Amphioxus – 8 Primitive Features of Amphioxus
The primitive features of Amphioxus can be summarized with this 8 concise descriptions:
1.The ciliary mode of feeding is undoubtedly primitive and has been lost completely in many metazoan phyla; it persists however in lamelibranch mollusc, in the urochordates and cephalochordates. The endostyle, as an accessory feeding organ is found only in urochordates, cephalochordates and in the larvae of cyclostomes craniates. In the ammocoete larva of the lamprey, petromyzon fluviatilis, the endostyle is present and functional, but in the adult it becomes the thyroid gland and that is its destiny in all vertebrates.
2. Amphioxus possesses a complete row of segmental myotomes from front to rear. This persistence of complete segmentation is associated with non-chordates and particularly annelids. In the vertebrates, though there is always segmentation of the mesoderm, it often disappear in adult, especially in the anterior region. We see in the dogfish a long series of myotomes but the first three pairs becomes modified to form the eye-muscles.
3. In the condition of its nervous system, Amphioxus is primitive. The primitive features of amphioxus can be clearly seen in the very little differentiation of the brain; there are no paired sense organs. The afferent nerve fibres originate in superficial sensory cells and proceed directly into the central nervous system without the mediation of dorsal ganglia on the afferent roots. This is the condition we find in annelids and arthropods.
4. Other primitive features of amphioxus is that the blood has no respiratory pigment and there is no heart.
5. The blood vessel do not show distinction into arteries and veins. Even by comparison with the great non-chordate phyla there is a startling lack of cephalization and of paired limbs.
6.The persistence of the notochord in an unmodified state throughout life is not found in other chordates. In vertebrates, traces of it are present though it becomes enveloped and in some cases almost obliterated by the vertebral column.
7. The primitive features of amphioxus can also be observed in the skin which presents a condition with which we are familiar in the annelids.
8.There is an epidermis one cell thick, lying over connective tissue. In all the vertebrate chordates, the epidermis becomes a stratified epithelium.
The slender fish-like animals called amphioxus are not easily recognized, as they lack a well-defined head. But they do have a dorsal neural cord, notochord and gill slits which distinguish them from the other chordates.
They also have a naked epidermis, unlike other animals with cuticles. The characteristics are reminiscent of their ciliated ancestral form.
Morphology of Amphioxus
The living fossil amphioxus or lancelet is a basal chordate and a model for understanding the evolution of vertebrates. It has a complex body plan with a notochord, nerve cord and gill-slits. It also has a straight digestive tract and no jaws or paired fins. Amphioxus is one of the few chordates to retain complete segmentation. Segmentation occurs in the block-like structures of muscle known as somites. This feature is also found in other chordates, but disappears in many modern vertebrates. It is essential for understanding the origin of vertebrates and their ancestors.
Amphioxus has been a subject of extensive scientific study for more than a century, with early research done by the famous Chinese zoologist Tung Tichou. It has been used for embryological, genetic and morphological studies. Modern molecular phylogenetic analyses place it at the base of the chordate family, with strong similarities to vertebrate ancestors. The genome of amphioxus provides a unique opportunity to study the evolution of nonchordate deuterostomes and invertebrate chordates as well as vertebrates.
Other Primitive Features of Amphioxus
It is a free-swimming marine mollusk with a segmented, muscular body that resembles a fish. Its genus Amphioxus is named after its distinctive hood or oral lips. It is of special zoological interest because it represents the last vestige of the primitive chordate phylum Chordata. Amphioxus is an important symbiotic partner for other marine species and has a wide distribution around the world.
The notochord in amphioxus extends through its entire length and has a nerve cord that runs along it. It also has a series of gill-slits on each side of its body. The gill-slits are used to feed and respire. Amphioxus also has a straight intestine and ciliary feeding. Its brain and sense organs are degenerate.
This symbiotic relationship with other animals has enabled amphioxus to survive for millions of years in a variety of habitats. It is especially abundant in shallow sandy environments. It is the most common marine invertebrate in some parts of the world. It is also an important food item for human beings and domestic animals.
A phylogenetic analysis based on mitochondrial genomes of Asymmetron, Branchiostoma and Epigonichthys indicates that these amphioxus genera split from the earliest chordate ancestor in the Early Cambrian, more than 405 million years ago. The phylogenetic tree shows that amphioxus is the closest living relative to vertebrates.
This is a significant advance because it allows for the estimation of divergence times with more accuracy than previous estimates using deep reference points that cannot be applied to mitochondrial genes due to substitution saturation9. In addition, the analysis of gene turnover in amphioxus has revealed a fast evolutionary rate in comparison to other chordates. The results of these analyses indicate that amphioxus is an important stepping stone on the path to the emergence of vertebrates.
Ecological Importance
Amphioxus are a major component of shallow sandy seafloor communities and play important roles in the food chain. Their symbiotic relationships with corals, other invertebrates, and fishes as well as their ability to withstand desiccation are all very significant. They are also important habitat modifiers and nutrient recyclers. They can increase the depth of a sand bank by compacting it and forming sediment bridges, and they can reduce turbidity and sedimentation, and improve water quality by removing excess nutrients from the water column. Amphioxus can also be a valuable tool in assessing environmental change. They can detect water quality changes by their respiration rate, which varies according to temperature and salinity. They can also help to monitor the effects of oil spills on coastal ecosystems by their response to contaminants.
In learning about the primitive features of amphioxus, we must understand that they have a long evolutionary history, from their Precambrian origin to the most recent diversification. This makes them a useful model system for investigating the evolutionary dynamics of bilaterians.
The last common ancestor (LCA) of extant amphioxus was likely small, with a ciliated body and left-right asymmetry. The LCA may have inhabited shoals of fine sand on the seafloor and acquired its modern development pattern with metamorphosis, similar to that of the Florida lancelet Branchiostoma floridae.
Today, amphioxus are abundant in many shallow sandy environments. In Discovery Bay, Jamaica, they can be found at a density of five thousand per square metre of sand. They can also be widely cultivated as ornamental aquarium species.
Amphioxus are an important food item for domestic animals and are eaten by humans in some parts of the world. They are also harvested as a source of fertilisers and aquaculture feed.
Phylogenetic analyses of molecular data have provided new estimates of divergence time between the extant genera of amphioxus. In studying the primitive features of amphioxus, we see that the current phylogenetic tree shows that the amphioxus crown lineage evolved in the Eocene at a rate comparable to that of vertebrates. The divergence times between the Asymmetron and Epigonichthys clades are particularly close, and they coincide with geological events such as the closure of the Neo-Tethys36 and the formation of the Isthmus of Panama37.
The symbiotic relationship between amphioxus and the corals they graze on is an example of coevolution. They are both adapted to the specific conditions of each other’s habitat and their interaction has shaped the evolution of coral reef communities. This type of evolutionary coevolution between a mutualistic organism and its host is an excellent example of natural selection. It has led to the development of complex symbiotic relationships and has been an important driving force in coral reef biodiversity. Moreover, it provides a key to understanding the process of biomagnification in corals, a phenomenon that threatens the sustainability of many marine resources. In the future, research on the interactions of amphioxus with other aquatic organisms will be essential to a better understanding of the impact of human activities on coral reefs.
Significance in the food chain
The little marine animals called amphioxus (from Greek “amphi” + “oxys” = sharp at both ends) are close relatives of vertebrates, as morphological and molecular evidence suggest. They are typically slender and barely more than 8 cm (3 inches) long, resembling a small echinoderm with gill slits and a dorsal tubelike nerve cord. They are abundant in the warmer sections of world’s coastal waters and are often an important food source for fishes, including humans.
A number of unique features distinguish amphioxus from other chordates and invertebrates. For example, their embryonic development is characterized by the presence of a notochord, dorsal tubular nerve cord and pharyngeal gill slits. They also have segmented somites and a well-developed heart-like circulatory system, both of which are shared by vertebrates.
These primitive traits probably arose in the common chordate ancestor. Molecular studies reveal that amphioxus and its closest living relatives, the Florida lancelets of the genus Branchiostoma, have retained these characteristics throughout their evolutionary history.
Amphioxus is one of the smallest surviving members of the chordate subphylum Cephalochordata. Its phylogenetic position as the most recent common ancestor of vertebrates, inferred from its anatomy and genome sequence, makes it an excellent model for understanding the evolution of early chordates.
As in other chordates, amphioxus breathes through gill slits on the sides of the head. The digestive tract, which consists of a short intestine and stomach, contains a hepatic cecum that appears to be homologous to the liver in vertebrates. It projects into the atrium on the right side of the pharynx. The excretory organ consists of three segments, each of which is specialized for its function: solenocytes secrete a salty substance called amphioxanin into the pharyngeal cavity; renal tubules secrete urea from the kidney into the hepatic cecum; and a gland called the endostyle produces iodinated tyrosine molecules that may serve as a hormone in vertebrates.
The amphioxus genome contains two copies of most genes. The genes are clustered into 17 distinct blocks, each corresponding to a ancient chordate linkage group in human, chicken and teleost fish genomes. The pattern of amphioxus-human synteny suggests that these gene clusters evolved from chromosome segments that were duplicated in the common chordate ancestor.
The top phylogenetic tree is based on the amino acid sequences of nuclear transcriptomes. The bottom phylogenetic tree is based upon the nucleotide sequences of whole mitogenomes. Bootstrap values and posterior probabilities are shown at nodes. The Florida lancelets are monophyletic with the Branchiostoma species B. floridae and the Epigonichthyes species Asymmetron clade.
