The ecology of rafting in the marine environment - Bedim

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MARTIN THIEL & LARS GUTOW frequently over these distances, could result in establishment and maintenance of meta-populations of these organisms. Finally, organisms capable of local recruitment may be able to establish viable demes on rafts and new populations after rafting voyages of >100 days, possibly extending over >1000 km. Only a limited set of species will be capable of surviving these long-distance voyages, namely, species that can hang on, persist and establish local populations on rafts (Figure 25). Given the multiple selective processes that rafters may have experienced during long-lasting voyages, suspension-feeders with asexual reproduction or with direct development appear to be the most likely candidates for long-distance dispersal. The results of this review provide support for the conceptual model developed herein (Figure 25), although it is admittedly very simplistic. Based on the presently available evidence, we have attempted to elucidate the selective processes to which rafting organisms are exposed during a rafting voyage. Furthermore we made a first intent to relate these processes and their outcome to potential dispersal distances. This model should be viewed as a first step in formulating specific hypotheses about rafting as a dispersal mechanism for marine (and terrestrial) organisms. As pointed out in the previous review (Thiel & Gutow 2004), successful rafting depends not only on the rafting organisms themselves but also on the quality and abundance of rafting substrata, on floating direction, and on inter- and intraspecific interactions during the journey and after debarkation. Occurrence of floating substrata may be highly variable, and periods with only a few items floating at the sea surface may alternate with periods when dense armadas of items are underway (see Thiel & Gutow 2004). There appears to be a linkage between high abundances of floating items of terrestrial origin and intense rainfalls (see, e.g., Heatwole & Levins 1972, Zarate- Villafranco & Ortega-García 2000) or hurricanes (Censky et al. 1998). Volkmann-Kohlmeyer & Kohlmeyer (1993) discussed that after storms large amounts of driftwood (carrying rafting fungi) reached Hawaii. During these events many terrestrial organisms (including groups of conspecifics) may be swept to sea on large and abundant floating items (Van Duzer 2004). After volcanic eruptions, high densities of floating pumice have been reported (Coombs & Landis 1966, Jokiel 1990b, Fushimi et al. 1991). While these events may be very rare and often irregular in occurrence, supply of floating items is high when they occur. During these events, rafting organisms are not homogeneously distributed over all floating items. Rather, many species feature a clumped distribution due to ecological interactions during the rafting journey. For example, an uneven distribution of different rafting species over available floating substrata had been observed by Ingólfsson (2000) in Icelandic waters (see also above). Highsmith (1985) also noted that rafters are not evenly distributed over floating substrata in a region but rather occur in clumped assemblages. He suggested that co-arrival of several potential colonisers should increase the probability of successful establishment. Outlook Rafting is a common dispersal process in present-day oceans. Coastal marine habitats harbour many species that have arrived in the past via rafting and that continue to be dispersed via rafting. Many authors pointed out that species with direct development are well suited for long-distance dispersal via rafting (e.g., Johannesson 1988, Castilla & Guiñez 2000) and this is confirmed by the comparisons conducted herein. Species with direct development are common in most marine environments, in particular in many coastal habitats (e.g., Levin 1984, Davenport & Stevenson 1998, Grantham et al. 2003) but also on seamounts (Parker & Tunnicliffe 1994). The motile fauna of littoral algae is dominated by peracarid crustaceans, brooding echinoderms, bivalves and also, among the gastropods, many species that feature direct development. Interestingly, the proportion of direct developers appears to be particularly high in macroalgae known to float, often for long time periods. In some of these habitats, connectivity via rafting could thus be similarly (or more) important than exchange via pelagic larvae. 394
RAFTING OF BENTHIC MARINE ORGANISMS Based on the results of this review, several hypothesis can be formulated, e.g., small organisms are better suited for rafting dispersal than are large organisms (rationale: holding on and food requirements); substratum generalists are better suited for rafting dispersal than are substratum specialists (rationale: probability to find a raft); food generalists are better suited to survive rafting journeys than are food specialists (rationale: probability to find food); suspension feeders are better suited for long-distance rafting than are grazing species (rationale: food supply); internal fertilisers are better suited to achieve successful reproduction on rafts and in new habitats than are external fertilisers (rationale: gamete concentrations); and brooding species are better adapted to establish persistent populations on rafts and in new habitats than are species with pelagic larvae (rationale: local recruitment and future mate finding). All these hypotheses are supported by the information presented herein, but a rigorous analysis must await compilation of further data. Some of these hypotheses are difficult to test, but once additional information becomes available or when specific experiments are conducted, thorough tests of most of these hypotheses are possible. Testing of these hypotheses will foster our understanding of biological traits that favour successful rafting. Available estimates of dispersal distances of marine organisms focus almost exclusively on species with pelagic larval stages (Kinlan & Gaines 2003, Shanks et al. 2003, Schiel 2004), probably because for many of these species the duration of the pelagic stage is known and, in combination with predominant current speeds and directions, one can calculate approximate dispersal distances. Presently, the prevailing perception is that species with pelagic larvae depend primarily on larval dispersal, yet almost all developmental stages (except pelagic larvae) of some of these species have also been reported rafting. Rafting may in fact also be important for these species, since recent studies have shown that dispersal distances of pelagic larvae may be relatively limited, possibly because larvae exhibit behaviours retaining them near native habitats (Swearer et al. 2002). Estimation of dispersal distances for species that rely on rafting is complicated since usually neither the duration, nor the speed, nor the direction of rafting voyages are well known. However, rafting as a dispersal mechanism provides great opportunities for researchers, because dispersal can be observed in situ. Rafting occurs at the sea surface, and floating items can be easily followed. Most rafters travel on relatively large items, which can be easily marked or deployed with transmitters that would permit tracking the rafting voyage. The usefulness of this approach has been demonstrated by studies conducted in Japan (Segawa et al. 1962) and California (Harrold & Lisin 1989). However, to our knowledge all these studies have been conducted with floating macroalgae in coastal waters and the succession of the rafting community was not examined. We suggest that future studies should mark floating items and track them over time in order to examine the succession of the rafting community in situ (Figure 26). Deployment of transmitters on rafting items in the open ocean would also permit the detection of these items when these are approaching nearshore waters and enable the arrival process to be studied. In summary, floating items that represent an important connecting agent for rafting organisms provide ideal opportunities to study connectivity between coastal habitats. Over a local scale of hundreds of metres and following floating seagrass shoots with rafting ascidians, Worcester (1994) demonstrated the validity and feasibility of in situ observations of the rafting process. Similar studies could be done with floating macroalgae, which can be tracked directly or using radio transmitters. In particular, we consider that additional studies of the arrival and colonisation processes are necessary in order to evaluate the importance of rafting in the marine environment. Studies of gene flow between populations of coastal marine invertebrates have often utilised a comparative approach, contrasting species with pelagic larvae with congeners that feature direct development (e.g., Russo et al. 1994, Edmands & Potts 1997, Hoskin 1997, Chambers et al. 1998, Kyle & Boulding 2000, Collin 2001). The hypothesis examined in these studies usually concentrated on the developing mode of the investigated species, but the results often led authors a posteriori to suggest rafting as an important and efficient dispersal mechanism for the directly developing 395
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Table 2 RAFTING OF BENTHIC MARINE O
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Table 2 (continued) Species Region
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Porifera RAFTING OF BENTHIC MARINE
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Cnidaria (Anthozoa) RAFTING OF BENT
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