Lancelets are small, translucent marine animals found on ocean floors worldwide. These fish-like creatures offer a unique glimpse into the diversity of aquatic life. Despite their unassuming appearance, lancelets provide valuable insights into early chordate evolution and the origins of vertebrates.
Defining a Lancelet
Lancelets, also known as amphioxi, are marine invertebrates belonging to the subphylum Cephalochordata. This classification places them within the phylum Chordata, the same broad group that includes all vertebrates, from fish to humans. The common name “lancelet” reflects their slender, elongated body shape, which tapers to a point at both ends, resembling a tiny lance.
Distinctive Physical Traits
Lancelets possess an elongated body, typically growing between 2 to 8 centimeters in length. Their bodies are translucent and lack paired fins or limbs. A defining characteristic is the notochord, a flexible, rod-like structure that runs the entire length of their body, providing internal support. Above the notochord lies a dorsal hollow nerve cord, a feature shared with all chordates.
Their body also exhibits numerous pharyngeal slits, which are openings in the throat region used for feeding. Gas exchange occurs directly through their skin, not the slits. A post-anal tail, an extension of the body past the anus, is another shared chordate trait. Their muscles are arranged in repeating, V-shaped blocks called myomeres, similar to those found in fish.
Where Lancelets Live and How They Survive
Lancelets are bottom-dwelling organisms, preferring shallow, warm temperate, and tropical marine environments. They are commonly found in sandy or gravelly substrates, often at depths of up to 30 meters. Their lifestyle involves spending much of their time buried in the sediment, with only their anterior end protruding into the water. This burrowing behavior is facilitated by their tapered body shape and rapid wiggling movements.
As filter feeders, lancelets draw water into their mouths using ciliary action. The water passes through their pharyngeal slits, where food particles like plankton, bacteria, and detritus are trapped in a mucus net. The trapped food is then transported to the digestive tract, while filtered water exits the body through an opening called the atriopore.
Evolutionary Significance
Lancelets hold a significant position in evolutionary biology due to their unique combination of invertebrate and chordate features. They are considered one of the closest living invertebrate relatives to vertebrates. Their simple body plan retains the four fundamental chordate characteristics—a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail—throughout their adult life. This provides a living model for early chordate evolution. This persistence of primitive features is particularly informative, as in many other chordates, some of these traits are only present during embryonic development.
Studying lancelets offers insights into the origins of the vertebrate body plan and how vertebrates diverged from their invertebrate ancestors. While lancelets lack a complex brain, true vertebrae, or paired fins, their genomic studies reveal many genes homologous to those in vertebrates, but with fewer duplications. This suggests that the lancelet genome may represent a more ancestral state before the extensive gene duplications that contributed to the complexity of vertebrates. The comparison between lancelets and vertebrates helps clarify which genetic attributes and control mechanisms were present in early chordates and which evolved later to shape vertebrate diversity.