Harmeria scutulata, a cheilostome bryozoan with a circum-Arctic distribution, is an important component of intertidal and shallow subtidal rock communities. In terms of numerical abundance, H. scutulata can reach over 50% of total individuals within Arctic bryozoan assemblages. It is an annual, fast-growing species that loses over 70% of interactions for space with other organisms. Uniquely for a bryozoan, the subcircular colonies have large zooids at the centre ringed by a marginal zone of up to six generations of small zooids. This pattern is shown here to be due to polymorphism, reflecting a functional differentiation between large, feeding zooids (autozooids) and small, non-feeding zooids that brood embryos (gonozooids). The switch from the budding of autozooids to gonozooids occurs more or less simultaneously around the circumferential growing edge of the colony and is irreversible. Colonies apparently produce embryos only near the end of the growing season. Strong wave action and/or ice scour during the winter months destroys most of the colony but it is inferred that some of the gonozooids, which are more thickly calcified than the autozooids, overwinter, surviving into the spring and releasing their larvae to found a new generation of colonies. A formal systematic redescription of H. scutulata is given.