Patterns of gammaridean amphipod abundance and species composition associated with dominant subtidal macroalgae from the western Antarctic Peninsula

The communities of gammaridean amphipods associated with eight dominant macroalgal species were examined near Palmer Station, Western Antarctic Peninsula. A total of 78,415 individuals belonging to 32 amphipod taxa were identified with mean densities ranging up to 20 individuals/g algal wet wt. The most abundant amphipod taxon, Metaleptamphopus pectinatus, was found to associate predominately with the brown alga Desmarestia menziesii, while the second most common taxon, Jassa spp. occurred primarily on the red alga Gigartina skottsbergii. Non-metric multidimensional scaling analysis demonstrated that the population densities of each amphipod species and amphipod species composition were similar on the same algal species but dissimilar on different species of algae. Comparisons of amphipod communities associated with a given algal species but from different sampling sites indicated that although the structure of species-specific macroalgal-associated amphipod communities can vary across spatial scales of 3 km, 50% of the macroalgal species examined showed no significant inter-site differences in associated amphipod community structure. Spearman rank correlation analyses showed that higher abundances of amphipods occurred on the macroalgae with the highest number of branches. As many Antarctic amphipods are known consumers of macroalgae, their remarkable abundances are likely to play a significant role in mediating energy and nutrient transfer in nearshore Antarctic Peninsular macroalgal communities.     

Amphipods exclude filamentous algae from the Western Antarctic Peninsula benthos: experimental evidence

Hard bottom, subtidal communities along the Western Antarctic Peninsula are dominated by forests of large, chemically defended macroalgae that support a very dense assemblage of amphipods. Free-living filamentous algae are rare in the subtidal, but filamentous algal endophytes are common in many of the larger macroalgae, both likely as the result of amphipod grazing pressure. Filamentous algae are common in the intertidal, but primarily in the upper intertidal and on high-energy shores where amphipods are likely to be excluded much of the time. We tested the hypothesis that free-living, filamentous algae would be rapidly consumed if transplanted from the intertidal to the subtidal, and our results clearly supported this hypothesis. The filamentous, intertidal green alga Cladophora repens was transplanted to the benthos in 6 different macroalgal habitats. Control algae were transplanted in 3 m deeper waters nearby (usually 12 m or less laterally) but suspended 3 m off the bottom where amphipods are absent or rare. Overall consumption during approximately 6 h on the bottom ranged from 22 to 98% of the initial biomass, while significantly less biomass loss occurred in the water column.     

The importance of shore height and host identity for amphipod assemblages

The spatial distribution of organisms associated with marine intertidal macroalgae may be a direct result of their tolerance to air exposure or an indirect consequence of the distribution of their host. We compared amphipod assemblages from five intertidal macroalgae to investigate their relationship with algal identity. To test the effect of height regardless of algal characteristics, we transplanted coralline algal turfs to three different levels within the intertidal zone and compared amphipod assemblages after 1 and 14 days. Interstitial volume was positively correlated to the abundance of amphipods, suggesting that this attribute may correspond better to the potential space for their occupation when compared to algal biomass, thallus volume or the ratio between thallus and interstitial volume. Algal level determined the structure of the amphipod assemblages. Upper-level (Acanthophora spicifera and Caulerpa racemosa) and intermediate-level (coralline) algae host similar amphipod assemblages dominated by Apohyale media, but different from lower-level algae (Padina gymnospora and Sargassum cymosum), which were dominated by Hyale niger. Ten of the 15 amphipod species reported from natural communities were found in the transplanted plots. Distinct pools of amphipod species colonized coralline transplants at upper and lower levels after 1 day. However, regardless of the position on the shore, transplanted coralline turfs supported similar assemblages after 14 days, indicating that algal identity is also important for species assemblages. Our results suggest that both height on the shore and host identity combine to determine the vertical structure of amphipod assemblages in the rocky intertidal.     

Feeding rates of common Antarctic gammarid amphipods on ecologically important sympatric macroalgae

Single species feeding trials employing both fresh algal tissues and alginate food pellets containing dried finely ground algal tissues were conducted to examine the relative palatability of sympatric Antarctic macroalgae (three brown and five red macroalgal species) to three common herbivorous gammarid amphipods (Prostebbingia gracilis Chevreux, Gondogeneia antarctica (Chevreux) Thurston, and Metaleptamphopus pectinatus Chevreux). In fresh algal tissue bioassays, both the amphipods P. gracilis and G. antarctica consumed significantly greater amounts of the red alga Palmaria decipiens over all other seven species of macroalgae. The amphipod M. pectinatus failed to consume measurable quantities of fresh thalli of any macroalgae and therefore is likely to feed on other resources. In food pellet bioassays, the consumption rates of amphipods fed with eight different species of macroalgae were compared with consumption rates on a highly palatable control green alga. Alginate pellets containing finely ground tissues of P. decipiens were consistently the most palatable of any of the macroalgae to P. gracilis and G. antarctica, while pellets containing the brown algae Desmarestia menziesii, D. anceps and the red alga Plocamium cartilagineum were not consumed by any of the three amphipod species. Regression analysis indicated that feeding rates of the amphipods P. gracilis and G. antarctica on alginate food pellets were not significantly correlated with known species-specific parameters of macroalgal nutritional quality (%N, %C, C:N ratio, soluble protein, soluble carbohydrate, and lipid). Therefore, differences in amphipod macroalgal palatability are most likely related to other factors including physical and/or chemical deterrents.     

IMPACTS OF MESOGRAZERS ON EPIPHYTE AND ENDOPHYTE GROWTH ASSOCIATED WITH CHEMICALLY DEFENDED MACROALGE FROM THE WESTERN ANTARCTIC PENINSULA: A MESOCOSM EXPERIMENT1

It has been hypothesized that the extensive mesograzer community along the western Antarctic Peninsula regulates epiphytic algae as well as emergent filaments from endophytic species. Should grazing limit growth of fouling or potentially pathogenic microphytes, then Antarctic macrophytes may actually benefit from the remarkably high densities of mesograzer amphipods that occur in these waters. Although initially counterintuitive, the negative impacts of epi/endophyte fouling may outweigh stresses caused by limited amphipod grazing on chemically defended macrophytes by reducing stress from endo/epiphyte biomass. If so, then alleviating mesograzing stress should result in significant increases in endo/epiphytic biomass. To test this hypothesis, a mesocosm experiment was conducted. Individuals representing four common species of Antarctic macroalgae were placed in flow‐through seawater mesocosms. Amphipods were added to five mesocosms at simulated natural densities, while the other five remained herbivore free. At the end of 7 weeks, endo/epiphytic growth on individual macrophytes was quantified. Most species of macroalgae demonstrated noticeably higher instances of endophyte coverage, epiphytic diversity, and diatom colonization in consumer‐free mesocosms than in the presence of amphipods. These data suggest that macroalgae along the western Antarctic Peninsula rely on grazers to control populations of potentially harmful epiphytes. We hypothesize that the chemically defended macroalgal flora lives in mutualism with high densities of mesograzers, providing amphipods with shelter from predation while continually being cleaned of potentially harmful endo/epiphytes.     

On the potential of fatty acids as trophic markers in Arctic grazers: feeding experiments with sea urchins and amphipods fed nine diets of macroalgae

To elucidate dietary preferences of benthic grazers at the Arctic Kongsfjorden by means of fatty acid trophic markers, ten different parallel treatments (starvation, 3 species of red macroalgae, 4 species of brown macroalgae and 2 species of green macroalgae) were offered as mono-algal diet to specimens of the dominant sea urchin Strongylocentrotus droebachiensis (Echinodermata, Echinoidea) and the gammarid amphipod Gammarellus homari (Crustacea, Amphipoda). At the end of the 3-week feeding experiments, amphipods and sea urchins (soft tissue) were deep-frozen and analysed for total lipid contents as well as fatty acid (FA) compositions. In addition, FA profiles of the algal species were determined and screened for specific FA patterns or single FAs qualifying as potential trophic markers in the grazers. Despite their diets of nine algal species with different FA compositions, FA patterns of the sea urchins and amphipods revealed a pronounced similarity between treatments. This strong similarity was also observed in the faecal pellets of the sea urchins. Hence, deviating FA compositions of the macroalgae were neither reflected in the FA patterns of the grazers’ tissue nor in their faecal pellets. Suitable algal FA trophic markers could thus not be identified in the two grazers from Kongsfjorden. The rather low lipid levels, especially in the amphipods, as well as a pronounced degradation and modification of FAs may explain that the FA trophic marker approach did not provide evidence of dietary preferences. Future experiments may obtain a higher resolution of potential FA trophic markers by analysing separate lipid classes or single tissues of lipid-poor grazers. Alternatively, different methods are needed to reveal high-resolution trophic relationships between macroalgae and herbivores in Kongsfjorden.     

Epibenthic amphipod abundance and predation efficiency of the pink shrimp Farfantepenaeus duorarum (Burkenroad, 1939) in habitats with different physical complexity in a tropical estuarine system

Amphipod abundance and biomass were determined in soft-bottom substrates (SBS), monospecific Thalassia testudinum patches and T. testudinum with attached macroalgae (SAV) from Términos Lagoon. Amphipods were absent in SBS, and their density and biomass were higher in SAV (3351 individualsm(-2), 1718 mg AFDWm(-2)) than in T. testudinum (1220 indm(-2), 625 mg AFDWm(-2)). Although macroalgae and seagrasses are recognised as an alternative refuge against predation for amphipods, the high abundance of amphipods in SAV suggests that macroalgae represent a habitat that provides greater food availability. Pink shrimp Farfantepenaeus duorarum (Burkenroad, 1939) consumption rate (Mo) of epibenthic amphipods was experimentally evaluated. Mo intensifies as prey density increases and varied from 0.39 to 2.39 mg AFDWh(-1). Predation efficiency of F. duorarum on epibenthic amphipods was also evaluated in four artificial habitats with different physical complexity: soft-bottom substrates (SBS), small woody debris (SWD), seagrasses with densities of 300 and 1200 shootsm(-2) (S300 and S1200, respectively), macroalgae (MA), and at two prey densities (962 and 2406 indm(-2)). Amphipod consumption rate by F. duorarum varied from 1.20 to 2.07 indh(-1) in S1200 and MA, respectively. Habitat complexity had a significant effect on consumption rate, but prey density did not. Habitat physical complexity and predation efficiency maintained an inverse and a non-linear relationship. Presumably, the decrease in predation efficiency in association with the habitat complexity is due to the differential refuge value of these habitats. However, predation efficiency may also be influenced by either the microhabitat use by amphipods, the shrimp's dependence on seagrasses, or by differences in habitat value caused by the diel behavioural distribution pattern of amphipods and shrimp. Both field and experimental results highlight the importance of evaluating the relative value of tropical estuarine habitats through the study of the relationship between habitat physical complexity and predator-prey interactions. They also emphasise that inherent biological and ethological factors of the predator and prey involved, coupled to spatial and temporal variations in the habitat, should also be considered.     

The ecology of south-east tasmanian phytal animal communities. IV. Factors affecting the distribution of ampithoid amphipods among algae

Adults of four abundant species of phytal ampithoid amphipod which occur sympatrically at Fancy Point, Tasmania, were found to partition the environment by size and plant species. A series of motility, predation, and algal selection experiments was carried out to help explain the observed distribution and life-history patterns of these animals. The restricted distributions among algae of two of these amphipods were exceptional for animals within the phytal community but followed a general pattern of increasing specialization of amphipods as the number of co-occurring congeneric species increased.

Sampling over a 24-h period indicated that a large proportion of the total amphipod population moved away from Zonaria turneriana J. Ag. at night but that no noticeable changes were evident in the densities of animals collected from Sargassum verruculosum (Mert.) C. Ag. Recolonization experiments, however, showed that some movement of animals occurred between S. verruculosum plants and also, presumably, between the other erect algae.

Selection experiments showed that large Cymadusa sp. 1 preferred Zonaria to the other algae but were forced onto Sargassum bracteolosum J. Ag. and then the more filamentous S. verruculosum and Cystophora retroflexa (Labill.) J. Ag. as crowding increased. Juveniles preferred Cystophora and Sargassum verruculosum to Zonaria and Sargassum bracteolosum.

The omnivorous fish Acanthaluteres spilomelanurus Quoy & Gaimard was found to capture significantly more amphipods from among the open branches of Cystophora than from the more compact Zonaria in the laboratory. The fish were also found to consume more large than small amphipods residing on Cystophora over a 2-h feeding period.

An analysis of the size-frequency distributions of the four amphipod species, in conjunction with the results of the laboratory experiments, provided circumstantial evidence that the ampithoid guild was structured by competitive constraints and differing predation pressures at two size levels.     

Increasing canopy shading reduces growth but not establishment of Elodea nuttallii and Myriophyllum spicatum in stream channels

In aquatic environments around the globe, rooted macrophytes have been replaced by filamentous macroalgae. The consequences of such shifts in vegetative habitat for fauna are poorly understood. Given differences in morphology and life history characteristics of rooted macrophytes and filamentous macroalgae, we hypothesized that these habitat types were not functionally redundant for small-bodied fishes and macroinvertebrates. We examined this hypothesis in spring-fed Florida rivers characterized by decreases in native rooted macrophytes and concomitant increases in filamentous macroalgae. Although faunal densities were generally greater in filamentous macroalgae than in rooted macrophytes, differences in the community assemblage structure suggest that the two types of vegetative habitat do not function interchangeably. Accordingly, continued replacement of rooted macrophytes with filamentous macroalgae is expected to affect the small fish and macroinvertebrate community, as well as higher trophic levels that depend on it.     

Do predatory fishes affect the structure of an epiphytal amphipod assemblage on a protected algal reef in Hawaii?

On a sheltered Hawaiian algal reef seaweeds are abundant and large herbivorous fishes are absent. Epiphytal amphipods are abundant on their seaweed habitats. Predator exclusion experiments were conducted to determine if predatory fishes affect the abundance, diversity, and size distribution of the epiphytal amphipod community living on plant substrata. Results suggested that amphipod abundance, expected number of species, and size distribution were all independent of predation pressure.     

Water current velocity as an environmental factor regulating the distribution of amphipod species in Gamo lagoon, Japan

The role of current velocity as an environmental factor affecting the distribution of amphipod species was investigated in the brackish Gamo lagoon, Japan. Two gammaridean amphipod species, Eogammarus possjeticus and Melita setiflagella, were found inhabiting stations of different water current with same substrates. E. possjeticus and M. setiflagella selected substrates at low and high water current, respectively. The lifestyles and anatomical features of these two free-living amphipods contributed to their adaptation to habitats with different water currents. The results of field and experimental observations suggest the existence of some type of interspecific competition between the two species. The presence of E. possjeticus caused a displacement of the fundamental niche of M. setiflagella. Received: October 10, 2000 / Accepted: August 17, 2001     

Partitioning mechanisms of Predator Interference in different Habitats

Prey are often consumed by multiple predator species. Predation rates on shared prey species measured in isolation often do not combine additively due to interference or facilitation among the predator species. Furthermore, the strength of predator interactions and resulting prey mortality may change with habitat type. We experimentally examined predation on amphipods in rock and algal habitats by two species of intertidal crabs, Hemigrapsus sanguineus (top predators) and Carcinus maenas (intermediate predators). Algae provided a safer habitat for amphipods when they were exposed to only a single predator species. When both predator species were present, mortality of amphipods was less than additive in both habitats. However, amphipod mortality was reduced more in rock than algal habitat because intermediate predators were less protected in rock habitat and were increasingly targeted by omnivorous top predators. We found that prey mortality in general was reduced by (1) altered foraging behavior of intermediate predators in the presence of top predators, (2) top predators switching to foraging on intermediate predators rather than shared prey, and (3) density reduction of intermediate predators. The relative importance of these three mechanisms was the same in both habitats; however, the magnitude of each was greater in rock habitat. Our study demonstrates that the strength of specific mechanisms of interference between top and intermediate predators can be quantified but cautions that these results may be habitat specific. An erratum to this article can be found at     

Habitat structure,resources and diversity: the separate effects of surface roughness and macroalgae on stream invertebrates

Habitat structure has pervasive effects on community composition and diversity, with physically complex habitats often containing more species than physically simple ones. What factors or mechanism drive this pattern is little understood, but a complicating problem is that different sources of habitat structure can be confounded in both surveys and experiments. In this study, we carried out an experiment in which two sources of habitat structure, attached macroalgae and substrate surface texture, were separately manipulated to discern their joint and separate effects upon the diversity and composition of colonizing macroinvertebrates in a stony, upland stream. Because stream algae vary markedly in abundance in both space and time, we also sampled the epilithon of stream stones at two spatial scales on eight dates over 2 years to gain some preliminary data on how stream algae vary between individual substrata over time. Experimental substrata had either a smooth (siltstones, sandstones, crystal-poor felsic volcanics, plain paving bricks) or rough (granodiorites, crystal-rich felsic volcanics, sand-blasted paving bricks) surface. We allowed these substrata to be colonized naturally by macroalgae, mostly the filamentous red alga Audouinella hermannii. Half of each of the rough and smooth substrata were selected at random and the macroalgae gently sheared off. All substrata were defaunated with a household insecticide with little field persistence, set out randomly through the study riffle, and invertebrates allowed to colonize them for 14 days. Some substrata were sampled immediately to check the efficacy of faunal and algal removals, which proved to be successful. Experimental results showed that both surface texture and macroalgae increase species richness independently of each other. Surface texture had no effect on densities, while macroalgae increased colonization densities, but rarefaction showed that both sources of habitat structure increased species richness above values expected simply on the basis of the numbers of colonists. However, reference stones with high macroalgal cover had the same species richness as those with low cover, suggesting that the effects of macroalgae on species richness are transient relative to those associated with surface texture. Epilithon samples taken at different times suggest that the magnitude of spatial variation in plant growth alters with time. If plants generally recolonize rough surfaces more quickly than smooth, then the effects of habitat structure on macroinvertebrates ought to be strongest after major disturbances during growing seasons of plants. Received: 1 September 1999 / Accepted: 10 January 2000     

Long-term changes in amphipod population dynamics in a temperate estuary following ecosystem restoration

The Mondego estuary (Portugal) has suffered severe ecological stress over the last two decades, as manifested in the replacement of seagrasses by opportunistic macroalgae, degradation of water quality and increased turbidity. A restoration plan was implemented in 1998, which aimed to reverse the eutrophication effects, and especially to restore the original natural seagrass (Zostera noltii) community. This article explores the long-term changes in Ampithoe valida and Melita palmata (Amphipoda) populations in response to eutrophication (with consequent seagrass loss and macroalgal proliferation) and to the subsequent restoration plan (with progressive seagrass recovery and macroalgal biomass decline). Until the early 1990s, high densities of A. valida and M. palmata were recorded in the Mondego estuary, especially during the occurrence of the macroalgal bloom and during all the periods in which green macroalgae were available. After the implementation of the restoration plan, species abundance, biomass and production levels decreased considerably due to the progressive decline of green macroalgae. This implied the virtual disappearance of the amphipod population, mainly A. valida. Distinct behaviours displayed by the two species could be related to different food strategies and habitat preferences. Ampithoe valida showed feeding preferences for ephemeral softer, filamentous or bladed algae (e.g. Ulva sp.) due to its high caloric content, using the Z. noltii bed only as a habitat for protection against predators or shelter from wave action. On the other hand, M. palmata did not suffer a strong decline in its population density, biomass and production, which may indicate that this species is probably not a primary consumer of green macroalgae and may readily shift to alternative ecological niches. Handling editor: P. Viaroli     

Feeding ecology and growth of age 0 year Platichthys flesus(L.) in a vegetated and a bare sand habitat in a nutrient rich fjord

A study was carried out to investigate the diet and feeding strategies of age 0 year juvenile flounder Platichthys flesus in two different micro‐tidal habitats in the nutrient enriched Mariager Fjord on the Danish east coast. Juvenile flounder and benthic macrofauna were sampled monthly from June to October 1999 in a bare sand habitat and a habitat covered by filamentous and mat forming macroalgae. The presence of the 'opportunistic' macroalgae created a shift in the dominance of surface dwelling prey such as epifaunal amphipods to more infaunal groups such as oligochaetes and polychaetes. The diet of the flounder varied considerably between the two habitats mainly reflecting prey availability relative to their abundance, prey spatial distribution, habitat structure and ontogenetic prey shifts as a function of total length. Flounder in the vegetated site fed on a diverse diet of copepods, polychaetes and oligochaetes, whereas those caught in the bare sand site fed primarily on the amphipod Corophium volutator which was numerically dominant at this site. During the growth season, two diet shifts were observed: from copepods early in the season to macrofauna organisms and, later in the season, the inclusion of more hyperbenthic prey such as Mysidea spp; Idotea spp. and the common goby Pomatoschistus microps .     

Effects of avian grazing on the algal community and small invertebrates in the rocky intertidal zone

In this study of a rocky intertidal habitat in northern Japan, feeding by avian consumers had significant effects on algal assemblages and small herbivorous invertebrates. The effects of the birds on algae were different from those of invertebrate grazers such as urchins and gastropods. The abundance of the dominant algal species decreased during the grazing period, increased again after the grazing period, and indirectly affected algal species richness and evenness. Avian grazing also decreased the density of tube-dwelling amphipods on the dominant alga, but did not change the density of mobile and free-living isopods. These results suggest that avian grazers may act as habitat modifiers rather than exploitative competitors for the small herbivorous crustaceans. Avian herbivores consumed only the upper parts of large algal fronds, apparently reducing the amount of suitable microhabitat for the small herbivorous crustaceans, which are subject to a variety of physical or biological stress. Thus, avian herbivores function as ecosystem engineers, regulating community structure in a manner different to invertebrate herbivores in rocky intertidal habitats.     

西湖湖西浮游与着生藻类季节变化及相互关系

2014年11月至2015年8月调查了西湖湖西沿岸带浮游藻类和不同基质(植物、石块和底泥)上着生藻类的群落结构及季节变化, 分析了着生丝状藻与浮游丝状藻的相互关系以及它们与环境因子的相关性, 探讨湖西生态修复过程中季节性暴发的丝状藻水华的原因。结果表明浮游藻类和植物、底泥、石头上着生藻类中均以硅藻门种类数(分别占52.5%、60.4%、86.7%和72.7%)最多, 蓝藻门(分别占10.1%、8.9%、6.7%和15.2%)和绿藻门(分别占26.3%、19.8%、5.6%和10.6%)次之, 其他门类相对较少, 浮游藻类与着生藻类优势种季节差异较大。附植丝状藻密度显著高于附泥和附石丝状藻, 且狐尾藻上着生丝状藻密度与浮游丝状藻密度呈显著正相关, 表明狐尾藻着生丝状藻可能是浮游丝状藻较重要的来源之一, 该结果可为西湖丝状藻水华的控制提供一些参考。相关性分析表明, 着生藻类和丝状藻与各理化因子(水深、透明度、溶解氧、水温、pH、TN、SRP、TP等)均无显著相关性。     

Amphipods on seaweeds: partners or pests?

Summary Herbivorous marine amphipods have been implicated as important grazers on filamentous and ephemeral algae, and thus as beneficial to macrophytes in reducing overgrowth by epiphytic competitors. In North Carolina, USA, amphipods comprise 97% of all macroscopic animals inhabiting the abundant brown seaweed Sargassum filipendula, and peak in abundance between late winter and early summer. I used outdoor tank experiments to test the species-specific impact of common phytal amphipods on the growth of Sargassum and its epiphytes. The results show that seaweed-associated amphipods are a trophically diverse group that could either increase or decrease host fitness depending on their feeding preferences. The amphipods Ampithoe marcuzii, Caprella penantis, and Jassa falcata each significantly reduced growth of epiphytes on Sargassum plants relative to amphipod-free controls, while Ericthonius brasiliensis had no significant effect on Sargassum or its epiphytes. However, amphipod grazing was not necessarily beneficial to Sargassum. A. marcuzii consumed Sargassum in one outdoor tank experiment, reducing its mass by 11%, while Sargassum plants without amphipods grew by 81%. Epiphytes (mostly diatoms and the filamentous brown alga Ectocarpus siliculosus) and detritus remained abundant on these plants suggesting that A. marcuzii preferred the host to its epiphytes. Similarly, when given simultaneous access to Sargassum and to several common foliose and filamentous epiphytes in the lab, A. marcuzii ate Sargassum almost exclusively. The other three amphipods ate no macroalgae. In contrast to A. marcuzii, C. penantis consistently reduced epiphytes with no negative effect on Sargassum. Thus the species composition of the amphipod fauna can determine whether these animals increase or decrease seaweed fitness.     

Palatability and Chemical Defenses of Macroalgae in the Antarctic Peninsula

We examined palatability of 37 species of nonencrusting macroalgae from the Antarctic Peninsula. This represents approximately 30% of the entire antarctic macroalgal flora and 75% of the 49 nonencrusting species we collected. Organic extracts from most species were also prepared and mixed into artificial foods. We examined palatability using feeding bioassays with three common, macroalga‐consuming animals (an omnivorous antarctic rockfish, Notothenia coriiceps; an omnivorous sea star, Odontaster validus; and a herbivorous amphipod, Gondogenia antarctica). Thallus pieces from 23 of 34 macroalgal species tested with the fish (68%) were rejected. Of the 23 species rejected as thallus, organic extracts of 16 were bioassayed using the fish with 9 (56%) unpalatable. Thallus pieces from 21 of 36 macroalgal species tested with the sea star (58%) were rejected. Of the 21 species rejected as thallus, organic extracts of 20 were bioassayed using the sea stars and 14 (70%) were unpalatable. Overall, 28 of the 37 species assayed as thallus (76%) were rejected by either or both the fish and sea stars. The amphipod assay was not suitable for use with thallus but was utilized with organic extracts of 23 macroalgal species that were rejected as thallus by either or both the fish and sea stars. Of these, 14 (61%) of the species' extracts were rejected by the amphipods. Unpalatability was highest among the brown algae examined with only an ephemeral, ectocarpoid species not rejected as thallus out of 10 species tested. Of the remaining nine brown algal species, six of seven tested were also unpalatable as extracts, including all the ecologically dominant, perennial species in the area. We conclude that unpalatability to herbivores is common in antarctic macroalgae and that chemical defenses may play an important role in the unpalatability of many algal species (NSF OPP9814538, OPP9901076).     

The specialized diet of Harpagifer bispinis:

The diet of Harpagifer bispinis (Pisces: Nototheniidae) from two localities of the South Shetland Archipelago was studied. Simultaneous to the capture of H. bispinis and at the same sites the availability of food was considered, and amphipod diversity was compared with the density of Harpagifer. It was found that three quarters of the fish fed only on amphipods (mainly Gondogeneia antarctica) and for the rest amphipods were also the main component, even when other prey species were available. The high selectivity of G. antarctica is due to its high mobility and to the fact that Harpagifer is an ambush feeder. At different predator densities the amphipod fraction of the community appears to be highly modified by the predator both numerically and in species evenness. We postulate that Harpagifer can be a key species in structuring the mobile epibenthic community, even when this environment is subject to strong physical stress.