Sea urchins are marine invertebrates belonging to the class Echinoidea (phylum Echinodermata), which also includes starfish and sea cucumbers. They populate the ocean floor in nearly every climate, from tropical coasts to polar regions. They possess a globular or dome-shaped body encased in a hard shell, or test, which is covered with numerous protective spines. Ranging typically from 3 to 10 centimeters across, sea urchins move slowly across the seabed using specialized hydraulic tube feet.
The Primary Diet of Sea Urchins
Sea urchins are primarily herbivores, meaning their diet consists mostly of plant material. Their main food sources are macroalgae, which is the general term for seaweed, and large brown algae, such as kelp. They also graze on benthic microalgae, which are thin layers of plant life growing on hard surfaces. Urchins are opportunistic feeders, readily consuming detritus and decaying organic material that settles on the ocean floor.
How Urchins Consume Food
Sea urchins use a specialized feeding apparatus located on their underside called Aristotle’s Lantern. This complex structure is composed of five hardened, calcareous plates that function like teeth or jaws. The teeth are used to effectively scrape algae off rocks, chew tough plant material like kelp, and grind down food before ingestion. Since the teeth are constantly worn down from scraping, the structure continuously grows new tooth material, reportedly at a rate of 1 to 2 millimeters per week. Although the mouth is on the bottom, sea urchins employ their numerous tube feet to maneuver food particles toward the opening.
Urchin Diet Variation
A sea urchin’s diet is not strictly uniform and can vary significantly based on the species, habitat, and the availability of preferred food. Deep-sea species, for example, rely less on large seaweeds and more on “marine snow,” the continuous shower of organic detritus sinking from the upper water column. When primary algal sources become scarce, urchins shift to an omnivorous diet. This involves consuming sessile invertebrates, such as sponges, barnacles, or mussels. Some specialized species, like the heart urchin, consume organic matter found within sand and mud.
Ecological Influence of Urchin Feeding
The feeding habits of sea urchins influence the structure of marine ecosystems. When populations are unchecked by natural predators, their constant grazing pressure can decimate large areas of kelp forests. This results in the formation of “urchin barrens,” which are vast, barren seafloors stripped of macroalgae. The loss of kelp, which provides shelter and food for countless fish and invertebrate species, triggers a significant negative effect throughout the marine food web. Recovering these forests often requires managing the urchin population to allow kelp to regrow and restore habitat complexity.