Grazer Feeding Research Articles

The effect of vorticity on the feeding of a freshwater grazer

AbstractThe transfer of energy through zooplankton grazing on phytoplankton is one of the most important interactions in aquatic ecosystems, yet the role of hydrodynamic forces is not fully understood. Factors that influence these interactions include algal size and shape and the interactions between grazer and alga dictated by hydrodynamic forces. Hydrodynamic forcing lead to unimodal relationships in grazer clearance rates (CR) because water motion and turbulence increases encounters but interferes with feeding at high levels. We expect higher CR on small algae due to increased encounter rates caused by vorticity generated in a horizontally oriented rotating cylinder to model increasing turbulence, whereas longer cells, tumbling in the flow, may be more difficult to handle. Ultimately, we expect that increased vorticity will interfere with zooplankton swimming. The CR of Daphnia magna followed the unimodal trend, with higher CR on the small, spherical Chlorella vulgaris than on larger, elongated, and colonial Scenedesmus quadricauda. This was likely due to lower handling time and higher concentration of the smaller cells in the feeding radius around D. magna. The normal hop‐sink swimming behavior of the Daphnia was affected by increasing vorticity; hop frequency decreased, displacement increased, and there was a loss of vertical orientation. This pattern in feeding and swimming were predicted by vorticity. These results for Daphnia differ from those obtained for copepods, which do not respond to vorticity. Given the importance of vorticity in nature, it is relevant to examine grazer feeding interactions under ecologically relevant hydrodynamic conditions.

Distinctive chemotactic responses of three marine herbivore protists to DMSP and related compounds.

Marine planktonic predator-prey interactions occur in microscale seascapes, where diffusing chemicals may act either as chemotactic cues that enhance or arrest predation, or as elemental resources that are complementary to prey ingestion. The phytoplankton osmolyte dimethylsulfoniopropionate (DMSP) and its degradation products dimethylsulfide (DMS) and acrylate are pervasive compounds with high chemotactic potential, but there is a longstanding controversy over whether they act as grazing enhancers or deterrents. Here, we investigated the chemotactic responses of three herbivorous dinoflagellates to point-sourced, microscale gradients of dissolved DMSP, DMS, and acrylate. We found no evidence for acrylate being a chemotactic repellent and observed a weak attractor role of DMS. DMSP behaved as a strong chemoattractor whose potential for grazing facilitation through effects on swimming patterns and aggregation depends on the grazer's feeding mode and ability to incorporate DMSP. Our study reveals that predation models will fail to predict grazing impacts unless they incorporate chemotaxis-driven searching and finding of prey.

Who Cares More about Chemical Defenses — the Macroalgal Producer or Its Main Grazer?

The consequences of defensive secondary metabolite concentrations and interspecific metabolite diversity on grazers have been extensively investigated. Grazers which prefer certain food sources are often found in high abundance on their host and as a result, understanding the interaction between the two is important to understand community structure. The effects of intraspecific diversity, however, on the grazer are not well understood. Within a single, localized geographic area, the Antarctic red seaweed Plocamium sp. produces 15 quantitatively and qualitatively distinct mixtures of halogenated monoterpenes ("chemogroups"). Plocamium sp. is strongly chemically defended which makes it unpalatable to most grazers, except for the amphipod Paradexamine fissicauda. We investigated differences in the feeding and growth rates of both Plocamium sp. and P. fissicauda, in addition to grazer reproductive output, in relation to different chemogroups. Some chemogroups significantly reduced the grazer's feeding rate compared to other chemogroups and a non-chemically defended control. The growth rate of Plocamium sp. did not differ between chemogroups and the growth rates of P. fissicauda also did not show clear patterns between the feeding treatments. Reproductive output, however, was significantly reduced for amphipods on a diet of algae possessing one of the chemogroups when compared to a non-chemically defended control. Hence, intraspecific chemodiversity benefits the producer since certain chemogroups are consumed at a slower rate and the grazer's reproductive output is reduced. Nevertheless, the benefits outweigh the costs to the grazer as it can still feed on its host and closely associates with the alga for protection from predation.

Antioxidant Responses in Copepods Are Driven Primarily by Food Intake, Not by Toxin-Producing Cyanobacteria in the Diet.

The association between oxidative processes and physiological responses has received much attention in ecotoxicity assessment. In the Baltic Sea, bloom-forming cyanobacterium Nodularia spumigena is a significant producer of various bioactive compounds, and both positive and adverse effects on grazers feeding in cyanobacteria blooms are reported. To elucidate the effect mechanisms and species sensitivity to the cyanobacteria-dominating diet, we exposed two Baltic copepods, Acartia bifilosa and Eurytemora affinis, to a diet consisting of toxin-producing cyanobacteria N. spumigena and a high-quality food Rhodomonas salina at 0–300 μg C L−1; the control food was R. salina provided as a monodiet at the same food levels. The subcellular responses to food type and availability were assayed using a suite of biomarkers – antioxidant enzymes [superoxide dismutases (SOD), catalase (CAT), and glutathione S-transferases (GST)] and acetylcholinesterase (AChE). In parallel, we measured feeding activity using gut content (GC) assayed by real-time PCR analysis that quantified amounts of the prey DNA in copepod stomachs. As growth and reproduction endpoints, individual RNA content (a proxy for protein synthesis capacity), egg production rate (EPR), and egg viability (EV%) were used. In both toxic and nontoxic foods, copepod GC, RNA content, and EPR increased with food availability. Antioxidant enzyme activities increased with food availability regardless of the diet type. Moreover, CAT (both copepods), SOD, and GST (A. bifilosa) were upregulated in the copepods receiving cyanobacteria; the response was detectable when adjusted for the feeding and/or growth responses. By contrast, the diet effects were not significant when food concentration was used as a co-variable. A bimodal response in AChE was observed in A. bifilosa feeding on cyanobacteria, with up to 52% increase at the lower levels (5–25 μg C L−1) and 32% inhibition at the highest food concentrations. These findings contribute to the refinement of biomarker use for assessing environmental stress and mechanistic understanding of cyanobacteria effects in grazers. They also suggest that antioxidant and AChE responses to feeding activity and diet should be accounted for when using biomarker profiles in field-collected animals in the Baltic Sea and, perhaps other systems, where toxic cyanobacteria are common.

Scraping and grazing herbivorous/detritivorous fish display opposite feeding behaviours under different protection regulations

ABSTRACT Herbivorous and detritivorous fish display complex feeding behaviour, and research into how feeding behaviour changes with environmental and social variables is lacking. Such knowledge is imperative to infer how herbivory/detritivory will differ, in light of shifting resources and communities and specifically whether reefs can recover from disturbance. Fish abundance, feeding rate, body size, diet and schooling feeding of three major functional groups (scrapers, grazers and browsers) were examined across reef types under different fishing regulations. Scrapers and grazers (parrotfishes and surgeonfishes) were more abundant and displayed the highest feeding rates on reef flats. Scrapers mainly resided inside the restricted zone, while more grazers were found in the general use zone, where macroalgal abundance was highest. Browsers (rabbitfishes) were seldom observed and patchily gathered on the reef flat and reef slope in both zones. Thus, fishing protection did not appear to benefit grazers and browsers, whereas more scrapers gathered on shallow reef flats in the protection zone. Scraper and grazer feeding rates increased from an individual to a pair and increased with body size, these factors led to variations in feeding behaviours across reef types and protection regulations. Protection appears to benefit scrapers and variations in feeding rates were largely related to school size. Lastly, grazer density was associated more with macroalgal coverage than protection status. The opposite feeding behaviours of scrapers and grazers indicates not only protection status, but fishing restrictions and size limit regulations are needed to maintain coral reef fisheries and functional diversity on coral reefs in Taiwan.

Comparative assessment of macroinvertebrate communities within three Afromontane headwater streams influenced by different land use patterns

Headwater streams in Afromontane ecoregions harbour locally adapted aquatic communities. However, across many regions in Africa, these ecosystems and their unique aquatic biodiversity have been severely impacted by unsustainable land use practices. We tested the hypothesis that land use disturbances were the primary drivers of community dynamics by comparing spatial and temporal dynamics together with trait-environment relationships of macroinvertebrate communities in three headwater streams influenced by different land use practices. The three headwater streams were distinguished based on high conductivity, total dissolved solids and alkaline pH in the agriculture-disturbed stream, and low temperature in a stream whose riparian zone was invaded by non-native vegetation compared to a near-natural stream. Macroinvertebrate taxonomic diversity was, nevertheless, comparable among these three streams. Constrained canonical ordination revealed that seasonality was a major driver of macroinvertebrate dynamics that was reflected mostly by the abundances of six macroinvertebrate taxa (Baetis, Dicentroptelum, Afronurus, Tricorythus, Simulium and Cheumatopsyche), whereas land use contributed a small but significant difference. Trait-environment relationships reflected seasonal changes that included the importance of benthic substratum in winter, the occurrence of collector-gatherer invertebrates in spring and aerial breathing traits in summer. Land use-related traits were, nevertheless, reflected by gill respiration and grazer feeding traits represented by Afronurus in the near-natural stream, predator traits represented by Aeshna and Lestes in the invaded stream, and aerial respiration represented by Enithares, Orectogyrus and Rhagovelia in the agriculture-disturbed stream. Our results suggest that environmental variability associated with seasonality probably played a deterministic role within which land use disturbances operated. Overall, our study suggest that importance of using multiple metrics to unpack the patterns associated with land use disturbances in headwater streams.

The Mammuthus-Coelodonta Faunal Complex at its southeastern limit: A biogeochemical paleoecology investigation in Northeast Asia

During the past several decades, the paleoecology of the Mammuthus-Coelodonta Faunal Complex in the Palearctic has been thoroughly explored, especially using stable isotope analysis. Numerous studies have documented high ecological plasticity and regional heterogeneities for this fauna. However, very limited attention has focused on Northeast Asia, at the southeastern edge of the distribution of the mammoth steppe biome. In the present study, we undertook radiocarbon dating, zooarchaeology by mass spectrometry (ZooMS), and stable isotope analysis on the fossil faunas from Yanjiagang Paleolithic site, Northeast (NE) China, and from the Geographical Society Cave in the nearby Russian Far East, to explore the paleoecology of this fauna in this middle-latitude region. Isotopic (δ13C, δ15N) data from these two sites suggested that the woolly mammoth (Mammuthus primigenius) was a grazer feeding on grasses/sedges, while the woolly rhinoceros (Coelodonta antiquitatis) and steppe bison (Bison priscus) utilized a wider range of food resources. All megaherbivores exhibited some ecological plasticity. Meanwhile, the mammal remains from Geographical Society Cave prevalently exhibited lower δ15N values than those from China, indicating variable environments and vegetation in Northeast Asia during MIS 3. Interestingly, through reevaluating the diachronic mammal distribution and ecology based on direct radiocarbon dating and ZooMS, we report the credible presence of Bubalus, typically interpreted as a mild climate faunal element in the Pleistocene, within the Mammuthus-Coelodonta Faunal Complex in Yanjiagang. This emphasizes the unique scenario of this biome in Northeast Asia, where the fauna exhibited sensitivity to synergic effects of climatic oscillations and local geographic traits. After comparing isotopic data of mammoth from different subregions of Asia, we found that NE China displayed both highest δ13C and δ15N values, possibly related to the higher temperature compared to the Arctic regions. This study reveals the characteristics and complexity at the southeastern limit of the range of the mammoth steppe biome and urges more systematic studies within and outside this region.

Dynamics of a discrete-time stoichiometric optimal foraging model

In this paper, we discretize and analyze a stoichiometric optimal foraging model where the grazer's feeding effort depends on the producer's nutrient quality. We systematically make comparisons of the dynamical behaviors between the discrete-time model and the continuous-time model to study the robustness of model predictions to time discretization. When the maximum growth rate of producer is low, both model types admit similar dynamics including bistability and deterministic extinction of the grazer caused by low nutrient quality of the producer. Especially, the grazer is benefited from optimal foraging similarly in both discrete-time and continuous-time models. When the maximum growth rate of producer is high, dynamics of the discrete-time model are more complex including chaos. A phenomenal observation is that under extremely high light intensities, the grazer in the continuous-time model tends to perish due to poor food quality, however, the grazer in the discrete-time model persists in regular or irregular oscillatory ways. This significant difference indicates the necessity of studying discrete-time models which naturally include species' generations and are thus more popular in theoretical biology. Finally, we discuss how the shape of the quality-based feeding function regulates the beneficial or restraint effect of optimal foraging on the grazer population.

Phytoplankton chlorophyll-breakdown pathway: Implication in ecosystem assessment

The chlorophyll-breakdown to pheophorbide is determined by biotic factors such as grazing (via chlorophyllide) or senescence (via pheophytin). So far, much of the information on chlorophyll-breakdown is available from sediments, but information from the water column is limited. This study addressed chlorophyll-breakdown-pathways (Chl-BP) on a seasonal basis from eight major ports (18–30 stations/port) representing freshwater, estuarine, and marine ecosystems. The distribution of chlorophyll and its breakdown fractions (pheophytin, pheophorbide) exhibited distinct spatial and seasonal variations. Fresh-water (except Haldia-port) and estuarine ports are characterized by high-biomass, high-pheophytin, and low-pheophorbide, whereas marine-ports by low-biomass (except Mangalore-port), low-pheophytin, and high-pheophorbide. Pheophytin and pheophorbide distribution were biomass independent and dependent, respectively. The pheophorbide: pheophytin ratio indicated a potential proxy for determining the dominant breakdown pathway, i.e., herbivory dominant (>1) or not dominant (<1). However, CHl-BP is taxa-specific and grazer's feeding habits. The ratios exhibited apparent differences between different ecosystems, i.e., the higher ratios in marine (up to 11.2) followed by estuarine (up to 0.9) and freshwater (up to 0.4; except Haldia) systems. The diatoms (preferred grazer diet) contribution to total phytoplankton was more in marine followed by estuarine and freshwater systems. The low and high ratios suggested the prevalence of chlorophyll-breakdown via senescence and grazing mode, respectively. We proposed that such scaling will have implications in the ballast water management – BWM (ballast tank conditions (eg. dark) during voyages, post-voyage discharge – including treated water using approved BWM systems, and the nature of ports, potential discharge point) and algal bloom research (e.g. understanding fate and in control measures).

Sewage contamination under water scarcity effects on stream biota: biofilm, grazers, and their interaction.

One of the most common anthropogenic impacts on river ecosystems is the effluent discharge from wastewater treatment plants. The effects of this contamination on stream biota may be intensified in Mediterranean climate regions, which comprise a drought period that leads to flow reduction, and ultimately to stagnant pools. To assess individual and combined effects of flow stagnation and sewage contamination, biofilm and gastropod grazers were used in a 5-week experiment with artificial channels to test two flow velocity treatments (stagnant flow/basal flow) and two levels of organic contamination using artificial sewage (no sewage input/sewage input). Stressors' effects were determined on biofilm total biomass and chlorophyll (Chl) content, on oxygen consumption and growth rate of the grazers (Theodoxus fluviatilis), and on the interaction grazer-biofilm given by grazer's feeding activity (i.e., biofilm consumption rate). The single effect of sewage induced an increase in biofilm biomass and Chl-a content, simultaneously increasing both grazers' oxygen consumption and their feeding activity. Diatoms showed a higher sensitivity to flow stagnation, resulting in a lower content of Chl-c. Combined stressors interacted antagonistically for biofilm total biomass, Chl-b contents, and grazers's feeding rate. The effect of sewage increasing biofilm biomass and grazing activity was reduced by the presence of flow stagnation (antagonist factor). Our findings suggest that sewage contamination has a direct effect on the functional response of primary producers and an indirect effect on primary consumers, and this effect is influenced by water flow stagnation.

PATTERNS OF UNREPAIRED SHELL DAMAGE IN RECENT BRACHIOPODS FROM FIORDLAND (NEW ZEALAND)

In order to provide quantitative data concerning patterns of shell breakage and repair in rhynchonelliform brachiopods, we studied undisturbed death assemblages from a New Zealand fiord complex where three species of terebratulide and one rhynchonellide occur in dense mixed patches on the near vertical walls. Proportions of damaged (both repaired and non-repaired) individuals varied between both taxa and sampling site. However, the general observation was that few individuals show signs of having been able to repair damage but the proportion of individuals showing unrepaired, and hence presumably fatal, breakages was higher (up to 76% in Magasella sanguinea from one sample from Tricky Cove in Doubtful Sound). Damage was mostly concentrated around the anterior margins and affected both valves and is consistent with having been clamped between a set of either jaws or claws. Potential culprits include fish (wrasse), rock lobsters and echinoids. As yet it is unclear whether the damage results from deliberate feeding activity or as collateral damage from grazers feeding on other organisms on the fiord walls which may allow secondary predation by asteroids. The net effect is, however, the same, in that the damage appears to have been fatal. More structured sampling is now required to understand the spatial variation in this damage and mortality, and also to establish the culprits with more certainty.

Diel feeding rhythms in marine microzooplankton: effects of prey concentration, prey condition, and grazer nutritional history

In this study, we aim at disentangling the causes and consequences of diel feeding rhythms in marine microzooplankton. We focused on the diel feeding activity of two heterotrophic dinoflagellate species, Gyrodinium dominans (one laboratory strain) and Oxyrrhis marina (laboratory cultivated and wild strains). We observed higher ingestion during the day in both dinoflagellate species. Feeding rhythms appeared to be independent of circadian changes in prey biochemical composition. Grazers fed with prey under stationary phase, with equivalent stoichiometric composition between day and night, showed 5 (G. dominans) and 10 (O. marina) times higher ingestion rates during the day. Previous grazer feeding history (starved vs well-fed) did not affect the feeding rhythm. However, prey concentration altered the rhythm; food limiting conditions reduced the amplitude of the rhythms. Our results establish a resource dependence of diel periodicity in microzooplankton grazing, which can have unanticipated consequences for standard field dilution grazing experiments.

A method for growing a monospecific epilithic cyanobacterial biofilm for use in marine ecological experiments

On most intertidal rocky shores, marine filamentous cyanobacteria Oscillatoriales are important components of natural biofilms. Some studies have already suggested the nutritional importance of benthic cyanobacteria for intertidal grazers; however, due to the complex nature and technical difficulties of observing and manipulating biofilm in situ, questions related to the relationship between what is available and what is actually consumed and assimilated by intertidal grazers remain unanswered. An alternative approach could be to use an isolated benthic cyanobacterial strain and grow it on surfaces resembling natural rocky shores under controlled laboratory conditions. Researchers can then test hypotheses about feeding behaviours and nutritional implications of grazers feeding on a monospecific biofilm. In this study a procedure to grow a blue-green cyanobacterial biofilm on experimental sandstone units (ESUs) was developed. Evidence of this growth is demonstrated using two established techniques in the analysis of chlorophyll a (Chl a) as a proxy for biomass. Destructive (spectrophotometric) and non-invasive (spectrometric) methods were used to measure the photosynthetic pigment in the benthic filamentous blue-green cyanobacterium Geitlerinema sp. strain CS-897 in a 26-day experiment. Amounts of Chl a were assessed every two days and a continuous and significant growth was observed through time (values ranging from 0.20μgcm−2 on day 0 to a maximum of 1.10μgcm−2 on day 20). A strong linear relationship (R2=0.87) was found between the spectrophotometric measurements of Chl a and the index NIR/red (R750/R672) obtained from reflectance spectrometry, providing for the first time evidence for the attachment and growth of a monospecific cyanobacterial biofilm on sterilised ESUs under controlled laboratory conditions. The linear regression equation obtained from this relationship can then be used to assess non-destructively the amount of food provided and consumed by key intertidal grazers in future manipulative feeding experiments. These results provide a platform for further studies focusing on the nutritional ecology and feeding behaviour of intertidal grazers. The development of experimental monospecific and/or mixed benthic biofilms on rocky substrata opens up new possibilities for experiments to improve understanding of key ecological processes such as competition, foraging behaviour and the effects that different benthic grazers have on the biofilm.

Complexity of ruminant masticatory evolution.

The evolution of robust jaws, hypsodont teeth, and large chewing muscles among grazing ruminants is a quintessential example of putative morphological adaptation. However, the degree of correlated evolution (i.e., to what extent the grazer feeding apparatus represents an evolutionary module), especially of soft and hard tissues, remains poorly understood. Recent generation of large datasets and phylogenetic information has made testing hypotheses of correlated evolution possible. We, therefore, test for correlated evolution among various traits of the ruminant masticatory apparatus including tooth crown height, jaw robustness, chewing muscle size, and characters of the molar occlusal surfaces, using phylogenetic and nonphylogenetic comparative methods as well as phylogenetic evolutionary model selection. We find that the large masseter muscles of grazing ruminants evolved with the inclusion of grass in the diet, an increase in the proportion of occlusal enamel bands oriented parallel to the chewing stroke, and possibly hypsodonty. We suggest that the masseter evolved under two evolutionary regimes: i) selection for higher masticatory forces during chewing and ii) flattening of the tooth profile, which resulted in reduced tooth guidance and, thus, a requirement for more chewing muscle activity during each chewing stroke, in agreement with previous research. The linear jaw metrics (depth of the mandibular angle, mandibular angle width, and length of the superficial masseteric scar) all show correlated evolution with hypsodonty and the proportion of enamel bands oriented parallel to the chewing stroke. We suggest that changes in the shape of the mandible represent the combined effects of selection for a reorientation of the chewing stroke, so as to emphasize horizontal translation of the teeth, and accommodation of high-crowned teeth. Our analyses show that the ruminant feeding apparatus is an evolutionary mosaic with its various components showing both correlated and independent evolution.

Crystal ball – 2013

Crystal ball – 2013

Trophic significance of the kelp Laminaria digitata (Lamour.) for the associated food web: a between-sites comparison

This study aimed at establishing the trophic significance of the kelp Laminaria digitata for consumers inhabiting two rocky shores of Northern Brittany (France), displaying contrasted ecological conditions. The general trophic structure did not vary between these two sites, with a wide diversity of filter-feeders and predators, and only 14% of the species sampled belonging to the grazers' trophic group. The diversity of food sources fueling the food web appeared also similar. The food webs comprised four trophic levels and the prevalence of omnivory appeared relatively low compared to previous studies in the same area. Conversely, to the food web structure, which did not differ, the biochemical composition of L. digitata differed between the two sites, and was correlated to a larger diversity of grazers feeding on this kelp in sheltered conditions. This indicated that the spatial variability occurring in the nutritive value of L. digitata is likely to deeply affect the functioning of kelp-associated food webs. The contribution of L. digitata-derived organic matter to the diet of filter-feeders inhabiting these two environments was assessed using the mixing model Isosource, which showed the higher contribution of kelp matter in sheltered conditions. These results highlight the spatial variability that may occur in the functioning of kelp-associated food webs. Moreover, this suggests that hydrodynamics is likely to control the availability of kelp-derived organic matter to local filter-feeders, probably through an increase of detritus export in exposed areas.

Stable isotope evidence for nutritional stress, competition, and loss of functional habitat as factors limiting recovery of rare antelope in southern Africa

A major focus in population ecology is understanding factors that limit rare species. We used stable isotope approaches to diet to determine whether remaining rare antelope populations in Kruger National Park (KNP), South Africa experience i) nutritional stress; ii) competition with sympatric bulk grazers; iii) reduced habitat heterogeneity. Rare species consumed near-pure C 4 grass-based diets throughout the seasonal cycle, in contrast to field observations that reported significant levels of C 3 consumption (browse) by these taxa. This finding, coupled with low faecal %N at the height of the dry season, may indicate nutritional stress, but recent isotopic studies of the same species elsewhere in Africa suggest that field observations overestimated levels of browse consumption. We find little evidence for diet niche overlap between rare antelope with bulk grazing species. This partitioning of resources (interpreted mainly as tall- versus short-grass grazing, respectively), is consistent with reported differences in observed diet, and comparative oral morphology. Last, we find less seasonal diet variations amongst bulk grazers feeding in rare antelope habitats compared with other landscapes. We propose that loss of functional heterogeneity, apparently brought about by high densities of artificial waterholes, limits recovery of diet- and habitat-selective rare antelope populations in KNP.

Age and nutrient limitation enhance polyunsaturated aldehyde production in marine diatoms

Skeletonema marinoi produces 2,4-heptadienal, 2,4-octadienal, and 2,4,7-octatrienal, the latter only in traces. In nutrient-replete cultures, the production of potentially defensive polyunsaturated aldehydes (PUA) increases from the exponential to the stationary phase of growth from 1.2 fmol cell −1 (±0.4 fmol cell −1 SD) to 4.2 fmol cell −1 (±1.0 fmol cell −1 SD), with 2,4-heptadienal as the dominant aldehyde. The plasticity of PUA production with age of the culture supports the hypothesis of a direct link between toxin production and cell physiological state. N- and P-limited cells in stationary phase produced 1.4 and 1.8 fold higher amounts of PUA than control cultures and 10.7 and 4.6 times higher PUAs when compared to their own exponential growth phase, respectively. The increase in PUA production in the nutrient-limited cultures was not paralleled by an increase in the total amount of precursor fatty acids indicating that physiological stress might trigger an enhanced expression or activity of the enzymes responsible for PUA production, i.e. chemical defense increase in aged and nutrient-stressed diatoms. If this holds true during blooms, grazers feeding at the end of a bloom would be more affected than early-bloom grazers.

Food habits of some deep-sea fish off South Africa's west coast. 2. Eels and spiny eels (Anguilliformes and Notacanthiformes)

The food habits of five species of eels and one species of spiny eel collected from the upper continental slope off the Cape west coast and Agulhas Bank, South Africa, are described. All a re embers of a defined demersal micro- or mesocarnivore feeding guild except Simenchelys parasitica, a scavenger. Two congrids, Bassanago albescens and Gnathophis capensis and the spiny eel Notacanthus sexspinis usually employ the demersal grazer feeding method and infrequently switch to picking nekton in the water column. Two synaphobranchids, Diastobranchus capensis and Synaphobranchus kaupii, are chiefly demersal mesocarnivores that stalk prey, but also pick nekton and scavenge. A significant amount of the prey ofthe three large eels, B. albescens and the two synaphobranchids, consisted mainly of mesopelagic stomiiform, myctophid and juvenile gadiform fish, penaeid and caridean shrimps and various squid taken at the bathyal/deep-pelagic interface, as was found previously with some grnadiers (Macrouridae. Five categories of fleshy remains are documented for the scavenging Simenchelys parasitica for the first time.

HOST USE BY AUSTRALASIAN SEAWEED MESOGRAZERS IN RELATION TO FEEDING PREFERENCES OF LARGER GRAZERS

Marine mesograzers use individual seaweeds as both habitat and food. Previous studies by M. E. Hay, J. E. Duffy, and others have shown that mesograzers can acquire enemy-free space by inhabiting and consuming seaweeds that are chemically defended against larger, more mobile consumers. However, their studies, carried out in North Carolina and two tropical regions, consider only a small number of mesograzer species and focus primarily on fishes, even though some other large consumers can destroy mesograzers' host seaweeds. We asked whether the previous findings could be generalized to (1) mesograzers at the community level, (2) large consumers other than fishes, and (3) a different ecosystem in another region, namely, diverse temperate rocky reef communities in Australia and New Zealand. Results from both Australasian regions were similar. In multiple-choice feeding assays larger grazers (three fishes, four gastropods, and three sea urchins) varied in breadth of preferences, but most often preferred to eat green algae of the family Ulvales and the red alga Pterocladiella capillacea, all of which were restricted to the intertidal zone. Most of the mesograzers surveyed (seven amphipods, three isopods, a gastropod, and a sea urchin) preferred to eat the seaweed species from which they were collected, many of which were either inaccessible or less palatable to many larger grazers. This difference in mesograzer vs. larger grazer feeding preferences was consistent with the findings of Hay and Duffy et al. However, community-level patterns of mesograzer distribution and abundance were not consistent with the extensions of the theory developed here. Seaweeds that were less palatable to the larger grazers did not tend to support more mesograzer individuals, mesograzer species, or specialist mesograzers (for either the entire mesograzer assemblage or just for sedentary mesograzers), but the evenness of entire mesograzer assemblages was significantly higher on seaweeds that were less palatable to larger grazers. We argue that spatial refuges on Australasian rocky reefs and differences in the biology of both the seaweed and animal species in Australasia vs. previously studied locations enable palatable seaweeds to persist in the face of herbivory by larger grazers, which reduces the selective pressure on mesograzers to inhabit unpalatable species.