DNA barcoding and morphological analyses revealed validity of Diadema clarki Ikeda, 1939 (Echinodermata, Echinoidea, Diadematidae)

A long-spined sea urchin Diadema-sp reported from Japanese waters was genetically distinct from all known Diadema species, but it remained undescribed. Extensive field surveys in Japan with molecular identification performed in the present study determined five phenotypes (I to V) in Diadema-sp according to the presence and/or shape of a white streak and blue iridophore lines in the naked space of the interambulacral area. All phenotypes were distinct from Diadema setosum (Leske, 1778) and Diadema savignyi (Audouin, 1829), of which a major type (I) corresponded to Diadema clarki Ikeda, 1939 that was questioned and synonymized with Diadema setosum by Mortensen (1940). The holotype of Diadema clarki has not been found, but three unlabeled dried tests of Diadema were found among Ikeda's original collection held in the Kitakyushu Museum of Natural History and Human History, Fukuoka, Japan. A short mtDNA COI fragment (ca. 350bp) was amplified from one of the tests, and the nucleotide sequence determined (275bp) was nearly identical with that of Diadema-sp. Arrangements of the primary tubercles on the coronal plates in Diadema-sp and the museum specimen also conformed with Diadema clarki, indicating that Diadema-sp is identical to Diadema clarki and a valid species. Narrow latitudinal distribution (31°N to 35°N) of Diadema clarki in Japan was observed, where it co-existed with abundant Diadema setosum and rare Diadema savignyi. No Diadema clarki was found in the southern islands in Japan, such as Satsunan Islands to Ryukyu Islands and Ogasawara Island, where Diadema setosum and Diadema savignyi were commonly observed.