Polymorphism and variation in modular animals: morphometric and density analyses of bryozoan avicularia

Avicularia are polymorphic zooids characteristic of most species of cheilostome bry- ozoans. We examined the morphological diversity of avicularia to elucidate morphometric patterns in 38 cheilostome species from 11 superfamilies collected from a range of depths (habitats) around New Zealand. Multivariate statistical analyses of 4 types of avicularia (adventitious, interzooidal, vicarious and vibracula) showed that vicarious and vibracula were morphometrically invariable, and vibracula morphometrically distinct, even though these 4 types are dissimilar in their relationships to proge- nitor zooids and colony budding pattern. Adventitious types showed extensive overlap in morpho- space with interzooidal and vicarious types. No depth-related effect on either morphometric variation or colony density of avicularia was discerned over the depth ranges studied (<8, 20-300, and 600-1000 m). Inter-colony variability in avicularian density existed in some species, whilst the main contributor to morphometric grouping was the type of avicularia involved. Greatest avicularian den- sity occurred in colonies of those species possessing 2 types of avicularia (e.g. Caberea spp. adventi- tious and vibracula). Adventitious avicularia were the most common type, were morphometrically diverse and occurred at higher densities within colonies than interzooidal and vicarious types. Such polymorphism, frequency and density of adventitious types in natural populations may represent an evolutionary shift towards maximisation of functional efficiency at a smaller spatial scale of the colony. By analysing polymorph variation we show how a modular development promotes flexibility in module production, arrangement and morphology.