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    Received 20 December 1991. Accepted 16 November 1992.

  • This research was supported by grant No. 103–88 from FON-DECYT-CHILE. We thank C. Retamales, J. Schulz, O. Hijerrra, J. Palma, A. Bravo, and S. Buitano for their invaluable help in the field and the laboratory. We appreciate the help of M. Werkmeister for preparing the figures and S. Angus for reviewing the English of the manuscript. Thanks also to D. López, G. Gajardo, A. Carvacho, and B. Santelices and for the constructive criticisms and comments made by S. Murray and an anonymous reviewer, which greatly improved the manuscript. Finally, we acknowledge the help of E. González in identifying the amphipod species.

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Evidence about the potential of mobile marine invertebrates to act as algal spore dispersal agents is presently circumstantial. Using a field correlational and experimental protocol, our study tested the hypothesis that amphipods can increase the spore recruitment of the red alga Iridaea laminarioides Bory. Iridaea laminarioides spore recruitment onto glass slides was measured at a site with high amphipod abundance and a site with low density of amphipods. To evaluate the effect of an Ulva canopy on recruitment, replicated glass slides with and without a surrounding Ulva canopy were installed at both sites. The number of I. laminarioides spores recruited on the glass slides was four to eight times higher at the high amphipod abundance site than at the low density site. However, the presence of an Ulva canopy covering the glass slides did not significantly increase the recruitment of I. laminarioides. Because the abundance of I. laminarioides, the proportion of cystocarpic plants, and the percentage of open cystocarps only differed slightly between the low and high abundance amphipod sites, we suggest that the variation in recruitment between the sites is due to the differences in amphipod abundance (and their movements) and not to differences in spore production. Moreover, the presence of I. laminarioides cystocarps showing amphipod grazing scars was significantly higher at the high amphipod density site than at the low density site.