The ecology of rafting in the marine environment - Bedim

MARTIN THIEL & LARS GUTOW
journeys? What are the distances over which organisms may become dispersed? How does colonisation
of new habitats proceed? Answers to these questions permit a better understanding of the
evolution of coastal marine and terrestrial organisms. Knowing the degree of exchange via rafting
within and between local populations is essential for estimates of connectivity between habitats.
Rafting has been in the past (e.g., Scheltema 1977, Ingólfsson 1992) and continues to be an
important dispersal mechanism in the world’s oceans (Barnes 2002).
Finally we wish to emphasise that there exist many other mechanisms by which organisms
(without or with limited pelagic stages) may become dispersed across wide oceanic distances (see,
e.g., Ruiz et al. 2000, Domaneschi et al. 2002, Winkworth et al. 2002). For many species, rafting
will be of no or only of minor importance. However, we believe that rafting is important on many
different spatial and temporal scales, and evidence for this has been mounting during the past
decades, in particular since the advent of molecular tools (e.g., Wares 2001, Wares & Cunningham
2001, Sponer & Roy 2002). Information on rafting in the marine environment is scattered over a
wide area of scientific fields (e.g., geology, herpetology, terrestrial biogeography), sometimes
making it difficult to evaluate this process. Our own background is marine biology and we cannot
pretend to cover all these fields exhaustively. Yet, we hope that this review will be a useful synopsis
in order to identify organisms for which rafting is (or has been) important. Based on the results of
the present revision we formulate hypotheses and questions about possible consequences of rafting
dispersal, which will be explored in a future contribution.
Rafting organisms
Taxa and species
Many different organisms have been found on floating substrata, among them unicellular organisms
(Masó et al. 2003) and large vertebrates (Censky et al. 1998), as well as fully marine (Hobday
2000a) or terrestrial species (Heatwole & Levins 1972). Rafting species can be facultative (e.g.,
Helmuth et al. 1994) or obligate (e.g., Gutow & Franke 2003) inhabitants of floating substrata.
Species may be highly selective with respect to the floating substrata they colonise or they may
settle on any floating item available. For example, stalked barnacles (Lepadidae) have been reported
from a wide diversity of different substrata including plastics (Carpenter & Smith 1972, Aliani &
Molcard 2003), pumice (Coombs & Landis 1966), wood (Nilsson-Cantell 1930), macroalgae
(Arnaud 1973, Hobday 2000a) and even tar balls (Horn et al. 1970, Minchin 1996). In contrast,
other species have only been reported from specific substrata, such as, e.g., the gastropod Littorina
saxatilis,
which exclusively rafts on floating macroalgae (Johannesson 1988, Ingólfsson 1995).
Herein we gathered information on a total of 1205 species, which have been either observed
rafting or for which rafting had been inferred based on other evidence (distributional pattern, found
on stranded items, genetic population structure). Completeness of this list is not claimed, yet we
believe that it provides a representative overview of the most common rafting species and illustrates
the general trends. Species from the phyla Cnidaria (Hydrozoa), Crustacea (Amphipoda) and
Ectoprocta (= Bryozoa) were most commonly reported as rafters (Table 1). A total of 959 species
are considered as facultative rafters, 41 species as obligate rafters and for 205 species information
was insufficient to identify their status (at least four of this last group of species are capable of
autonomous floating).
In this review we will present a taxonomic account of species found on floating substrata and
whenever possible we will briefly discuss the propensity/capability of the respective taxa to settle
and survive on floating substrata. Herein we primarily focus on organisms that depend on floating
items to travel along the surface of the sea, i.e., organisms with negative buoyancy, which would
otherwise sink to the sea floor.
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