a1 Alfred Wegener Institute for Polar and Marine Research, Box 12 01 61, 27515 Bremerhaven, Germany
a2 School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Anglesey, LL59 5AB, UK
a3 Biologische Anstalt Helgoland, Foundation Alfred Wegener Institute for Polar and Marine Research, Marine Station, Box 180, 27483 Helgoland, Germany
Rafting on floating macroalgae is a common dispersal mechanism of marine benthic invertebrates. Most benthic macroalgae are inhabited by diverse epifaunal communities but not all organisms may be adapted to live on floating algae. In particular, knowledge about the immediate effects of algal detachment on the associated biota is limited. Herein, we studied the composition of the communities of mobile invertebrates on benthic thalli of Ascophyllum nodosum and compared it with detached thalli that had floated for short periods. The community of the mobile invertebrates changed significantly within the first minute after detachment of the algae and showed decreased diversity and a tendency towards reduced abundances in most taxa. However, during the subsequent two hours of floating at the sea surface the species composition did not change further. A comparison of the size-spectra of the gastropod Littorina obtusata from attached and detached algae did not reveal differential migratory activity among size-classes of these gastropods. Most of the species encountered in this study are common rafters in coastal and offshore waters, which are well capable of holding onto floating seaweeds. Therefore, our results indicate that the animals actively abandoned the macroalgae immediately after detachment. A benefit of this behaviour may be to avoid increased predation risk in the open water. The fact that individuals remain associated with their algal host after detachment indicates the importance of rafting dispersal for a great variety of phytal species that might lead to range expansion and regional population persistence through metapopulation effects.
(Received February 07 2008)
(Accepted July 23 2008)
(Online publication November 17 2008)