The ecology of rafting in the marine environment - Bedim

RAFTING OF BENTHIC MARINE ORGANISMS
Table 9 (continued) Arthropoda Malacostraca reported or inferred as rafting
Species Region
(Davenport & Rees 1993, M. Thiel, personal observation). Also juvenile portunid crabs may occur
in high abundances on floating macroalgae (Tanigawa 2001).
Adults from the genus Plagusia can frequently be found on floating items (Schubart et al.
2001). The species P. depressa and P. squamosa from the Atlantic and Indo-Pacific, respectively,
are very similar in morphology and only recent molecular studies revealed that they are two distinct
species (Schubart et al. 2001). The authors concluded that despite the high dispersal capabilities
of these species, exchange between the Indo-Pacific and Atlantic oceans around the southern tips
of the major continents has not occurred.
Some decapods are obligate rafters. Species from the genus Planes colonise a variety of floating
objects ranging from floating Sargassum (Hitchcock 1941, Sano et al. 2003) over tree trunks and
buoys (Chace 1966, Jara & Jaramillo 1979) to sea turtles (Dellinger et al. 1997). Davenport (1992)
reported high abundances of Planes minutus on Sargassum rafts. Population densities of Planes
minutus are linked to the quantity of floating substrata because this crab has never been collected
from waters free of floating items. The fact that crabs on wood and other flotsam are usually small
and non-reproductive suggests that these items represent secondary habitats and that turtles are the
preferred habitat of these crabs. Another obligate rafter is Pachygrapsus marinus. This species is
found on flotsam in the Pacific (Chace 1951). It has been found among a dense growth of stalked
barnacles (Lepas sp.) on a large glass fishing float (Dell 1964). Van der Baan et al. (1972) explained
the occurrence of Macropodia sp. and Hyas araneus in the neuston off the Dutch coast with an
association of the animals with floating seaweed accumulating at the hull of a lightship. On the
Great Barrier Reef crabs of the species Thalamita stimpsoni have been found rafting on buoyant
corals (DeVantier 1992).
Genetic distance between Hawaiian and Panamanian populations of the brachyuran crab Trapezia
ferruginea did not differ significantly from zero (Huber 1985) indicating either that the eastern
population had recently immigrated from the central Pacific, or that there exists gene flow between
the two regions. Long-distance dispersal between the two locations was considered impossible
because the duration of larval development of this species is too short. Even though the author
does not mention the possibility of population connection by rafting, this mechanism is not unlikely
because Trapezia ferruginea is a common symbiont of corals from the genus Pocillopora, which
have repeatedly been observed rafting (Jokiel 1984).
Caridean and peneid shrimp are also frequently found on floating macroalgae (Hirosaki 1964,
Kingsford & Choat 1985, Hobday 2000a). Some species from the Sargassum community spend
their entire life cycle on floating algae (Butler et al. 1983, Hacker & Madin 1991). Most species
are grazers that feed on their floating substratum. While marine decapods are frequently found on
floating items, Marcus (1926) and Wittmann (1934) precluded large-scale dispersal of freshwater
crabs over entire oceans for physiological reasons.
325
Rafting
inference
Floating
substratum Reference
Tanais dulongii Azores C M Morton & Britton 2000
Tanais stanfordi Galapagos D n.i. Peck 1994
Notes: B = stranded floating item on beach; C = circumstantial; D = distributional inference; G = genetic inference; M =
macroalgae; n.i. = no information; O = other; P = plastics; R = rafting; S = seeds; T = tar lumps; W = wood.
* References refer to this species under a different name.