The ecology of rafting in the marine environment - Bedim

More documents

Recommendations

Info

Mean No. of Fish (+1 SE) 35 30 25 20 15 10 MARTIN THIEL & LARS GUTOW Figure 14 Numbers of the fish Abudefduf troschelii under artificial floats that were in clean state or colonised by a diverse fouling community. Figure modified after Nelson 2003. Iceland were also found to depend on the algal species composition (Ólafsson et al. 2001). Pavia et al. (1999) remarked that epiphytic algae have a significant effect on the distribution of crustacean mesograzers inhabiting attached Ascophyllum nodosum. They suggested that the animals might benefit from increasing habitat complexity of fouled macroalgae providing better shelter from predators. Furthermore, epiphytes might be more palatable than the floating macroalgae for some mesograzers. The rafters Cyclopterus lumpus, Gammarellus angulosus and Dexamine thea apparently preferred Ascophyllum nodosum that was overgrown by the red alga Polysiphonia lanosa since their numbers were correlated positively with the weight of this epiphyte (Ingólfsson 2000). Most of these studies have been conducted in regions with high abundance of floating macroalgae, and consequently contact between different algal patches may commonly occur (see also Ingólfsson 2000), offering rafters the chance to select between items. Hobday (2000a) coined the term ‘rafthoppers’ referring to organisms that can switch between floating items. The highest selectivity with respect to substratum characteristics can be expected for fishes, which are capable to move autonomously between floating items. This is demonstrated by the rapid colonisation of floating items by fish. Subsequent reshuffling of fish among floating items results within short time periods in a significant relationship between substratum size and fish number (e.g., Nelson 2003). Many fish associates appear rather selective about substratum size, concentrating under large floating items (e.g., Moser et al. 1998), but they also show a preference for items colonised by a diverse rafting community (Figure 14). Regardless of their preference criteria, many, and in particular large fish, show a relatively high selectivity for large, complex and already colonised rafts. The high degree of selectivity among these fish probably is a reflection of their high motility. In summary, selectivity with respect to floating substrata appears to be a good reflection of the motility of rafting organisms, with sessile rafters being least selective and motile fish most selective (Figure 15). The limited degree of selectivity among sessile rafters may be an expression of limited encounter chances with floating substrata. Influence of rafters on floating substrata 5 0 Clean Fouled 9-Sep 10-Sep 11-Sep 12-Sep Rafting organisms can also exert a strong influence on floating substrata. For example, feeding by rafters on their floating substratum contributes to the continuous destruction of their raft. Proceeding disintegration of holdfasts of floating Macrocystis pyrifera from Tasmanian waters was mainly ascribed to the boring activity of isopods from the genus Phycolimnoria (Edgar 1987). Strong 364 Abudefduf troschelii
Substratum selectivity Lepadidae RAFTING OF BENTHIC MARINE ORGANISMS Suspension-feeder Grazer Predator Bryozoa Spirorbidae Hydrozoa Bivalvia Polychaeta & Peracarida Sedentaria Holothuroidea Ophiuroidea Fully sessile Semi-sessile Walking/Crawling Swimming Figure 15 Schematic relationship between motility of principal rafting taxa and their substratum selectivity. boring activity has also been observed for Limnoria stephenseni on attached holdfasts of Durvillaea antarctica at Macquarie Island (Smith & Simpson 1995). As Stoner & Greening (1984) mentioned that only few species of the Sargassum community directly consume their host plant, destruction of the algal raft might be a feature of facultative rafters originating from benthic habitats. Furthermore, floating Sargassum might be well protected from feeding by the active synthesis of toxic polyphenolic substances (Sieburth & Jensen 1969). For floating Macrocystis pyrifera in Californian waters, decomposition of algal fronds was strongly temperature dependent (Hobday 2000b). While at temperatures below 20°C the decomposition rate was quite low, a dramatic increase in the aging rate of the algae (measured as loss of algal blade length) was observed at higher temperatures. The relationship between longevity of floating algae and water temperature can also be seen from the good condition of algal fronds floating in cold Icelandic waters. Even after about 6 wk of floating, plants of Ascophyllum nodosum hardly exhibited any sign of decomposition (Ingólfsson 1998). In contrast, plants of this species incidentally found near the equator did not appear ‘particular robust’ (John 1974). However, since John (1974) did not mention any grazer associated with the plants it is not known whether this lack of robustness is a result of grazing or of temperatures far above the physiological optimum of this rather northern species. Boring of associated rafters is also responsible for decomposition of floating wood (Emery 1963), which in the case of large trees, though, is more resistant to destruction than macroalgae and may persist sufficiently long to carry rafters over distances >1000 km. Another important effect, which rafting organisms have on their substratum, is that they contribute to an increase in weight of a floating item. Hentschel (1922) and Parr (1939) suggested that the weight addition of calcareous skeletons might contribute to the sinking of macroalgae (Sargassum). In a personal communication to Johnson & Richardson (1977) N.J. Blake reported on small fragments of Sargassum in the Gulf of Mexico that were so heavily encrusted by Membranipora sp. and Lepas sp. that it was only barely buoyant. Zaitsev (1970) measured the 365 Motility Caridea Polychaeta & Peracarida Errantia Echinoidea Brachyur Brachyura, Gastropoda, Asteroidea Pisces
Page 1 and 2:
1 Oceanography and Marine Biology:
Page 3 and 4:
RAFTING OF BENTHIC MARINE ORGANISMS
Page 5 and 6:
Page 7 and 8:
Page 9 and 10:
Table 2 RAFTING OF BENTHIC MARINE O
Page 11 and 12:
Table 2 (continued) Species Region
Page 13 and 14:
Page 15 and 16:
Page 17 and 18:
Porifera RAFTING OF BENTHIC MARINE
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
Cnidaria (Anthozoa) RAFTING OF BENT
Page 25 and 26:
Page 27 and 28:
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36: RAFTING OF BENTHIC MARINE ORGANISMS
Page 85: RAFTING OF BENTHIC MARINE ORGANISMS
Page 93 and 94: B A Organic Matter Small Predators
Page 101 and 102: Abundance (number of individuals) 0
Page 103 and 104: Table 18 Comparison between benthic
Page 105 and 106: Proportion of Debris Items Colonize
Page 109 and 110: Fall off (scraped off) Clonal growt
Page 113 and 114: Species number Proportion Holding o
Page 137 and 138:
Page 139 and 140:
show all

The ecology of rafting in the marine environment - Bedim

Create successful ePaper yourself

Delete template?

Save as template?