The ecology of rafting in the marine environment - Bedim

MARTIN THIEL & LARS GUTOW
Consequently, this species depends at least during part of its life cycle on a floating algal substratum
that serves a nest function making Parathalestris croni a temporarily obligate rafter.
Temporary use of floating items has also been reported for many fish species, in particular for
juvenile stages (Safran & Omori 1990, Druce & Kingsford 1995). Segawa et al. (1961) reported
that the Pacific Saury Cololabis saira and flying fishes lay their eggs on floating seaweeds, which
is underlined by observations from Dooley (1972) and Hobday (2000a) who mentioned that the
flying fishes Cypselurus heterurus and C. californicus attach egg masses to floating macroalgae.
Juvenile lumpsuckers Cyclopterus lumpus associate during their first year with diverse floating
substrata where they find shelter and food (Davenport & Rees 1993, Ingólfsson & Kristjánsson
2002). Shaffer et al. (1995) described that the function of floating macroalgae change seasonally
for associated juvenile splitnose rockfish Sebastes diploproa in coastal waters of Washington, U.S..
During spring and summer, floating macroalgae serve as spatial refuge for S. diploproa from
vertebrate predators, while in autumn floating vegetation serves increasingly as food resource.
Larger fishes associate with floating objects for protection from predators and to find food themselves
(Gooding & Magnuson 1967, Mitchell & Hunter 1970, Moser et al. 1998, Castro et al. 2002,
Nelson 2003). Kingsford (1995) suggested that floating algae increase the habitat complexity of
the pelagic environment, thereby making them attractive for fish associates.
Most rafting organisms reported herein can be considered as facultative rafters that are only
occasionally found on floating items. In the majority of cases, these species usually inhabit benthic
substrata in the coastal zone. Rafting is generally viewed as a non-intentional excursion of these
species onto floating items, which might then occasionally result in dispersal. Adults of facultative
rafters need to be well adapted to survive both in benthic habitats and on floating items. For this
reason it is not surprising that most facultative rafters are reported from floating items that are
already densely colonised while still attached in coastal habitats. Many facultative rafters are
reported from macroalgae that first develop in benthic environments where also initial colonisation
proceeds. Upon detachment, these algae carry with them a diverse fauna of facultative rafters
(Kingsford & Choat 1985, Bushing 1994, Ingólfsson 1998, Hobday 2000a). Many facultative rafters
also colonise floating items after these are detached (see, e.g., Ingólfsson 1995).
Interestingly, rafting has rarely been considered as an integral component in the life history of
facultative rafters, even though some species appear well adapted to rafting (see above) and are
found frequently and in large numbers on floating items. Some of these species may actively
contribute to their substratum going afloat. For example, burrowing isopods and amphipods excavate
holdfasts and stipes of common floating macroalgae (Edgar 1987, Chess 1993, Smith & Simpson
1995, Thiel 2003a) and may thereby facilitate detachment and subsequent dispersal on these algae.
Similarly root-boring isopods may cause breakage and subsequent floating of entire mangrove trees
or parts thereof (Svavarsson et al. 2002). Since these peracarid borers feature direct development
and local recruitment within the maternal patch, they may face a high risk of local inbreeding and
it could be advantageous to increase dispersal frequencies by causing detachment of the positively
buoyant burrowing substrata (Figure 11). Whether these or other species actively induce detachment
(by strategic burrowing) and subsequent floating dispersal should be examined in future studies. It
could be hypothesised that local demes that are detached and rafting on floating macroalgae contain
a higher proportion of juveniles and subadults (i.e., individuals prone to colonise new microhabitats,
see also Brooks & Bell 2001a) than do demes in firmly attached plants.
Brooding in the past has primarily been viewed either as a consequence of morphological
constraints (Strathmann & Strathmann 1982) or as a mechanism to improve offspring survival
(Ebert 1996, Chaparro et al. 2001). Since brooding in combination with local recruitment appears
highly advantageous in the rafting environment, it cannot be completely discounted that in some
facultative rafters, brooding has evolved as an adaptation to rafting dispersal. Rafting appears to
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