Lancelet (Branchiostoma spp.)

MarineChordateInvertebrate

Overview

The lancelet (genus Branchiostoma, also called amphioxus) is a small, slender, almost transparent marine animal, usually only a few centimetres long. It looks a little like a tiny fish, but it is neither a fish nor a vertebrate — it has no backbone, no jaws, no skull, and no true brain or heart in the way vertebrates do.

What makes the lancelet remarkable is its anatomy. It possesses the defining features of the chordates: a stiff supporting rod called a notochord, a hollow nerve cord running along its back, and a pharynx pierced by slits. Lancelets are cephalochordates, an invertebrate group that sits very close to the ancestry of all backboned animals — making them a living window onto how vertebrates may have begun.

Note: there are several lancelet species; details here describe the group broadly.

Habitat & Range

Lancelets live on the seabed in shallow, clean, sandy or gravelly coastal waters in temperate and tropical seas around the world. They spend most of their lives buried tail-down in the sediment, with only the front end protruding into the water, and they prefer well-sorted, oxygen-rich sand.

Diet

The lancelet is a filter feeder. Buried with its mouth end exposed, it draws a current of water in through its mouth and over its slitted pharynx, straining out tiny food particles — microscopic algae, plankton, and organic debris — and trapping them in mucus before passing them to its gut.

Behavior

Lancelets are mostly sedentary, lying buried and filter-feeding, but they can swim in quick, darting bursts with side-to-side, fish-like movements if disturbed or to relocate, then quickly burrow back into the sand. Despite the lack of a true brain, they respond to light and touch, and they breed by releasing eggs and sperm into the water, where fertilisation and development take place.

Human Interaction & Conservation

Lancelets are hugely important to science: because their simple bodies show the basic chordate body plan so clearly, they are key model animals for understanding the origin and evolution of vertebrates, including ourselves. In a few places, such as parts of coastal China, lancelets are also harvested and eaten. Their dependence on clean, undisturbed sandy seabeds makes them sensitive to coastal pollution and dredging.

More photos of the lancelet