Amphioxus, also known as lancelet, is a group of small marine animals that belong to the subphylum Cephalochordata of the phylum Chordata. They are considered to be close relatives of the vertebrates and provide evolutionary insight into the origins of vertebrate features. In this article, we will explore the characteristics, behavior, ecology and diversity of amphioxus.

Amphioxus are slender, fish-like animals that lack eyes, a definite head, a brain and a cranium. They have a notochord (a stiffening rod), a dorsal nerve cord and gill slits that persist throughout their life. They also have a segmented body with muscle blocks called myotomes that enable them to swim and burrow. They have a dorsal fin that runs along their entire back and becomes a caudal fin around the tip of their tail. They also have a ventral fin but no paired fins.

Amphioxus have a simple circulatory system with no distinct heart. Blood flows forward along the ventral side and backward along the dorsal side. They have no red blood cells or hemoglobin for transporting oxygen. Instead, they rely on diffusion across their thin body wall and gill slits.

Amphioxus have a mouth surrounded by a hood with fringelike structures called cirri that filter out large particles from the water. They feed by creating a ciliary current that passes water through their gill slits and traps food particles in a mucous sheet. The food is then transported to the gut by cilia. Amphioxus can feed on particles ranging from microbial to small phytoplankton size.

Amphioxus are mostly benthic animals that live in sandy or gravelly bottoms of shallow subtidal waters. They burrow into the substrate with their posterior end first and expose only their anterior end to the water for feeding. They can quickly withdraw into their burrow when disturbed or threatened by predators.

Amphioxus are also capable of swimming by contracting their myotomes in a side-to-side motion. They usually swim near the bottom at night or when searching for new burrows. They are not buoyant and sink quickly when they stop swimming.

Amphioxus are dioecious (having separate sexes) and reproduce by external fertilization. The eggs and sperm are released into the water where they fuse to form zygotes. The zygotes develop into free-swimming larvae that have a tail fin, an adhesive organ and sensory organs called Hatschek’s pit and Joseph cells. The larvae undergo metamorphosis into adults after several weeks or months.

Amphioxus are widely distributed in temperate, subtropical and tropical seas around the world. They are found in various habitats such as sand flats, coral reefs, seagrass beds and mangrove swamps. They are tolerant of low salinities and temperatures ranging from 3 to 37 °C.

Amphioxus play an important role in marine ecosystems as filter feeders that consume plankton and organic detritus. They also serve as food for many predators such as fish, crustaceans, echinoderms and cephalopods.

There are about 30 to 35 species of amphioxus in two genera: Branchiostoma (also called Amphioxus) and Epigonichthyes (also called Asymmetron). The two genera differ mainly in the number and arrangement of gill slits, gonads and nephridia (excretory organs). Branchiostoma has more than 15 pairs of gill slits, gonads arranged in metamerically repeated clusters and nephridia located on both sides of the body. Epigonichthyes has less than 15 pairs of gill slits, gonads arranged in two longitudinal rows and nephridia located only on the left side of the body.

The species of amphioxus vary in size