Biology/Natural History: Simultaneous hermaphrodites, though fertilization is external. Animals alternately release sperm and eggs. Shell often grows to conform to the hole or crevice it is in; or if not constrained, the anterior end grows narrow and short and the posterior end is swollen but abruptly truncate and gaping. Can attach by byssal threads to its crevice. Predators include Pycnopodia helianthoides. May contain a symbiotic pea crab Pinnixia faba.