The ecology of rafting in the marine environment - Bedim

RAFTING OF BENTHIC MARINE ORGANISMS
neustonic organisms. Ekman (1967, cited in Adams 1960) and Friedrich (1969) provided corresponding
divisions of the Sargassum fauna into four functional groups represented by (1) motile
epifauna, (2) sessile epifauna, (3) haptic or clinging fauna and (4) nektonic fauna hiding in or
otherwise associated with seaweed. Adams (1960) suggested an additional group termed ‘associated
organisms’ that can be identified in spite of significant overlap with the other groups. Associated
organisms may be further subdivided into (1) self-buoyant invertebrates such as the siphonophores
Physalia sp. and Velella sp. or the gastropod Janthina sp. that become entangled with floating
seaweed caught in the same current, front or eddy and (2) those species (mainly fish) that seek
shelter or food in seaweed, or lay their eggs on it, or whose surface-dwelling larvae are typically
found among floating seaweed. The adults of the latter are usually independent of floating algae
and show no particular adaptation to the rafting lifestyle.
These categories have been developed for fauna on floating macroalgae without or with tiny
holdfasts (genera Ascophyllum, Fucus, Sargassum) and are not necessarily applicable to macroalgae
with extensive holdfasts (e.g., genera Macrocystis, Durvillaea) harbouring a wide diversity of
coastal organisms. Herein, we distinguish two major groups of rafters, namely the obligate rafters
that depend at least during one stage of their life history on floating items and the facultative rafters
that only occasionally occur on floating objects and can become passively dispersed by these.
Species from several phyla have assumed an obligate rafting lifestyle. Crustaceans, followed
by gastropods, dominate in this group (see also Table 1). Not surprisingly most representatives
from this group have been reported from the Sargasso Sea. Most species from the Sargassum fauna
complete their entire life cycle on floating substrata. Stoner & Greening (1984) mentioned several
species that are considered to be endemic to the Sargassum community. Besides the grapsid crab
Planes minutus they listed the amphipods Sunampithoe pelagica and Biancolina brassicacephala,
the polycladid platyhelminth Hoploplana grubei, the nudibranch gastropod Scyllaea pelagica and
the Sargassum fish Histrio histrio. Interestingly, most of these species maintain a limited motility:
they can cling to a Sargassum patch but they can also let go and actively swim over short distances
to adjacent patches.
The oceanic crabs from the genera Planes and Plagusia (Dellinger et al. 1997, Schubart et al.
2001, Donlan & Nelson 2003) also possess the capability to move on and between floating items.
Planes minutus can swim rapidly over short distances but has limited endurance. After about half
an hour of swimming, limb movement slows down and the crabs start to sink (Davenport 1992).
This author also mentioned that crabs rarely move >5 cm away from their raft. The dependence
on floating objects also becomes evident from the absence of this species in waters without flotsam.
Other obligate rafters that retain high motility during their adult life are the isopod Idotea metallica
and nudibranch gastropods. These species may also seek out new floating items when conditions
on their original raft deteriorate.
Typical obligate rafters with a sessile lifestyle are most stalked barnacles. These feature internal
fertilisation in the mantle cavity (e.g., Patel 1959), brooding of embryos during early development
and pelagic larval stages of variable duration. Stalked barnacles achieve their entire life cycle in
the pelagic environment and members of this group often dominate the rafting community, impressively
demonstrating that successful reproduction and colonisation of floating items is possible
even with inclusion of a pelagic larval stage.
Some pelagic species utilise floating items for reproductive purposes but are not dependent on
them during the rest of their lives. The ocean-skaters from the genus Halobates attach their egg
masses to floating objects (Stoner & Humphris 1985, Andersen & Cheng 2004). Similarly, the large
harpacticoid copepod Parathalestris croni has a high affinity to floating Ascophyllum nodosum for
reproductive purposes (Ingólfsson & Ólafsson 1997). Naupliar larvae, unable to swim, hatch from
eggs deposited by females on the plants. The larvae remain crawling on the alga until they
metamorphose to the first copepodite stage, which is able to leave the algal substratum by swimming.
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