The ctenostome bryozoan Alcyonidium diaphanum is widespread and abundant in the coastal waters of the British Isles. During the Centre for Environment, Fisheries and Aquaculture Science groundfish surveys some atypical colonies were found and subsequent histological and genetic analyses have shown that these represent two previously undescribed species of erect subtidal Alcyonidium. They are described here fully and named Alcyonidium condylocinereum and Alcyonidium hydrocoalitum. Comparisons with further museum specimens of erect Alcyonidium species have shown that A. proliferans, A. topsenti and A. gelatinosum form lacourti can be synonymized with A. diaphanum. The northern and southern limits of A. diaphanum are here more clearly defined; however, the identities of Arctic erect Alcyonidium species have yet to be fully investigated using histological and genetic methodologies. It is clear from this study that gross colony morphology is not a good indicator of species identity, but that tentacle number and reproductive mode can be useful taxonomic characteristics in these species.