ABSTRACT
Many species of Alcyonidium Lamouroux, 1813 (Ctenostomatida) are superficially very similar in both colonial and zooidal morphology. For example, the European A. duplex Prouho, 1892, A. gelatinosum (Linnaeus, 1761), A. hirsutum (Fleming, 1828), A. mytili Dalyell, 1847, A. polyoum (Hassall, 1841) and A. variegatum Prouho, 1892, all form a firmly gelatinous, whitish-grey to pale brown encrusting layer over various living or inert substrata. The absence of calcification, as characterizes cheilostomates, deprives taxonomists of the use of skeletal features such as ooecia, morphometry of the orifice, and the form and arrangement of heterozooids (avicularia, vibracula and spines). Therefore, reliance must be placed on characters of the polypide, such as tentacle number and morphology of the gut, and of reproductive biology, which may be apparent only at particular times of the year (for example, the presence of an inter-tentacular organ and numerous small oocytes in A. mytili (Cadman & Ryland 1996)). The utility of zooid morphometrics has not been investigated in Alcyonidium but in widely distributed species, if we extrapolate from other bryozoans and non-modular poikilotherms, is likely to be complicated by the effect of
environmental temperature on size (Atkinson 1994). In this paper we investigate zooid size across the geographic range of two especially similar species, A. gelatinosum and A. polyoum.
