segmented gonads are found in them.
• They reproduce only during their spawning season, which varies slightly between species, usually in the spring and summer months.
• All lancelets species spawn shortly after sunset, either synchronously e.g. Branchiostoma floridae, about once every 2 weeks during spawning season, or asynchronously e.g. Branchiostoma lanceolatum, gradual spawning through the season.

Anatomy of lancelets (Body Parts related facts):

The length of amphioxus varies depending on species, 2.5 to 8 cm. Except for the size, the species are very similar in general appearance, differing mainly in the number of Myotomes and the pigmentation of their larvae.

• They have a translucent, somewhat fish-like body but the absence of any paired fins or other limbs.
• A relatively poorly developed tail fin is present, so they are not especially good swimmers.
• They possess some cartilages material stiffening the gill slits mouth and tail.
• The true skeleton is not found in them.

Nervous system and Notochord of Amphioxus:

lancelets have a hollow nerve cord running along the back, a pharyngeal slit, and a tail that run past the anus (found tip to tip notochord).

• Myomeres: In lancelets the muscles are arranged in blocks called myomeres.
• The Notochord is not protected by bone, it is made up of a cylinder of cells.
• The Notochord extends into the head.
• The Lancelets do not possess a true brain.

The sensory organs in Amphioxus:

Lancelets have four kinds of light-sensing structures i.e. Joseph cells, Hesse organs, an unpaired anterior eye, and lamellar body. The organs use opsins as light receptors.

All of these sense organs and structures are located in the neural tube, with the frontal eye at the front, followed by the lamellar body, the Joseph cells, and the Hesse organs.

• Opsins are a group of proteins made light-sensitive via the chromophore retinal found in photoreceptor cells of the retina.
• Both the Joseph cells and Hesse organs are in the neural tube

Joseph cells: These are bare photoreceptors surrounded by a band of microvilli. These cells bear the opsin melanopsin. Joseph cells forming a dorsal column of the neural tube. The Joseph cells extend from the caudal end of the anterior vesicle to the boundary between myomeres 3 and 4

The Hesse organs: also known as dorsal ocelli consist of a photoreceptor cell surrounded by a band of microvilli and bearing melanopsin, but half enveloped by a cup-shaped pigment cell. the Hesse organs in the ventral part along the length of the neural tube. Hesse organs begin and continue nearly to the tail.

Frontal Eye: The frontal eye consists of a pigment cup, a group of putative photoreceptor cells, three rows of neurons, and glial cells.

Fluorescent proteins: The amphioxus contains Green Fluorescent proteins (GFP ) near the eyespot and inside the oral tentacles.

• It is suspected GFP plays multiple roles with lancelets such as attracting plankton towards their mouth. Considering the lancelets are filter feeders, the natural current would draw nearby plankton into the digestive tract.

Development of Amphioxus:

The Egg of Amphioxus:

• The egg of Amphioxus is microlecithal, and isolecithal type.
• The nucleus is almost centric, the yolk content is very less and does not affect the nucleus.
• The egg can be differentiated into an animal (upper) pole and vegetative pole (Lower).

The sperm of Amphioxus:

The amphioxus’s sperm consists of a beak or acrosome, a head comprising a large compact nucleus, a neck or middle piece, and a long vibratile tail.

• Acrosomal sperm

Fertilization in Amphioxus

Only one sperm can fuse with the egg. After the entry of sperm, the membrane becomes fibrous, and it is called the fertilization membrane. The fertilization membrane prevents the entry of more sperms. The egg and sperm come close and form a nuclear membrane around them and form the zygote.

Cleavage in Amphioxus

Cleavage in amphioxus is complete, i.e. holoblastic.

1. A first cleavage is a meridional plane, which is passing through the animal pole to the vegetal pole axis forming two equal blastomeres.
2. The second plane is also meridional but at the right angle to the first one forming four equal-sized blastomeres.
3. The 3^rd plane of cleavage is latitudinal, slightly above the equatorial plane, the product is the 8-cell stage of which four upper smaller cells called micromere and four lower larger are called megameres.
4. 4^th stage of cleavage is the meridional forming 16 cell stage.
5. 5^th cleavage is latitudinal and forms 32 cells in four tiers.
6. 6^th set of cleavage is the meridional form 64 cell stage.
7. now the cleavage is synchronous
8. The cleavage plane on the 7^th and onward is asynchronous, i.e. cells do not divide at a particular time.
9. Now morula formation is a solid ball-like formation.
10. A small cavity appears in the interior of the embryo. It becomes fluid-filled and expands gradually pushing the cells on the periphery and as a result, a hollow ball of cells is formed having a spacious fluid-filled cavity called blastocoel surrounded by a single layer of cells. This is called Blastula.

Fate Map

The different parts of Amphioxus embryo at 32 cell stage have got following fates

• The vegetal hemisphere is responsible for the formation of Endoderm
• Above this is a crescent-shaped area responsible for the formation of mesoderm
• Above the notochord area, lies the neural ectoderm area

Gastrulation in Amphioxus

Gastrulation is a process by which the monoblastic blastula is converted into a structure containing well-defined three germinal layers, from which different organs can be formed. It can be dealt with under the following stages;

1. Invagination of the prospective endoderm and formation of the blastopore:
   1. Invagination is a process in which the layer of cells itself goes into the interior of the developing embryo.
   2. This process starts at the vegetal pole.
   3. Initially, the invaginating cavity is very small but gradually it deepens into the blastocoel.
   4. The new cavity is called Archenteron. As the archenteron advances, the old cavity (blastocoel) starts Obliterating
   5. The archenteron communicates to the outside by a pore called the blastopore.
   6. Initially, the blastopore is a small pore but very is soon it becomes a well-developed structure and can be divided into three lips- one dorsal, two lateral, and one ventral.

2. Involution of chord –mesoderm;
   1. In the advance distance of invagination that that’s the lip of the blast to poor is flanked by the prospect of notochord cells, while data when trailing by the prospective mesodermal cells.
   2. Then auto chordal cells move inside the process of involution to take the dorsal position in the developing embryo.
   3. The mesodermal cells move inside by the process of involution through the ventral lip of the blaster pore and take the ventral position in the developing embryo.
3. Stretching of the embryo in the anterior-posterior direction:

Now the embryo is stretched in the anterior-posterior direction, which causes the stretching of the notochord in the axis of the body and it occupies the roof of the archenteron from anterior to posterior end. Mesodermal mass is also stretched so that it occupies the laterodorsal position in the developing embryo.

4. Closing of the blastopore:

After the completion of the invagination and involution, the blastopore starts diminishing and finally leaves a narrow crevice.

Structure of a fully formed gastrula:

A fully formed gastrula of Amphioxus contains well-defined three-terminal layers, the ectoderm, mesoderm, and endoderm. The upper portion is covered by neural ectoderm and all sides by the epidermal ectoderm. A spacious cavity called archenteron is present in the interior of the embryo which contains various cell types at various portions:

• The roof is formed of notochord cells.
• Sides contain mesodermal cells.
• Rest is endodermal.

Tubulation:

It is the formation of organ rudiments from three germinal layers so that organs can develop from them.

• Neurulation- formation of neural tube
• Notogenesis- formation of the notochord.
• Mesogenesis- formation of mesoderm.
• Development of endoderm and formation of the alimentary canal

Notogenesis

The roof of the Archenteron is invaginated to form a closed structure called a notochordal cord. Later on, it develops into a notochord just below the neural tube and above the alimentary canal.

Mesogenesis

The lateral portions on both sides are evaginated to form a cord of cells from the anterior to the posterior end containing a space derived from the archenteron called Coelom.

Later on the cord is cut off in different segments to form somites.

Development of endoderm and alimentary canal:

After the cutting of cellular masses of the notochord and mesoderm, the remaining portion of the inner layer of the embryo flanks the cavity and is called the endoderm, while the cavity, later on, develops into the alimentary canal.

 

Reference: NCERT Text Books, ANC notes of UG part 2, https://en.wikipedia.org/wiki/Lancelet

You can also read:

• Characteristics and Classification of Phylum- Chordata
• Animal kingdom – Classification
• Phylum – Porifera
• Phylum – Coelenterata (Cnidaria)
• Phylum- Ctenophora
• Phylum- Platyhelminthes
•  Aschelminthes,
• Phylum-Annelida
• Phylum-Arthropoda
• Phylum-Mollusca
• Phylum – Echinodermata

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