Sea urchins are marine invertebrates belonging to the class Echinoidea, found across all oceans and at various depths. These spiny, globular creatures play a significant role in marine ecosystems, influencing the health and structure of underwater habitats.
Main Food Sources
Sea urchins primarily consume algae, including various types of seaweed and kelp. They feed on macroalgae, coralline algae, and microalgae that grow on rocky surfaces. While their diet largely consists of plant matter, sea urchins are also opportunistic feeders and can be omnivores.
Their diet can expand to include detritus, decaying organic matter, as well as sessile or slow-moving invertebrates such as sponges, bryozoans, and mussels. Some species have even been observed consuming carrion, like dead fish. The specific diet of a sea urchin can vary depending on its species, habitat, and the availability of food sources.
How Urchins Eat
Sea urchins possess a specialized feeding apparatus known as Aristotle’s lantern, a complex structure used for processing food. This unique mechanism consists of five hard, tooth-like plates made of calcium carbonate, surrounded by a sophisticated arrangement of muscles. The lantern allows them to scrape, cut, and chew their food efficiently.
The mouth, located on the underside of the sea urchin’s body, is where Aristotle’s lantern is situated. These teeth can be extended to scrape algae off rocks or to bite into larger food items. Sea urchins also use their numerous tube feet to slowly move across surfaces and position themselves over food.
Ecological Role of Their Diet
The grazing activities of sea urchins significantly impact marine ecosystems, particularly in kelp forests. Their consumption of algae helps maintain the balance of these underwater habitats by preventing the overgrowth of certain algal species. However, an imbalance in sea urchin populations can lead to significant ecological shifts.
When sea urchin populations become too dense, often due to a lack of natural predators, their intense grazing can decimate kelp forests, leading to the formation of “urchin barrens.” These areas are characterized by a lack of kelp and other large algae, replaced by crustose coralline algae or small turf-like algae. The loss of kelp forests reduces habitat and food for numerous marine species, negatively impacting biodiversity.
Sea otters are known predators of sea urchins, and their presence helps control urchin populations, allowing kelp forests to thrive. In areas where sea otter populations have declined, urchin numbers can increase, accelerating the conversion of kelp forests into barrens. While often viewed as destructive grazers, sea urchins also play a role in nutrient cycling by processing kelp detritus, making it available to other detritivores in the ecosystem.