The cephalochordates, or "lancelets", comprise one of the three subphyla in the phylum Chordata (the other two being the vertebrates and tunicates). Lancelets are small fishlike animals that are generally less than 5 cm in length, although some reach 6 to 9 cm. Lancelets are found most commonly in shallow subtidal tropical, subtropical, and temperate sand flats, where they burrow in clean gravel or sand (not silty sediments) with just the head exposed. There are three cephalochordate genera: Branchiostoma, Asymmetron, and Epigonichthys (Nishikawa 2004; Kon et al. 2007 and references therein; Zhong et al. 2009). Currently, 32 lancelet species are recognized: 24 Branchiostoma species, 7 Asymmetron species, and a single Epigonichthys species (Holland and Holland 2010 and references therein).
In some places, lancelet populations can be quite large. Population densities as high as several thousand individuals per square metre have been recorded for Branchiostoma floridae. When disturbed, lancelets leave their burrows and swim quickly for a short distance, then rapidly burrow again with the posterior end in the substrate. Lancelets swim using lateral body undulations that drive water posteriorly and provide forward thrust. The body wall muscles form chevron-shaped blocks known as myotomes arranged along much of the length of the body. The notochord--an elastic, flexible rod characteristic of one or more life stages of all chordates--extends beyond the myotomes both anteriorly and posteriorly, providing support and apparently helping to hold the body rigid when burrowing. (Brusca and Brusca 2003; Lambert 2005a,b and references therein)
Cephalochordates are suspension-feeders. Water is driven into the mouth and pharynx and out through the pharyngeal gill slits into the surrounding atrium and exits the body through the ventral atriopore. Currents are generated by the beating of pharyngeal cilia, not by muscles. The feeding system includes several structures that prevent large particles from even entering the mouth. Food particles are captured in mucus and moved by cilia to the mouth. (Brusca and Brusca 2003)
Cephalochordates have separate sexes. Spawning occurs around or after sunset. Eggs and sperm are released into the water flowing out of the atrium and fertilization is external. The eggs contain little yolk and and embryos develop rapidly into free-swimming larvae. Larvae swim upward in the water column where they remain as plankton for weeks (the duration varying with temperature). When feeding, larvae swim upward, then hover or slowly sink passively with the body held horizontally and the mouth directed downward, feeding on plankton and other suspended material. Development from the larval to juvenile stage is generally gradual. (Brusca and Brusca 2003) Lancelet embryology is reviewed by Whittaker (1997). Lancelet life spans are surprisingly long, 2 to 5 years depending on the species, with a spring and summer breeding season (Stokes 1996, Whittaker 1997).
There has been considerable debate regarding relationships among the chordate subgroups. For some time, the traditional view has been that the cephalochordates are the sister group to the vertebrates, with the tunicates falling just outside this grouping. This tree topology was favored by Cameron et al. (2000) based on their analysis of 18S ribosomal RNA and by Stach (2008) based on a morphological analysis. However, a number of more recent studies have recovered vertebrates and tunicates as sister taxa (e.g., see Philippe et al. 2005; Dunn et al. 2008; Putnam et al. 2008; review by Stach 2008 and references therein). Swalla and Smith (2008) note that mitochondrial and ribosomal evidence seems to place cephalochordates as the sister group to the vertebrates, whereas genomic analyses have placed tunicates in this position.
In China, lancelets are harvested by humans as food. They are also an important food item for bottom-feeding fish and small crustaceans. Lambert (2005a,b) reviewed the history of research on hemichordates, cephalochordates, and tunicates as well as diverse aspects of their biology.
Putnam et al. (2008) report on the draft genome sequence of Branchiostoma floridae and compare its structure with the genomes of other animals. Yu et al. (2008) report on their analysis of an EST library from this same lancelet species .
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