The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding
chordates
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...
in the order Amphioxiformes. They are the modern representatives of the subphylum
Cephalochordata
A cephalochordate (from Greek: κεφαλή ''kephalé'', "head" and χορδή ''khordé'', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called lancelets. Cephalochordates possess 5 synapomorphies, or prima ...
. Lancelets closely resemble 530-million-year-old ''
Pikaia
''Pikaia gracilens'' is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway ...
'', fossils of which are known from the
Burgess Shale
The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest foss ...
.
Zoologists
This is a list of notable zoologists who have published names of new taxa under the International Code of Zoological Nomenclature.
A
* Abe – Tokiharu Abe (1911–1996)
* Abeille de Perrin, Ab. – Elzéar Abeille de Perrin (1843–1910)
* ...
are interested in them because they provide evolutionary insight into the origins of
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s. Lancelets contain many organs and organ systems that are closely related to those of modern fish, but in more primitive form. Therefore, they provide a number of examples of possible evolutionary
exaptation
Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common ...
. 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. Lancelet
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
s hold clues about the early
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
of vertebrates: by comparing genes from lancelets with the same genes in vertebrates, changes in gene expression, function and number as vertebrates evolved can be discovered. The genome of a few species in the genus ''
Branchiostoma
''Branchiostoma'' is one of the few living genera of lancelets (order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name me ...
'' have been sequenced: ''B. floridae,'' ''B. belcheri'', and ''B. lanceolatum''.
In Asia, lancelets are harvested commercially as food for humans and
domesticated animals. In Japan, amphioxus (''B. belcheri'') has been listed in the registry of “Endangered Animals of Japanese Marine and Fresh Water Organisms”.
Ecology
Habitat
Amphioxi are distributed in shallow
subtidal
The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth.
From the point of view of marine biology it forms a relatively stable and well-illuminated ...
sand flats in
temperate
In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
(as far north as
Norway
Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...
),
subtropical
The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Geographical z ...
and
tropical
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referred to ...
seas around the world.
The only exception is ''Asymmetron inferum'', a species known from the vicinity of
whale fall
A whale fall occurs when the carcass of a whale has fallen onto the ocean floor at a depth greater than , in the bathyal or abyssal zones. On the sea floor, these carcasses can create complex localized ecosystems that supply sustenance to deep- ...
s at a depth of about .
Although they are able to swim, adult amphioxi are mostly
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
. They live in sandy bottoms whose granulometry depends on the species and the site,
and they are usually found half-buried in sand.
When disturbed, they quickly leave their burrow, swim a short distance, and then rapidly burrow again, posterior end first, into the sand. Adults (''B. floridae'') can tolerate salinities as low as 6‰ and temperatures from .
Feeding
Their habitat preference reflects their feeding method: 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 ''B. lanceolatum'' preferentially traps bigger particles (>4 µm).
Reproduction and spawning
Lancelets are
gonochoric
In biology, gonochorism is a sexual system where there are only two sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric.
Gonochorism c ...
animals, i.e. having two sexes, and they reproduce via external
fertilization
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
. They only reproduce during their
spawning
Spawn is the eggs and sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of releasing the eggs and sperm, and the act of both sexes is called spawning. Most aquatic animals, except for aquat ...
season, which varies slightly between species — usually corresponding to spring and summer months.
All lancelets species spawn shortly after sunset, either synchronously (e.g. ''Branchiostoma floridae'', about once every two weeks during spawning season
) or asynchronously (''Branchiostoma lanceolatum'', gradual spawning through the season).
Nicholas and
Linda Holland
Linda Zimmerman Holland is a research biologist at Scripps Institution of Oceanography known for her work examining the evolution of vertebrates.
Education and career
Holland has a B.A. (1962) and an M.A. (1964) from Stanford University. She wo ...
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.
History
Taxonomy
The first representative organism of the group to be described was ''
Branchiostoma lanceolatum
''Branchiostoma lanceolatum'', the European lancelet or Mediterranean amphioxus, is a 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 evoluti ...
''. It was described by
Peter Simon Pallas
Peter Simon Pallas Fellow of the Royal Society, FRS FRSE (22 September 1741 – 8 September 1811) was a Prussian zoologist and botanist who worked in Russia between 1767 and 1810.
Life and work
Peter Simon Pallas was born in Berlin, the son ...
in 1774 as
mollusc
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
an slugs in the genus ''
Limax
''Limax'' is a genus of air-breathing land slugs in the terrestrial pulmonate gastropod mollusk family Limacidae.
The generic name ''Limax'' literally means "slug".
Some species, such as the leopard slug (''L. maximus'') and the tawny garden ...
''. It was not until 1834 that Gabriel Costa brought the phylogenetic position of the group closer to the agnathan vertebrates (
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, a ...
and
lampreys
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
), including it in the new genus ''
Branchiostoma
''Branchiostoma'' is one of the few living genera of lancelets (order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name me ...
'' (from the Greek, branchio = "gills", stoma = "mouth").
In 1836, Yarrel renamed the genus as ''Amphioxus'' (from the Greek: "pointed on both sides"), now considered an obsolete
synonym
A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
of the genus ''Branchiostoma''. Today, the term "amphioxus" is still used as a
common name
In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...
for the Amphioxiformes, along with "lancelet," especially in the English language. The order Amphioxiformes was apparently named in 1886 in the ''Jahresbericht und Abhandlungen des Naturwissenschaftlichen Vereins in Magdeburg''.
Anatomy
Observations of amphioxus anatomy began in the middle of the 19th century. First, the adult then the embryonic anatomy were described.
Alexander Kowalevsky first described the key anatomical features of the adult amphioxus (hollow dorsal nerve tube, endostyle, segmented body, postanal tail).
De Quatrefages first completely described the nervous system of amphioxus. Other important contributions to amphioxus adult anatomy were given by Heinrich Rathke and John Goodsir.
Kowalevsky also released the first complete description of amphioxus embryos,
while Schultze and Leuckart were the first to describe the larvae. Other important contributions to amphioxus embryonic anatomy were given by Hatschek, Conklin and later by Tung (experimental embryology).
Anatomy
![Lancelet_Anatomy](https://upload.wikimedia.org/wikipedia/commons/7/71/Lancelet_Anatomy.png)
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
myotome
A myotome is the group of muscles that a single spinal nerve innervates. Similarly a dermatome is an area of skin that a single nerve innervates with sensory fibers. Myotomes are separated by myosepta (singular: myoseptum). In vertebrate embryon ...
s 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
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
material stiffening the
gill slit
Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In con ...
s, mouth, and tail, they have no true skeleton.
Nervous system and notochord
In common with vertebrates, lancelets have a hollow nerve cord running along the back,
pharyngeal slit
Pharyngeal slits are filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth and out the phary ...
s and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called
myomere
Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by connective fascia (myosepta) and most easily seen in larval fishes or in the olm. Myomere c ...
s.
Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler
notochord
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...
made up of a
cylinder
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infin ...
of
cells that are closely packed to form a toughened rod. The lancelet notochord, unlike the vertebrate
spine, extends into the head. This gives the subphylum 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
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...
indicate the presence of a
diencephalic
The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in ol ...
forebrain
In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the Anatomical terms of location#Directional terms, rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hin ...
, a possible
midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
, and a
hindbrain
The hindbrain or rhombencephalon or lower brain is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes. Metencephalo ...
.
Recent studies involving a comparison with vertebrates indicate that the vertebrate
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
,
pretectum
In neuroanatomy, the pretectal area, or pretectum, is a midbrain structure composed of seven nuclei and comprises part of the subcortical visual system. Through reciprocal bilateral projections from the retina, it is involved primarily in mediati ...
, and
midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
areas jointly correspond to a single, combined region in the amphioxus, which has been termed ''di-mesencephalic primordium'' (DiMes).
Visual system
Lancelets have four known kinds of light-sensing structures: Joseph cells, Hesse organs, an unpaired anterior eye and lamellar body, all of which utilize
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
s as light receptors. All of these 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.
Joseph cells and Hesse organs
Joseph cells are bare photoreceptors surrounded by a band of
microvilli
Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, ...
. These cells bear the opsin
melanopsin. 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 peak sensitivity of both cells is ~470 nm
(blue).
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 3 and 4, where the 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 (termed ''Row 1''), three rows of neurons (''Rows 2–4''), and
glial cells
Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form myel ...
. The frontal eye, which expresses the
PAX6
Paired box protein Pax-6, also known as aniridia type II protein (AN2) or oculorhombin, is a protein that in humans is encoded by the ''PAX6'' gene.
Function
PAX6 is a member of the Pax gene family which is responsible for carrying the geneti ...
gene, has been proposed as the homolog of vertebrate paired eyes, the pigment cup as the homolog of the RPE (
retinal pigment epithelium
The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual ce ...
), the putative photoreceptors as homologs of vertebrate
rods and
cones
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines conn ...
, and Row 2 neurons as homologs of the
retinal ganglion cells
A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina ...
.
The pigment cup is oriented concave dorsally. Its cells contain the pigment
melanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
.
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
serotonergic
Serotonergic () or serotoninergic () means "pertaining to or affecting serotonin". Serotonin is a neurotransmitter. A synapse is serotonergic if it uses serotonin as its neurotransmitter. A serotonergic neuron ''produces'' serotonin. A substance is ...
neurons in direct contact with Row 1 cells. Row 3 and 4 cells are also neurons. Cells of all four rows have
axons
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, th ...
that project into the left and right ventrolateral nerves. For Row 2 neurons, axon projections have been traced to the
tegmental neuropil
Neuropil (or "neuropile") is any area in the nervous system composed of mostly unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies. The most prevalent anat ...
. The tegmental neuropil has been compared with
locomotor control regions of the vertebrate
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
, where
paracrine Paracrine signaling is a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse ove ...
release modulates locomotor patterns such as feeding and swimming.
Fluorescent proteins
![Lancelet GFP](https://upload.wikimedia.org/wikipedia/commons/e/ec/Lancelet_GFP.png)
Lancelets naturally express
green fluorescent protein
The green fluorescent protein (GFP) is a protein that exhibits bright green fluorescence when exposed to light in the blue to ultraviolet range. The label ''GFP'' traditionally refers to the protein first isolated from the jellyfish ''Aequorea ...
s (GFP) inside their oral tentacles and near the eye spot. Depending on the species, it can also be expressed in the tail and
gonad
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
s, though this is only reported in the Asymmetron genus. Multiple fluorescent protein
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s 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 by
copepod
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s than jellyfish (''
Aequorea victoria
''Aequorea victoria'', also sometimes called the crystal jelly, is a bioluminescent hydrozoan jellyfish, or hydromedusa, that is found off the west coast of North America.
The species is best known as the source of two proteins involved in biolu ...
'').
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. GFP is also expressed in
larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The ...
e, signifying it may be used for
photoprotection
Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative stre ...
by converting higher energy blue light to less harmful green light.
![Lancelet GFP GIF](https://upload.wikimedia.org/wikipedia/commons/3/3e/Lancelet_GFP_GIF.gif)
The fluorescent proteins from lancelets have been adapted for use in molecular biology and microscopy. Th
yellow fluorescent proteinfrom ''
Branchiostoma lanceolatum
''Branchiostoma lanceolatum'', the European lancelet or Mediterranean amphioxus, is a 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 evoluti ...
'' exhibits unusually high
quantum yield The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.
Applications
Fluorescence spectroscopy
The fluorescence quantum yield is defined as the ratio of the numb ...
(~0.95). It has been
engineered into a
monomer
In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
Mo ...
ic green fluorescent protein known a
mNeonGreen which is the brightest known monomeric green or yellow fluorescent protein.
Feeding and digestive system
Lancelets are inactive
filter feeders,
spending most of the time half-buried in sand with only their frontal part protruding. They eat a wide variety of small
plankton
Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
ic organisms, such as bacteria,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
,
diatom
A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s,
dinoflagellate
The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...
s and
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
, and they will also take
detritus
In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commun ...
.
[ Little is known about the diet of the lancelet ]larvae
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The ...
in the wild, but captive larvae of several species can be maintained on a diet of phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
, although this apparently is not optimal for ''Asymmetron lucayanum''.[
Lancelets have oral cirri, thin ]tentacle
In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
-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 pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove called the endostyle
The endostyle is an anatomical feature found in invertebrate chordates and larval lampreys. It is an organ which assists chordates in filter-feeding. It is found in adult urochordates and cephalochordates, as well as in the larvae of the vertebra ...
, which, connected to a structure known as Hatschek's pit In the lancelet, Hatschek's pit, not to be confused with Hatschek's nephridium, is a deep ciliated fossa on the dorsal midline of the buccal cavity (the region of the gut behind the mouth). Among other things, it secretes mucus which entraps food p ...
, produces a film of mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
. Cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
ry 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 caecum
The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, to which it is joined). The wo ...
, branches off from the underside of the gut, with a lining able to phagocytize
Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
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
In biology, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats and birds, the arms of prima ...
of the vertebrate liver.
Other systems
Lancelets have no respiratory system, breathing solely through their skin, which consists of a simple epithelium
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
. 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 a heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
. There are no blood cells, and no hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
.
The excretory system consists of segmented "kidneys" containing protonephridia
The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...
instead of nephron
The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure ...
s, and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmented gonad
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
s.
Model organism
Lancelets became famous in the 1860s when Ernst Haeckel
Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist and artist. He discovered, described and named thousands of new sp ...
began promoting them as a model for the ancestor of all vertebrates. By 1900 lancelets had become a model organism
A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
. 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.
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
The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...
and West Atlantic ''Branchiostoma floridae'' and the circumtropical (however, genetic evidence suggest the Atlantic and Indo-Pacific
The Indo-Pacific is a vast biogeographic region of Earth.
In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
populations should be recognized as separate[) ''Asymmetron lucayanum''.] They can reach an age of up to 7–8 years.[
]
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 Cephalochordata were traditionally seen as the sister lineage to the vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s; in turn, these two groups together (sometimes called Notochordata) were considered the sister group to the Tunicata (also called Urochordata and including sea squirts). Consistent with this view, at least 10 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
Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
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
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
(shared, derived characters) of vertebrates and lancelets.
Among the three extant (living) genera
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
, ''Asymmetron'' is basal and it is placed in its own family. Genetic studies have come to separate conclusions on their divergence, with some suggesting that ''Asymmetron'' diverged from other lancelets more than 100 million years ago and others (with higher statistic support) that it occurred about 42 million years ago.[ The two remaining genera, ''Branchiostoma'' and ''Epigonichthys'' diverged from each other about 36 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 ]ITIS
The Integrated Taxonomic Information System (ITIS) is an American partnership of federal agencies designed to provide consistent and reliable information on the taxonomy of biological species. ITIS was originally formed in 1996 as an interagen ...
. Other sources recognize about thirty species.[ It is likely that currently unrecognized ]cryptic species
In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...
remain.[
*Order Branchiostomiformes
**Family Asymmetronidae
*** Genus '' Asymmetron'' Andrews 1893 Gill 1895">'Amphioxides'' Gill 1895****'']Asymmetron inferum
''Asymmetron'' is a genus of lancelets belonging to the family Branchiostomatidae.
The species of this genus are found in world oceans.
Species:
*'' Asymmetron inferum''
*''Asymmetron lucayanum
''Asymmetron'' is a genus of lancelets belongi ...
'' Nishikawa 2004
****''Asymmetron lucayanum
''Asymmetron'' is a genus of lancelets belonging to the family Branchiostomatidae
Branchiostomatidae is a family of lancelets belonging to the class Leptocardii, order unknown.
Genera:
* ''Asymmetron'' Andrews, 1893
* '' Branchiostoma'' Costa, ...
'' Andrews 1893 (Sharptail lancelet)
**Family Epigonichthyidae Peters 1876
*** Genus '' Epigonichthys'' Peters 1876 [''Amphipleurichthys'' Whitley 1932; ''Bathyamphioxus'' Whitley 1932; ''Heteropleuron'' Kirkaldy 1895; ''Merscalpellus'' Whitley 1932; ''Notasymmetron'' Whitley 1932; ''Paramphioxus'' Haekel 1893; ''Zeamphioxus'' Whitley 1932]
****''Epigonichthys australis'' (Raff 1912)
****''Epigonichthys bassanus'' (Günther 1884)
****''Epigonichthys cingalensis'' (Kirkaldy 1894); nomen dubium
****'' Epigonichthys cultellus'' Peters 1877
****'' Epigonichthys hectori'' (Benham 1901) (Hector's lancelet)
****'' Epigonichthys maldivensis'' (Foster Cooper 1903)
**Family Branchiostomatidae
Branchiostomatidae is a family of lancelets belonging to the class Leptocardii, order unknown.
Genera:
* ''Asymmetron'' Andrews, 1893
* '' Branchiostoma'' Costa, 1834
* ''Epigonichthys'' Peters, 1876
* '' Epigonopterus'' Jordan & Gilbert, 1882
* ...
Bonaparte 1841
*** Genus ''Branchiostoma
''Branchiostoma'' is one of the few living genera of lancelets (order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name me ...
'' Costa 1834 non Newport 1845 non Banks 1905 [''Amphioxus'' Yarrell 1836; ''Limax'' Pallas 1774 non Linnaeus 1758 non Férussac 1819 non Martyn 1784; ''Dolichorhynchus'' Willey 1901 non Mulk & Jairajpuri 1974]
****''Branchiostoma africae'' Hubbs 1927
****''Branchiostoma arabiae'' Webb 1957
****''Branchiostoma bazarutense'' Gilchrist 1923
****''Branchiostoma belcheri'' Gray 1847 (Belcher's lancelet)
****''Branchiostoma bennetti
''Branchiostoma bennetti'', the mud lancelet, is a lancelet of the genus ''Branchiostoma'' endemic to the Gulf of Mexico
The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surro ...
'' Boschung & Gunter 1966 (Mud lancelet)
****'' Branchiostoma bermudae'' Hubbs 1922
****'' Branchiostoma californiense'' Andrews 1893 (Californian lancelet)
****'' Branchiostoma capense'' Gilchrist 1902
****'' Branchiostoma caribaea'' Sundevall 1853 (Caribbean lancelet)
****'' Branchiostoma clonasea''
****''Branchiostoma elongata
''Branchiostoma'' is one of the few living genus, genera of lancelets (order (biology), order Amphioxiformes). It is the type (zoology), type genus of family (biology), family Branchiostomatidae.
These small vaguely eel- or snake-like animals a ...
'' Sundevall 1852
****'' Branchiostoma floridae'' Hubbs 1922 (Florida lancelet)
****'' Branchiostoma gambiense'' Webb 1958
****'' Branchiostoma indica'' Willey 1901
****'' Branchiostoma japonica'' Willey 1896 (Pacific lancelet)
****'' Branchiostoma lanceolata'' Pallas 1774 (European lancelet)
****'' Branchiostoma leonense'' Webb 1956
****''Branchiostoma longirostra
''Branchiostoma'' is one of the few living genera of lancelets ( order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name ...
'' Boschung 1983 (Shellhash lancelet)
****''Branchiostoma malayana
''Branchiostoma'' is one of the few living genera of lancelets (order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name me ...
'' Webb 1956
****'' Branchiostoma moretonense'' Kelly 1966; nomen dubium
****''Branchiostoma nigeriense
''Branchiostoma'' is one of the few living genera of lancelets (order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name me ...
'' Webb 1955
****''Branchiostoma platae
''Branchiostoma'' is one of the few living genera of lancelets (order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name me ...
'' Hubbs 1922
****'' Branchiostoma senegalense'' Webb 1955
****''Branchiostoma tattersalli
''Branchiostoma'' is one of the few living genera of lancelets (order Amphioxiformes). It is the type genus of family Branchiostomatidae.
These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name me ...
'' Hubbs 1922
****'' Branchiostoma virginiae'' Hubbs 1922 (Virginian lancelet)
References
Further reading
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External links
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{{Taxonbar, from1=Q1156226, from2=Q20722244, from3=Q21286775
Cephalochordata