The ecology of rafting in the marine environment - Bedim

RAFTING OF BENTHIC MARINE ORGANISMS
reproductive habitat (Ingólfsson & Ólafsson 1997). Females of the large pelagic species Parathalestris
croni deposit their offspring (non-swimming nauplii) in floating macroalgae that serve as a
nest for this species (Ingólfsson & Ólafsson 1997). This nest function of algal clumps could not
be observed for Harpacticus chelifer, which was found by Ólafsson et al. (2001) to be by far the
most abundant harpactocoid species on floating algae near Iceland, occurring in virtually all floating
fronds collected during the summer months.
Rafting on macroalgae may also serve as an efficient dispersal mechanism between distant
locations (Yeatman 1962, Ólafsson et al. 2001). Since harpacticoid copepods are able to cling very
tightly to algae (Monk 1941 cited in Yeatman 1962) they seem to be well equipped for rafting on
macroalgae. The same behaviour has been described for nauplii of Tisbe furcata by Johnson &
Olson (1948). This species produces benthic larvae that use their prehensile hooks and second
antennae to cling tightly to detritus particles. This indicates that generations of harpacticoid copepod
species might be able to cover large distances by rafting because even juvenile stages do not get
lost during the journey. Harpacticoid copepods represented more than 90% of the total fauna
collected from sediment patches floating in the surface water of the Pauatahanui Inlet (Wellington,
New Zealand) while cyclopoids made up only a very small portion (~0.1%) (Hicks 1988). A total
of 21 harpacticoid species were found with more than 95% belonging to the epibenthic species
Parastenhelia megarostrum, which occurred in all samples. Even though all developmental stages
were found on sediment rafts, the overwhelming majority consisted of juveniles (nauplii and
copepodites) among which the larval stages dominated. However, on adjacent benthic (non-floating)
substrata, proportions of larvae were much lower indicating that larger or older individuals are less
likely to raft probably due to their burrowing habit in contrast to the nauplii, which crawl on the
sediment surface and cling to sand particles. A survivorship of more than 75% after a period of
18 h following sampling and handling in the laboratory suggested that nauplii were not negatively
affected by sediment rafting.
Arthropoda (Crustacea, Cirripedia)
Barnacles are some of the most typical rafting organisms (Table 8). Stalked barnacles are found
on macroalgae (Helmuth et al. 1994, Adams 1960, Nilsson-Cantell 1930), wood (Hodgson 1896,
Nilsson-Cantell 1930), floating corals (Kornicker & Squires 1962, DeVantier 1992), empty Nautilus
shells (Chirat 2000), volcanic pumice (Richards 1958, Jokiel 1984, 1989), tar balls (Horn et al.
1970, Wong et al. 1974, Minchin 1996), glass fishing floats (Dell 1964) and plastic items (Arnaud
1973, Aliani & Molcard 2003). Three species of the genus Lepas (L. anserifera, L. pectinata and
L. anatifera) and one species of the genus Conchoderma (C. virgatum) are listed as members of
the Sargassum community (Hentschel 1922, Adams 1960, Butler et al. 1983). Lepadid barnacles
can form dense assemblages on floating substrata (Figure 3). Nilsson-Cantell (1930) provided
images of (balanomorph as well as lepadomorph) barnacles growing on the skin and on baleen
plates of whales. These barnacles inevitably become rafters during postmortem floating of their
hosts. Lepas anatifera was the most abundant rafter found on floating kelp near St. Helena (Arnaud
et al. 1976). Near Bermuda, Butler (1975) counted 150 individuals of L. pectinata (2–8 mm in
length) from only four tar balls. From the size and the growth rates of the barnacles, he estimated
the tar balls to be at least 2 months old. Lepadids possess pelagic larvae that colonise any floating
item during the early stages of succession (Tsikhon-Lukanina et al. 2001b). Concentrations of
lepadid larvae in general appear to be relatively low but exposed items nevertheless are colonised
rapidly and in high densities (Il’in et al. 1977). Most Lepadidae are obligate rafters and their
populations are entirely pelagic. For L. fascicularis rafting is an obligate but only transient developmental
stage (Thörner & Ankel 1966). Floating objects (mainly macroalgae but also terrestrial
objects such as wood and seeds of vascular plants) serve only for a short time period after attachment
315