This half-inch-long jellyfish is a common inhabitant of the northern Atlantic, Arctic and northern Pacific coastal waters during spring and early summer. It is the sexually reproductive life history phase of a hydroid that grows on floats, piling and other hard surfaces. During the late summer, fall and winter, the hydroid phase grows and replicates asexually. Then it buds off a series of jellyfish that swim away.
Hydroids grow up to 6 inches tall with many branches. They feed with a tentacle-bearing structure called a polyp. The hydroid is eaten by sea slugs (nudibranchs).
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This jellyfish feeds primarily on crustacean plankton such as copepods and barnacle larvae. Its general feeding behavior consists of intermittent swimming, then holding motionless with its four tentacles extended. The tentacles are covered with numerous weapons called nematocysts. When a prey contacts one of the fishing tentacles, the nematocysts discharge, adhering to the exoskeleton of the prey. After capture, the tentacle bends inward towards the mouth and the prey is engulfed and digested.
Source: David W. Jamison, a marine biologist of Boston Harbor