''Branchiostoma lanceolatum'', the European lancelet or Mediterranean amphioxus, is a lancelet in the subphylum Cephalochordata. It is a marine invertebrate with a

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 ...

but no backbone and is used as 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 ...

to study the evolutionary development of vertebrates.

Anatomy

''Branchiostoma lanceolatum'' has an elongated body, flattened laterally and pointed at both ends. A stiffening rod of tightly packed cells, the

, 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 Giovanni Battista Cirri (1 October 1724 – 11 June 1808) was an Italian cellist and composer in the 18th century. Biography Cirri was born in Forlì in the Emilia-Romagna Region of Italy. He had his first musical training with his brother ...

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

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 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 south to the Mediterranean Sea and east to the Black Sea. Its range has expanded through the

Suez Canal The Suez Canal ( arz, قَنَاةُ ٱلسُّوَيْسِ, ') is an artificial sea-level waterway in Egypt, connecting the Mediterranean Sea to the Red Sea through the Isthmus of Suez and dividing Africa and Asia. The long canal is a popular ...

to the northerly parts of the Indian Ocean and the coasts of

East Africa East Africa, Eastern Africa, or East of Africa, is the eastern subregion of the African continent. In the United Nations Statistics Division scheme of geographic regions, 10-11-(16*) territories make up Eastern Africa: Due to the historical ...

. 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, 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 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 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 ...

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 and

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 ...

as they descend. While in these surface waters they drift with the current. The larval stage lasts for up to 200 days.

Research

The

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

l genome of ''Branchiostoma lanceolatum'' has been sequenced, and the species serves as a model organism for studying the development of vertebrates. The way the coding genes and the two rRNA genes are organised is the same as the organisational method used by the sea lamprey (''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 Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

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