Sea urchins, those spiny denizens of the ocean floor, are more than just prickly decorations. These creatures play a crucial role in marine ecosystems, and their diet is a key factor in maintaining the delicate balance of these environments. Understanding what sea urchins eat, their feeding habits, and how their diet impacts the world around them reveals their importance in the ocean's health. They are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, sand dollars, brittle stars, and crinoids.
What Sea Urchins Eat: An Omnivorous Appetite
Sea urchins are omnivores with a highly varied diet. They are opportunistic feeders, consuming both plant and animal matter. Their food choices depend on what is available in their habitat. Sea urchins have among the most diverse feeding modes within the Echinodermata.
- Algae and Kelp: The primary component of most sea urchin diets is algae, particularly kelp and seaweed. Several species are found in cold-temperate water habitats like California, New Zealand, and Chile. Sea urchins use a specialized jaw-like structure known as Aristotle's lantern to scrape algae off rocks and other surfaces.
- Plankton: Sea urchins also consume plankton, small organisms that drift in the water column. They catch plankton using their tube feet, moving the food to their mouths.
- Decomposing Matter: Sea urchins act as underwater vacuum cleaners, feeding on dead organisms and helping to recycle materials into usable energy for other organisms.
- Small Invertebrates: Sea urchins will eat slow-moving or sessile animals such as crinoids and sponges.
- Dead Fish: Videos have shown them feasting on dead fish, demonstrating their opportunistic feeding habits.
Feeding Mechanisms: Aristotle's Lantern and More
Sea urchins possess a unique feeding apparatus called Aristotle's lantern. This complex structure consists of five calcium carbonate teeth or plates that come together like a beak. They use their beak-like mouth to scrape rocks clean of algae. The plates wear down, but sea urchin teeth grow to replace worn-down ones.
Irregular urchins, such as sand dollars and heart urchins, have evolved different feeding modes that involve plowing through sediment/mud/sand in order to obtain food.
- Deposit or Sediment Feeding: Sand dollars, sea biscuits, and spatangoid sea urchins are deposit feeders.
- Filter Feeding: Some sea urchins, like Dendraster excentricus, use their tube feet, pedicellariae, and spines to pass along food caught from water currents to their mouths. Dermechinus horridus is another species that is thought to capture food from the water.
- Predation: Some cidaroid urchins have been observed preying on other invertebrates.
Ecological Importance: Keystone Species in Kelp Forests
Sea urchins play a crucial role in maintaining the health of kelp forests. They are herbivores that graze on kelp, preventing it from overgrowing and shading out other marine life. In a balanced ecosystem, sea urchins help maintain a healthy balance of algae.
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However, when sea urchin populations grow unchecked, they can decimate kelp forests, creating what are known as "urchin barrens."
- Urchin Barrens: Removal of predators can lead to a circumstance known as "urchin barrens" where sea urchin abundance goes out of control. Urchin barrens are devoid of macroalgae and associated fauna. Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem.
- Kelp Forest Recovery: Mass mortality of sea urchins may cause the rapid return of a kelp forest, as was observed in the Southern California Bight, where the ecosystem returned to a "kelp-dominated state" within 6 months of a disease outbreak.
Predators and Population Control
Sea urchin populations are controlled by a variety of predators, including:
- Sea otters (Enhydra lutris)
- California sheephead (Semicossyphus pulcher)
- Sunflower stars (Pycnopodia helianthoides)
- Wolf eels
- Lobsters
- Crabs
- Triggerfish
- Cod
- Foxes
- Birds
- Sea stars
- Humans
Sea otters are a keystone species in kelp forest ecosystems, playing a critical role in keeping sea urchin populations in check.
Threats to Sea Urchins: Disease and Climate Change
Sea urchins face a number of threats, including:
- Sea Star Wasting Syndrome (SSWS): SSWS causes urchins’ spines and sea stars’ legs to be degraded.
- Black Spot Disease: Black spot disease causes lesions on the test where a portion of the spines fall off completely, leaving the test exposed.
- Ocean Acidification: The lowering pH of the ocean can cause the spines and test of the urchin to break down leaving the urchin vulnerable to predation and starvation.
- Climate Change: Rising ocean temperatures and ocean acidification threaten many of the species that sea otters rely on for food.
Sea Urchins as Food for Humans: A Culinary Delicacy
Sea urchins are harvested for their gonads, also known as uni. These bright yellow to orange lobes are “stockpiles” of sugars, amino acids, and salts: a trifecta of sweet, salty, and umami. It’s been dubbed as the foie gras of the sea given its buttery texture and delicate ocean flavors.
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- Global Consumption: The greatest consumption of sea urchin occurs in Japan, France, and Korea. Japanese consumption, however, wins by a landslide: the country consumes 80-90% of the current global supply.
- Preparation: Sea urchin is usually served raw as sushi, commonly seen in Japanese cuisine, but it has a variety of applications. Mediterranean cuisines have used urchin in sauces, pastas, and on breads for centuries. Modern day chefs are even transforming the ingredient into foams and mousses.
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