Interspecific interactions have been shown to play critical roles in determining the realized niches of species in marine rocky intertidal (ie Connell 1961, Paine 1966,, Dayton 1971) and terrestrial (i.e~ Harper 1969, Pianka 1973, Cody 1974) communities. In contrast, such interactions have not been clearly demonstrated as determinants of the distribution and abundance of marine intertidal soft-substrate organisms (however, see Levinton 1977). There are great physical differences between the rocky and mud-sandflat intertidal habitats, Some of these are wave exposure, sediment size (boulders to silts), penetrability and organic content of the substrate, frequency of log damage, and exposure to sunlight. Considering these physical differences and the fact that soft-substrate environments often support diverse communities, it is interesting to assess the role of biological interactions in' structuring these intertidal communities. Previous work on this system primarily consists of descriptions of animal-sediment · relationships and of food partitioning (i.e~ Sanders et al.. 1962, Mangum 1964, Reid and Reid 1968, Johnson 1971, Rhoads and Young 1971, Young and Rhoads 1971, Fenchel et al, 1975, Warren 1977). The role of interspecific interactions in establishing and maintaining infaunal distribution patterns on a sandflat in northern California is reported here.

This paper is primarily a study of the factors contributing to the distribution of Tegula funebralis and Tegula brunnea in the vicinity of Bodega Bay, California. The two species seldom appear extensively together in the same zone. Tegula brunnea is found on the exposed outer coast of Tomales Point, but not in the immediate zone of Tegula funebralis. Tegula funebralis is found at some points north of Dillon Beach, but it is rarely in the presence of abundant Tegula brunnea. The writer hopes that his efforts may lay a basis for more ecological studies of this genera on the Pacific Coast.

This study deals with the reproductive ecology and larval development of Mytilus californianus. Mytilus californianus is the common mussel found along the more exposed west coast of North America from the Aleutian Islands south Isla Socorro, Mexico (Soot-Ryen, 1955) and was chosen because it is an important species in rocky intertidal communities (being the most abundant if not the most conspicuous) and forms the basis for diverse associations of numerous other species. The reproductive cycle, larval development to settlement, comparisons of reproduction in high and low populations and field observations on larval settlement are described.

Life history patterns of individual organisms are the result of natural selection and should be correlated with particular physical and biological characteristics of habitats in which they live. The littoral fringe is known to be the most severe physical habitat in the intertidal zone, and it is expected that organisms in it should have population characteristics associated with selective response to physical adversity rather than biological interaction. Ecological studies of populations of Littorina planaxis were made in the Dillon Beach and Bodega Head areas. The study localities differed principally in extent of wave exposure rather than geographical location.