Spatial variation in the recruitment of sessile marine invertebrates with planktonic larvae may be derived from a number of sources: events within the plankton, choices made by larvae at the time of settlement, and mortality of juvenile organisms after settlement, but before a census by an observer. These sources usually are not distinguished.
A study of the recruitment of four species of sessile invertebrates living on rock walls beneath a kelp canopy showed that both selection of microhabitats by settling larvae and predation by fish may be important. Two microhabitats were of interest; open, flat rock surfaces, and small pits and crevices that act as refuges from fish predators.
The polychaete Spirorbis eximus and the cyclostome bryozoan Tubulipora spp. showed no preference for refuges, but settled apparently at random on the available substrata. Tubulipora was preyed upon heavily by fish, while Spirorbis was relatively unaffected. The bryozoans Celleporaria brunnea and Scrupocellaria bertholetti both recruited preferentially into refuges. Scrupocellaria were preyed upon, while Celleporaria juveniles seemed unaffected. Predation by fish modified the spatial distribution (Tubulipora), abundance (Tubulipora), or size distribution (Scrupocellaria) of the juvenile population, or had relatively little effect (Celleporaria, Spirorbis).
All of the above events occur within three weeks of settlement. Since inferences about the effect of larval events on the population dynamics of adult organisms are often based on observations of the patterns of recruitment after one or two months, they are therefore likely to be misleading.