Lancelets are small, fish-like marine invertebrates that inhabit coastal waters around the world. These translucent animals are typically slender and elongated, tapering to a point at both ends, which gives them their common name, resembling a lance. They are also widely known by their older scientific name, amphioxus. While unassuming in appearance, these creatures represent a distinct group of animals with a unique body plan.
Anatomy and Habitat
Lancelets possess a streamlined, blade-like body, typically ranging from 2.5 to 8 centimeters (1 to 3 inches) in length. Their transparent bodies allow for the observation of internal organs. A defining feature is the notochord, a flexible, rod-like structure that extends along the entire length of the body, providing internal support.
Running above the notochord is a dorsal hollow nerve cord, a tube of nervous tissue. The pharynx, a large section behind the mouth, contains numerous narrow pharyngeal slits, which are lined with cilia. A post-anal tail extends beyond the anus. These animals are typically found burrowed in the sand, gravel, or coarse shelly sand on the floor of shallow, coastal marine waters. They are common in temperate and tropical regions globally, often living in dense populations of several thousand individuals per square meter.
Feeding and Behavior
Lancelets are suspension feeders, filtering small particles from the water. They use cilia, hair-like structures, that line their pharyngeal slits and oral hood to create a water current. This current draws water, along with small organisms and organic debris, into their mouth. Tentacle-like cirri around the mouth prevent larger particles from entering.
Food particles become trapped in a mucus sheet produced by a groove called the endostyle. The cilia then push this mucus sheet towards the digestive tract, where digestion and absorption occur. Lancelets typically remain mostly buried in the sand, often with only their anterior end protruding. While they spend most of their time burrowed, they are capable of simple, fish-like swimming motions, moving rapidly forward and backward through water or wet sand by contracting segmented muscle blocks along their bodies.
Significance in Vertebrate Evolution
Lancelets are a significant example of a primitive chordate, a group that includes vertebrates like humans. They possess several anatomical features that are considered defining characteristics of the phylum Chordata, providing insight into the evolutionary origins of animals with backbones. The notochord, for instance, is a flexible rod that provides support, largely replaced by the vertebral column in vertebrates during embryonic development.
The dorsal hollow nerve cord in lancelets is homologous to the neural tube in vertebrates, which develops into the brain and spinal cord. While lancelets have only a slight enlargement at the anterior end of this nerve cord rather than a complex brain, its presence indicates a shared evolutionary lineage. The pharyngeal slits are thought to be precursors to gill structures in fish and other structures in land vertebrates.
The post-anal tail, another chordate feature, is present in lancelets and is also found in the embryonic stages of many vertebrates, even if it is not retained in adulthood. By studying lancelets, scientists can observe these fundamental chordate traits in a relatively simple, invertebrate form. Their genome, which has been sequenced, offers clues about the genetic changes that led to the development of more complex vertebrate features, such as a head, paired limbs, and a more elaborate nervous system. Lancelets essentially serve as a living model, offering a window into what the distant ancestors of all vertebrates may have resembled over 500 million years ago.
Classification Within Chordates
Lancelets are classified within the subphylum Cephalochordata, part of the phylum Chordata. The phylum Chordata encompasses all animals that possess a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their life cycle. Beyond Cephalochordata, the phylum Chordata also includes two other subphyla: Urochordata and Vertebrata.
Urochordata, commonly known as tunicates or sea squirts, are marine filter feeders that, in their larval stage, exhibit all four chordate characteristics. However, adult tunicates are sessile and lose most of these features, including their notochord and post-anal tail. In contrast, Vertebrata comprises all animals with a true backbone, where the notochord is surrounded or replaced by cartilage or bone during development. Lancelets are unique among chordates because they retain all four defining chordate characteristics throughout their adult life. Their lack of a true backbone, unlike vertebrates, and their retention of primitive features, unlike most adult tunicates, position them distinctly within the chordate family tree.