We investigated small scale spatial structure in Mastocarpus papillatus, a red alga with sexual and apomictic life history variants. Spatial structure and habitat partitioning can facilitate the coexistence of ecologically similar organisms, and in M. papillatus a large scale geographic spatial division is well documented. Variants of M. papillatus display geographic parthenogenesis where sexual fronds are common south of San Francisco Bay and apomicts dominate north of San Francisco Bay. Although geographic parthenogenesis in M. papillatus is well documented small scale spatial structure was not previously known due to the similar appearance of sexual and apomictic upright fronds. We investigated whether sexual and apomictic fronds differed in their spatial distribution by sampling for vertical differences in intertidal height and horizontal patchiness. Fronds were sampled when reproductive and cultured for life history. We found sexual variants were significantly aggregated at lower intertidal heights while apomicts were found throughout the intertidal range. Sexual fronds were strongly seasonal and reproduced October-December whereas apomictic fronds reproduced throughout the year. These results parallel the large scale geographic distribution with apomicts inhabiting marginal areas, and suggest dispersal capability may also facilitate geographic parthenogenesis.

Key words: apomict, mastocarpus, sexual reproduction, spatial distribution