Cephalochordata

Abstract

Cephalochordates, which together with vertebrates and tunicates constitute the Chordata, were initially thought to be mollusks by Pallas (1774), who described Limax lanceolatus. The name was revised to Branchiostoma lubricum by Costa (1834), who recognized its affinity to fishes, and to Amphioxus lanceolatus by Yarrell, who considered it most closely allied to the agnathans among the vertebrates (Yarrell 1836). Due to precedence of the name Branchiostoma, the European species came to be classified as Branchiostoma lanceolatum, and the term “amphioxus” has become a common name for cephalochordates, which are also called lancelets. The name “Branchiostoma” means “gill mouth” and is an apparent reference to the motile sensory cirri around the mouth which keep too large particles from being eaten. The terms “lanceolatum” and “amphioxus” refer to the elongated shape of the animal, which reaches a maximum length of about 6 cm and is pointed at both ends. Today, about 25 species of Branchiostoma are recognized. There are two additional genera, Asymmetron and Epigonichthys, which are similar to Branchiostoma, but have a series of gonads only on the right side compared to both sides in Branchiostoma (Fig. 3.1B). A single species of Epigonichthys (E. maldivensis) and two of Asymmetron (A. lucayanum and A. interferum) have been described; however, there may be additional cryptic species of Asymmetron (Kon et al. 2007). In phylogenetic analysis with whole mitochondrial genome sequences, Asymmetron is basal in the cephalochordates with a divergence time of about 162 my from the Branchiostoma and Epigonichthys clade (Fig. 3.1A; Kon et al. 2007; Nohara et al. 2005). Although virtually nothing is known about any aspect of the biology of Epigonichthys, much more is known about Asymmetron lucayanum, which was first described from Bimini, Bahamas, by Andrews (1893). He obtained a few embryos, which developed to the gastrula stage when they “were destroyed by an accident.” It was over 100 years before more embryos of A. lucayanum were obtained and the embryology described (Holland and Holland 2010). It was discovered that animals spawn a few days before the new moon (Holland 2011), but to date, there are no studies of developmental genes.

Chapter vignette artwork by Brigitte Baldrian.© Brigitte Baldrian and Andreas Wanninger.