Lancelets diverged from other chordates during or prior to theCambrian period. A number of fossil chordates have been suggested to be closely related to lancelets, includingPikaia andCathaymyrus from the Cambrian andPalaeobranchiostoma from thePermian, but their close relationship to lancelets has been doubted by other authors.[11][12]Molecular clock analysis suggests that modern lancelets probably diversified much more recently, during theCretaceous orCenozoic.[13][14]
They are of interest tozoologists as lancelets contain many organs and organ systems that arehomologous to those of modern fish. Therefore, they provide a number of examples of possible evolutionaryexaptation. 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 havered blood cells orhemoglobin for transporting oxygen.
Comparing thegenomes of lancelets and vertebrates and their differences in gene expression, function and number can shed light on the origins of vertebrates and theirevolution.[15] The genome of a few species in the genusBranchiostoma have been sequenced:B. floridae,[16]B. belcheri,[17] andB. lanceolatum.[18]
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".[19]
Adult amphioxus typically inhabit the seafloor, burrowing into well-ventilated substrates characterized by a soft texture and minimal organic content. While various species have been observed in different types of substrate, such as fine sand, coarse sand, and shell deposits, most exhibit a distinct preference for coarse sand with low levels of fine particles. For instance,Branchiostoma nigeriense along the west coast of Africa,Branchiostoma caribaeum inMississippi Sound and along the coast from South Carolina to Georgia,B. senegalense in the Atlantic Ocean on theshelf region offNorth West Africa, andB. lanceolatum along the Mediterranean coast of southern France all demonstrate this preference.[20][21][22][23][24][25] However,Branchiostoma floridae fromTampa Bay, Florida, appears to be an exception to this trend, favoring fine sand bottoms instead.[26]
Their habitat preference reflects their feeding method: they only expose the front end to the water and filter-feed onplankton 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,[27] whileB. lanceolatum preferentially traps bigger particles (>4 μm).[28]
Lancelets aregonochoric animals, i.e. having two sexes, and they reproduce via externalfertilisation. They only reproduce during theirspawning season, which varies slightly between species — usually corresponding to spring and summer months.[29] All lancelets species spawn shortly after sunset, either synchronously (e.g.Branchiostoma floridae, about once every two weeks during spawning season[30]) or asynchronously (Branchiostoma lanceolatum, gradual spawning through the season[31]). Rare instances ofhermaphroditism have been reported inBranchiostoma lanceolatum andB. belcheri, where a small number of female gonads were observed within male individuals, typically ranging from 2 to 5 gonads out of a total of 45–50.[citation needed]
Nicholas andLinda Holland were the first researchers to describe a method of obtaining amphioxus embryos by induction of spawning in captivity and in vitro fertilization.[32] Spawning can be artificially induced in the lab by electric or thermal shock.[33]
The first representative organism of the group to be described wasBranchiostoma lanceolatum. It was described byPeter Simon Pallas in 1774 asmolluscan slugs in the genusLimax.[34] It was not until 1834 thatOronzio Gabriele Costa brought the phylogenetic position of the group closer to theagnathan vertebrates (hagfish andlampreys), including it in the new genusBranchiostoma (from the Greek,branchio- = "gills",-stoma = "mouth").[35][36] In 1836,William Yarrell renamed the genus asAmphioxus (from the Greek: "pointed on both sides"),[37] now considered an obsoletesynonym of the genusBranchiostoma. The term "amphioxus" is still used as acommon name along with "lancelet", especially in the English language. All extant lancelets are all placed in the family Branchiostomatidae, class Leptocardii, and subphylum Cephalochordata.[38]
Observations of amphioxus anatomy began in the middle of the 19th century.Alexander Kovalevsky first described the key anatomical features of the adult amphioxus (hollowdorsal nerve tube,endostyle, segmented body, postanal tail).[39]Armand De Quatrefages first completely described the nervous system of amphioxus.[40]
Kovalevsky also released the first complete description of amphioxus embryos,[39] whileMax Schultze was the first[specify] to describe the larvae.[41]
The larvae are extremely asymmetrical, with the mouth and anus on the left side, and the gill slits on the right side.[42][43] Organs associated with the pharynx are positioned either exclusively on the left or on the right side of the body. In addition, segmented muscle blocks and parts of the nervous system are asymmetrical.[44] After metamorphosis the anatomy becomes more symmetrical, but some asymmetrical traits are still present also as adults, such as the nervous system and the location of the gonads which are found on the right side in Asymmetron and Epigonichthys (in Branchiostoma gonads develop on both sides of body).[45][46]
Depending on the exact species involved, the maximum length of lancelets is typically 2.5 to 8 cm (1.0–3.1 in).[47][48]Branchiostoma belcheri andB. lanceolatum are among the largest.[47] Except for the size, the species are very similar in general appearance, differing mainly in the number ofmyotomes and the pigmentation of their larvae.[47] 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 somecartilage material stiffening thegill slits, mouth, and tail, they have no true complex skeleton.[49]
In common with vertebrates, lancelets have a hollow nerve cord running along the back,pharyngeal slits and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks calledmyomeres.[50]
Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simplernotochord made up of acylinder ofcells that are closely packed in collagen fibers to form a toughened rod. The lancelet notochord, unlike the vertebratespine, extends into the head. This gives the subphylum, Cephalochordata, its name (κεφαλή,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[51]
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 andtransmission electron microscopy indicate the presence of adiencephalicforebrain, a possiblemidbrain, and ahindbrain.[52][53] Recent studies involving a comparison with vertebrates indicate that the vertebratethalamus,pretectum, andmidbrain areas jointly correspond to a single, combined region in the amphioxus, which has been termeddi-mesencephalic primordium (DiMes).[54]
Lancelets have four known kinds of light-sensing structures: Three are respectively calledJoseph cells,Hesse organs andlamellar body.[further explanation needed] The fourth is an unpaired anterior eye. All of them utilizeopsins as light receptors. All of these organs and structures are located in the neural tube, with the frontal eye at the front, followed by thelamellar body, theJoseph cells, and theHesse organs.[55][47][56]
Joseph cells are bare photoreceptors surrounded by a band ofmicrovilli. These cells bear the opsinmelanopsin. TheHesse 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 peak sensitivity of both cells is ~470 nm[57] (blue).
Both theJoseph cells andHesse organs are in the neural tube, theJoseph cells forming a dorsal column, theHesse organs in the ventral part along the length of the tube. TheJoseph cells extend from the caudal end of the anterior vesicle (or cerebral vesicle) to the boundary between myomeres three and four, where theHesse organs begin and continue nearly to the tail.[58][59]
The frontal eye consists of a pigment cup, a group of photoreceptor cells (termedRow 1), three rows of neurons (Rows 2–4), andglial cells. The frontal eye, which expresses thePAX6 gene, has been proposed as the homolog of either the paired eyes or thepineal eye on vertebrates, the pigment cup as the homolog of the RPE (retinal pigment epithelium), the putative photoreceptors as homologs of vertebraterods andcones, and Row 2 neurons as homologs of theretinal ganglion cells.[60] The pigment cup is oriented concave dorsally. Its cells contain the pigmentmelanin.[60][61]
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 opsinc-opsin 1, except for a few which carryc-opsin 3.[60][62]
The Row 2 cells areserotonergic neurons in direct contact with Row 1 cells. Row 3 and 4 cells are also neurons. Cells of all four rows haveaxons that project into the left and right ventrolateral nerves. For Row 2 neurons, axon projections have been traced to thetegmentalneuropil. The tegmental neuropil has been compared withlocomotor control regions of the vertebratehypothalamus, whereparacrine release modulates locomotor patterns such as feeding and swimming.[60]
Green fluorescence in Lancelets. (a.Branchiostoma floridae GFP near the eye spot and in the oral tentacles.) (b.Asymmetron lucayanum green fluorescence in the gonads.)
Lancelets naturally expressgreen fluorescent proteins (GFP) inside their oral tentacles and near the eye spot.[63] Depending on the species, it can also be expressed in the tail andgonads, though this is only reported in theAsymmetron genus.[64] Multiple fluorescent proteingenes have been recorded in lancelet species throughout the world.Branchiostoma floridae alone has 16 GFP-encoding genes. However, the GFP produced by lancelets is more similar to GFP produced bycopepods than jellyfish (Aequorea victoria).[citation needed]
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 inlarvae, signifying it may be used forphotoprotection by converting higher energy blue light to less harmful green light.[citation needed]
Live lancelet (B. floridae) under a fluorescent microscope.
Lancelets are passivefilter feeders,[14] spending most of the time half-buried in sand with only their frontal part protruding.[66] They eat a wide variety of smallplanktonic organisms, such as bacteria,fungi,diatoms, andzooplankton, and they will also takedetritus.[67] Little is known about the diet of the lanceletlarvae in the wild, but captive larvae of several species can be maintained on a diet ofphytoplankton, although this apparently is not optimal forAsymmetron lucayanum.[67]
Lancelets have oral cirri, thintentacle-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 largepharynx, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove called theendostyle, which, connected to a structure known asHatschek's pit, produces a film ofmucus.Ciliary 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 theepipharyngeal 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.[49]
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.[68][69]
The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. Thehepatic caecum, a single blind-endingcaecum, branches off from the underside of the gut, with a lining able tophagocytize 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 ahomolog of the vertebrate liver.[70][71][72]
Lancelets have no respiratory system, breathing solely through their skin, which consists of a simpleepithelium. Despite the name, little if any respiration occurs in the "gill" slits, which are solely devoted to feeding. The circulatory system does resemble that of primitive fish in its general layout, but is much simpler, and does not include aheart. There are no blood cells, and nohemoglobin.[49]
The excretory system consists of segmented "kidneys" containingprotonephridia instead ofnephrons, and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmentedgonads.[49]
Lancelets became famous in the 1860s whenErnst Haeckel began promoting them as a model for the ancestor of all vertebrates. By 1900, lancelets had become amodel organism. By the mid-20th century they had fallen out of favor for a variety of reasons, including a decline of comparative anatomy and embryology, and due to the belief that lancelets were more derived than they appeared, e.g., the profound asymmetry in the larval stage.[73][74] More recently, the fundamental symmetric and twisted development of vertebrates is the topic of theaxial twist theory. According to this theory, there is a deep agreement between the vertebrates and cephalochordates, and even all chordates.[75][76]
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.[77][36]
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 EuropeanBranchiostoma lanceolatum, but later also the West PacificBranchiostoma belcheri andBranchiostoma japonicum, theGulf of Mexico and West AtlanticBranchiostoma floridae and the circumtropical (however, genetic evidence suggest the Atlantic andIndo-Pacific populations should be recognized as separate[14])Asymmetron lucayanum.[67][78] They can reach an age of up to 7–8 years.[78]
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.[citation needed] When their gonads start to ripen in the spring it affects their flavor, making them taste bad during their breeding season.[79]
The lancelet is a small, translucent, fish-like animal that is one of the closest living invertebrate relatives of the vertebrates.[80][81]
The lancelets were traditionally seen as the sister lineage to thevertebrates; in turn, these two groups together (sometimes called Notochordata) were considered the sister group to theTunicata (also called Urochordata and includingsea squirts). Consistent with this view, at least ten morphological features are shared by lancelets and vertebrates, but not tunicates.[82] Newer research suggests this pattern of evolutionary relationship is incorrect. Extensivemolecular phylogenetic analysis has shown convincingly that the Cephalochordata is the mostbasal subphylum of the chordates, with tunicates being the sister group of the vertebrates.[83][84] This revised phylogeny of chordates suggests that tunicates have secondarily lost some of the morphological characters that were formerly considered to besynapomorphies (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 likeexon shuffling and domain combination.[17]
Among the three extant (living)genera,Asymmetron is basal.Molecular clock studies have come to different conclusions on their divergence, with some suggesting thatAsymmetron diverged from other lancelets more than 100 million years ago[13] while others have suggested that it occurred about46 million years ago.[14] According to the younger estimation,Branchiostoma andEpigonichthys have been estimated to have diverged from each other about38.3 million years ago.[14] Despite this deep separation,hybrids betweenAsymmetron lucayanum andBranchiostoma floridae are viable (among the deepest split species known to be able to produce such hybrids).[67]
The following are the species recognised byWoRMS. Other sources recognize about thirty species.[74][14][85] It is likely that currently unrecognizedcryptic species remain.[67]
GenusBranchiostomaCosta 1834 non Newport 1845 non Banks 1905 [AmphioxusYarrell 1836;LimaxPallas 1774 non Linnaeus 1758 non Férussac 1819 non Martyn 1784;DolichorhynchusWilley 1901 non Mulk & Jairajpuri 1974]
Thecladogram presented here illustrates thephylogeny (family tree) of lancelets, and follows a simplified version of the relationships found by Igawa and colleagues (2017):[74][14][85]
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