Sea urchins live on the seafloor, where they scavenge algae, including giant kelp. In dense kelp forests, sea urchins shelter from predators in the cracks and crevices of the rocky, temperate reef.
Sea urchins are spiny marine invertebrates that live in the sea on the ocean floor. They have a spherical shape and hollow needle-like spines that can grow up to 12 inches long.
Sea urchins are among the most diverse groups of echinoderms, with about 900 species. Most sea urchins live in shallow water, but some live deep underwater.
What Zone Do Sea Urchins Live In
Sea urchins are often seen inhabiting tide pools. Still, because of the wide diversity of species, they can be in many depth zones, from the seashore inter-tidal zone to the Bathypelagic zone. The Bathypelagic zone extends to a depth of 4000 metres below sea level.
At 4000 meters depth, the water temperature is about 40 degrees F, and sunlight does not reach, which is why the Bathypelagic zone has the nickname “Midnight zone.” Some sea urchins also inhabit the layer above the Midnight zone called the Twilight zone.
The twilight zone, or Mesopelagic zone, spans sea depths between 200 and 1000 meters. Sunlight is dim in this zone.
One species that live in these deep seas is the green sea urchin (scientific name Strongylocentrotus droebachiensis). It has been found as deep as 1,150 meters.
The green sea urchin has short, sharp spines that are non-poisonous and more delicate than the purple sea urchin’s. This species varies from a pale green colour to greenish mottled with purple or brown spines.
The Abyssal Zone
Some species of sea urchin specialize in living in the Abyssopelagic zone, also called the abyssal zone. This ocean zone, extending from 4,000 to 6,000 metres deep (13,000 to 20,000 ft), stays in perpetual darkness.
Urchins that live at this depth have had to evolve to withstand the sheer cold and intense pressure found at this level. Among them are the cactus urchin (Dermechinus horridus), some species of pencil urchin, and many in the family Pourtalesiidae, like the wonderful sea urchin (Pourtalesia miranda), which is a pale brown coloured “irregular” urchin.
An irregular sea urchin is symmetrical from one side to the other, whereas regular urchins have five-fold symmetry. Another difference is that the regular sea urchins use their tube feet to attach themselves to rocks and coral and move around. Irregular urchins use their tube feet to dig burrows in the sand and maintain them.
Where Can Sea Urchins Be Found?
Sea urchins cannot live on land, so they need to live in the ocean. They are usually found on sandy sea floors, rocks, temperate reefs, and coral reefs.
The preferred spot where sea urchins live can vary with the population level of sea urchin predators. One study from 2012 looked at sea urchins in the Mediterranean off the coast of Catalan. The researchers detailed twenty years of scientific studies monitoring sea urchins (Paracentrotus lividus and Arbacia lixula) in the Medes Islands.
Mediterranean sea urchins, whose predators are fish such as white sea bream and gilthead sea bream, consume seaweeds and other algae and play an essential part in coastal ecosystems. You might think that in protected conservation areas where fish proliferate, the sea urchins population would be decreased. But this effect was not seen. The urchins were holed up in crevices and pits.
“If there are many fish, sea urchins hide, and they are less accessible to predators. This explains why, in protected areas, despite the presence of predators, there are large populations of sea urchins that live hidden to avoid them,”
said first author Bernat Hereu. As the sea urchins stay hidden, they do not move much, and they do not eat so many algae.
But on more vertical walls, in contrast to flat rocky seabeds,
“sea urchins cannot find refuge. Here differences between protected and unprotected areas are found. There is also a positive association between juvenile and adult sea urchin populations in protected areas. Outside the marine reserve, this effect is not seen due to the lower control predators have on recruiting sea urchins,”
according to Hereu.
Sea Urchins Habitats
Where do sea urchins live? Well, sea urchins inhabit all climates, ranging from warm tropical seas to frigid polar oceans. The majority of species, however, are found along temperate and tropical coastlines, between the surface and some tens of meters deep, near sunlight-derived food sources such as plankton and algae.
One example of a sea urchin living in a non-typical habitat is the Antarctic sea urchin (Sterechinus neumayeri), which looks like a red pincushion, about 5 inches in diameter, with long spines extending from its round shell. This sea urchin can live on the seabed in the waters around Antarctica, to a depth of about 800 feet.
The Antarctic sea urchin has been used for research in reproductive biology, embryology, ecology, physiology and toxicology. One 2001 study found that it’s larvae use energy 25 times more efficiently than other organisms.
- Apprill A (2017) Marine Animal Microbiomes: Toward Understanding Host–Microbiome Interactions in a Changing Ocean. Front. Mar. Sci. 4:222. doi: 10.3389/fmars.2017.00222
- Hereu B, Linares C, Sala E, Garrabou J, Garcia-Rubies A, Diaz D, et al. (2012) Multiple Processes Regulate Long-Term Population Dynamics of Sea Urchins on Mediterranean Rocky Reefs. PLoS ONE 7(5): e36901.
- Lidz, Franz. Rise of the Sea Urchin. Smithsonian Magazine. July 2014
- Marsh, A et al. High Macromolecular Synthesis with Low Metabolic Cost in Antarctic Sea Urchin Embryos. Science 15 Feb 2001, Vol 291, Issue 5510 pp. 1950-1952 DOI: 10.1126/science.1056341
- Nichols, D., 1967. Echinoderms. Hutchinson University Library, London.
- Tyler, Paul A. Ecosystems of the Deep Oceans: Ecosystems of the World. Elsevier (2003). ISBN 978-0444826190.
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