Echinothrix calamaris
(1: Lighthouse, Lanai / 2: Shark Fin, Lanai / 3-4: Pawai, Kona / 5: Kua Bay, Kona / 6: Harbor Ledges, Kauai / 7 : Kahalu'u Beach Park, Kona / 8: Sea Tiger, Oahu)
Blue-Black Urchin
Echinothrix diadema
(1-2: Kamaole Beach Park III, Maui)
Collector Urchin
Tripneustes gratilla
(1: First Cathedral, Lanai / 2: Mala Pier, Maui / 3: Kahalu'u Beach Park, Kona / 4: Hammerhead Point, Kona / 5: Garden Eel Cove, Kona / 6: Turtle Canyons, Oahu / 7: Olowalu, Maui)
In the 2nd picture, the urchin is inhabiting an old bicycle. The urchin in the 4th picture has been particularly clever, and is camouflaging itself inside of pieces of shell from another urchin.
Keeled Heart Urchin
Brissus latacarinatus
These usually live under the sand. I've only seen their skeletons:
(1: Pipe Dreams, Kona / 2: Turtle Canyons, Oahu)
This one has been partially eaten by a horned helmet snail:
(1: Amber's Arches, Kauai)
Long-Spined Urchin
Diadema paucispinum
(1: Mala Pier, Maui / 2-3: Harbor Ledges, Kauai / 4: Kahalu'u Beach Park, Kona / 5: Garden Eel Cove, Kona / 6-7: Hale O Honu, Kauai / 8: Pahoehoe Beach, Kona)
Needle-Spined Urchin
Echinostrephus aciculatus
(1: The Dome, Kona / 2-3: Sheraton Caverns, Kauai / 4: Tortugas, Kauai / 5: Stone House, Kauai / 6: Turtle Canyons, Oahu / 7: End of the World, Molokini)
Oblong Urchin
Echinometra oblonga
(1: Black Rock, Maui / 2: Kahalu'u Beach Park, Kona)
These like to hide in crevices during the day.
Pebble Collector Urchin
Pseudoboletia indiana
(1: Shark Fin, Lanai / 2: St. Anthony Wreck, Maui / 3: Kaloko Arches, Kona)
Note the fine white tentacles in the first picture; that's how pebble collector urchins grab pebbles and move them to their back for camouflage.
Red Pencil Urchin
(aka Slate Pencil Urchin)
Heterocentrotus mamillatus
(1: Kaloko Arches, Kona / 2: Pentagon, Kona / 3: Horseshoe Reef, Oahu / 4: Mala Pier, Maui / 5-6: Olowalu, Maui / 7-8: Black Rock, Maui)
Common at shallower depths.
This is a juvenile, less than an inch across:
When they die their spines fall off, and what's left is the test (skeleton):
Rock-Boring Urchin
Echinometra mathaei
(1-2: Kealia Beach, Maui / 3: Hoover's, Kona / 4: Kahalu'u Beach Park, Kona / 5: Mario World, Oahu / 6: Black Rock, Maui / 7: Dog Beach, Kona / 8: Sheraton, Kona)
These fluoresce under UV light:
(1: Kahalu'u Beach Park, Kona)
Rough-Spined Urchin
Chondrocidaris gigantea
(1: First Cathedral, Lanai / 2-3: Garden Eel Cove, Kona / 4: Hoover's, Kona / 5: Sheraton, Kona / 6: Turtle Heaven, Kona)
In the second picture, the rough-spined urchin is to the right; the smoother-spined urchin to the left is a red pencil urchin.
Thomas's Urchin
Actinocidaris thomasi
(1: Barge Harbor North, Lanai)
