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 photos copyright John P. Hoover unless otherwise credited

Some interesting sea urchins not in Hawaii's Sea Creatures
Here's an (unofficial?) Bishop Museum list of Hawaiian sea urchins
Chris Mah's Echinoblog post on Hawaiian deepwater urchins:
http://echinoblog.blogspot.com/2010/08/hawaiian-deep-sea-urchins-below-surface.html

Diadema savignyi (Audouin, 1809)
family Diadematidae
     When viewed under bright light, this urchin has spectacularly brilliant electric-blue lines on its upper surface. Although formerly rare in Hawai`i, in the last few years it has become common in some areas. The urchin above, photographed off Maui in 2009 by veteren diver and biologist Pauline Fiene, was the first Hawaiian specimen she had ever seen. At first she thought it was a juvenile Diadema paucispinum because of its very long spines, but when she saw it again 3 months later in the same location and noticed that it hadn't grown, she began to wonder. Eventually she found a photo of savignyi in Debelius and Kuiter's massive marine invertebrate atlas and realized what it was.

Diadema sp. juvenile
    Jim Petruzzi (http://www.myhawaiianimages.com/) photographed this unusual juvenile urchin during a night dive at Ulua Beach, Maui, at a depth of about 30 ft. in December 2011. Given the very long spines it would have to be either Diadema paucispinum or Diadema savignyi. The latter is very rare in Hawaii. According to urchin expert Dr. Richard Mooi, because of the banded spines it might be a juvenile Diadema savignyi, but not enough is known at present to be able to tell for sure from a photograph. More photos of juvenile long-spined urchins with size and depth information would be helpful. Dr. Mooi says a photo from the top directly down using flash would be best.

Centrostephanus asteriscus Agassiz and Clark, 1907 ???
family Diadematidae
     I found this urchin under a slab off the Lana`i Lookout, O`ahu, at about 40 ft. Sea urchin specialist Dr. Richard Mooi of the California Academy of Sciences tentatively identified it from a slide as possibly belonging to the genus Centrostephanus. The Bishop Museum species list lists Centrostephanus asteriscus as the only member of the genus present in Hawai`i. Since it is seen so infrequently at scuba diving depths it's possible that its center of population lies in deep water. I saw another one once, off Makua, at about 60 ft.. It had an urchin shrimp in it.

Cory Pittman writes:

I was scanning the recent entries on your site and noticed the photo of Centrostephanus asteriscus. I've also seen it twice. Once was a lone individual under rubble at Honolua bay (around 40-50 ft). The other time was a pair in a hole in rubble that the black coral divers had knocked off a freshly collected colony at the Lahaina docks. Presumably, those came from around 200 ft which would support the suggestion of a deeper distribution.

Metalia sternalis Lamarck, 1816
family Brissidae
    This irregular urchin test was sitting on the sand at about 50 ft. at Makua, O`ahu. It was larger than most other Hawaiian species except the common Brissus latecarinatus, but it appeared to be different. I sent the photo to Dr. Richard Mooi at the California Academy of Sciences. He replied:

The reason that the big urchin is different from B. latecarinatus is that it seems to be a good match for Metalia sternalis. Since some of the identification depends on things like pedicellariae and pore-pair counts, it's a little difficult to wing it from the photo. However, gross matching with sources other than Mortensen (who has remarkably poor image coverage of this important echinoid) strongly suggests M. sternalis (Lamarck, 1816).