Calliopius laeviusculus is an amphiboreal species found on rocky shores in the western Atlantic from Hudson Bay south to Long Island Sound. Fifty percent of the females were calculated to be mature at 5.6 and 6.7 mm in the summer and winter respectively in Newfoundland waters, but, in Labrador, maturity was not achieved in females up to 10 mm in length. In Newfoundland several generations are produced per year and the females may have more than one brood. However, the females are in the resting stage between August and November–December and young are only released from spring until late summer. The overwintering animals are larger than those found in the summer and the population reaches a peak in late summer. In Labrador, where only one generation is produced per year, young are released in the spring but probably do not reach maturity until the autumn. In Newfoundland fecundity does not vary seasonally.