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

Christian M. Ibáñez*, Javiera Bravo, Sergio A. Carrasco, Mauricio J. Carter, Moisés A. Aguilera

*Corresponding author:

ABSTRACT: Biotic interactions can determine species distribution and range limits, but few theoretical background there exist on variation on commensalistic associations across latitudes. We estimated the geographic variation of the epibiont limpet Scurria parasitica association with its obligate host chiton species Enoplochiton niger across their distribution from Peru to northern Chile (12 to 30°S), to test the influence of the host size variation on the epibiont occurrences, individual spatial distribution, and body size. We first analyzed the contribution of chiton body size and the pattern of abundance and distribution of occupancy of the epibiont limpet. Second, we examined the relationship of the limpet shell size, coverage, and incidence probability function, with the chiton’s body size across latitudes. For some localities across the range edge of the host grazer distribution (i.e. 28°S to 30°S), incidence and densities of the epibiont limpet were higher on larger chitons. Unoccupied host chiton proportion diminished at the poleward range edge of the host-epibiont species 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 on the host species at southern latitudes. Therefore, local and large-scale processes seem to contribute to modify the host-epibiont association pattern, and further studies are necessary to disentangle if this association may shift from commensalistic to antagonistic across the distributional range of both species.