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The lancelets ( ), also known as amphioxi (: amphioxus ), consist of 32 described species of somewhat fish-like benthic filter feeding in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae.
Lancelets diverged from other chordates during or prior to the Cambrian period. A number of fossil chordates have been suggested to be closely related to lancelets, including Pikaia and Cathaymyrus from the Cambrian and Palaeobranchiostoma from the Permian, but their close relationship to lancelets has been doubted by other authors. Molecular clock analysis suggests that modern lancelets probably diversified much more recently, during the Cretaceous or Cenozoic.
They are of interest to Zoologists as lancelets contain many organs and organ systems that are homologous to those of modern fish. Therefore, they provide a number of examples of possible evolutionary exaptation. For example, the gill-slits of lancelets are used for feeding only, and not for respiration. The circulatory system carries food throughout their body, but does not have red blood cells or hemoglobin for transporting oxygen.
Comparing the genomes of lancelets and vertebrates and their differences in gene expression, function and number can shed light on the origins of vertebrates and their evolution. The genome of a few species in the genus Branchiostoma have been sequenced: B. floridae, B. belcheri, and B. lanceolatum.
In Asia, lancelets are harvested commercially as food for humans. In Japan, amphioxus ( B. belcheri) has been listed in the registry of "Endangered Animals of Japanese Marine and Fresh Water Organisms".
Nicholas and Linda Holland were the first researchers to describe a method of obtaining amphioxus embryos by induction of spawning in captivity and in vitro fertilization. Spawning can be artificially induced in the lab by electric or thermal shock.
Kovalevsky also released the first complete description of amphioxus embryos, while Max Schultze was the first to describe the larvae.
Depending on the exact species involved, the maximum length of lancelets is typically . Branchiostoma belcheri and B. lanceolatum are among the largest. Except for the size, the species are very similar in general appearance, differing mainly in the number of and the pigmentation of their larvae. They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, so they are not especially good swimmers. While they do possess some cartilage material stiffening the , mouth, and tail, they have no true complex skeleton.
Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are closely packed in collagen fibers to form a toughened rod. The lancelet notochord, unlike the vertebrate Vertebral column, extends into the head. This gives the subphylum, Cephalochordata, its name (, kephalē means 'head'). The fine structure of the notochord and the cellular basis of its adult growth are best known for the Bahamas lancelet, Asymmetron lucayanum
The nerve cord is only slightly larger in the head region than in the rest of the body, so that lancelets do not appear to possess a true brain. However, developmental gene expression and transmission electron microscopy indicate the presence of a diencephalic forebrain, a possible midbrain, and a hindbrain. Recent studies involving a comparison with vertebrates indicate that the vertebrate thalamus, pretectum, and midbrain areas jointly correspond to a single, combined region in the amphioxus, which has been termed di-mesencephalic primordium (DiMes).
Both the Joseph cells and Hesse organs are in the neural tube, the Joseph cells forming a dorsal column, the Hesse organs in the ventral part along the length of the tube. The Joseph cells extend from the caudal end of the anterior vesicle (or cerebral vesicle) to the boundary between myomeres three and four, where the Hesse organs begin and continue nearly to the tail.
The putative photoreceptor cells, Row 1, are arranged in two diagonal rows, one on either side of the pigment cup, symmetrically positioned with respect to the ventral midline. The cells are flask-shaped, with long, slender ciliary processes (one cilium per cell). The main bodies of the cells lie outside of the pigment cup, while the cilia extend into the pigment cup before turning and exiting. The cells bear the opsin c-opsin 1, except for a few which carry c-opsin 3.
The Row 2 cells are Serotonin neurons in direct contact with Row 1 cells. Row 3 and 4 cells are also neurons. Cells of all four rows have axons that project into the left and right ventrolateral nerves. For Row 2 neurons, axon projections have been traced to the Tegmentum neuropil. The tegmental neuropil has been compared with locomotor control regions of the vertebrate hypothalamus, where paracrine release modulates locomotor patterns such as feeding and swimming.
It is suspected GFP plays multiple roles with lancelets such as attracting plankton towards their mouth. Considering that lancelets are filter feeders, the natural current would draw nearby plankton into the digestive tract. GFP is also expressed in , signifying it may be used for photoprotection by converting higher energy blue light to less harmful green light. The fluorescent proteins from lancelets have been adapted for use in molecular biology and microscopy. The yellow fluorescent protein from Branchiostoma lanceolatum exhibits unusually high quantum yield (~0.95). It has been engineered into a green fluorescent protein known as mNeonGreen, which is the brightest known monomeric green or yellow fluorescent protein.
Lancelets have oral cirri, thin tentacle-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body. Water passes from the mouth into the large human pharynx, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of mucus. Cilium action pushes the mucus in a film over the surface of the gill slits, trapping suspended food particles as it does so. The mucus is collected in a second, dorsal groove, known as the epipharyngeal groove, and passed back to the rest of the digestive tract. Having passed through the gill slits, the water enters an atrium surrounding the pharynx, then exits the body via the atriopore.
Both adults and larvae exhibit a "cough" reflex to clear the mouth or throat of debris or items too large to swallow. In larvae the action is mediated by the pharyngeal muscles while in the adult animal it is accomplished by atrial contraction.
The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. The hepatic caecum, a single blind-ending cecum, branches off from the underside of the gut, with a lining able to phagocytosis the food particles, a feature not found in vertebrates. Although it performs many functions of a liver, it is not considered a true liver but a homolog of the vertebrate liver.
The excretory system consists of segmented "kidneys" containing nephridium instead of , and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmented .
With the advent of molecular genetics lancelets are once again regarded as a model of vertebrate ancestors, and are used again as a model organism.
As a result of their use in science, methods of keeping and breeding lancelets in captivity have been developed for several of the species, initially the European Branchiostoma lanceolatum, but later also the West Pacific Branchiostoma belcheri and Branchiostoma japonicum, the Gulf of Mexico and West Atlantic Branchiostoma floridae and the circumtropical (however, genetic evidence suggest the Atlantic and Indo-Pacific populations should be recognized as separate) Asymmetron lucayanum. They can reach an age of up to 7–8 years.
Among the three extant (living) genera, Asymmetron is basal. Molecular clock studies have come to different conclusions on their divergence, with some suggesting that Asymmetron diverged from other lancelets more than 100 million years ago while others have suggested that it occurred about Lutetian million years ago. According to the younger estimation, Branchiostoma and Epigonichthys have been estimated to have diverged from each other about Bartonian million years ago. Despite this deep separation, hybrids between Asymmetron lucayanum and Branchiostoma floridae are viable (among the deepest split species known to be able to produce such hybrids).
The following are the species recognised by WoRMS. Other sources recognize about thirty species. It is likely that currently unrecognized cryptic species remain.
As human food
The animals are edible and harvested in some parts of the world. They are eaten both fresh, tasting like herring, and as a food additive in dry form after being roasted in oil. When their gonads start to ripen in the spring it affects their flavor, making them taste bad during their breeding season.
Phylogeny and taxonomy
The lancelets were traditionally seen as the sister lineage to the ; in turn, these two groups together (sometimes called Notochordata) were considered the sister group to the Tunicate (also called Urochordata and including sea squirts). Consistent with this view, at least ten morphological features are shared by lancelets and vertebrates, but not tunicates.Michael J. Benton (2005). Vertebrate Palaeontology, Third Edition 8. Oxford: Blackwell Publishing. . Newer research suggests this pattern of evolutionary relationship is incorrect. Extensive molecular phylogenetic analysis has shown convincingly that the Cephalochordata is the most basal subphylum of the chordates, with tunicates being the sister group of the vertebrates. This revised phylogeny of chordates suggests that tunicates have secondarily lost some of the morphological characters that were formerly considered to be synapomorphies (shared, derived characters) of vertebrates and lancelets. Lancelets have turned out to be among the most genetically diverse animals sequenced to date, due to high rates of genetic changes like exon shuffling and domain combination.
The cladogram presented here illustrates the phylogeny (family tree) of lancelets, and follows a simplified version of the relationships found by Igawa and colleagues (2017):
See also
Further reading
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