Growth and Mortality of the Bryozoan Alcyonidium Hirsutum (Fleming) on Fucus Serr Atus L.

Extract

As the processes by which epiphytic bryozoans become established in aggregations on intertidal algae become more clearly understood, it is necessary to consider some of the ecological consequences of non-random settlement. The adaptive significance of aggregation in epiphytic bryozoans has been discussed recently by Hayward & Ryland (1974), who also showed how the worst effects of overcrowding are mitigated. Acorn barnacles and spirorbine tubeworms have evolved a specific behavioural pattern which results in regular, spaced-out settlement within aggregations, thus allowing room for each individual to become established and begin growing. By contrast, epiphytic bryozoans do not exhibit this kind of behaviour (Harvey, Ryland & Hayward, in preparation). Al-though a minimum size is probably required before the colony is able to breed, the pattern of growth observed for Alcyonidium hirsutum shows that this can be achieved through lability of growth direction. The linear relationship found to exist between colony size and fecundity shows that no advantage is conferred by increased size; the smaller colony is as effective in reproduction as the larger. In the present paper two other ecological consequences of aggregated settlement are considered: mortality and size.