The creature commonly known as a starfish is more accurately called a sea star, a name that reflects its true biological classification. These animals belong to the phylum Echinodermata, which includes other familiar marine organisms like sea urchins and sea cucumbers. Sea stars are characterized by their unique pentaradial symmetry, where body parts are arranged in five sections around a central axis. The common name often leads to confusion about their physical structure and how they navigate the ocean floor.
Starfish Are Not Fish
Sea stars do not possess fins because they are invertebrates, a biological distinction that separates them from true fish. The term “fish” refers to a vertebrate animal that possesses a backbone, gills, and fins for aquatic movement. Unlike fish, which use fins for propulsion, steering, and balance, sea stars lack all these features. They also do not have gills or scales, highlighting the vast taxonomic difference between the two groups.
The phylum Echinodermata is entirely marine and relies on a different system for movement and survival. This group of animals evolved without the need for streamlined bodies or fin-based locomotion seen in fish. Their physical form, often featuring five or more arms radiating from a central disk, is built for crawling and clinging rather than swimming. This body plan necessitates a distinct and specialized method for traveling across the substrate.
How Starfish Move Across the Seafloor
Sea stars accomplish locomotion using hundreds of extensions called tube feet, or podia, located on the underside of their arms. These tube feet line the ambulacral groove that runs along the oral surface of each arm. Movement begins as a coordinated wave of extension and retraction, allowing the animal to slowly glide across a surface. Tube feet function by extending, gripping the substrate, and then contracting to pull the body forward.
While they appear to use suction, the primary gripping action is achieved through the secretion of specialized adhesive chemicals. Once attached, the feet shorten, and the body moves a short distance. This cycle repeats in a continuous, deliberate manner, which makes a sea star’s movement notably slow. Tube feet are also used for securing a strong hold on rocks and manipulating prey, such as prying open clam shells.
The Water Vascular System
The movement of sea stars is powered by an internal hydraulic network known as the water vascular system (WVS). Seawater is drawn into this system through a sieve-like plate on the animal’s upper surface called the madreporite. From the madreporite, the water passes through the stone canal, which connects to a ring canal encircling the mouth. This system functions as an engine for locomotion and other life processes.
Five radial canals branch off the ring canal, extending down the length of each arm above the rows of tube feet. Each tube foot is connected to a muscular sac inside the body cavity called an ampulla. When the ampulla muscles contract, they squeeze water into the tube foot, causing it to elongate and extend outward. Conversely, when the tube foot muscles contract, water is forced back into the ampulla, causing the foot to retract and pull the sea star forward.