In the spring I used to collect small ones in the sand of streams and creeks in sonoma county, they were the non parasitic types, they filter feed until they are mature then spawn and die. Once while walking the creek behind my house I found hundreds of them all tied up in a huge ball right above some small riffles and a few days later in the calm water they were all dead and rotting. Back then I didn't know the life cycle of them and didn't understand what they were doing and what happened. I tried several times to keep them, but they just burried in the gravel of my tanks and starved as back then there wasn't the available foods that probably would have kept them alive, now I bet I could keep them, though they never come out of the sand.
There are also parasitic ones, they feed on fish as adults, but the juvs are the same, cryptic filter feeders that burry in the sand.
