amphioxus
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What is an amphioxus?
An amphioxus, also known as lancelets, are invertebrates belonging to the subphylum Cephalochordata within the phylum Chordata. These marine animals inhabit coastal waters, particularly in warmer regions.
Reaching lengths of up to 8 cm, they possess features like a notochord, gill slits, and a dorsal nerve cord that persist from the larval stage into adulthood. Amphioxi filter food particles from the water and swim by contracting muscle blocks along their body. Reproduction occurs sexually, with separate sexes in amphioxus.
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Which animal group does amphioxus belong to?
Amphioxus belongs to the subphylum Cephalochordata within the phylum Chordata. These marine invertebrates are closely related to vertebrates and share key features such as a notochord, dorsal nerve cord, and pharyngeal gill slits. Unlike vertebrates, amphioxus retains its notochord into adulthood. The informal term Amphioxus refers to the genus Branchiostoma, with about two dozen species distributed in warmer coastal waters. They filter feed while partially buried in the ocean bottom and can swim using muscle blocks along their body.
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Where can you find amphioxus in nature?
Amphioxi, also known as lancelets, are marine invertebrates belonging to the subphylum Cephalochordata. These creatures inhabit coastal waters, preferring the warmer regions of the world, but can also be found in temperate areas.
They often bury themselves in gravel or mud on the ocean floor, with only the anterior part of their body exposed to filter food particles from the water. While feeding, water passes through their gill slits. At night, they may swim near the bottom. They use rapid body movements to burrow into the sand. In certain coastal areas of China, amphioxi are abundant enough to support a fishing industry.
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What features make amphioxus different from other sea animals?
Amphioxi, belonging to the subphylum Cephalochordata, possess unique features that set them apart from other sea animals. They are small, seldom exceeding 8 cm, and resemble slender fishes but lack eyes or a distinct head.
Key distinctions:
* Chordate features: The notochord, gill slits, and dorsal nerve cord persist from the larval stage into adulthood.
* Lack of a heart: Instead of a single heart, amphioxus blood is transported via independent contractions in vessels like the sinus venosus.
* Blood composition: Amphioxus blood is nonpigmented and lacks cells; oxygen is transported via simple solution.
* Phagocytosis: Unlike other chordates, amphioxi can engulf food particles via phagocytosis.
* Unique swimming motion: They swim via side-to-side movement from staggered muscle blocks.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why are cephalochordates important for scientists studying evolution?
Cephalochordates, such as amphioxus, are crucial to understanding the evolution of chordates because they are likely the closest living invertebrate relatives of the vertebrates. Amphioxus possess key chordate features such as a notochord, dorsal nerve cord, and pharyngeal gill slits, which are also found in vertebrates.
Amphioxus retain the notochord throughout their life, whereas in vertebrates, the embryonic notochord is replaced by bony vertebrae or cartilaginous tissue. Studying amphioxus helps scientists understand how the vertebrate brain may have evolved from a simple nerve cord. The existence of fossilized amphioxus dating back 525 million years provides insights into early chordate evolution.
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How does the body structure of amphioxus help us understand the development of vertebrates?
Amphioxus, belonging to the subphylum Cephalochordata, are considered the closest living relatives of vertebrates. Possessing a notochord, dorsal nerve cord, and gill slits, similar to vertebrates, the amphioxus body structure provides insights into vertebrate development.
The notochord in amphioxus extends throughout the entire body and is retained into adulthood, unlike in most vertebrates where it is replaced by vertebrae. The similarities in basic body plan, along with the presence of Hox genes, crucial for body development, offer valuable clues about the evolutionary origins of vertebrates. Studying amphioxus helps in understanding the development of the nervous system and the evolution of the vertebrate brain.
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amphioxus, any of certain members of the invertebrate subphylum Cephalochordata of the phylum Chordata. Amphioxi are small marine animals found widely in the coastal waters of the warmer parts of the world and less commonly in temperate waters. Both morphological and molecular evidence show them to be close relatives of the vertebrates. A brief treatment of amphioxi follows. For full treatment, see cephalochordate.
Amphioxi are seldom more than 8 cm (3 inches) long and resemble small, slender fishes without eyes or definite heads. They are grouped in two genera—Branchiostoma (also called Amphioxus) and Epigonichthyes (also called Asymmetron)—with about two dozen species. The chordate features—the notochord (or stiffening rod), gill slits, and dorsal nerve cord—appear in the larvae and persist into adulthood.
Amphioxi spend much of their time buried in gravel or mud on the ocean bottom, although they are able to swim. When feeding, they let the anterior part of the body project from the surface of the gravel so that they can filter food particles from water passing through their gill slits. At night they often swim near the bottom. They burrow into sand using rapid movements of the body, which is tapered at both ends and is covered by a sheath (the cuticle).
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The animals swim by contracting the muscle blocks, or myotomes, that run from end to end on each side of the body. The blocks on each side are staggered, producing a side-to-side movement of the body when swimming. Amphioxi are not buoyant, and they sink quickly when they stop swimming. A dorsal fin runs along the entire back, becomes a caudal fin around the tip of the tail, and then continues as a ventral fin; there are no paired fins.
The notochord runs through the body from tip to tip, providing a central support. A slight bulge distinguishes the anterior end of the nerve cord. Although there is no brain or cranium, growing evidence suggests that the vertebrate brain evolved from a portion of the nerve cord in a lancetlike ancestor. Blood flows forward along the ventral side and backward along the dorsal side, but there is no distinct heart.
The oral cavity of amphioxi is furnished with a hood whose edges are lined with cirri; these are fringelike structures that form a coarse filter to screen out particles too large to be consumed. Water is directed through the small mouth into the pharynx by the action of cilia on the gill slits. Food particles in the passing water are caught by the mucous lining of the gill basket and passed into the gut, where they are exposed to the action of enzymes. Unlike other chordates, amphioxi are capable of a digestive process called phagocytosis, in which food particles are enveloped by individual cells.
Above the pharynx is the excretory system made up of the nephridia, which opens into an excretory canal leading to the atrium. The endostyle corresponds to the thyroid in vertebrates, since it seems to produce iodinated, tyrosine molecules, which may function as regulatory substances, much like hormones, in amphioxi.
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Male and female amphioxi are identical in outward appearance and differ internally only in the nature of the gonads, or reproductive glands, which form in rows on the wall of the atrial cavity. Breeding takes place several times a year in tropical regions but only once in temperate areas. Sacs containing eggs or sperm burst and discharge their contents into the water through an opening on the underside of the body. Eggs are fertilized in the water, and after about two days microscopic ciliated larvae develop from the fertilized eggs. The larvae are carried with ocean currents for several weeks before metamorphosing into juvenile amphioxi and taking up life in sandy sediments.
Along parts of the coast of China, amphioxi are so numerous that they constitute the basis of a fishing industry. What appear to be fossilized amphioxi have been found recently in sediments about 525 million years old.