Grazer commensalism varies across the species range edge: host chiton size influences epibiont limpet incidence and spatial segregation

Biotic interactions can determine species distributions and range limits, but little theoretical background exists regarding variation in commensalistic associations across latitudes. We estimated the geographic variation of the epibiont limpet Scurria parasitica’s association with its obligate host chiton species Enoplochiton niger across their distribution from Peru to northern Chile (12−30°S), to test the influence of host size variation on epibiont occurrence, individual spatial distribution, and body size. We analyzed the contribution of chiton body size to the pattern of abundance and distribution of occupancy of the epibiont limpet. We also examined the relationship between limpet shell size, coverage, and incidence probability function and chiton body size across latitudes. For some localities across the range edge of the host grazer’s distribution (i.e. 28−30°S), incidence and densities of the epibiont limpet were higher on larger chitons. Unoccupied host chiton proportions decreased at the poleward edge of the host−epibiont species range compared with sites located to the north. Increased variation in the epibiont limpet distribution on the host chiton plates suggests that limpets’ spatial segregation may have a role in lessening intraspecific interference competition with the host species at southern latitudes. Therefore, local and large-scale processes seem to contribute to modify the host−epibiont association pattern. Further studies are necessary to determine if this association shifts from commensalistic to antagonistic across the distributional range of both species.