The patterns of microhabitat use were investigated in three coexisting epizoic algae on an intertidal gastropod, Lunella correensis in an attempt to enhance understanding of the niche relations among ecologically similar species in a space-limited environment. Our analysis of field-derived data showed clear spatiotemporal partitioning of shell microhabitats by epizoic algae with different degrees of resource specialization. As a substrate-specializing, obligate epizoic alga, Pseudocladophora conchopheria occupied wider micro-niches on the host of different sizes than facultative epizoic algae (encrusting rhodophytes, crustose Corallina and Gelidium). While Pseudocladophora demonstrated uniform use of all shell microhabitats on hosts of all sizes, crustose Corallina and Gelidium showed more varied microhabitat use with shifting positions on hosts of different sizes. Furthermore, in addition to slight differences in microhabitat use, crustose Corallina and Gelidium demonstrated differences in their interspecific relationship with Pseudocladophora. Our generalized linear models (GLM) analyses indicated that the occurrence/abundance of crustose Corallina was negatively affected by Pseudocladophora but that of Gelidium was positively affected, while Pseudocladophora appeared to be competitively inferior to both rhodophytes when they co-occurred on hosts of relatively large sizes. The present study, therefore, points to variable niche partitioning among algae even on a small spatial scale, which may result from complex interactions between spatio-temporal substrate heterogeneity on a live host and interspecific interactions.
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