Threshold effects of food concentration on the skeletal morphology of the marine bryozoan Electra pilosa (Linnaues, 1767).

Many paleontological studies rely heavily on characteristics of the preserved phenotype, i.e., morphology of skeletal hard parts. Although the potential for environmental influences on the phenotype is expected, rarely is the magnitude of the effects quantifiable relative to genetic factors. The clonal/colonial body plan of Bryozoa allows for the partitioning of morphological variance into its genetic and environmental factors and addressing of the question, “how much phenotypic variation is induced in a population by changing a single environmental factor?” The effects of variation in food concentration on whole-colony growth rate and on zooid size/morphology can be profound in bryozoans. Here we experimentally test food effects on the skeletal phenotype of Electra pilosa (Linnaeus, 1767), a particulate suspension feeding bryozoan. A threshold effect was observed for the relationship between zooecium size and food concentration. Very low concentrations resulted in stunted colonies with small zooecia, but at low to intermediate concentrations a close relationship existed with zooecium size. Maximum zooecium size occurred at sub-maximal food concentration and sub-maximum zooecium size occurred at higher food concentrations. Previous studies, which have reported no effect of food availability on zooecium size, assessed food concentration effects at levels higher than were effective in the present study. In the absence of other factors, variation in zooecium size is minimal and unchanging at moderate to high food concentration levels. Greater variation in zooecium size is expected at and below threshold food concentrations. We show that the preservable phenotype of these specimens subjected to controlled and induced environmental variation also records information with genetic significance.