Puget dwarf brittle star (Amphipholis pugetana)
This little fellow is one of the smallest of a group of animals called the brittle stars, or Ophiuroids. It has a central disk of about three-eighths of an inch and arms that are six to eight times as long. Its disk color ranges from gray to whitish and the arms from tan to white. A close cousin appears similar, but the arms are shorter.
Brittle stars are a diverse group of animals related to sea stars. Their arms have many joints that tend to break easily when the animal is handled, hence the name brittle star.
However, this species does not fragment as easily as others. Unlike sea stars, brittle stars move by using their arms to push and pull the disk along the surface, which allows for very rapid movement. The tube feet are mainly used for feeding and burrowing in sand.
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Like most brittle stars, the Puget dwarf reproduces by releasing sperm and eggs into the water, but a few species brood the eggs in a special pouch in the disk next to the arm joints.
Puget dwarf brittle stars are found under rocks or on kelp from the high intertidal zone down into the subtidal zone from Alaska to northern Baja California. They can occur in large numbers in favorable habitats.
They feed on diatoms and small organic matter. Brittle stars pass the fine particles to the mouth and then to the stomach, which is held within the disk.
David W. Jamison is a marine biologist and Boston Harbor resident.