Genetic and phenotypic variation in two marine bryozoans
Chronic copper toxicity was found to result in growth hormesis in both C. hvalina and E. pilosa. Tolerance levels of the bryozoans to copper contamination were found to be high in comparison with those of other marine organisms. Generally, both genotype and copper dosage affected growth and sexual functions, but genotype had no affect on organism response to copper. Nonetheless, large differences in responses were detected between the two E. pilosa populations examined. Variation in tentacle number of C. hyalina colonies was found to be very limited and dietary regime was not observed to have a marked influence upon this trait. In contrast, colonies of E. pilosa were found to produce more tentacles per lophophore in optimal dietary conditions. Significant differences were detected between two E. pilosa populations in astogeny, sexual maturity and tentacle number. Laboratory experiments designed to identify the cue for induction of extended (long) medium proximal spines in E. pilosa colonies were unsuccessful. Differences in spine growth were again identified between the two E. pilosa populations from contrasting sites. Reciprocal transplantation demonstrated that `long' spine formation was triggered in colonies previously possessing only `short' spines and vice versa. Flume observations of particle path trajectories imply that spine formation may result in near-colony flow conditions which are more favourable to feeding in high flow velocities. Demographic analysis using the RAPD technique for both C. hyalina and E. pilosa indicate population structuring corresponding to their contrasting modes of larval dispersal. Populations of C. hyalina appear to exhibit considerable genetic differentiation over distancesa s small as 100 m, whereasE . pilosa is characterisedb y high levels of genetic heterogeneity over much larger spatial scales. E. pilosa population differentiation is observed at a site some 80 Km distance, which may be a consequenceo f hydrographic features. Evidence from analysis of mtDNA (COI) and observations on reproductive isolation and morphological differentiation indicate high levels of cryptic speciation amongst globally distributed populations of C. hyalina. It is suggested that the Chilean C. hyalina population is sufficiently different from all other populations examined to be considered a separate species.