Phragmatopoma californica (Fewkes, 1889)
Phylum Annelida, Class Polychaeta, Order Sabellida, Family Sabellariidae
Tubes 3-4 mm diameter of cemented sand grains with short purple tentacles & operculum protruding; worm to 5 cm; often in large aggregations (from gregarious larval settlement, not asexual division).
Common; can form large reefs several meters across & >50 cm thick; mid- to low intertidal zones & subtidal; protected outer coast.
Geo Range: central California to Baja
Similar species: Sabellaria cementarium is mostly subtidal, has golden tentacles instead of purple and doesn't form large reef-size aggregations.
Image: Cladophora (green tuft, top center), Mazzaella leptorhynchos (top, left of center), Anthopleura sola (right side)