Are Antarctic suspension-feeding communities different from those elsewhere in the world?

This paper reviews the trophic ecology of benthic suspension feeders in Antarctic shelf communities, studied within SCAR's EASIZ Programme, in comparison with published information from other seas. Dense benthic suspension-feeder communities capture large quantities of particles and may directly regulate primary, and indirectly, secondary production in littoral food chains. Most work has been performed in temperate and tropical seas; however, little is known about suspension feeders in cold environments. Recent studies on Antarctic littoral benthic suspension feeders suggest the period of winter inactivity may last only a few weeks. This contrasts with the hypothesis that in Antarctic communities there is a prolonged period of minimal activity lasting at least 6 months during the austral winter. Results from other oceans may explain how dense benthic communities could develop under such conditions. Alternative food sources, i.e. the "fine fraction", sediment resuspension, lateral advection and efficient food assimilation may play a significant role in the development of suspension-feeder dominated, very diversified, high biomass and three-dimensionally structured communities on the Antarctic shelf.     

A New Angle on Microscopic Suspension Feeders near Boundaries

Microscopic sessile suspension feeders are a critical component in aquatic ecosystems, acting as an intermediate trophic stage between bacteria and higher eukaryotic taxa. Because they live attached to boundaries, it has long been thought that recirculation of the feeding currents produced by sessile suspension feeders inhibits their ability to access fresh fluid. However, previous models for the feeding flows of these organisms assume that they feed by pushing fluid perpendicular to surfaces they live upon, whereas we observe that sessile suspension feeders often feed at an angle to these boundaries. Using experiments and calculations, we show that living suspension feeders (Vorticella) likely actively regulate the angle that they feed relative to a substratum. We then use theory and simulations to show that angled feeding increases nutrient and particle uptake by reducing the reprocessing of depleted water. This work resolves an open question of how a key class of suspension-feeding organisms escapes physical limitations associated with their sessile lifestyle.     

A New Angle on Microscopic Suspension Feeders near Boundaries

Microscopic sessile suspension feeders are a critical component in aquatic ecosystems, acting as an intermediate trophic stage between bacteria and higher eukaryotic taxa. Because they live attached to boundaries, it has long been thought that recirculation of the feeding currents produced by sessile suspension feeders inhibits their ability to access fresh fluid. However, previous models for the feeding flows of these organisms assume that they feed by pushing fluid perpendicular to surfaces they live upon, whereas we observe that sessile suspension feeders often feed at an angle to these boundaries. Using experiments and calculations, we show that living suspension feeders (Vorticella) likely actively regulate the angle that they feed relative to a substratum. We then use theory and simulations to show that angled feeding increases nutrient and particle uptake by reducing the reprocessing of depleted water. This work resolves an open question of how a key class of suspension-feeding organisms escapes physical limitations associated with their sessile lifestyle.     

Seasonal and long-term variation in body-water content of an Antarctic springtail – a response to climate change?

Body-water content of field-fresh samples of the springtail, Cryptopygus antarcticus Willem (Collembola, Isotomidae) was measured at monthly intervals over 11 years (1984-1995) at Signy Island, in the maritime Antarctic. A clear annual cycle of variation in water content was observed, with maxima in the austral spring and autumn, and minima in midwinter and midsummer. There was no overall trend during the 11-year study, in contrast to an earlier analysis of the initial 1984-1987 period, which demonstrated a significant increase in body-water content (from 56.6 to 66.0% fresh weight). It is suggested that, between 1984 and 1987, water stress on C. antarcticus in its environment declined, and thereafter stabilised between 1988 and 1995. Springtail body-water content between 1984 and 1995 showed significant increases in several months, particularly in autumn and early winter, with decreases in midsummer. This was consistent with the predicted consequences of the pattern of regional climatic warming in the maritime Antarctic, where small increments in temperature have effectively increased the length of the potential biologically active period. C. antarcticus responds rapidly to local and short-term variations in environmental conditions and will be able to take advantage of increases in the thermal energy budget and growing season length. It is predicted that climate warming could lead to a reduction in life-cycle duration, an increase in population density and extension of geographical range.     

Ascidian suspension feeding

Ascidians are a diverse group of benthic suspension feeders. This review presents and discusses the current literature on ascidian suspension feeding including the different structures involved in feeding as well as how feeding responds to variation in environmental parameters like water temperature and particle concentration. It is concluded that clearance rates in different species at identical conditions will not vary more than within the same species of different sizes, and that variation in clearance rate in ascidians in relation to temperature and particle concentration involves different regulatory mechanisms. Finally ascidian and mussel suspension feeding is compared. It is concluded that the two pumps are very alike with regard to pump performance and specific clearance rate, but whereas ascidian suspension feeding is characterised by high efficiency in terms of particle range and costs of pumping, mussel suspension feeding is more adapted to turbid conditions.     

Seasonality in coastal benthic ecosystems

For historical reasons, knowledge about seasonality in the dynamics of marine benthic suspension feeders from temperate areas comes mainly from studies of cold temperate seas. Recent surveys of Mediterranean taxa show different patterns from those observed in cold temperate seas, which are characterized by winter dormancy. In the Mediterranean, summer dormancy predominates among taxa and appears to be related to energetic constraints. Temperature and food availability are crucial to the dynamics of benthic suspension feeders. However, because these factors tend to be positively correlated in cold temperate seas, it is difficult to distinguish between their effects. Such correlation does not occur in Mediterranean ecosystems. The contrast between recent studies in the Mediterranean and in other areas can help to disentangle confounded environmental controls.     

Biodiversity and biogeographic affiliation of Bryozoa from King George Island (Antarctica)

King George Island (KGI), which is located between the Antarctic and South American continents, may play a crucial role in the exchange of Bryozoa amongst the various Antarctic sectors and across the Polar Front. Knowledge regarding the biological diversity of this area could help us understand the evolution of the Antarctic ecosystem and its connectivity to the South American continent as well as the colonization ability of particular species. Here, we investigate the patterns of diversity and biogeographic affiliation of the cheilostome Bryozoa from KGI and the surrounding areas. Of 114 identified taxa from a depth range of 6–492 m, 26 species were reported for the first time in KGI. The most speciose genera were Camptoplites, Osthimosia, Smittina, and Cellarinella. Species richness at KGI consisted of 70% of the total bryozoans at the South Shetland Islands (SSI). Fifty-nine per cent of the bryozoans from KGI are endemic to Antarctica, which closely reflects the previously estimated endemism rate for bryozoans and other Antarctic taxa. Cluster analysis indicated that the strongest faunal links of SSI bryozoans were with Antarctic Peninsula assemblages, corresponding to the physical distance between both locations. The biogeographic similarities between SSI and South America confirm the broad trend of existing Antarctic–South American faunal links previously observed in bryozoans and many other taxa and indicate that SSI might be an important transitional zone between Antarctica and South America.     

Dispersal of moss propagules on Signy Island, maritime Antarctic

Aerobiological studies were conducted for >1 year using arrays of rotorod samplers at three sites on Signy Island, South Orkney Islands, maritime Antarctic. Spores of five bryophyte taxa were identified, all of which are known to be widely distributed and fruit frequently on the island. Spore size did not appear to influence potential for dispersal, with spores of all five taxa being captured 0.5–1 km from their nearest known source plants. Spores were more abundant than plant fragments, although the occurrence of both propagule types in the air was small in comparison to the ground cover of mosses, and the occurrence of lichen propagules in the air. Spores were captured over a much longer period of the year (including the winter months) than that in which sporophyte dehiscence is thought to occur on Signy Island; possible reasons for this are discussed. Received: 20 February 1997 / Accepted: 7 June 1997     

Seasonality in Antarctic Airborne Fungal Spores

Airborne fungal spores were monitored over periods of up to 131/2 months at three sites on Signy Island in the maritime Antarctic. Fungal spore concentrations in the air were much lower than in other parts of the world. Concentrations were very low during the austral winter but increased during the austral summer. Chlamydospores were the most abundant fungal spore type found. Spores of Cladosporium spp. were the second most frequently trapped form. All spore types samples were most abundant in the summer months, except for chlamydospores, which were most numerous during the winter. The concentration of Cladosporium spores in the air at Signy Island was compared with the concentrations of this spore type found in the air in other parts of the world. It was evident that Cladosporium loses its dominance as the most abundant component of the air spora with increasingly high latitude. The peak concentration of fungal spores occurred at two sites following the start of the thaw; at the third site, the peak occurred with the arrival of spores by long-distance transport from more northerly regions.     

The seasonal occurrence of chroococcoid cyanobacteria at an Antarctic coastal site

Summary The chroococcoid cyanobacterium Synechococcus was found at 5 m depth at a site 6 km off the Antarctic coast where the water depth was 30 m. These cells reached a peak concentration of 140 cells ml^-1 in August at the time of maximum rate of development of seaice. They were accompanied in the water column by detritus consisting of diatom frustules and sediment particles. A smaller peak of abundance occurred at the time the sea-ice was breaking up. Cyanobacteria were seen only in very low numbers at one, and were absent at the other, of two deep water sampling sites. We propose that the two peaks of cyanobacterial abundance in the water column result from the suspension of a benthic population by ice crystals during the winter and their release from the sea-ice in summer.     

Invaders are not a random selection of species

We assembled information on 119 species of freshwater macroinvertebrate invaders in North America and Europe, and compared them to all native freshwater species in North America and Europe. We tested whether the invaders were a random or selected group among taxa (phylum or class), water quality requirements, and feeding habit. We found that freshwater macroinvertebrate invaders are not a random selection of species, and are over-represented by molluscs and crustaceans, while taxa richness of native communities are dominated by insects. Over 35% of native species of aquatic invertebrates in North America are only able to live in areas with excellent or very good water quality, and are intolerant of organic pollution. In contrast, all invaders are tolerant of at least moderate amounts of organic pollution. There was a significant difference in the distribution of feeding habits between native species and invaders: collector-filterers (including suspension feeders) were 2.5–3 times more abundant, and predators were 3–4 times less abundant among invaders than among native invertebrates. The ongoing spread of exotic species affects the biodiversity of selected taxa, shifts communities toward greater tolerance of organic pollution and increases the numbers of suspension feeders, thereby enhancing benthic pelagic coupling in waterbodies with high densities of invaders. Because these processes are very similar in Europe and North America, we suggest that the observed patterns may have a common global effect.     

Spatial and temporal variations in the diet of nototheniid fish in McMurdo Sound,Antarctica

Specimens of 4 species of Antarctic fish were captured at different locations in McMurdo Sound during the early summer, and for one species also during late winter. Stomach contents were analysed to examine resource utilization across species, at different locations, and between late winter and early summer. The results are consistent with earlier findings that there is a gradation in resource utilization across these species.T. pennelli andT. bernacchii tend to take predominantly benthic crawling prey, though they also take prey from the water column.T. hansoni andT. nicolai tend to take more prey from the water column, a tendency which can be related to the visual feeding vector of these species. Substantial differences in diet for the same species captured in different locations indicated significant flexibility in prey selection which would allow utilization of spatial and temporal fluctuations in prey availability. Successful feeding byT. bernacchii in late winter is a further indication that this species can feed non-visually and supports the notion that non-visual feeding mechanisms are likely to be of importance in the biology of the Antarctic fishes.deceased     

Response of avian nectarivores to the flowering of Aloe marlothii : a nectar oasis during dry South African winters

In southern Africa, Aloe marlothii flowers during the dry winter season and offers copious dilute nectar to a variety of birds. Avian abundance and community composition were monitored at an A. marlothii forest at Suikerbosrand Nature Reserve, South Africa. Sampling occurred during two summer months (February–March) when no flowers were present, and six months (May–October) that spanned the winter flowering. We hypothesized that an influx of occasional nectarivores to the A. marlothii forest during flowering would lead to significant changes in the avian community. Overall bird abundance increased 2–3 fold at the peak of nectar availability (August). We recorded 38 bird species, of 83 species detected during transects, feeding on A. marlothii nectar; this diverse assemblage of birds belonged to 19 families, including Lybiidae, Coliidae, Pycnonotidae, Sylviidae, Cisticolidae, Muscicapidae, Sturnidae, Ploceidae and Fringillidae. Surprisingly, only two species of sunbird (Nectariniidae) were observed feeding on A. marlothii nectar, and both occurred in low abundance. We predicted that competition for nectar resources would be high, but few aggressive inter- and intra-specific interactions occurred between birds while feeding on inflorescences. During peak flowering, insect feeders (insectivores, omnivores, nectarivores) fed on nectar during the cold morning when insect activity was low, whilst non-insect feeders (frugivores and granivores) fed on nectar in the middle of the day. Our study highlights the importance of A. marlothii nectar as a seasonal food and water source for a diverse assemblage of occasional nectarivores.     

In situ monitoring of microclimate and metabolic activity in lichens from Antarctic extremes: a comparison between South Shetland Islands and the McMurdo Dry Valleys

Lichens are the dominant organisms in terrestrial Antarctic ecosystems and show a decline in species number, coverage, and growth rate from the maritime Antarctic (62°S) to the McMurdo Dry Valleys (78°S). While Livingston Island (maritime Antarctica) is a hot spot for lichen biodiversity, the McMurdo Dry Valleys (continental Antarctica) are known as one of the most extreme environments for life. Previous studies suggest the biodiversity gradient to be linked to water availability acting through length of active period, but no activity data are available for the Dry Valleys. The work presented here compares metabolic activity of lichens at Livingston Island and the Dry Valleys for 4½ months from continuous monitoring that involves concurrent measurements of chlorophyll fluorescence and microclimate. The latitudinal comparison involves two contrasting habitats for plant physiological activity and microclimate. Two species of the foliose genus Umbilicaria were monitored in both regions plus one sample of the crustose Caloplaca in the Dry Valleys. The results showed a very large difference in the duration of activity over the monitoring period, and this supports the different coverage, species abundance, and growth rates already reported for lichens between both regions. Despite this large difference in activity, and in habitat conditions, analysis of the activity behaviour of the two Umbilicaria species shows interesting common features, while the crustose Caloplaca had additional strategies to improve hydration. This offers one explanation for the abundance of crustose lichens inside the Valleys, indicating better adaptation strategies to a polar desert.     

Assessing methods for collecting and transferring viable algae from Signy Island, maritime Antarctic, to the United Kingdom

. Acquisition and transfer of algae from distant polar locations to research facilities in the northern hemisphere may take several months, at which point the algae may be non-viable and in a deteriorated state. This study explores the effectiveness of 3% (w/v) Benomyl fungicide (Benlate) pre-treatments, on fungal "spoilage", viability and biochemical status of algae collected from terrestrial and freshwater habitats at Signy Island, maritime Antarctic. The results allow recommendations concerning sampling procedures and species selection for the ex-situ study of Antarctic algae to be made. The three algal taxa examined showed that although the fungicide was not necessary for successful transfer of viable specimens, it had a positive effect on viability. Prasiola crispa is suggested as a target organism for future ex-situ physiological studies.     

Stream insects as passive suspension feeders: effects of velocity and food concentration on feeding performance

Benthic suspension feeders are important components of aquatic ecosystems, often dominating the use of space and influencing patterns of material cycling between the water column and benthos. Biomechanical theory predicts that feeding by these consumers is governed by the flux (i.e., product of food concentration and velocity) of particulate material to their feeding appendages. We performed a laboratory flume experiment to test how feeding by larval black flies (Simulium vittatum Zett.) responds to independent manipulations of flow and food concentration. We quantified larval body posture, flick rate of the labral fans, and ingestion rate as a function of two concentrations of a baker's yeast/chalk suspension (0.96 and 4.44 mg l^-1) and five water velocities (20, 30, 45, 60, and 90 cm s^-1). Using analysis of covariance, we found that both flick rate and ingestion rate increased in a decelerating manner with increasing velocity, while fan height decreased linearly with increasing velocity. In contrast, food concentration had no effect on any aspect of feeding behavior. Thus, although both velocity and food concentration contribute to particle flux, our results indicate that the two were not substitutable under the range of conditions tested here.     

Cinefilms of particle capture by an induced local change of beat of lateral cilia of a bryozoan

A local disruption of the metachronal wave always accompanies capture of algal cells by tentacles of Flustrellidra hispida (Fabricius). Beat changes for ≈0.2 s over ≈100μm of the ciliated band during capture of a 10-μm particle. The halted parcel of water is therefore larger than the particle of food but much smaller than the flow that continues past the tentacles elsewhere. These events are consistent with the hypothesis that an induced local reversal of beat concentrates particles for those suspension feeders that retain particles upstream from a band of simple cilia (adults or larvae of bryozoans, brachiopods, phoronids, hemichordates, and echinoderms). These events are not explained by other hypotheses that have been advanced for concentration of particles by these suspension feeders. Aerosol filtration models of direct interception are not applicable to this type of ciliary suspension feeder because retention depends on the magnitude of a stimulus and response to it. The stimulus will not be the same function of diameter of the food particle, and response is unlikely below a threshold stimulus.     

Appearance and evolution of marine benthic communities in the Early Palaeozoic

The development of marine benthic communities in the Early Palaeozoic occurred mainly in the shallow water epicontinental seas. It included those stages of the Cambrian and Ordovician evolutionary radiations that were dominated by a high rate of morphogenesis, when new food and territory resources were not limited. This provided many opportunities for coadaptation of emerging organisms. At the time of the Cambrian radiation, the body plans of all animals were formed, while in the Ordovician, the maximum rank of emerging taxa did not exceed the level of class. The beginning of each radiation was explosive. Vendian benthic communities developed in cold seas and in the shallowest areas of warm seas, where organic matter from the surface layers was available at the bottom because of the absence of a thermocline. The Cambrian radiation began with the appearance of pelagic suspension feeders, because of which much of the primary production could penetrate the thermocline and settle at the bottom. This allowed the occupation of warmer seas and greater depths. At the same time, the productivity of the pelagic region sharply increased because of the emergence of positive feedback between the producers and consumers in the water, leading to increased water transparency and elongation of trophic chains. Arthropods, the first suspension feeders, were the launch group of the Cambrian radiation. Cambrian benthic suspension feeders could seize only the smallest particles, mostly bacteria, and dissolved organic matter. This food resource was contained in the thin bottom water layer. Therefore, the food grasping structures of all the Cambrian suspension feeders were near the bottom, without forming tiers. The Ordovician evolutionary radiation began with the launch of the Pelmatozoan echinoderms, which were the first benthic suspension feeders to begin feeding on plankton. The exploitation of this resource led to the creation of a 1-m tier above the bottom and an increase in their calcite productivity. Positive feedback emerged between the grounds and the community of its inhabitants and considerably changed the composition and diversity of grounds, which sharply increased the diversity of benthos. The appearance of positive feedback between different components of ecosystems resulted in explosive evolution in both the Cambrian and Ordovician.     

Analysis of dietary overlap in Antarctic fish (Notothenioidei) from the South Shetland Islands: no evidence of food competition

A dietary overlap analysis between notothenioid species was carried out among three fish assemblages in the South Shetland Island area. Using Tyler's (1972) method, the re-occurrence of main and secondary prey among fish predators was 33% in summer and 37% over the year at Potter Cove, 25% in summer and 7% in winter at Admiralty Bay, and 20% in autumn/winter around Elephant Island. Likewise, using the "S" index of Linton et al. (1981), the diet similarity between most species pairs is <50%. This relatively low dietary overlap may be explained by the about equally divided occurrence of generalised feeders and specialised feeders, with no evidence of competition among them. The fishes' trophic niches seem to be separated by depth and prey taxa. These findings are compared with other Antarctic fish communities and those in similar non-Antarctic cold marine ecosystems.     

Feeding behavior in freshwater bryozoans: function,form, and flow

Bryozoans are impressively active suspension feeders, with diverse feeding behaviors. These have been studied extensively in marine bryozoans, but less so in their freshwater counterparts. Here we identified 16 distinct behaviors in three phylactolaemate species and classified them into behaviors involving separate tentacles, groups of tentacles, lophophore arms, the introvert, or multiple zooids. We examined (1) the repertoire of behaviors in each species, and each behavior's (2) absolute frequency, (3) relative frequency and (4) duration in each of the three species, at two flow velocities (0 and 0.2 cm s^?1). Nine feeding behaviors were shared by all three species, but the occurrence of other behaviors in a given species was limited by its morphology. Behaviors involved in particle capture were the most frequent, and were often faster than the reactions involved in particle rejection. By contrast, the absolute frequency of behaviors varied widely among species without clear associations with species form, or function of the behavior. Flow velocity had only minor effects on the feeding behaviors exhibited by a species, or their frequencies or durations. Our results show that phylactolaemates have the same key feeding behaviors of the individual polypides (especially involving separate tentacles) as previously described in gymnolaemate and stenolaemate bryozoans, although their behaviors tend to be carried out more slowly than those of stenolaemates or gymnolaemates. Feeding behaviors involving multiple zooids were nearly absent in the studied phylactolaemates, but are common in gymnolaemates. Freshwater bryozoans appear to be intermediate between stenolaemate and gymnolaemate bryozoans in terms of richness of the repertoire of feeding behaviors.