Abstract

Many seaweed species (primary rafters) float at the sea surface and travel with marine currents after detachment from benthic habitats. Various studies have confirmed that dispersal via floating sporophytes and/or gametophytes influences the phylogeography and genetic population structure of these buoyant seaweeds. In addition, non-buoyant seaweeds (secondary rafters) that grow attached to or intermingled with these primary floaters may also become dispersed by rafting on their floating hosts. Here, we examine reports of non-buoyant seaweed species associated with buoyant seaweeds and discuss potential consequences for their phylogeography and/or genetic population structure. We found that mostly red and brown algae have been reported with floating seaweed rafts, most of them growing as epiphytes and some as obligate parasites (e.g. endophytes) that travel with their hosts. Molecular evidence suggests dispersal associated with primary floaters in 16 non-buoyant seaweeds, although colonization of distant sites could also have occurred via other floating substrata such as wood, buoys, and other man-made materials. Transoceanic dispersal has been inferred for non-buoyant seaweeds (for example, Gracilaria chilensis and Capreolia implexa) based on low levels of genetic structure and shared haplotypes among populations separated over vast distances of open ocean (e.g. New Zealand–Chile). Some non-buoyant species suspected or shown to be dispersed by rafting are from intertidal habitats, and these algae can resist physiologically stressful conditions during long trips at the sea surface. However, subtidal and low intertidal non-buoyant species have higher potential to be transported because they cohabit with common raft-forming kelps, often growing on them as epiphytes. We conclude that buoyant seaweeds play an important role in driving the phylogeography, evolution, connectivity and distribution of non-buoyant associated seaweeds. Dispersal of non-buoyant seaweeds via these floating seaweeds may have been underestimated in the past.