Phormosoma placenta

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Phormosoma placenta is a deepwater sea urchin that lives on the Atlantic continental slope on both sides of the ocean. It is yellowish-brown and can grow up to about 12 cm across. Its shell, or test, is flexible and dome-shaped on the top with a flattened bottom. The plates of the test overlap and are held together by a membranous connection. When taken from the water, the urchin often collapses into a disc.

On the upper (aboral) surface there are few spines, but the lower (oral) surface is densely covered with perforated tubercles that hold slender, club-shaped spines. These spines sit in membranous sacs and help support and move the animal. Some live spines on the aboral surface are also enclosed in sacs, though those sacs are usually damaged when the animal is collected.

Phormosoma placenta is found in the North Atlantic from Iceland and Greenland down to the Caribbean in the west and the Gulf of Guinea in the east. It normally lives at depths of 500 to 3,700 meters (1,600 to 12,100 feet), though sometimes it is found shallower. There are three subspecies: P. p. placenta in the northern range, P. p. sigsbei in the Caribbean, and P. p. africana off Africa.

This species often occurs in groups on sandy bottoms or coral rubble. It is likely an omnivore, eating algae fragments and other detritus. Its stomach has been found to contain small pellets of mud bound with mucus. Juvenile cusk-eels sometimes hide under or between its long spines, gaining protection and a place to feed where refuges are scarce.

Phormosoma placenta has separate male and female individuals. The eggs are large and yolky and float after fertilization, rising at about 25 cm (9.8 inches) per minute for two days before sinking back to the seabed. The embryos feed on yolk and develop directly without a free-swimming larval stage. Spawning can occur at any time of year, and the group living together helps fertilization and dispersal.

The species was first described in 1872 by Charles Wyville Thomson, from specimens collected in deep waters of the Butt of Lews and the Rockall Channel.

Conservation status: Secure.