The Functional Morphology of Avicularia in Cheilostome Bryozoans
Avicularia are modified zooids characteristic of cheilostome bryozoans. Through evolutionary time the functional capacity of the polypide has evolved and is now a vestigial feature within the avicularium. The functional role of avicularia in the colony is unclear. Unable to feed, avicularia are dependent on nutrients from the parental or neighbouring zooids and therefore constitute a significant metabolic cost in production and maintenance, a cost which must be met by some reciprocal function. Details on the functional morphology of avicularia are severely lacking with avicularia constituting a large gap in the knowledge of cheilostome biology. By examining their biology, this study will enhance our understanding of the evolution and functional capacity of avicularia. Fundamentally this study will provide insight into the degree of morphological and morphometric divergence among avicularia. The objective of this study was to: (1) conduct a morphological survey of the putative sensory structures associated with the palate (or orificial structures) of avicularia (SEM); (2) conduct an ultrastructural-level study (TEM) of the vestigial polypide from two species of bugulids from disparate habitats; and (3) incorporate a holistic study on the behaviour and functional evolution of character traits in Bugula flabellata. The avicularia from 38 species of cheilostome bryozoans were examined covering 11 superfamilies, 18 families and 29 genera. The results indicate that avicularia are morphologically diverse and display extensive variation in avicularian orifice morphology. Structures associated with the orifice varied from tufts of cilia, simple pores, tubular protuberances, and pores with an organic exude. The adventitious avicularia dominated (83%) and displayed the greatest morphometric diversity compared with interzooidal, vicarious and vibracular types. Within superfamilies, only the catenicellids displayed a significant level of morphometric and anatomical congruence. The density of avicularia within a colony was greatest by combining adventitious and vibracular types (e.g. Caberea). This finding may represent an evolutionary shift towards maximisation of function within a smaller spatial scale. The ultrastructure of the vestigial polypide of the subtidal B. flabellata and deep-sea Nordgaardia cornucopioides revealed disparities suggestive of differing functional roles either as a result of habitat differences or differing stages of evolutionary development. The avicularium of B. flabellata has undergone significant modification in character traits from the plesiomorphic autozooid. The current anatomy and behaviour of this avicularium provides strong evidence for a mechanoreceptive function whereby detection of tactile stimuli from passing invaders is important in the functional role of the bird's-head avicularium of B. flabellata. This study is the first to examine the functional morphology of avicularia in such detail. The techniques used in this study have never been applied to avicularia before allowing numerous features such as the muscles, orificial structures and vestigial polypide anatomy to be discerned. The results indicate that avicularia are morphologically diverse and anatomically equipped to perform a variety of functions. The morphological diversity of avicularia may be of immense ecological importance and represent differing responses to various selective pressures in the environment.