The bell is marked by 4 narrow radial canals and 4 tentacles. At the base of each tentacle is an easily seen ocellus. The manubrium is relatively long and surrounded by gonad tissue. The bell is transparent and colorless, but other parts of the body may have tinges of red, green, blue or orange. Sarsia tend to hang motionless with tentacles extended to prey on small planktonic crustaceans.
Hydroids form distinctive colonies during the spring in harbors, on floats and in rocky areas.
Sarsia is found from central California to the Bering Sea, but is not particularly common in the southern part of the range. Farther north it can be very abundant in nearshore surface waters from spring through fall.