This is an unpurchased blog post for a tourism industry client.
Sea urchins are found throughout the waters of Hawaii from the shorelines into deeper ocean waters. Though most are herbivores and non-venomous, two common sea urchin species are
Urchins belong to the same Phylum—Echinoderms—as
If you’re lucky, these are some common species of sea urchin you may see on your water adventures:
Rock Boring Urchin
Two main species of rock-boring urchin grow together in Hawaiian waters. The most common shallow water urchin, Echinometra mathaei, ranges in color from greenish-gray to tan or pink and grows to about 6cm in diameter. The less common Echinometra oblonga is typically dark purple to black and has shorter, thicker spines.
Found under or between rocks in areas protected from the waves, these sea urchins use their sharp spines and jaws to enlarge their living spaces, carving out bits of rock as they grow. When seaweed pieces wash inland and get stuck on their spines, the urchins use their long sucker feet to transfer this food to their mouths.
Watch out that you don’t step on them as their tough, sharp spines will hurt the bottoms of your feet.
These urchins, Tripneustes gratilla, look like the darker rock-boring urchins, Echinometra oblonga, but they grow larger (10cm) and they have much thinner, shorter spines that are sometimes tipped in white or pink. Their name comes from the pebbles and shells that get trapped in their spines by the shifting tides and camouflage them from predators.
Voracious eaters, collector urchins serve a critical purpose in the Hawaiian reefs flats by eating the invasive algae that block the sunlight from the reefs below. So if you see them in the water, know that they are working hard, and be sure to say mahalo nui!
Shingle Urchin (Helmet Urchin)
These urchins, Colobocentrotus atratus, love to ride the rough waves and they’re built for it. With paddle-shaped spines that lie like scales (or roof singles) around its surface, these armored urchins live on rocks in surge zones. Mostly dark purple, they grow to 4-6cm, though they can be up to 9cm in diameter.
Shingle urchins’ food source is the hard, cement-like, red coralline algae (Porolithon) which grow at tide level on the rocks where these urchins live. Watch out for them when walking along the shorelines.
Slate Pencil Urchin
One of the larger sea urchins, Heterocentrotus mammillatus, can grow to 20cm in diameter. Their size alone would make them hard to miss, but they really stand out in the reefs due to their bold red color. Long, blunt spines grow as thick as fingers and are typically lighter than the body of the urchin.
If you’re ever in the water at night and see a slate pencil urchin, don’t be surprised to find it looks a bit different as the bold red spines turn a lighter pink in the dark.
Rough Spined Urchin
A primitive urchin, Chondrocidaris gigantea, has two rows of spines. Its larger spines are much like the red slate pencil urchin’s—long, blunt, and thick as fingers. The secondary spines are smaller, flatter and ring the larger ones, acting as protection for the body between the sparse, larger spines. Unlike other urchins, the spines do not contain living tissue, which allows algae, debris, and invertebrates like snails and sponges to collect on them, creating a rough, dirty appearance that blends in with the reefs.
Larger specimens, 8-10cm, can be found in deeper water, and though they are also closer inland, they hide under coral slabs and in crevices, so you’ll have to search carefully to find them.
Banded Sea Urchin
One of the venomous urchins in Hawaiian water, Echinothrix calamaris, can be found in the holes of reefs. Varying from light to dark green, the banded sea urchin has long, needle-like spines that are striped kind of like a porcupine. Each spine is covered in smaller spines that sting to the touch.
Luckily, these urchins are quite large (up to 15cm), so they’re easy to avoid.
Blue-Black Sea Urchin
As an adult, Echinothrix diadema is black with a blue tinge in the light, but when younger, it can be greenish and has bands like Echinothrix calamaris, so is often mistaken for them. Closely related to, but more common than the banded sea urchin, it is also venomous, so beware and keep a wide berth.
If you’re in the ocean during the day, you’ll likely only see the tips of its long 10-12cm spines as it hides under rocks and in crevices, only coming out at night to eat as many algae as it can find.These beautiful (and sometimes shy) sea urchins are some of the many wonders you’ll see when you join us on one of our submarine tours in the Hawaiian waters.