''Branchiostoma lanceolatum'', the European lancelet or Mediterranean amphioxus, is a

lancelet The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochord ...

in the

subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used th ...

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 ...

. It is a marine

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

with a notochord but no backbone and is used as a model organism to study the evolutionary development 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, with c ...

Anatomy

''Branchiostoma lanceolatum'' has an elongated body, flattened laterally and pointed at both ends. A stiffening rod of tightly packed cells, the notochord, extends the whole length of the body. Unlike vertebrates, the notochord persists in the adult, in form of simple dorsal neural tube slightly thickened in the anterior part (the cerebral vesicle). Above it is a nerve cord with a single frontal eye. The mouth is on the underside of the body and is surrounded by a tuft of 20 or 30 cirri or slender sensory appendages. The gut runs just below the notochord from the mouth to the anus, in front of the tail. There is a flap-like, vertical fin surrounding the pointed tail. Gas exchange takes place as water passes through gill slits in the mid region, and segmented gonads lie just behind these. The animal is pearly white and semi-transparent which enables the internal organs to be seen from outside. Its appearance is similar to a "primitive fish". It can grow up to 6 cm (2.5 in) long.''Branchiostoma lanceolatum''
Marine Species Information Portal. Retrieved 2011-11-14.

Distribution and habitat

''Branchiostoma lanceolatum'' is found in shallow seas in the north-east Atlantic Ocean, from 67°N in

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 the ...

south to the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...

and east to the

Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...

. Its range has expanded through the Suez Canal to the northerly parts of the Indian Ocean and the coasts of East Africa. It burrows in soft substrates such as sand, gravel and shell fragments and is quite particular as to the size of the particles. It occurs from the low tide mark down to about 40 metres (130 ft).

Biology

In the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...

, breeding takes place in June and July. The mature adult ''Branchiostoma lanceolatum'', aged 2 to 3 years, congregate in masses on the sea floor. Individuals are either male or female and spawn once a year. The eggs are laid and fertilisation takes place externally. The early larval stages take place in the substrate but a little later, the larvae become pelagic. They are elongated and flattened laterally and have a swollen region around the

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

slits. These slits number 6 to 19, the number increasing as the larva passes through its various stages. The larvae have a vertical daily migration. Each evening they rise to near the surface of the sea and in the morning they sink through the water column, feeding on phytoplankton, copepods and detritus as they descend. While in these surface waters they drift with the current. The larval stage lasts for up to 200 days.

Research

The mitochondrial

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 g ...

of ''Branchiostoma lanceolatum'' has been

sequenced In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which suc ...

, and the species serves as a model organism for studying the development of

s. The way the coding genes and the two

rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosoma ...

genes are organised is the same as the organisational method used by the

sea lamprey The sea lamprey (''Petromyzon marinus'') is a parasitic lamprey native to the Northern Hemisphere. It is sometimes referred to as the "vampire fish". Description The sea lamprey has an eel-like body without paired fins. Its mouth is jawless, ...

(''Petromyzon marinus''). These data, among others, suggest a close relationship between ''Branchiostoma lanceolatum'' and the vertebrates. Adults can be induced to spawn in the laboratory with a thermal shock several times per year. Metamorphosis in the lab takes place in 1 to 3 months. Since 2015, a merged effort from different labs working on this species, the Amphiencode consortium, provides a centralized platform on which genomic data is publicly accessible.

References

{{Taxonbar, from=Q2267046 Cephalochordata Animals described in 1774 Taxa named by Peter Simon Pallas