The identification, distribution and biology of encrusting species of Alcyonidium (Bryozoa: Ctenostomatida) around the coasts of Ireland

Species of Alcyonidium occur commonly on rocky shores and in the shallow sublittoral. Five of them, A. diaphanum, A. gelatinosum, A. hirsutum, A. mytili and A. polyoum, are described, including new taxonomic information. A. gelatinosum has been the subject of extensive nomenclatural confusion, its name until recently having been incorrectly applied to two other species (A. diaphanum and A. polyoum). Three species, A. gelatinosum, A. mytili and A. polyoum, have been consistently misidentified. However, A. mytili is oviparous, whereas A. gelatinosum and A. polyoum are larviparous. A. gelatinosum and A. polyoum can be distinguished by, for example, gut shape, tentacle number and number of embryos per brood chamber, but breeding season is the main distinguishing characteristic, as A. gelatinosum is a winter breeder (like A. hirsutum) and A. polyoum reproduces in summer (or in autumn in some places outside Ireland). New data on settlement seasons are presented and the variation found between localities is discussed. A key to species is included. Irish distributions are given, corrected from the literature where possible, and based on a recent survey by the authors. A. gelatinosum and A. hirsutum are by far the most widespread and abundant species.