We describe the cribrimorph cheilostome bryozoan Cribrilina mutabilis n. sp., which we detected as an epibiont on eelgrass (Zostera marina) at Akkeshi, Hokkaido, northern Japan. This species shows three distinct zooid types during summer: the R (rib), I (intermediate), and S (shield) types. Evidence indicates that zooids commit to development as a given type, rather than transform from one type to another with age. Differences in the frontal spinocyst among the types appear to be mediated by a simple developmental mechanism, acceleration or retardation in the production of lateral costal fusions as the costae elongate during ontogeny. Colonies of all three types were identical, or nearly so, in partial nucleotide sequences of the mitochondrial COI gene (555–631 bp), suggesting that they represent a single species. Zooid types varied temporally in overall frequency in the population: colonies contained nearly exclusively R-type zooids in mid-June; predominantly I-type, or both R- and I-type, zooids in mid-July; and I-type, S-type, or both I- and S-type zooids (interspersed or in discrete bands) in mid- to late August. Reproduction occurred throughout the season, but peaked in July, with only R- and I-type zooids reproducing. Reproductive zooids bear a vestigial compound (tripartite) ooecium and brood internally; S-type zooids, first appearing in August, were non-reproductive, which suggests that they may serve as an overwintering stage. As this species is easily accessible, common, and simple in form, it is potentially useful as a model system for studying polyphenism at multiple levels (zooid, colony, and population) in the context of life-history adaptations.