Critter of the Week: Histocidaris – The explosive urchins?

The Cidaroida is an order of very spiky and robust regular sea urchins, which can resemble something like a sputnik satellite or an underwater mine to the uninitiated.

An unlucky snorkeler from Sydney, Australia misidentified a species of shallow water cidarid as an unexploded underwater mine and alerted local police. The story quickly became national news in Australia, fortunately experts were able to tell it was nothing to be worried about. Specialists from an echinoderm forum think that what the snorkeler saw was likely a species of Phyllacanthus.

An unidentified species of Histocidaris sea urchin at 700 m deep on Diamondhead seamount east of the Chatham Rise, New Zealand. [DTIS camera, NIWA Seamounts programme]

The stunning purple urchin

In New Zealand we have three families, 13 genera, and 18 described species of Cidaroida. One particularly stunning deepsea variety is Histocidaris purpurata (Thomson, 1872). This species is supposedly quite widely distributed from the North Atlantic to the South West Pacific (Kroh, 2016), but is rare in our collection. We have five records of H. purpurata from 788–1105 m deep from the Southern Kermadec Seamounts to the lower East Coast of the North Island of New Zealand.

This stunning purple sea urchin, Histocidaris purpurata (Thomson, 1872), was found on Clark Seamount on the southern end of the Kermadec Ridge at 990 m. Credit: Rob Stewart, NIWA. Ocean Survey 20/20 Mapping the Mineral Resources of the Kermadec Arc Project (LINZ, GNS, NIWA, WHOI).

The oral view of an unidentified species of Histocidaris collected from 700 m deep on Diamondhead Seamount east of the Chatham Rise. [Owen Anderson, NIWA. NIWA Seamounts programme]