Biology Reference
In-Depth Information
can stay and proliferate on algal rafts, other effective rafters are sessile organisms
such as hydrozoans and bryozoans, which can form dense colonies on algal blades
where they overgrow each other when competing for space (Thiel and Gutow
2005b
and references therein). Some of these sessile epibionts colonize the rafting
substrata at a later stage via the propagules from the water column.
The most common marine invertebrates on floating algae are peracarid
crustaceans, which incubate their embryos in a marsupium, from which fully
developed juveniles emerge. Direct development has been considered an important
life history trait for rafters, because offspring can immediately recruit onto the algae
(Ing´ lfsson
1998
; Gutow
2003
; Thiel and Haye
2006
; Vandendriessche et al.
2006
).
Thus, their populations might not only persist but also increase over time. This
mode of local recruitment is advantageous for rafters, since local populations on the
algae can be maintained, even when the duration of the rafting journey exceeds the
lifetime of the species (Thiel and Gutow
2005b
). Direct development also favors
successful colonization of new habitats.
17.4.1 Successional Changes of Epibiont Communities
on Algal Rafts
Rafting communities can experience substantial changes during their journey.
These changes are immediately initiated with the detachment of the algae from
the benthic substratum (Kingsford and Choat
1985
; Miranda and Thiel
2008
;
Gutow et al.
2009
) and continue throughout the entire floating period. For instance,
Helmuth et al. (
1994
) reported that with distance from their benthic source
populations, kelp rafts had more sessile epibionts. V
´
squez (
1993
), who tethered
kelp holdfasts to a main line, demonstrated that densities of associated peracarid
crustaceans increased towards the end of the experiment. In natural algal rafts
around Iceland, Ing´ lfsson (
1995
) observed that the diversity of associated
organisms decreased with increasing distance from source regions.
These temporal changes of rafting communities can be influenced by species
interactions. Floating seaweeds attract fish (Kingsford
1992
,
1995
) and seabirds
(Vandendriessche et al.
2007b
), which prey upon associated organisms, thereby
suppressing the populations of these organisms (including small grazers) on the
rafts. In addition, rafters may compete with each other for food and space, which
also affects species succession during a long voyage (Tsikhon-Lukanina et al.
2001
).
17.5 Rafting Dispersal of Seaweeds and Associated Organisms
The dispersal of algae and their associated organisms strongly depends on the
persistence of the rafts at the sea surface and on the capability of the organisms to
withstand the rafting conditions. Under favorable conditions, algae can continuously
