Arbacia is a genus of sea urchin

Sea urchin

Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

s.
Arbacia
by Dave Grant
"Sometimes you see queer things...spiny sea urchins, for instance…in a
slow motion parade. In the magic of the night the wooden soldiers have
come to life, though it is a stiff, hardly perceptible life."
(Marston Bates - The Forest and the Sea)

Sea urchins are an ancient and diverse group of invertebrates that, with the noteworthy exception of Sandy Hook Bay (NJ), have filled many niches in the world's oceans. There are about 800 species worldwide; interestingly, more than appear in their long fossil record.

This link is to an article about sea urchins in the NY Harbor area:
http://ux.brookdalecc.edu/staff/sandyhook/dgrant/field/Urchins.htm

Species

• Arbacia crassispina Mortensen, 1910
• Arbacia dufresnii (Blainville, 1825)
• Arbacia lixula
  Arbacia lixula
  The Black Sea Urchin, Arbacia lixula is a Sea Urchin found on the coast of the Mediterranean Sea and Macaronesian Islands , and less commonly, on the Atlantic coast of Western Africa and the Brazilian coast....
  
  (Linnaeus, 1758)
• Arbacia punctulata
  Arbacia punctulata
  Arbacia punctulata is a species of Arbacia genus of purple-spined sea urchins. Its natural habitat is in the Western Atlantic Ocean. Arbacia punctulata can be found in shallow water from Massachusetts to Cuba and the Yucatan Peninsula, from Texas to Florida in the Gulf of Mexico, the coast from...
  
  (Lamarck, 1816)
• Arbacia spatuligera (Valenciennes, 1846)
• Arbacia stellata (Blainville, 1825; ?Gmelin, 1788)