By Obiabin Onukwugha
Sea urchins are globe-shaped creatures that live on the ocean floor. Sea urchins belong to a group of marine invertebrates called Echinodermata, which means spiky-skinned.
About 950 species of sea urchin are distributed on the seabeds of every ocean and inhabit every depth zone from the intertidal seashore down to 5,000 meters.
Sea urchins cannot swim. They live and move along the seafloor, favouring hard surfaces such as coral and rocks.
Sea urchins move slowly, crawling with tube feet, and also propel themselves with their spines.
They are found in every ocean and in every climate, from the tropics to the polar regions, and inhabit marine benthic habitats, from rocky shores to hadal zone depths.
Although algae are their primary diet, sea urchins also eat slow-moving animals. Predators that eat sea urchins include a wide variety of fish, starfish, crabs, marine mammals, and humans.
However, adult sea urchins are usually well protected against most predators by their strong and sharp spines, which can be venomous in some species.
The closest echinoderm relatives of the sea urchin are the sea cucumbers, which like them are deuterostomes, a clade that includes the chordates.
Like other echinoderms, sea urchin early larvae have bilateral symmetry, but they develop five-fold symmetry as they mature.
This “irregular” body form has evolved to allow the animals to burrow through sand or other soft materials.
If left unchecked by predators, sea urchins can devastate their environments, thus creating what biologists call an urchin barren, 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.
Sea urchins reproduce by sending clouds of eggs and sperm into the water. Millions of larvae are formed, but scientists note that only a handful make it back to the shoreline to grow into adults. They term this scenario as a risky life strategy.
Sea urchins were originally thought to live 7 to 10 years but recent studies have shown that they can live for more than 100 years. This is attributed to the fact that sea urchins are highly regenerative organisms.
It is reported that adults can regenerate external appendages and can maintain their regenerative abilities throughout life. They grow indeterminately and reproduce throughout their entire adult life.
It can also take them years to die of starvation.
The survival of sea urchins is vital for maintaining balance within marine ecosystems. Sea urchins are responsible for grazing around 45% of algae on coral reefs. Without sea urchins, coral reefs can become overgrown with macroalgae, which can limit the growth of corals.