Coprophagy: a supplementary food source for two freshwater gastropods?

1. The freshwater pulmonate snail Radix peregra voluntarily and regularly fed on its faeces, whereas Bithynia tentaculata (Prosobranchia) did not.
2. With high-quality faeces as food (faeces with C : N-values below ten) Radix could grow and survived for 11 weeks. Growth was 24–30% of that achieved with control food (lettuce, Chlamydomonas and Tetramin), and body mass was close to the expected value.
3. As with real food, the percentage organic content of faeces was reduced during gut passage, with the exception of very low-quality faeces.
4. Faeces contained living cells of green algae ( Chlamydomonas reinhardii ) and diatoms ( Achnanthes lanceolata, Eunotia pectinalis ), which survived the gut passage of R. peregra and B. tentaculata . Faecal material was also rich in bacteria, with up to 150 × 10⁶ cells mg^–1 dry mass. Bacteria increased in abundance in faeces which had been evacuated for 7 days or more.
5. A further degradation of partly digested food in the faeces was indicated by the activity of the extracellular digestive enzymes cellobiase and chitobiase. These enzymes hydrolysed material with an activity of 0.2–14.5 pkat mg^–1 faecal dry mass in 1–12-day-old faeces. In faeces of Bithynia fed control food or incubated sycamore leaves, chitobiase activity was correlated with bacterial abundance.
6. Coprophagy is considered to be a suitable strategy for further degradation and utilization of refractory material for which one gut passage is too short and/or too inefficient.     

Transformation of algal turf by echinoids and scarid fishes on French Polynesian coral reefs

The respective roles of regular echinoids and scarid fishes in the transformation of turf algae, the main food resource for reef herbivores, were investigated on French Polynesian coral reefs. The role of one species of parrotfish (Scarus sordidus) was compared with that of four species of echinoids. The degree and ways of degradation of the algal matter were determined by the organic matter percentage, the composition of the sugar fraction, and the concentration and composition of chlorophylltype pigments as assayed by HPLC analysis. Chemical analyses were performed on anterior and posterior intestines for scarids, intestinal contents and faeces for echinoids, and on fresh algal turf as a control of initial food quality. A decrease in mean percentage of organic matter in gut content was observed from intestine (9.7%) to faeces (7%) in sea urchins, but not in parrotfishes. The total sugar fraction decreased from fresh algal turf (32% of total organic matter) to echinoid (28%) to scarid (18%) gut contents. The ratio of insoluble to soluble sugars (I/S ratios) was higher in echinoids (2.6) than in scarid gut contents (1.0). A decrease in the total pigment concentration was measured from fresh algal turf to echinoid and it was found to be even lower in scarid gut contents. Chromatograms showed that the composition of chlorophyll-type pigments in scarid intestines was very similar to fresh algal turf, with a dominance of native forms, mainly chlorophyll a and b. On the contrary, degraded pigment forms dominated in echinoids. The main degraded products were pheophorbides in sea urchins, and chlorophyllides in parrotfishes. These results provided evidence for differentiation in digestive processes occurring in the two types of grazers. Echinoids released higher degraded algal material than did scarids. Thus, these two types of grazers play different roles in the recycling of organic matter on coral reefs.     

Gut fluorescence in herbivorous copepods: an attempt to justify the method

Recently the gut fluorescence technique has been critisized because of the possible degradation of chlorophyll into nonfluorescent derivatives during passage through copepod guts and changes of the gut passage time with food concentration. Here pigment budgets have been calculated in 6 experiments with Calanus finmarchicus CIV caught 2 km offshore of the Murmansk Marine Biological Institute (the Barents Sea, Dalnije Zelentsi) in September 1992. Copepods were fed with culture of Platymonas viridis at different concentrations. Gut pigment and ingestion rate increased with food concentration in a similar way. On average between 78% and 89% of ingested chlorophyll was recovered in the guts and faecal pellets. No trend for a greater loss of fluorescence at low food concentration than at high was observed. Pigment content of faecal pellets incubated in filtered seawater decreased by 20–30% in the first 7–12 h and by up to 60% in 48 h. The decline of pigment content was accompanied by a rapid bacterial growth (by a factor of 3 in 48 h). Gut passage time increased with decreasing food concentration (from 40 min at 9 µg pigm l^–1 to 64 min at 0.9 µg pigm l^–1). These results together with some data by other authors suggest that the gut fluorescence method can be used to estimate in situ grazing rate providing gut passage time is measured properly and there are no losses of faecal material. However, careful consideration should be given to the previous feeding history of copepods.     

Gut clearance and pigment destruction in a herbivorous copepod, Acartia tonsa, and the determination of in situ grazing rates

Gut clearance rates of starving and continuously feeding Acartiatonsa were estimated. During the initial 30 min the rates weresimilar (0.045 and 0.048 min^–1, respectively; 14°C)but thereafter starving animals expelled the remains of theirgut contents at half the rate (0.019 min^–1) of fed ones(0.048). Pigment destruction was estimated by (i) incubationexperiments over 3–4 days, (ii) silica to pigment ratioin algae and faeces and (iii) by gut filling experiments. Theincubations showed that 8% of the ingested pigments were destroyedto nonfluorescent residues during gut passage. The silica topigment ratio method gave an average of 11 % (1 –24) destructionand gut-filling experiments showed no systematic differencebetween ingestion measured as gut filling rate (fluorescence)and particle reduction. ¹Present address: Kristineberg Marine Biological Station, S-45034 Fiskebäckskil, Sweden     

Arctic sea ice as feeding ground for amphipods – food sources and strategies

The amphipod species Gammarus wilkitzkii, Apherusa glacialis, Onisimus nanseni and O. glacialis live permanently associated with the Arctic sea ice. Qualitative and semi-quantitative investigations of gut contents and faeces showed that all four species use detritus as the main food source. Detrital lumps from the underside of sea ice had the same item composition as amphipod gut contents and faeces. Crustacean remains and ice algae were additional food items, but overall they were quantitatively less important. All species are omnivorous; however, differences in gut contents, behavioural observations and functional–morphological studies of the mandibles suggest a differentiation within this feeding strategy. G. wilkitzkii is a detritivorous-carnivorous-necrophagous-suspension-feeding species and shows the most complex feeding strategy. O. nanseni and O. glacialis are predominantly detritivorous-necrophagous, whereas A. glacialis is characterised as a more herbivorous-detritivorous species. By using a variety of the available food sources under Arctic sea ice, the amphipods are well adapted to the under-ice habitat and are less influenced by temporal and spatial variations. Furthermore, the wide food spectrum of all four species reduces the intra- and interspecific competition in a habitat where certain food sources are limited or only seasonally available. Accepted: 30 June 2000     

Copepod feeding behavior and the measurement of grazing rates in vivo and in vitro

In this paper some aspects of the use of the gut fluorescence method for estimating ingestion rates have been, examined. One assumption is that gut turnover time in feeding copepods is equal to the gut clearance time in filtered seawater. When arctic Pseudocalanus were pre-fed on Thalassiosira weisflogii, and then given a trace addition of the same C¹⁴-labelled culture, or were transferred to filtered seawater, results suggested that this assumption was probably justified. In another experiment in which Pseudocalanus were fed at the same concentration of either melted ice algae, or pelagic under ice algae, there were significant differences in both gut clearance times and gut pigment levels in the two cases.Pigment: biogenic silica ratios in epontic algae were higher than those in faecal pellets produced by Pseudocalanus feeding on the algae, suggesting that pigment destruction was occurring during grazing. In a 28 hr time course experiment ingestion rates determined by rate of disappearance of particulate chlorophyll were higher than those simultaneously determined by the gut fluorescence method, which also supports the idea of pigment destruction in copepods guts.     

取食聚苯乙烯塑料对黄粉虫幼虫肠道微生物群落的影响北大核心

随着塑料在人类社会中的普及,越来越多的废弃塑料及其前体物质被遗留在环境中,且其在自然环境中的降解速度十分缓慢,为此寻找有效的降解途径成为亟待解决的科学问题。【目的】探究利用黄粉虫(Tenebrio molitor)幼虫取食聚苯乙烯对其肠道微生物种群及其代谢路径的响应,以期通过食物诱导寻找一条生物降解和利用聚苯乙烯的有效途径。【方法】以聚苯乙烯为唯一食物来源喂饲黄粉虫幼虫,通过测量幼虫存活率和个体的体重来测定其生长发育情况;通过对其肠道内容物进行16S rRNA基因测序,分析其肠道菌群结构的变化;采用京都基因与基因组百科全书(Kyoto encyclopedia of genes and genomes,KEGG)分析法来预测相关功能基因。【结果】取食聚苯乙烯黄粉虫幼虫存活率和体重均下降,聚苯乙烯塑料明显减少;取食聚苯乙烯的黄粉虫幼虫肠道菌群丰度与多样性明显减少,在门水平,取食聚苯乙烯的黄粉虫幼虫肠道的优势菌为变形菌门(Proteobacteria)、软壁菌门(Tenericutes)和厚壁菌门(Firmicutes);在属水平,取食聚苯乙烯的黄粉虫幼虫肠道优势菌为螺旋体菌属(Spiroplasma)、肠杆菌(Enterobacillus)和大肠埃氏-志贺氏菌(Escherichia-Shigella);通过KEGG功能预测,找到与芳香类和烷烃降解功能相关的基因共18种,取食聚苯乙烯组黄粉虫幼虫肠道菌群降解聚苯乙烯相关通路丰度升高,相关基因表达增强。【结论】聚苯乙烯可以为黄粉虫幼虫生长发育提供一定的物质和能量,且能够使其完成一个世代过程;幼虫长时间取食单一食物后,其肠道菌群结构会发生目标性变化,利用KEGG预测能够找到与聚苯乙烯代谢相关的基因,为后续研究工作提供了有价值的依据。     

Growth of Corbicula fluminea (bivalvia) fed artificial and algal diets

Laboratory experiments were conducted to assess the growth of Corbicula fluminea Müller (1774) fed mixed diets of the green alga Ankistrodesmus and five artificial foods (nine-grain cereal, rice flour, rye bran, denatured brewer's yeast and Ralston Purina trout chow). Clams lost weight after 30 days on each artificial diet; however, weight losses were significantly less than those of control clams which were not fed (p < 0.05). In contrast, clam growth was positive in each treatment which included Ankistrodesmus. There was no significant difference in the tissue growth of clams fed on an Ankistrodesmus monoculture and Ankistrodesmus supplemented with each artificial diet (p > 0.2). Therefore, we conclude that all of the artificial diets evaluated were of nutritional value. However, the alga Ankistrodesmus appears to be a superior food item. In subsequent experiments, clams were fed on diets consisting of various combinations of six genera of green algae (Pedinomonas, Ankistrodesmus, Chlamydomonas, Chlorella, Scenedesmus, and Selenastrum). All trialgal combinations of Chlamydomonas, Chlorella, Scenedesmus, and Ankistrodesmus and a dialgal culture of Chlamydomonas and Chlorella resulted in strong positive tissue growth. No other dialgal or monoalgal culture resulted in similar growth. Cultures of Selenastrum were found to be toxic, and their filtrate was determined to inhibit Corbicula filtration.     

Resource allocation in Tomocerus flavescens (Insecta,Collembola): a study with C-14-labelled food

Summary Individuals of the collembolan species Tomocerus flavescens from a beech wood on limestone near Göttingen (West Germany) were fed with C-14-labelled algae in the laboratory. On an average, T. flavescens exchanged almost all of its endogenous carbon after 3.4 moults. In periods of starvation the interval between two ecdyses increased from 5.2 to 6.1 days (17°C). The assimilation efficiency was determined in three independent ways (carbon use of different algal components; A=C-FU; C=P+R+FU, A=P+R): it ranged from 0.34 to 0.40 (converted to energy values 0.30 to 0.45). It could be raised in phases of high mobility and in periods of food shortage by lowering the gut passage rate. The decrease in feeding activity (demonstrated by gut contents analyses) in case of a detoriation of the food quality and the food availability can be partly compensated in this way. For individuals which did not reproduce, the ratio of production to assimilation was 0.24 (converted to energy values 0.31). T. flavescens showed an increased body growth in connection with reaching sexual maturity. Specific demands of nutritive substances in periods of physiological changes could possibly be compensated by a more efficient resource use as well as by differences in resource allocation. The studied population of T. flavescens was compared both with that of a Danish beech forest and general data from the literature. The production of few eggs rich in energy, the high weight of newly hatched individuals, the high growth as well as the storage of carbon connected with egg production, the comparatively low respiration metabolism and the high amount of energy invested in search for food have to be regarded as an adaptation to the habitat. According to conservative estimates climbing individuals of this species consume 0.83 g of the algal dry mass growing on one beech tree during one summer. T. flavescens mainly uses the ethanol-soluble components of the algae.     

Trophic roles of tadpoles in tropical Australian streams

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Ingestion by deposit-feeding macro-zoobenthos in the aphotic zone does not affect the pool of live pelagic diatoms in the sediment

The ability of deposit feeders to utilise the pool of live planktonic diatoms in the sediment was investigated after the spring bloom in 2001 and 2002 at four sediment sites in the aphotic zone in the Kattegat. Seven species of deposit-feeding bivalves, gastropods and polychaetes were allowed to defecate in containers with filtered seawater. A total of 22 containers were set up holding between 3 and 32 animals, grouped by station and taxa. When defecation was completed, the animals were re-introduced to different stocks of homogenised sediment which were manipulated by addition of luminophores and spores of the diatom Chaetoceros diadema, and the faeces again collected and analysed. Our analysis included the number of germinable planktonic diatoms, measured by the dilution extinction method, and the concentrations of the algae pigments fucoxanthin and chl a, in the faeces samples, in the sediment stocks and in the sediment at the sampling location. Comparison between the sediment composition at the sampling locations, and the composition of the corresponding faeces, showed that the concentration of germinable diatoms averaged about 50,000 g⁻¹ dry weight (DW) in the surface sediment as well as in the faeces in 2001, while in 2002, the average concentration was about 390,000 g⁻¹ DW in both faeces and in the sediment. A similar comparison in the second part of the experiments also showed that the diatoms in the sediment were unaffected by gut passage and the added spores of C. diadema germinated in the same quantity in the faeces as in the sediment. The taxonomic composition of the diatoms in the faeces matched the composition at the respective stations and sediment stocks. Finally, the concentration of pigments did not change significantly during gut passage. The biomass of live planktonic diatoms in the area of study was estimated to be 2-10 g DW/m² or 0.2-5% of the total organic content in the top 3 cm of the sediment. It is concluded that this pool of diatoms is largely unaffected by deposit feeders and seems to be unimportant as a food source. It is hypothesised that the input of the spring bloom should be considered as composed of two fractions playing different roles for the benthic ecosystem. One fraction of dead organic material may be utilised immediately, while the fraction of live planktonic diatoms serve as a stable food buffer, which gradually become available to deposit feeders after the diatoms die and degradation starts.     

Relative importance of competition with Daphnia (Cladocera) and nutrient limitation on Anuraeopsis (Rotifera) population dynamics in a laboratory study

• 1 In the laboratory, the growth and reproduction of Anuraeopsis fissa were measured when fed on Scenedesmus species grown in nutrient‐sufficient, nitrogen‐limited and phosphorus‐limited media and in the presence or absence of one adult Daphnia longispina per vial.
• 2 Poor food quality may reduce the effect of competition on rotifers. Competition from Daphnia was stronger with nutrient sufficient algae than with nutrient limited algae. P‐limitation significantly reduced Anuraeopsis population growth rate and fecundity. The effect of nutrient limitation on Anuraeopsis was stronger than that of competition with Daphnia. The Anuraeopsis population declined with P‐limited food in both the presence and absence of Daphnia.
• 3 Exploitative competition by Daphnia on Anuraeopsis was stronger in the nutrient‐sufficient treatment than in the N‐limited one. Density, fecundity and population growth rate of Anuraeopsis were negatively affected by Daphnia in the nutrient‐sufficient treatment, while only fecundity was reduced by Daphnia in the N‐limited treatment. Consequently, in the N‐limited treatment, mortality should be lower in the presence of Daphnia. This result could suggest that Anuraeopsis lives longer when short of nitrogen.
• 4 Nutrient limitation may affect to the competitive interactions between zooplankton species. P‐limitation decreased the quality of algae as food for Anuraeopsis while N‐limitation decreased the susceptibility of this rotifer species to exploitative competition by Daphnia.

     

Application of 13C NMR to investigate the transformations and biodegradation of organic materials by wood- and soil-feeding termites, and a coprophagous litter-dwelling dipteran larva

Solid-state 13C nuclear magnetic resonance spectroscopy has been used to characterize the C in samples of the food (wood), gut contents and faeces from the wood-feeding termite, Microcerotermes parvus; soil in the guts and mound material from the soil-feeding termite, Thoracotermes macrothorax; and the food and faeces from the litter-feeding, coprophagous larvae of the dipteran fly, Bibio marci. Spectra from the wood-feeding termite indicated preferential loss of polysaccharide and accumulation of lignin with some modification to the O-aromatic-C and methoxyl-C (O-methyl-C) components during passage through the gut. Spectra for the soil-feeding termite indicated little change in the distribution of 13C between resonances following passage through the gut, except for some evidence of preferential polysaccharide loss. Interpretation of the spectra from these organisms was restricted by the relatively low C content of the soils and mound material, and by the large contribution to the NMR spectra from the gut tissue rather than the gut contents. Spectra for the litter-feeding dipteran larvae indicated preferential feeding on the polysaccharide-rich component of the litter and then overall loss of polysaccharide-C and accumulation of both aromatic-C and methoxyl-C in the gut. These changes were greater for the second passage than for the first passage through the gut, suggesting that principally mechanical and physical changes occurred initially and that chemical digestion was prevalent during the second passage.     

Shoreline algal wash as a factor in reed decline in Lake Constance-Untersee

This paper assesses the chemical and mechanical impact of algal wash (Cladophora, Spirogyra, Chara) upon the lakeside reed belt (Phragmites australis) using field mapping methods, bioassays with Scenedesmus acutus in batch culture, and field experiments. Heavy mats of filamentous algae are correlated with a reduction in number of the outermost reed stalks. The water pressed from decaying heaps of Cladophora and Spirogyra reduced the growth rate of Scenedesmus significantly, but mats from Chara did not. It is assumed that the toxic substance is an organic compound. In field experiments the detrimental effect could not be clearly evidenced. The reasons for this are discussed. It is concluded that mechanical impact is of major importance.     

Seed dispersal effectiveness of the western lowland gorilla (Gorilla gorilla gorilla) in Gabon

The quantitative and qualitative aspects of seed dispersal by the western lowland gorilla (Gorilla gorilla gorilla) were investigated in Gabon. Fresh faeces were collected and washed to identify and count the seeds. Seed germinability after gut passage was estimated with trials in a nursery at the study site. To assess the impact of gut passage on germination success and delay, comparative trials were run with four treatments: (i) gut passed seeds cleaned of faeces, (ii) gut passed seeds within a faecal matrix, (iii) seeds from fresh fruits surrounded by pulp, and (iv) seeds from fresh fruits cleaned of pulp. The analysis of 180 faecal units resulted in the identification of 58 species of seed. Germination trials were realized for 55 species and the mean germination success reached 46%. The impact of gut passage was investigated for Santiria trimera and Chrysophyllum lacourtianum; both species displayed higher germination success after ingestion. This study shows that gorillas effectively disperse seeds of numerous plant species, many of which provide timber or nontimber forest products or are typical of Gabonese forests. Considering the high‐quality of gorilla deposition sites, gorillas is thought to play a unique role in the dynamics of Central African forest.     

Gut pigment destruction by the copepod Acartia clausi

The extent of chlorophyll (Chi) a degradation into colorlessproducts by the copepod Acartia clausi was determined by measuringgut fluorescence prior to fecal pellet production. The efficiencyof pigment degradation of animals fed at different concentrationsof Thalassiosira weiss-fiogii varied between 30.56 and 94.05%,and the quantity of pigments lost during gut passage was directlyrelated to total ingestion. The role of feeding history in pigmentdestruction was tested. The fraction of ingested Chi a degradedby copepods acclimated to a high food concentration (15 ng Chla ml^–1) was greater than the fraction degraded by thoseacclimated to a low food concentration (1.5 ng Chl a ml^–1).The percentage of ingested Chl a that A.clausi transformed intofluorometrically undetected compounds was not constant. At present,knowledge of the pigment destruction process indicates thatChi a and Chi --derived pigments are not considered useful quantitativetracers of the feeding activity of copepods. The results ofthis study suggest a reassessment of the application of thegut fluorescence method to evaluate grazing activity of thecopepod A.clausi     

Comparative lipidomic studies of Scenedesmus sp. (Chlorophyceae) and Cylindrotheca closterium (Bacillariophyceae) reveal their differences in lipid production under nitrogen starvation

Microalgae are a promising resource for the highly sustainable production of various biomaterials (food and feed), high‐value biochemicals, or biofuels. However, factors influencing the valued lipid production from oleaginous algae require a more detailed investigation. This study elucidates the variations in lipid metabolites between a marine diatom (Cylindrotheca closterium) and a freshwater green alga (Scenedesmus sp.) under nitrogen starvation at the molecular species level, with emphasis on triacylglycerols using liquid chromatography–electrospray ionization mass spectrometry techniques. A comprehensive analysis was carried out by comparing the changes in total lipids, growth kinetics, fatty acid compositions, and glycerolipid profiles at the molecular species level at different time points of nitrogen starvation. A total of 60 and 72 triacylglycerol molecular species, along with numerous other polar lipids, were identified in Scenedesmus sp. and C. closterium, respectively, providing the most abundant triacylglycerol profiles for these two species. During nitrogen starvation, more triacylglycerol of Scenedesmus sp. was synthesized via the “eukaryotic pathway” in the endoplasmic reticulum, whereas the increase in triacylglycerol in C. closterium was mainly a result of the “prokaryotic pathway” in the chloroplasts after 96 h of nitrogen starvation. The distinct responses of lipid synthesis to nitrogen starvation exhibited by the two species indicate different strategies of lipid accumulation, notably triacylglycerols, in green algae and diatoms. Scenedesmus sp. and Cylindrotheca closterium could serve as excellent candidates for the mass production of biofuels or polyunsaturated fatty acids for nutraceutical purposes.     

Feedback between zebra mussel selective feeding and algal composition affects mussel condition: did the regime changer pay a price for its success?

1. We investigated the role of algal composition on pumping, clearance, assimilation, pseudofaeces and faeces production, feeding time budgets, and condition of zebra mussels from spring to autumn at two sites in Saginaw Bay (Lake Huron) and one site in western Lake Erie. Size‐fractioned chlorophyll was used to distinguish between feeding on small (<53 μm) and large (>53 μm) size fractions, and mussel feeding behaviour was quantified by video observations. 2. Mussel pumping, clearance and assimilation rates varied among sites, particularly during summer, when phytoplankton composition varied considerably among sites. Lowest values were seen at the inner‐bay site of Saginaw Bay, low to moderate values at the outer‐bay site of Saginaw Bay, and high values at the Lake Erie site. Clearance, pumping and assimilation rates were all highly positively correlated (R² = 0.76) with per cent contribution of flagellates to total algal biomass and negatively correlated with per cent of Microcystis aeruginosa (R² = 0.63). The negative effects on pumping rate (as determined by clearance rate on the <53 μm fraction) of Microcystis, which occurred in the >53 μm fraction, could be mitigated by the presence of flagellates in the <53 μm fraction. 3. Visual observations of mussel feeding showed evidence for poor seston quality during summer negatively affecting feeding rates. High faeces production during times of low assimilation rate was suggestive of poor assimilation efficiency and/or viable gut passage of grazing resistant algae. Long periods of time not filtering by the mussels during some Microcystis blooms and lack of production of a filtering current during one experiment were suggestive of intoxication from microcystin or other secondary compounds. 4. Clearance and feeding rates of the mussels in Saginaw Bay were high during spring and autumn and very low in summer, particularly at the inner‐bay site. Condition of the mussels (mass : length ratio) was highest in spring and lowest during summer. This seasonal variation probably reflected high food assimilation rate during autumn and spring and low assimilation rate and reproduction during summer. The condition of mussels throughout the year was higher at the outer‐bay than the inner‐bay site, reflecting better feeding conditions at the former. Mussel selective feeding may have been responsible for the poor quality of food at the inner bay site; therefore, we postulate that a regime shift in phytoplankton composition promoted by the mussels fed back into lowered condition of the mussels.     

Ingestion and digestion of epilithic algae by larval insects in a heavily grazed montane stream

1. Epilithic algae grown on elevated or non-elevated ceramic tiles were exposed (to produce assemblages with different grazing histories) in a heavily grazed, montane stream in New Mexico, U.S.A. to Ameletus nymphs (Ephemeroptera) and Ecclisomyia larvae (Trichoptera) and the algal composition in insect faeces was compared to that on the tiles. Differences in grazing and digestion efficiency between grazers were then assessed and also differences in susceptibility to ingestion and digestibility among common algae. 2. Ordination of tile and faecal samples, using the relative abundance of common algae, revealed that: (i) algal assemblages on elevated vs. non-elevated tiles differed only slightly; (ii) the taxonomic composition of algae in faeces of both caddis and mayflies differed substantially from that on the tiles, indicating low grazing efficiency for some algal taxa; and (iii) the algal composition of faeces produced by caddis larvae and mayflies was similar, indicating little difference in grazing efficiency between them. However, some algal taxa were more susceptible to ingestion by caddisfly larvae when occurring on elevated tiles than on non-elevated tiles, suggesting that previous exposure to caddis grazing influenced assemblage attributes. 3. Although Ameletus and Ecclisomyia differed little in grazing efficiency, the percentage of diatoms that were dead after passage through the gut was greatest in the mayfly treatment, suggesting that mayflies digested diatoms more efficiently than the caddis. Analyses of differences in the condition of chloroplasts within diatoms in tile and faecal samples showed that losses of ’live‘ diatom cells (i.e. those containing full chloroplasts) during gut passage through mayflies equalled the increase, in faeces, of ’dead‘ (empty frustules) cells of all common diatoms. In contrast, some diatoms were digested inefficiently by caddis larvae. 4. Algae on elevated tiles contained a higher proportion of dead diatoms than those on non-elevated tiles, possibly because mayflies visited raised tiles more often and, consequently, ingested and defaecated cells at a higher rate in the absence of caddis larvae. Moreover, diatom taxa differed in the percentage of cells that were dead within tile assemblages, with populations of typically grazer-resistant taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum and Cocconeis placentula var. euglypta) containing significantly more dead cells than grazer-susceptible taxa [e.g. small, chain-forming Fragilaria (= Staurosirella)]. This result suggests that a trade-off exists between ingestion vs. digestion resistance of microalgae. Both the ingestion and digestion efficiency of algivorous macroinvertebrates could influence the structure and function of algal assemblages. In heavily grazed systems, where algal cells are probably processed through grazer guts repeatedly, differential resistance to digestion among algae may be particularly important.