The ecology of rafting in the marine environment - Bedim

RAFTING OF BENTHIC MARINE ORGANISMS
spp.) has been held responsible for the often wide geographic distribution of many bivalve species
that lack planktonic larvae (Martel & Chia 1991).
Geographical evidence has been used to infer rafting for many small and brooding bivalves
(Buroker et al. 1983, Ó Foighil 1989, Helmuth et al. 1994, Ó Foighil & Jozefowicz 1999, Castilla &
Guiñez 2000). Vermeij et al. (1990) attributed the spread of several brooding mollusc species
(bivalves as well as gastropods) along the chain of the Aleutian Islands to rafting. However, no
rafting specimens were reported because sampling was limited to the benthos.
Wood-boring bivalves (Teredinidae) are among the most common rafting organisms on/in wood.
Edmondson (1946, 1962) suggested that the extensive distribution of the species Teredo gregoryi
in the North Pacific and T. clava in the Indian Ocean is the consequence of rafting dispersal in wood.
Mollusca (Cephalopoda)
Dooley (1972) listed cephalopods as one of the major groups of the Sargassum community. In
Tasmanian waters, however, octopuses quickly vanished from Macrocystis pyrifera holdfasts after
the plants were detached from their substratum (Edgar 1987).
Echinodermata
Echinoderms were primarily collected from floating macroalgae (Mortensen 1933, Hirosaki 1964,
Edgar 1987, Bushing 1994, Hobday 2000a) (Table 11). The brooding seastar Leptasterias hexactis
has been reported from the brown alga Nereocystis luetkeana, and Henricia leviuscula from another
floating alga (Bushing 1994). Arnaud et al. (1976) collected brittle stars as well as sea urchins from
the haptera of floating kelp at St. Helena. Holdfasts of Macrocystis pyrifera in Tasmanian waters,
too, are inhabited by echinoderms (Edgar 1987). The brittle star Amphipholis squamata is a common
species on floating macroalgae (Highsmith 1985, Edgar 1987, Bushing 1994, O’Hara 1998). Rafting
dispersal might explain the cosmopolitan distribution of this species (Sponer & Roy 2002).
Mortensen (1933) concluded from the absence of a pelagic larval stage during development of
Asterina exigua (= Patiriella exigua) that this species reached St. Helena from South Africa via
rafting most likely with floating Ecklonia maxima (see also Waters & Roy 2004).
Considering geographic distribution, depth range, habitat and life history characteristics of 42
echinoderm species from Macquarie Island in the Southern Ocean, O’Hara (1998) inferred for
about 20% of the species that rafting on floating macroalgae from the Kerguelen Islands is the
most probable mechanism of transport to that remote island. Due to the lack of morphological
differentiation between echinoderms from the Macquarie and Kerguelen islands, the author assumed
that there is still ongoing gene flow between these locations. Suggested rafters were exclusively
holothurians and asteroids, which are probably able to secure themselves within the holdfasts of
floating macroalgae by using comparatively large suctorial tube feet. Due to the minor efficiency
of these specialised structures in ophiuroids and echinoids, rafting was considered an unlikely
dispersal mechanism for these taxa even though they were quite abundant in kelp beds around the
island. This assumption was supported by Vásquez (1993) who experimentally revealed that asteroids
survived better on detached macroalgae than ophiuroids and echinoids did. Echinoderms are
also found on anthropogenic debris. The sea urchin Arbacia lixula is a member of a diverse fauna
on floating debris in the Mediterranean (Aliani & Molcard 2003).
Juveniles suspended in the water column have also been mentioned for the directly developing
seastar Leptasterias hexactis by Martel & Chia (1991) who found some specimens in intertidal offbottom
collectors. In contrast to young bivalves, floating of juvenile echinoderms is not facilitated
by mucus threads. Nevertheless, suspension in the pelagial enhances the probability of encountering
other floating objects and subsequent rafting.
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