DO CONSTANT ENVIRONMENTS PROMOTE COMPLEXITY OF FORM?: THE DISTRIBUTION OF BRYOZOAN POLYMORPHISM AS A TEST OF HYPOTHESES.

The distribution of cheilostome bryozoans on the Caribbean reefs of Panama was surveyed to test the hypothesis that physically constant environments favor increased morphologic complexity, expressed as the number of zooid types within a colony. The proportion of species within defined grades of complexity did not vary significantly with locality, depth, or substratum. Some differences were found in grade-specific ecological success, measured by colony abundance and spatial cover, but these were not consistently related to habitat type. There was no inverse correlation between morphologic complexity and range of distribution: morphologically specialized cheilostomes were not more stenotopic than generalized forms. Patterns of distribution and total space occupation indicate a sensitivity to local habitat conditions, but relative success of species was not correlated with level of polymorphism. In a bryozoan fauna from Florida, the frequency of polymorphic species was weakly associated with constancy of habitat. In estuaries, polymorphic cheilostomes are almost absent at salinities below 18‰, but this pattern is strongly confounded taxonomically. All species tolerant of low salinities are encrusting anascans; within this group, polymorphism does not decrease significantly with declining salinity. Bryozoan faunas from different biogeographic zones may vary in frequency of avicularian polymorphism, but not along a simple latitudinal cline. These large-scale comparisons may be strongly biased historically and taxonomically. The distribution of cheilostome polymorphism on a local and geographic scale provides no evidence for a causal relationship between habitat constancy and morphologic specialization at the zooidal level. This is in striking contrast to the strong habitat dependence of colony form, which suggests that selective processes may operate differently at the zooidal and colonial levels.