Sea urchins are marine invertebrates belonging to the class Echinoidea, part of the phylum Echinodermata, which also includes starfish and sand dollars. Found in all the world’s oceans, these animals inhabit diverse environments, from the intertidal zone down to the deep sea. As grazers, sea urchins play a significant role in marine ecosystems by feeding primarily on algae.
The Core Body Structure
The fundamental appearance of a sea urchin is defined by its rigid, spherical shell, known as the test. This test is the animal’s internal skeleton, or endoskeleton, and is composed of numerous interlocking plates made of calcium carbonate. In most common species, the test is globular, resembling a slightly flattened sphere or ball.
The size of the test ranges from 3 centimeters up to about 10 centimeters across, though some species are larger. While the spines often obscure the surface, the shell itself comes in a variety of colors, including purples, reds, greens, and blacks. The test is covered by a thin layer of muscle and skin, and its surface features tiny knobs called tubercles where the spines articulate.
The Defining Feature: Spines and Pedicellariae
The most distinct visual characteristic of a sea urchin is its dense covering of spines, which vary between species in length, thickness, and sharpness. These spines are movable, connecting to the test by a ball-and-socket joint that allows them to pivot. They serve a dual purpose: acting as the primary defense against predators and assisting in locomotion by pushing the body along the seafloor.
In some species, such as the long-spined black urchins, the spines are long, needle-like, and can reach up to 30 centimeters in length. Other types, like the slate-pencil urchin, have thick, blunt, widely spaced spines specialized for anchoring and burrowing. Interspersed among the spines are numerous tiny, stalked appendages known as pedicellariae. These pincer-like structures are operated by muscles to actively grasp and remove debris, parasites, or small organisms that settle on the urchin’s surface.
Hidden Features: Mouth and Locomotion
The parts of the sea urchin that facilitate movement and feeding are located on the underside, or oral surface, of the body. Here is the mouth opening, which is surrounded by a specialized, complex chewing mechanism. This five-sided structure is called Aristotle’s lantern and consists of five hard, protruding teeth used to scrape algae and other food sources off rocks.
Movement is achieved by hundreds of flexible, suction-cupped appendages called tube feet. These feet extend through minute pores in the test and are powered by a hydraulic water vascular system. By manipulating water pressure, the urchin can extend and retract the tube feet, allowing it to pull itself across surfaces and adhere firmly to the substrate.