Lancelet

Lore & Background

All extant lancelets are placed in the family Branchiostomatidae. Adult lancelets typically inhabit the seafloor, burrowing into well-ventilated substrates like coarse sand. They filter-feed by exposing their front ends and using a branchial ciliary current to pass water through a mucous sheet. They are gonochoric, reproducing via external fertilization during spring and summer, usually spawning shortly after sunset. Nicholas and Linda Holland first described a method of obtaining embryos by induced spawning in captivity. Lancelets have a hollow nerve cord, pharyngeal slits, and a post-anal tail, but lack a true brain and complex skeleton. Their notochord extends into the head, giving the subphylum its name. They possess light-sensing structures including Joseph cells, Hesse organs, a lamellar body, and an unpaired anterior eye.

Reader's Guide

Lancelets are significant because they diverged from other chordates during or prior to the Cambrian period, and their anatomy preserves features homologous to those of modern fish, such as gill-slits used for feeding rather than respiration. Their genomes have been sequenced for several Branchiostoma species, offering insights into vertebrate origins and evolution. The circulatory system lacks red blood cells and hemoglobin, carrying food but not oxygen. Fossil chordates like Pikaia and Cathaymyrus have been suggested as close relatives, though this is disputed. Molecular clock analysis suggests modern lancelets diversified during the Cretaceous or Cenozoic. In Asia, they are harvested commercially as food; in Japan, Branchiostoma belcheri is listed as endangered.

Did You Know?

• Lancelets have gill-slits used only for feeding, not respiration.
• Their circulatory system lacks red blood cells and hemoglobin.
• The larvae are extremely asymmetrical, with mouth and anus on the left side and gill slits on the right.
• Spawning can be artificially induced in the lab by electric or thermal shock.