Foundation species like macroalgae provide habitat for large numbers of animals. The spatial structure between branches or thalli can act as a refuge from larger predators and can affect the number and distribution of inhabitant species. Most metrics for habitat architecture are based on 2-dimensional measurements, but habitats are 3-dimensional. We report a new method, spherical space analysis, for characterizing the 3-dimensional volume distribution by size of interstitial spaces for 3 species of macroalgae (seaweed) with distinct architectures. This analysis gives the distribution of volumes within a foundation species that are inaccessible by an idealized spherical organism—an ‘inaccessible volume curve’. A second product is an ‘inaccessible surface area curve’. We incorporated abundances and size ranges of meso-invertebrates into spherical space analysis to predict predator-prey interactions as a function of the relationship between inaccessible volume and area and the size of predators and prey. The results show that filamentous forms of macroalgae have more smaller interstitial volume and area than branched or blade forms of macroalgae that support a larger number of smaller meso-invertebrates. The model suggests that the spatial structure of macroalgae affects predator-prey interactions with a greater number of smaller spaces providing more refuge. This was particularly apparent for kelp. Spherical space analysis provides a mechanism for understanding how the spatial architecture of a macroalgal environment mediates the network of feeding interactions occurring within it. This can have implications for restoration efforts, as the morphology and density of foundation species are integral in the maintenance of communities.