Sea urchins are marine animals related to starfish and sea cucumbers that are classified under the phylum Echinodermata. They have spiny, round bodies and five sided symmetry.
Sea urchins eat algae and other plant life on the ocean floor. Since they cannot move about in much range, chasing down animals that can- fish, crabs or shrimp, for example – is impossible for a sea urchin.
Instead, they adapted to a plant-based diet. Their preference for feeding on algae makes them valuable additions to salt-water aquariums or reef tanks, as they eat constantly and can keep the tank nice and clean.
Sea urchins are omnivores, so they feed on slower-moving or stationary non-plant sea life such as coral, sea cucumbers, mussels, sponges, bristle worms, feather stars, and brittle stars. From time to time, they will eat the remains of dead fish and crustaceans that fall to the sea floor.
How Do Sea Urchins Eat
You wouldn’t know it from a casual glance, but a sea urchin has a mouth, lips and teeth, all located in the middle of its underside. They eat using a specialized jaw-like apparatus of five strong triangular plates known as pyramids. The bottom surface of each pyramid has a hard tooth pointing towards the centre of the mouth.
Muscles control the protrusion of the jaw, known as Aristotle’s lantern, allowing the sea urchin to snatch, scrape, pull and tear what it is feeding on. A quick aside: Aristotle, the well-known 4th-century philosopher, also was an accomplished naturalist.
In his description of sea urchins, he saw a novel similarity with how horn lanterns looked. Horn lanterns, a widely used light source in his day, consisted of five window panes. The five-part arrangement of a Horn lantern made him think of a sea urchin mouth, except with the panes of horn left out.
Sea urchins have five different ways of feeding. Dr. Christopher Mah, a researcher who studies sea stars and other Echinodermata, detailed how sea urchins eat, in a 2013 post. The five “feeding modes”, he writes, are:
- Scavenging (herbivorous)
- Scavenging (omnivorous)
- Sediment Feeding
- Suspension Feeding
Grazing on kelp is the main food for sea urchins. Kelp grows in dense forests with high biodiversity and has many ecological functions.
Sea Urchins Diet
Sea urchins’ diet includes lots of algae, which play an important role in coral reef ecosystems. A 2011 study of Kenya’s coral reefs found that overfished reef systems have more sea urchins – organisms that eat coral algae which builds up tropical reef systems.
In comparison, reef systems closed to fishing have fewer sea urchins, because predatory fish keep urchins under control. They also had higher coral growth rates and more structure.
The study found that reefs having large numbers of grazing sea urchins diminished the abundance of crustose coralline algae, a species of algae that produce calcium carbonate. Coralline algae contribute to reef growth, especially for the kind of massive flat reefs that surround most of the world’s tropical reef systems.
“These under-appreciated coralline algae are known to bind and stabilize reef skeletons and sand as well as enhance the recruitment of small corals by providing a place for their larvae to settle,”
said Dr. Tim McClanahan of the Wildlife Conservation Society. So having too many sea urchins is bad for coral reefs.
But there is a fine balance. Not enough sea urchins in a coral reef can also have a bad outcome.
The black sea urchin was almost wiped out in a one-year period starting in 1983 due to an epidemic disease killing 95 percent of the population throughout the Florida Keys. What do sea urchins eat when there is no food? Since then, as corals have died due to bleaching, storm damage, predation, ship groundings and diseases, they have left more reef substrate open for reef algae to grow on.
Without the large numbers of sea urchins to graze the algae down, the reef substrate became progressively overgrown by fleshy seaweeds instead of the short turfs and crustose coralline algae that characterize healthy, heavily grazed coral reefs. The thick cover of seaweeds has prevented tiny coral larvae from recolonizing the reef substrate.
Do Sea Urchins Eat Phytoplankton
Phytoplankton is the base of both marine and freshwater food chains. These photosynthesizing microscopic life forms come in many shapes and sizes, including microalgae, diatoms, dinoflagellates, and cyanobacteria.
Sea urchins do eat plankton when they are not feeding on kelp and other seagrasses. Some species of sea urchin – for example, the Eccentric Sand Dollar and the Dermechinus horridus feed on phytoplankton by catching them from the ocean current passing. Most urchins, however, simply feed on whatever plankton they come across as they scavenge the ocean floor, seagrass meadows, or coral beds
In addition to phytoplankton, zooplankton is part of a sea urchin’s diet. Zooplankton is the animal counterpart to phytoplankton, which are the plant form of plankton. Plankton are all water organisms which can’t propel themselves and just drift with the currents.
What do sea urchins consume besides kelp and plankton? Since they need to eat a lot, they can be fairly adaptable when it comes to their diet. They will feed on dead fish, small fish larvae, mussels, small jellyfish, sea cucumbers and sponges.
Do Sea Urchins Eat Seaweed?
Sea urchins eat seaweed, in fact, it is their main food. Their seaweed of choice is kelp.
Kelp is an order of large brown algae seaweed with about 30 species. It grows in shallow sea waters in kelp forests. In waters where population growth of sea urchins has gone unchecked, due to declines in numbers of natural predators, destructive grazing of kelp forests results in what is called an urchin barren.
Sea urchins are not inherently bad for kelp forests, however.
How Do Sea Urchins Eat Kelp
In kelp forests, sea urchins mainly live in crevices among rocky seafloor reefs where they are protected against predators. Pieces of kelp drift down onto the reef similar to leaves falling in a forest, directly conferring food to the urchins in their shelters.
Kelp flourishes where cold, nutrient-rich water wells up along the coast from ocean depths. The giant kelp species that dominates on the Central Coast can grow more than a foot per day in good conditions.
But in 2014 an unusual marine heatwave hit the Northeast Pacific. Known as “the blob,” it spread over the West Coast from Alaska to Central California. Around the same time, a major El Niño event brought warm water up the coast from the south.
With masses of warm water suffusing the coast, kelp growth rates dropped like crazy. There were fewer kelp detritus drifting into the crevices of the reefs, and sea urchins began to come out in search of food. With no starfish around to prey on them, the urchins mowed down the living kelp fronds, turning kelp forests into urchin barrens.
- Barboza, Tony (2013-08-16). Purple sea urchins spoiling kelp forest. Los Angeles Times
- Christopher Mah. What (and How) do Sea Urchins Eat? Sea Urchin Feeding Roundup! The Echinoblog, July 9, 2013
- Jennifer K. O’Leary, Timothy R. McClanahan. Trophic cascades result in large-scale coralline algae loss through differential grazer effects. Ecology, 2010; 91 (12): 3584 DOI: 10.1890/09-2059.1
- Joshua G. Smith, Joseph Tomoleoni, Michelle Staedler, Sophia Lyon, Jessica Fujii, M. Tim Tinker. Behavioral responses across a mosaic of ecosystem states restructure a sea otter–urchin trophic cascade. Proceedings of the National Academy of Sciences, 2021; 118 (11): e2012493118 DOI: 10.1073/pnas.2012493118
- Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning. ISBN 978-81-315-0104-7
- Sardet, Christian (2015-06-05). Plankton: Wonders of the Drifting World. University of Chicago Press. ISBN 978-0-226-26534-6
- University Of North Carolina At Wilmington. Lab-Raised, Algae-Eating Sea Urchins May Reverse Coral Reef Decline. ScienceDaily, 30 July 2001
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