Common Sea Urchins of Hawaii Reefs and Tide Pools

September 23, 2017

If you love exploring tide pools and coral reefs to discover the many fascinating inhabitants, Maui is a great place to be. Whether you join us aboard a Molokini Snorkel Boat Tour, or you opt to stick to Maui's coastlines, the reefs and tide pools won't disappoint. But before you go exploring, it's a good idea to know what's what in terms of certain creatures. Particularly the pricklier ones. By that we mean sea urchins. Some are harmless, some are venomous, and some are just poky enough to cause trouble if you step on them. Here's a basic guide to the most common sea urchins of Hawaii.

Echinometra mathaei - Rock Boring Urchin: This is the most common urchin that you'll find in tide pools all around Maui. They come in one of two colors. One is a olive green, while the other is mauve. They aren't venomous, but their short, tapering spines are sharp enough at the tips that they would be painful to step on, so tread carefully. On the plus side, they carve their way into the rocky pools so each one is nestled into its own depression. This helps protect you from them, and them from you! They're also among the smaller urchins, normally growing to just 4-6 cm, but 15 cm specimens have been found.

Echinometra oblonga - Black Boring Urchin: This species is almost identical to the last one. Their size range is exactly the same, and so is their shape. They are found in the same kind of habitat and bore their way into rocks the same way. In fact, the only difference is that this species is black in color, or a very dark purple. They are also non venomous.

Tripneustes gratilla - Collector Urchin: This species could almost be mistaken for the Black Boring Urchin, but they have some very distinct differences. Although they share a very similar black/dark purple color, their spines are much shorter and finer, sometimes tipped with white or pink. Despite their short spines, they are larger on average, growing to about 10 cm. You're most likely to see them in reef flats, but they can be found in a variety of other places. They earned their name because of their tendency to pick up objects like pebbles and shells

Colobocentrotus atratus - Shingle Urchin: This species is unmistakable and very easy to identify. Its spines aren't sharp and pointing in all directions. Instead, they're shaped like little paddles, and lay flat like protective scales. These urchins are armored for surge zones, so you'll often find them on rocks along rough shorelines. They tend to be dark purple in color, and generally range from 4 to 6 cm in diameter, but can grow up to 9 cm.

Heterocentrotus mammillatus - Slate Pencil Urchin: This species is delightful to behold. They boast long, blunt, red spines that can be as thick as fingers. Because their spine tips are so blunt, they're especially benign, and their bold reddish hues add some extra vibrancy to the reefs where they live. It's not just their color that stands out, but their size as well. This species grows to a remarkable 20 cm in diameter.

Chondrocidaris gigantae - Rough Spined Urchin: These are somewhat similar to the Slate Pencil Urchin in that they both feature long, blunt spines like pencils. In this species, however, the spines are covered with rough, thorny projections, and they grow somewhat larger, at 25 cm in diameter. They are also found in holes on the reef, but sometimes at a greater depth. Their coloration is mottled red and cream on the body, which comprises the exoskeleton known as the "test," while the spines tends to match whatever is growing on the reef, as a camouflage strategy.

Echinothrix calamaris - Banded Sea Urchin (Wana): These last two species are the venomous kind, so you'll want to take care to steer clear and admire them from a safe distance. Typically found in holes on the reef, this species can be distinguished by the rich green hues of its long needle-thin spines. The green color varies from light to dark shades, and they bear distinct bands that make each spine look striped. The spines are also covered with small spinelets, and they sting. They can also grow up to 15 cm in diameter, which helps to make them especially easy to spot.

Echinothrix diadema - Black Sea Urchin (Wana): This species is closely related and therefor similar to the last, but more common. Its coloration is black in adulthood, but younger specimens also bear bands on their spines, and often share the green hues of their relatives, so the two species are often mistaken for each other at a young age. Although their adult color is black rather than green, they have the very long, needle-thin spines in common. Both species grow to 15 cm in diameter, and both can be found in holes in the reef.

We hope these descriptions are enough to get you started as you learn about the many fascinating creatures that can be found while snorkeling Maui and exploring the tide pools. We hope you have a safe and memorable experience! If you join us aboard a snorkeling boat tour, you can count on us to share our knowledge with you. Mahalo!

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