The Tube Foot Structure of a Sea Urchin

Sea Urchins

• Sea urchins are exclusively marine creatures that are distributed worldwide, preferring to inhabit shallow water depths and rocky bottoms. Sea urchins feed on a wide array of animal and plant matter, including algae, plankton and mussels; with a few species consuming mud and sand. Sea urchins have a network of teeth and jaws used to grind their food called Aristotle's lantern. This network was named after the Greek philosopher, Aristotle, due to his work as a naturalist.

  Sea urchins range in size from 2.3 to 4.7 inches in diameter. Their skeletons -- called tests -- are composed of calcium carbonate enriched plates. A sea urchin's spines are attached to the test and are used for protection, burrowing and to assist their tube feet in mobility.

Tube Feet

• Tube feet are extensions that protrude through the pores of the sea urchin's skeleton/test, and are used for movement and respiration. Tube feet also serve as receptors to light (photosensitivity) and chemicals (chemoreception). There are approximately 2,000 tube feet on each sea urchin.

Composition

• A tube foot is comprised of three parts: the podia, ampulla and suction cup. The podia is the external structure of the tube foot. The term podia is sometimes used synonymously with tube foot. While "ampulla" has various definitions, in a sea urchin, the ampulla is an internal canal-like structure within the podia that controls the fluid that gives substance to the podia. A suction cup is situated at the tip of the podia.

  The tube foot is primarily composed actin. Actin is a type of protein within various cells -- especially within muscle cells -- that facilitates a cell's mobility and structure. Actin is not unique to sea urchins, but is found in most vertebrate and invertebrate organisms.

  Tube feet contract and extend one-tenth of an inch to 6 inches, depending on the species. This flexing motion is made possible through a sea urchin's water vascular system.

Water Vascular System

• A sea urchin's -- and other echinoderms -- water-vascular system consists of a series canals that function like a network of hydraulic valves. Seawater enters the sea urchin's water-vascular system through an opening called the madreporite, which is controlled by the sea urchin. That water is funneled to the ampulla. When the system valve closes via muscles, the ampulla contracts and water is secreted into the podia. This causes the tube foot to extend. When the foot's muscle contracts, it squeezes water back into the ampulla, causing the tube foot to contract.