Sea urchins are common and ecologically significant marine invertebrates integral to the structure and function of tropical ecosystems worldwide. Belonging to the phylum Echinodermata, which includes sea stars and sea cucumbers, these spiny creatures are characterized by their globe-shaped body and radial symmetry. They are abundant inhabitants of coral reef systems globally, playing an active role in the complex dynamics of the reef environment. Their interactions with coral and algae define their importance to the ecosystem.
Where Sea Urchins Live on the Reef
Sea urchins on coral reefs exhibit a distinct behavioral pattern, primarily governed by their need to avoid predators like triggerfish and large wrasses. During the day, they typically seek refuge within the reef’s complex three-dimensional structure. This includes hiding deep in crevices, under large coral heads, or among the rubble and debris at the reef base.
When darkness falls, they emerge from their shelters and begin active foraging across the reef surface. This nocturnal movement allows them to graze with reduced risk of predation. The specific microhabitat chosen varies by species; some, like the long-spined Diadema, favor areas with high heterogeneity, while others are found in more exposed locations or adjacent sand flats.
The Functional Role of Grazing
The primary ecological function of sea urchins is herbivory, a continuous grazing activity essential for reef health. They primarily consume turf algae and macroalgae, which are fast-growing plant life that can quickly smother and kill slow-growing corals. By constantly “mowing” the reef surface, sea urchins maintain open, clean substrate.
This grazing is accomplished using a unique and complex feeding apparatus located on their underside, known as Aristotle’s lantern. This structure is a muscular, five-sided arrangement of calcium-based plates that function as five separate teeth. The teeth are continuously scraped against the hard reef surface to remove attached algae and micro-organisms. This action prevents the dominance of algae, which, if left unchecked, would lead to a phase shift from a coral-dominated to an algae-dominated reef system.
Shaping the Coral Reef Ecosystem
The constant feeding activity of sea urchins extends beyond consumption of algae to physically altering the reef structure itself. The removal of algal cover maintains clear patches on the hard substrate, which is a necessary condition for new coral larvae to settle and grow, promoting coral recruitment. This space-clearing service is a positive contribution to the reef’s ability to recover.
The intensive scraping action of the Aristotle’s lantern also leads to bioerosion, where small amounts of the calcium carbonate reef skeleton are inadvertently scraped away and ingested along with the algae. This is a natural part of reef turnover, but when urchin populations become overly dense, this erosion can become excessive. High densities can suppress the reef’s net carbonate production, meaning the rate of reef growth cannot keep pace with the rate of erosion.