The pinnotherid crab Pinnixa chaetopterana inhabits marine polychaete tubes (e.g., Amphitrite ornata and Chaetopterus variopedatus) along the Atlantic and Gulf of Mexico coasts of the United States. This crab occurred in 58.5% of Amphitrite tubes in New Jersey. The polychaete Lepidametria commensalis was found in 64.6% of Amphitrite tubes, often (41.7%) with the crab. In North Carolina, the occurrences of P. chaetopterana and the porcellanid crab Polyonyx gibbesi in Chaetopterus tubes were 69.1% and 30.9%, respectively. In addition, juveniles of the pinnotherid Tumidotheres maculatus occurred with 13.6% of these worms, nearly all (95.1%) of which were in tubes with P. chaetopterana. In Florida, 82.6% of Chaetopterus tubes harbored crabs, but P. gibbesi was dominant with a prevalence of 79.7% versus 2.9% for P. chaetopterana. The tubes of other species of polychaetes and bivalve burrows, however, were used as habitats by the pinnixid. The pinnixid crabs living with Amphitrite were significantly smaller in carapace width (CW) than those with Chaetopterus in North Carolina, and females living with both species of worms were significantly larger than males. Maximum CW for Floridian crabs was less than those to the north. The breeding season for P. chaetopterana in New Jersey was from May to September, from April to October in North Carolina, and possibly year-round in Florida. Brood size was positively correlated with CW in New Jersey; maximum brood size was >4000 in New Jersey crabs and >10,000 in North Carolina. Brood development at ca. 22°C was ca. 14 days. In New Jersey, P. chaetopterana may produce four to five broods/season. Crabs may molt between broods, during which time sperm remain viable in the seminal receptacles. Longevity of P. chaetopterana in the laboratory is at least two years. Known hosts of P. chaetopterana, as well as ecto- and endosymbionts of this species, are reviewed.