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cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Deuterostomia - Chordata - Cephalochordata - Branchiostomidae - Branchiostoma - Branchiostoma floridae
General descriptionLancelet, traditionally known as amphioxus (from the Greek for "both ends pointed"), was first described by Peter Pallas as a molluscan slug in 1774. Lancelets are flattened, about 1-2 inches long, marine organisms usually found in shallow parts of temperate or tropical seas buried in sand with only the head protruding above the sand surface. They are capable of swimming briefly and borrow into sand head first. Lancelets are harvested for human consumption in some parts of Asia.
Ecology and distributionThere are about 30 lancelet species, most belonging to the genus Brachiostoma. Lancelets populate a broad range of sandy and shell-gravel habitats in shallow (8 to 117 m, with 20 m being optimal depth) coastal marine waters across the globe. Although most lancelets species have a wide geographic distribution, they are typically rare through much of their range and occur at high densities in only a few scattered localities: Lagos Lagoon, Nigeria (B. nigeriense), Xiamen and Qingdao, China (B. belcheri), southern France (B. lanceolatum), Tampa Bay Florida and Discovery Bay, Jamaica (B. floridae).
Earliest chordate and not closest to vertebratesFor the last two centuries amphioxus was considered the closest living invertebrate relative to vertebrates. As vertebrates amphioxus have a hollow dorsal nerve cord, notochord, segmented muscles, pharyngeal gill slits and a post-anal tail that develops from tailbud. Unlike the vertebrates, the lancelet dorsal nerve cord is protected not by bone but by notochord, a flexible rodlike structure made of cells that are closely-packed and form a cylinder. The lancelet notochord extends into the head. This gives the subphylum its name (cephalo- i.e. "related to the head").
The notochord exists transiently during the life of most vertebrates. In ascidians (urochordates) these characteristics are maintained only on larval stage. The complete loss of protovertebrate features in adult ascidians has generated a lot of controversy. After years uncertainty, through the analysis of large amount of molecular data it was finally determined that not amphioxus but tunicates are the closest extant invertebrates to vertebrates.
Tunicate Ciona intestinalis at GeoChemBio
GenomeAmphioxus has 19 chromosome pairs. Among almost 22,000 protein-coding loci, it has many single members for most vertebrate multigenic families. The amphioxus genome analysis confirms two full genome duplication events leading from invertebrates to mammals. This makes amphioxus an excellent model for the ancestral chordate genome, not only in gene content but also with respect to exon-intron distribution and chromosomal organization. It also demonstrates that the urochordates secondarily lost many genes. After the two genome duplications, approximately 20-25% of the duplicates were maintained in the vertebrate genome, with a strong bias towards genes involved in transcriptional regulation, signal transduction and neuronal processes.
Sexes are separate, and each individual breeds at about 12-day interval throughout the summer. In nature spawning is induced by a drop in light about 30 min. after sunset, in lab settings it can be induced by mild electrical shock. During spawning period, population densities of adults reach up to 1,200 individuals per square meter in Old Tampa Bay, Florida.
- Unfertilized egg Gametes (eggs and spermatozoids) are broadcast into the seawater with single female producing 1,000-5,000 eggs at each spawning; the lancelet egg is miolethical i. e. contains little amount of yolk which is uniformly dispersed throughout the egg.
- Fertilized egg Egg is protected by a membrane. Male and female pronuclei meet at ~16 min after insemination.
- Cleavage Dividing egg, 2-16-cell embryo; cell division without growth.
- Morula the morula is solid ball of cells; All of the cells of a lancelet morula are approximately the same size because they have little yolk.
- Blastula the morula becomes a blastula when an internal cavity, the blastocoel, appears ~5-6 h after fertilization
- Gastrula gastrulation begins when embryo is Approximately 400 cells in size; during gastrulation cells migrate inward producing a hollow embryo with an opening to the space in the center and two layers of cells (ectoderm and endoderm) surrounding this cavity; the embryo swims about as a ciliated gastrula.
- Neurula ~24-26 h after fertilization; embryo assumes the form of a ciliated flattened cylinder, whith both ends alike; notochord now appeared and extends to the front end, beyond the end of the brain.
- Larval Eggs hatch as ciliated neurulae; development proceeds with the perforation of the first gill-slit and a growth period involving an increase in gill-slit and myotome numbers; the subsequent metamorphosis to the bottom living adult involves re-arrangement of the gill apparatus; during their development, lancelet larvae are capable of swimming using either their epidermal cilia or muscular body undulations.
- Juvenile Jiveniles look like miniture adults; their gonads are underdeveloped. B. floridae with body length between 5 mm (newly settled) and 18 mm have no detectable gonads. At a body length of 18 mm, the gonads begin to enlarge in both sexes.
- Adult Age of sexual reproductive maturity at about 6 weeks of age at minimum body length !23 mm; life span is estimated to be about 3 years; all species are less than 10 cm in length.
Garcia-Fernàndez J. Amphioxus: a peaceful anchovy fillet to illuminate Chordate Evolution (I). Int J Biol Sci. 2006;2(2):30-1.
For the photo and the song please see original article.
Original drawing (fig No. 11) and description of the amphioxus, Branchiostoma lanceolatum, originally classified as a mollusk. From ref. Pallas PS., 1774
- Holland ND, Holland LZ. Stage- and tissue-specific patterns of cell division in embryonic and larval tissues of amphioxus during normal development. Evol Dev. 2006 Mar-Apr;8(2):142-9.
- Cory RL and Pierce EL. Distribution and Ecology of Lancelets (Order Amphioxi) Over the Continental Shelf of the Southeastern United States. Limnology and Oceanography Vol. 12, No. 4 (Oct., 1967), pp. 650-656. (.pdf)
- Stokes MD and Holland ND. Reproduction of the Florida Lancelet (Branchiostoma floridae): Spawning Patterns and Fluctuations in Gonad Indexes and Nutritional Reserves. Invertebrate Biology Vol. 115, No. 4 (Autumn, 1996), pp. 349-359.
- Garcia-Fernàndez J, Benito-Gutiérrez E. It's a long way from amphioxus: descendants of the earliest chordate. Bioessays. 2009 Jun;31(6):665-75.
- Holland LZ, Laudet V, Schubert M. The chordate amphioxus: an emerging model organism for developmental biology. Cell Mol Life Sci. 2004 Sep;61(18):2290-308.
- Gibson-Brown JJ, Osoegawa K. et al. A proposal to sequence the amphioxus genome submitted to the Joint Genome Institute of the US Department of Energy. J Exp Zoolog B Mol Dev Evol. 2003 Dec 15
- PubMed: free full text articles about Branchiostoma