The ecology of rafting in the marine environment - Bedim

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MARTIN THIEL & LARS GUTOW Table 4 (continued) Cnidaria reported or inferred as rafting Species Region Rafting inference different species from floating Sargassum collected in the Sargasso Sea. He remarked that the species found on Sargassum are comparatively small and he suggested that this might be due to the limited availability of food in the oligotrophic Sargasso Sea. Niermann (1986), observing lower abundances of hydroids Clytia noliformis (and of bryozoans) on Sargassum spp. south of the thermal front, i.e., in the Sargasso Sea proper, also suggested that this might be due to food limitations of these species, which feed on suspended microalgae. This agrees with the findings of Smith et al. (1973) that respiration of the entire Sargassum community was higher in the nutrient-rich continental Slope Water than in the oligotrophic Sargasso Sea. Calder (1995) found a total of 10 hydroid species on floating Sargassum collected near Bermuda. In accordance with Smith et al. (1973) he found that species of the genus Clytia and Aglaophenia were among the numerically and functionally most important animals on floating Sargassum in that region. The two algal species S. natans and S. fluitans differed from each other in the composition and community structure of associated hydroids. Gonophores were present in different stages of development indicating that the hydroids reproduce and produce medusae while attached to floating algae. The hydroid fauna of the Azores, however, is dominated by species without medusae, which together with frequent observations on floating items, led Cornelius (1992a) to conclude that most species had reached the oceanic islands of the Azores via rafting. He reported hydroids from macroalgae and floating plastics. From floating objects in the Ligurian Sea (western Mediterranean) consisting mainly of anthropogenic debris but also of natural floats such as wood and leaves, Aliani & Molcard (2003) recorded five different hydroid species. The Hydrozoa ranked among the most diverse taxa with more than 20% of the total number of species found on floating items in that region. Arnaud et al. (1976) found some colonies of a single hydroid species on floating kelp (most probably Ecklonia maxima) off the island of St. Helena. Gregory (1978) even reported hydroids growing on very small substrata such as plastic granules collected on beaches of New Zealand (see also Carpenter & Smith 1972). 300 Floating substratum Reference Montipora sp. Fiji to Australia R V Jokiel 1990a Phyllangia americana Florida B P Winston et al. 1997 Pocillopora damicornis Hawaii B S, W Crossland 1952, cited in Richmond 1987 Pocillopora sp. Hawaii R P, V Jokiel 1984 Pocillopora sp. Fiji to Australia R V Jokiel 1990a Pocillopora spp. Marshall Islands B V Jokiel 1989 Pocillopora verrucosa Lizard Island B W Jokiel 1989 Porites lutea Marshall Islands B V Jokiel 1989 Porites sp. Fiji to Australia R V Jokiel 1990a Seriatopora sp. Fiji to Australia R V Jokiel 1990a Stylophora sp. Fiji to Australia R V Jokiel 1990a Scyphozoa Aurelia aurita Sagami Bay, Japan R M Hirosaki 1964 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; V = volcanic pumice; W = wood. * References refer to this species under a different name.
Cnidaria (Anthozoa) RAFTING OF BENTHIC MARINE ORGANISMS Sea anemones and corals have been reported from different floating substrata including macroalgae, volcanic pumice and plastics (Jokiel 1984, 1990a, Hobday 2000a) (Table 4). The sea anemone Anemonia alicemartinae can be frequently found on floating macroalgae off the Pacific coast of Chile (M. Thiel, personal observation). Individuals of the sea anemone Epiactis prolifera with brooded offspring were collected on floating macroalgae Macrocystis pyrifera (Bushing 1994). The same author also reported the solitary scleractinian coral Balanophyllia elegans. Demersal planula larvae of this species crawl over the benthic substratum at a speed of 1–4 mm min –1 (Gerrodette 1981), which cannot explain the widespread distribution of this species. The author suggested that rafting of adults on stones held by the haptera of detached kelps might be a possible dispersal mechanism for this species. However, rafting of Balanophyllia elegans has not yet been observed directly. Ottaway & Kirby (1975) suggested asexual reproduction or apomictic parthenogenesis for the brooding sea anemone Actinia tenebrosa, which might be a useful trait of a rafting organism upon reaching new habitats (see below). Corals may float themselves, but usually it is only the air-filled skeletons of dead specimens that are able to float (Kornicker & Squires 1962). Live corals found growing on volcanic pumice have apparently rafted over large distances as was suggested by colony sizes (Jokiel 1984, 1990a). Coral colonies of Pocillopora sp. growing on floating pumice at Hawaii were reproductive and had sizes that even indicated an age of 2–3 yr (Jokiel 1984). During this time the colonies could have covered a distance of about 20,000–40,000 km, corresponding to several circuits around the tropical and subtropical Pacific basin (Jokiel 1984). Common reports of stranded glass buoys lost by Japanese fishermen that were covered by corals led the author to the conclusion that some corals colonising Hawaii have their origin in the Western Pacific. Floating pumice, however, most likely came from San Benedicto (Mexico) indicating that the Hawaiian archipelago is colonised from several sources. Furthermore, P. damicornis has been found attached to coconuts and logs (Crossland 1952 cited in Richmond 1987). Richmond (1987), however, doubts that P. damicornis colonised the eastern Pacific region by rafting. In a personal communication to Richmond (1987) G.M. Wellington reported on histological observations he made on P. damicornis from that region: even though he observed gonadal development, the animals never reached full sexual maturity. Thus, larval recruits that are encountered in that region must have arrived there as planktonic larvae (see also Scheltema 1988, Glynn & Ault 2000). In addition to sexual reproduction, self-fertilisation and asexual production of planula larvae has been described for corals from this genus (Stoddart 1983), making them highly suited to colonise new habitats after arrival on a floating item. Ctenophora Bushing (1994) collected a single undetermined ctenophore with floating macroalgae. Since ctenophores are typically pelagic it remains questionable whether these animals can be classified as rafters. Davenport & Rees (1993) analysed the stomach content of ctenophores (Pleurobrachia pileus) in the Irish Sea without mentioning whether these animals were associated with floating algae. They revealed that portunid larvae are an important component of the diet of ctenophores. Larvae and megalopae of swimming crabs are quite abundant in floating weed patches from that region. Similar to other pelagic predators, ctenophores might at least loosely (or passively) associate with floating algae where food is easily available. Platyhelminthes Turbellarians were mentioned as abundant epifauna on floating items in the Northwest Pacific (Tsikhon-Lukanina et al. 2001a) (Table 5). Platyhelminthes are also found quite frequently on 301
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RAFTING OF BENTHIC MARINE ORGANISMS
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Substratum selectivity Lepadidae RA
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B A Organic Matter Small Predators
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Abundance (number of individuals) 0
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Table 18 Comparison between benthic
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Proportion of Debris Items Colonize
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Fall off (scraped off) Clonal growt
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Species number Proportion Holding o
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The ecology of rafting in the marine environment - Bedim

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