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Seasonal dispersal of gammaridean amphipods away from Sargassum beds in relation to host macroalgal defoliation

Masafumi Kodama*, Jun Hayakawa, Satoki Oba, Tomohiko Kawamura

*Corresponding author:

ABSTRACT: Epibenthic invertebrate assemblages in macroalgal beds generally show remarkable seasonal fluctuations in abundance and species composition, reflecting host macroalgal growth and defoliation. Notably, they often drastically decrease during the host macroalgal defoliation. Though this reduction has been explained in terms of dispersal away from the macroalgal bed in relation to the host macroalgal defoliation, this hypothesis has not previously been experimentally detailed. We conducted a field experiment using an imitation of Sargassum that was set 1 m away from a Sargassum yezoense bed, to confirm whether the epibenthic gammarids disperse away from the Sargassum bed according to the Sargassum defoliation. The imitations were replaced monthly throughout the Sargassum defoliation season and subsequently gammarids associated with the imitation (=assumed as migrants from the Sargassum beds) as well as the gammarids that occurred in the Sargassum bed were investigated. Seasonal patterns of gammarid abundance in the Sargassum bed and their dispersal to the imitations were found to be quite different among gammarid species. Especially, 2 dominant gammarids, the suspension-feeding Jassa morinoi and Podocerus sp., drastically decreased in the Sargassum bed and coincidently increased in the imitations during the Sargassum defoliation season, indicating their defoliation-associated dispersal from the Sargassum bed. This is the first field-experimental study demonstrating the defoliation-associated dispersal of epibenthic invertebrates in macroalgal beds. The dispersal would be caused by a reduction of habitat space and quality due to thalli defoliation. Since many macroalgae show seasonal growth and defoliation, the concept of defoliation-associated dispersal is potentially applicable to various macroalgal species.