Amphioxiformes

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The lancelets ( LA(H)N-slit), also known as amphioxi (sing.: amphioxus AM-fee-OK-səs), consist of 32 described species of somewhat fish-like benthic filter-feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae. Lancelets diverged from other chordates during or prior to the Cambrian period. A number of fossil chordates have been suggested to be closely related to lancelets, including Pikaia and Cathaymyrus from the Cambrian and Palaeobranchiostoma from the Permian, but their close relationship to lancelets has been doubted by other authors. Molecular clock analysis suggests that modern lancelets probably diversified much more recently, during the Cretaceous or Cenozoic. They are of interest to zoologists as lancelets contain many organs and organ systems that are homologous to those of modern fish. Therefore, they provide a number of examples of possible evolutionary exaptation. 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. Comparing the genomes of lancelets and vertebrates and their differences in gene expression, function, and number can shed light on the origins of vertebrates and their evolution. The genome of a few species in the genus Branchiostoma have been sequenced: B. floridae, B. belcheri, and B. lanceolatum. In Asia, lancelets are harvested commercially as food for humans. In Japan, the amphioxus (B. belcheri) has been listed in the registry of "Endangered Animals of Japanese Marine and Fresh Water Organisms".