ZOOID SIZE VARIATION IN CHEILOSTOME BRYOZOANS THROUGH 100 MILLION YEARS

Modules called zooids are the fundamental functional units of the colonial phylum Bryozoa. Autozooids are those zooids with a primarily feeding function which protrude a crown of tentacles for feeding and can retract this lophophore into safety within the zooid to avoid predation and physical damage. We have previously shown that encrusting cheilostome bryozoan species with zooids of larger size tend to overgrow species with smaller zooids. Although ‘bigger is better’, there are likely constraints on how large cheilostome autozooids can become, given the lack of a circulatory system and specialized organs for gaseous exchange. This research asks several questions pertaining to cheilostome autozooid size. First, what is the variability of autozooid size within colonies and does mean and variance of autozooid size change through time? In other words, how constrained is autozooid size at the species level? Second, do erect and encrusting cheilostomes have significantly different autozooid sizes? In other words, does the general ecology of a bryozoan have any influence on the manifestation of autozooid size? Third, have there been any evolutionary trends in cheilostome autozooid size? In other words, do constraints (if any) at species level extend to constraints at a macroevolutionary level? Fourth, regardless of what large-scale macroevolutionary trends might be discerned, is there any directionality in the evolution of autozooid size within lineages? In other words, do small ancestors with small autozooids tend to give rise to descendents with similarly small autozooids, or do they tend to give rise to descendants with larger autozooids? By answering these questions, we hope to shed-light on the macroevolutionary patterns of a key phenotypic trait.