ENVIRONMENTAL AND GENETIC COMPONENTS OF VARIATION IN SEXUAL ALLOCATION BY AN EPIALGAL BRYOZOAN

Celleporella hyalina colonies growing on Fucus serratus were collected from the field and induced to release larvae in the laboratory. From these larvae, colonies were established on glass slides and each clone d by fragmentation. Growth and sexual activity of genotypic replicates in each of 4 environments were monitored for 12 wk. The environments, representing seasonal contrasts in the field, were created from the 4 combinations of lower/higher temperature and lower/higher food supply . Growth, measured as the production of basal autozooids (responsible for feeding), and sexual activity, measured separately as the producti on of basal male, frontal male and female zooids, were significantly a ffected by genotype and environment. Genotypes differed in colonial gr owth rate and in the numbers of male and female zooids produced per co lony. Within genotypes, growth was fastest at higher temperature and f ood supply. Higher food supply encouraged the production of autozooids and sexual zooids. Higher temperature strongly encouraged female prod uction, but not male. Male production was greatest at higher food supp ly combined with lower temperature, when female production was Very lo w. Colonies maturing in 'summer' conditions therefore invested heavily in females whereas those maturing in 'winter' conditions invested hea vily in males. This phenotypic response might promote fitness by ensur ing that quantities of females are not produced under environmental co nditions that would prolong embryogenesis and brooding beyond a critic al time limit set by risks of colonial mortality. On the other hand, o verlapping generations and sperm storage would provide opportunity for cross fertilization even during winter, when it would pay to invest i n males. The genetically based component of variation in sexual alloca tion in C. hyalina may reflect polymorphism sustained by varying matin g opportunities.