Amphioxus, or lancet, is an invertebrate species of approximately two to three inches length commonly used for research as a model organism, in that it is used to study biological phenomena in itself that can be translated or applied to other organisms. For example, human disease research and experimentation can be impossible or unethical, but due to the common genetic makeup, the model organism can be tested. The amphioxus is commonly found in shallow seas in the Atlantic Ocean, Mediterranean Sea, Black Sea, Indian Ocean and the North Sea; however, most use of the species as a model organism is done through laboratory breeding.
The species looks like very thin fishes without eyes and has a mouth on the underside of its body. Feeding and breathing is accomplished by filtering food particles and gas using gill slits along the bottom.
Use of the lancet as a model organism is due to the understanding of the mitochondrial genome. Even though the species is invertebrate, the two ribosomal ribonucleic acid (rRNA) function in the same way as the vertebrate sea lamprey; this means the evolutionary relationship between the lancet and vertebrates is close. Scientists believe the research into the amphioxus brings new insight into the origin of vertebrates.
Humans are in a class of animals called chordates, along with all vertebrates and the amphioxus. The amphioxus contains the "body plan" of a vertebrate with a dorsal nerve cord as well as other traits shared by humans, fish, reptiles and amphibians like a notochord and a post-anal tail. Understanding how diseases affect the amphioxus species are only part of the useful research being done with these model organisms. Research into a liver culture for the human hepatitis B virus (HBV) is being developed using knowledge gained from the amphioxus (which is the first organism with a liver).