Branchiostomidae: The Closest Relatives of Vertebrates
Branchiostomidae, also known as lancelets or amphioxus, are a family of small, fish-like, marine animals that belong to the subphylum Cephalochordata. They are considered to be the closest living relatives of vertebrates, as they share many features with them, such as a notochord, a dorsal hollow nerve cord, gill slits, and segmented muscle blocks. However, they lack a true head, eyes, brain, and other organs that are typical of vertebrates.
Lancelets have an almost transparent body that is flattened from side to side and tapered at both ends. They have a mouth surrounded by tentacles that filter food particles from the water. They have no jaws or teeth, but use a muscular tongue-like structure called the velum to create a suction force. They have a single median nostril on the top of their head that connects to an endostyle, a glandular organ that secretes mucus and iodine-containing compounds. They have a simple circulatory system with a contractile vessel that pumps blood through the body. They have a large coelom that acts as a hydrostatic skeleton and provides space for the internal organs.
Lancelets are found in coastal waters throughout the world, usually buried in sand or mud with only their anterior end exposed. They are mostly sedentary and feed on plankton and organic detritus. They can swim by undulating their body from side to side, but they do so rarely and only for short distances. They are dioecious and reproduce by external fertilization. The eggs and sperm are released into the water through pores near the anus. The fertilized eggs develop into free-swimming larvae that metamorphose into adults after several weeks.
Lancelets are of great interest to biologists because they provide insights into the evolution of vertebrates. They are thought to resemble the ancestral chordates that gave rise to vertebrates more than 500 million years ago. By comparing their anatomy, physiology, development, and genetics with those of vertebrates, scientists can infer how vertebrate traits evolved and diversified over time.
Some of the questions that lancelets can help answer are:
- How did the vertebrate head evolve from a simple anterior end?
- How did the vertebrate brain and nervous system evolve from a simple nerve cord?
- How did the vertebrate endocrine system evolve from a simple endostyle?
- How did the vertebrate immune system evolve from a simple innate response?
- How did the vertebrate skeleton evolve from a simple notochord?
Lancelets are also useful models for studying basic biological processes, such as regeneration, stem cell differentiation, gene regulation, and embryonic development. They have a relatively simple genome that is similar to that of vertebrates but with less complexity and redundancy. They also have a high regenerative capacity that allows them to heal wounds and restore lost body parts.
Lancelets are not only fascinating animals in their own right, but also valuable allies for understanding our own origin and nature.