<Emphasis Type="Italic">Lacuna vincta</Emphasis> (Mollusca,Neotaenioglossa) herbivory on juvenile and adult <Emphasis Type="Italic">Nereocystis luetkeana</Emphasis> (Heterokontophyta,Laminariales)

Herbivory can be an important factor structuring coastal algal communities. Herbivores may preferentially graze particular algal species or tissue types. Mesograzers, despite their small size, can critically weaken kelp thalli and impact entire kelp beds. We propose that when kelp beds are composed of several kelp cohorts, mesograzers will selectively choose to inhabit younger plants and grazing activities will have a greater impact on younger plants. This study investigated the effects of grazing by the littorinid gastropod, Lacuna vincta, on different age classes of the bull kelp, Nereocystis luetkeana by (1) testing food preference of L. vincta on juvenile, first-year adult, and second-year adult Nereocystis blades in the laboratory, (2) determining substrate (blades of different ages) preference of L. vincta in the laboratory, and by (3) estimating in-situ herbivore abundances and densities on juvenile and adult Nereocystis. Results demonstrated that grazing by L. vincta produced greater damage on juvenile than older Nereocystis tissues. Although L. vincta did not select juvenile versus older kelps as substrate in the laboratory, in situ surveys showed that differences existed between age classes with higher L. vincta densities on juvenile than adult kelp. We conclude that at a local scale, L. vincta can be an important structuring factor in Nereocystis populations due to its high density and grazing ability. Handling editor: K. Martens     

Grazing damage and encrustation by an invasive bryozoan reduce the ability of kelps to withstand breakage by waves

Increased breakage of macroalgal fronds during large wave events can significantly reduce canopy cover and biomass. We examined the effects of encrustation by the invasive bryozoan Membranipora membranacea and damage by the snail Lacuna vincta on the ability of kelp blades (Saccharina longicruris, Laminaria digitata, and Laminaria complanata) to withstand wave forces. Using standard materials testing procedures, we documented significant reductions in the maximum stress before breakage, toughness, and extensibility of blade material following bryozoan encrustation. Histological sections of blade tissue indicated a significant degradation of the outer layers of cells following prolonged encrustation by M. membranacea as a likely cause of weakening. Full-thickness perforations and partial-thickness grazing scars also reduced blade strength, suggesting that grazing damage can initiate cracks that lead to blade breakage. Our findings provide a mechanistic link between the damaging effects of mesograzers and encrusting bryozoa on their algal hosts and the export of detrital material from subtidal kelp beds.     

Specificity in Mesograzer-Induced Defences in Seagrasses

Grazing-induced plant defences that reduce palatability to herbivores are widespread in terrestrial plants and seaweeds, but they have not yet been reported in seagrasses. We investigated the ability of two seagrass species to induce defences in response to direct grazing by three associated mesograzers. Specifically, we conducted feeding-assayed induction experiments to examine how mesograzer-specific grazing impact affects seagrass induction of defences within the context of the optimal defence theory. We found that the amphipod Gammarus insensibilis and the isopod Idotea chelipes exerted a low-intensity grazing on older blades of the seagrass Cymodocea nodosa, which reflects a weak grazing impact that may explain the lack of inducible defences. The isopod Synischia hectica exerted the strongest grazing impact on C. nodosa via high-intensity feeding on young blades with a higher fitness value. This isopod grazing induced defences in C. nodosa as indicated by a consistently lower consumption of blades previously grazed for 5, 12 and 16 days. The lower consumption was maintained when offered tissues with no plant structure (agar-reconstituted food), but showing a reduced size of the previous grazing effect. This indicates that structural traits act in combination with chemical traits to reduce seagrass palatability to the isopod. Increase in total phenolics but not in C:N ratio and total nitrogen of grazed C. nodosa suggests chemical defences rather than a modified nutritional quality as primarily induced chemical traits. We detected no induction of defences in Zostera noltei, which showed the ability to replace moderate losses of young biomass to mesograzers via compensatory growth. Our study provides the first experimental evidence of induction of defences against meso-herbivory that reduce further consumption in seagrasses. It also emphasizes the relevance of grazer identity in determining the level of grazing impact triggering resistance and compensatory responses of different seagrass species.     

Seasonal variation in epifaunal communities associated with giant kelp (Macrocystis pyrifera) at an upwelling‐dominated site

Kelp forests are highly productive and species‐rich benthic ecosystems in temperate regions that provide biogenic habitat for numerous associated species. Diverse epifaunal communities inhabit kelp sporophytes and are subject to variations in the physical environment and to changes experienced by the kelp habitat itself. We assessed seasonal variations in epifaunal invertebrate communities inhabiting giant kelps, Macrocystis pyrifera, and their effects on this seaweed. Six seasonal samplings were conducted over a year at an upwelling‐dominated site in northern‐central Chile where physical conditions are known to fluctuate temporally. More than 30 taxa were identified, among which peracarid crustaceans stood out in both diversity and abundance. Species richness and abundance differed among sporophyte sections (holdfast and fronds) and throughout the year. The frond community was dominated by two grazers (the amphipod Peramphithoe femorata and the isopod Amphoroidea typa), while suspension feeders, grazers, and omnivores (the amphipod Aora typica, the isopod Limnoria quadripunctata, and polychaetes) dominated the holdfasts. Abundances of the dominant species fluctuated throughout the year but patterns of variation differed among species. The most abundant grazer (P. femorata) had highest densities in summer, while the less abundant grazer (A. typa) reached its peak densities in winter. Interestingly, the area of kelp damaged by grazers was highest in autumn and early winter, suggesting that grazing impacts accumulate during periods of low kelp growth, which can thus be considered as ‘vestiges of herbivory past.’ Among the factors determining the observed seasonal patterns, strong variability of environmental conditions, reproductive cycles of associated fauna, and predation by fishes vary in importance. Our results suggest that during spring and early summer, bottom‐up processes shape the community structure of organisms inhabiting large perennial seaweeds, whereas during late summer and autumn, top‐down processes are more important.     

Temporal dynamics of inducible anti‐herbivory defenses in the brown seaweed Ascophyllum nodosum (Phaeophyceae)1

Anti‐herbivory defenses support persistence of seaweeds. Little is known, however, about temporal dynamics in the induction of grazer‐deterrent seaweed traits. In two induction experiments, consumption rates of the periwinkle Littorina obtusata (L.) on the brown seaweed Ascophyllum nodosum (L.) Le Jolis were measured in 3‐d intervals. Changes in palatability of directly grazed A. nodosum were tested every 3 d with feeding assays using fresh and reconstituted seaweed pieces. Likewise, assays with fresh A. nodosum assessed changes in seaweed palatability in response to water‐borne cues from nearby grazed conspecifics. Consumption rates of L. obtusata varied significantly during the 27‐d induction phase of each experiment. Direct grazing by L. obtusata lowered palatability of fresh and reconstituted A. nodosum pieces to conspecific grazers after 15 d as well as after 6 and 12 d, respectively. After 12, 18, and 24 d, fresh A. nodosum located downstream of L. obtusata‐grazed conspecifics was significantly less palatable than A. nodosum located downstream of ungrazed conspecifics. Changes in L. obtusata consumption rates and A. nodosum palatability during both induction experiments suggest temporal variation of grazer‐deterrent responses, which may complicate experimental detection of inducible anti‐herbivory defenses.     

Effect of Detachment on the Palatability of two Kelp Species

Many species of macroalgae survive after becoming dislodged from their primary substratum, but little is known about their capacity to express anti-herbivore defences after detachment. We examined the effect of detachment on the relative palatability of the two kelp species Lessonia nigrescens and Macrocystis integrifolia to mesograzers. Laboratory and field experiments were conducted on the northern-central coast of Chile to investigate whether (i) time after detachment and (ii) grazing on detached and attached algae could trigger internal defence mechanisms in the algae, which may have acted as deterrents to grazing. In order to examine palatability, feeding assays were run after each experiment using fresh algal pieces and artificial food. Time after detachment had a significant influence on palatability of L. nigrescens but not of M. integrifolia. During the first 12 days of detachment, detached L. nigrescens held in grazer-free laboratory tanks were not significantly more palatable than attached conspecifics from the field but thereafter detached individuals became more palatable. Floating individuals of M. integrifolia showed no effect of detachment, indicating that this alga maintains its defence after detachment. An experiment conducted in the field confirmed these results for M. integrifolia. An additional laboratory experiment confirmed that attachment status plays an important role on algal defence reaction for L. nigrescens when exposed to grazers. Detached and previously grazed individuals of this species were less palatable than grazer-free control algae, but grazing had no effect on palatability of attached algae. Our results indicate that kelps have varying capacities for development of anti-grazing responses once they become detached, possibly depending on their capacity to float and survive after detachment.     

Large mobile versus small sedentary herbivores and their resistance to seaweed chemical defenses

Summary Small, relatively sedentary herbivores like amphipods and polychaetes (mesograzers) often live on the plants they consume and should therefore view plants as both foods and living sites. Large, relatively mobile herbivores like fishes commonly move among, and feed from, many plants; they should view plants primarily as foods and rarely as potential living sites. In marine communities, fishes that consume plants are also important predators on mesograzers. Since seaweeds avoided by fishes should represent safer living sites for small herbivores, mesograzers living on and consuming seaweeds that are not eaten by fishes should have higher fitness than mesograzers living on plants preferred by fishes. In previous work, we demonstrated that seaweed secondary metabolites that deterred feeding by a fish and sea urchin had no effect on feeding by a common amphipod (Hay et al. 1987a). We then hypothesized that mesograzers would, in general, be less affected by seaweed chemical defenses than larger, more mobile herbivores like fishes. In this investigation, we evaluate the generality of this hypothesis by comparing the feeding of an omnivorous fish (Lagodon rhomboides) with that of an omnivorous, tube-building polychaete (Platynereis dumerilii) to see if the mesograzer prefers seaweeds avoided by the fish and if it is less affected by seaweed chemical defense. Platynereis dumerilii fed almost exclusively on Dictyota dichotoma, the seaweed eaten least by Lagodon rhomboides. The diterpene alcohols (dictyol-E and pachydictyol-A) produced by Dictyota significantly deterred feeding by Lagodon but did not affect, or at one concentration stimulated, feeding by Platynereis. Our data support the hypothesis that small, relatively sedentary herbivores that live on plants are more resistant to chemical defenses than are large, relatively mobile herbivores that move among many plants.     

Response of a native,herbivorous snail to the introduced seaweed <Emphasis Type="Italic">Sargassum muticum</Emphasis>

The role of native consumers in mediating biological invasions is poorly understood. In theory, there are reasons to expect both strong and weak effects of native consumers on non-native species. However, non-native ranges may include multiple regions or even continents, each with its own suite of consumers and invader–consumer interactions may play out differently in different places and times. In this Washington State (USA) study we found that the common herbivorous snail Lacuna vincta was 2–9 times more abundant on the non-native seaweed Sargassum muticum, compared to native kelps. Choice feeding trials with fresh tissue and artificial foods both suggest that S. muticum is a preferred food for Lacuna vincta. Lab experiments indicated that L. vincta did not experience diminished predation by two common predators on Sargassum muticum compared to native kelp hosts. Our results suggest that Sargassum experiences considerable herbivory by Lacuna vincta in our study region, a conclusion that is consistent with previous work and our own field observations. In our system, L. vincta and S. muticum have been coexisting in the same habitats for at least 50 years and available data suggest that it acquired a preference for S. muticum more than 30 years after the initial invasion. Comparison of our results to recent work on Sargassum–herbivore interactions in Europe suggests that the response of native consumer communities to S. muticum varies both within and among regions. Geographic and temporal variation in the response of native consumers are likely to be hallmarks of many large-scale invasions.     

Chemical Deterrence of a Marine Cyanobacterium against Sympatric and Non-sympatric Consumers

This study investigates the influence of mesograzer prior exposure to toxic metabolites on palatability of the marine cyanobacterium, Lyngbya majuscula. We examined the palatability of L. majuscula crude extract obtained from a bloom in Moreton Bay, South East Queensland, Australia, containing lyngbyatoxin-a (LTA) and debromoaplysiatoxin (DAT), to two groups: (1) mesograzers of L. majuscula from Guam where LTA and DAT production is rare; and (2) macro- and mesograzers found feeding on L. majuscula blooms in Moreton Bay where LTA and DAT are often prevalent secondary metabolites. Pair-wise feeding assays using artificial diets consisting of Ulva clathrata suspended in agar (control) or coated with Moreton Bay L. majuscula crude extracts (treatment) were used to determine palatability to a variety of consumers. In Guam, the amphipods, Parhyale hawaiensis and Cymadusa imbroglio; the majid crab Menaethius monoceros; and the urchin Echinometra mathaei were significantly deterred by the Moreton Bay crude extract. The sea hares, Stylocheilus striatus, from Guam were stimulated to feed by treatment food whereas S. striatus collected from Moreton Bay showed no discrimination between food types. In Moreton Bay, the cephalaspidean Diniatys dentifer and wild caught rabbitfish Siganus fuscescens were significantly deterred by the crude extract. However, captive-bred S. fuscescens with no known experience with L. majuscula did not clearly discriminate between food choices. Lyngbya majuscula crude extract deters feeding by most mesograzers regardless of prior contact or association with blooms.     

Major consequences of minor damage: impacts of small grazers on fast-growing kelps

Damage by small herbivores can have disproportionately large effects on the fitness of individual plants if damage is concentrated on valuable tissues or on select individuals within a population. In marine systems, the impact of tissue loss on the growth rates of habitat-forming algae is poorly understood. We quantified the grazing damage by an isopod Amphoroidea typa on two species of large kelps, Lessonia spicata and Macrocystis pyrifera, in temperate Chile to test whether non-lethal grazing damage could reduce kelp growth rates and photosynthetic efficiency. For L. spicata, grazing damage was widespread in the field, unevenly distributed on several spatial scales (among individuals and among tissue types) and negatively correlated with blade growth rates. In field experiments, feeding by A. typa reduced the concentration of photosynthetic pigments and led to large reductions (~80 %) in blade growth rates despite limited loss of kelp biomass (0.5 % per day). For M. pyrifera, rates of damage in the field were lower and high densities of grazers were unable to reduce growth rates in field experiments. These results demonstrate that even low per capita grazing rates can result in large reductions in the growth of a kelp, due the spatial clustering of herbivores in the field and the selective removal of photosynthetically active tissues. The impacts of small herbivores on plant performance are thus not easily predicted from consumption rates or abundance in the field, and vary with plant species due to variation in their ability to compensate for damage.     

Plant genotype identity and diversity interact with mesograzer species diversity to influence detrital consumption in eelgrass meadows

Seagrass meadows are among the world's most productive ecosystems, and as in many other systems, genetic diversity is correlated with increased production. However, only a small fraction of seagrass production is directly consumed, and instead much of the secondary production is fueled by the detrital food web. Here, we study the roles of plant genetic diversity and grazer species diversity on detrital consumption in California eelgrass Zostera marina meadows. We used three common mesograzers—an amphipod, Ampithoe lacertosa, an isopod, Idotea resecata, and a polychaete, Platynereis bicanaliculata. Each grazer consumed eelgrass detritus at rates greater than live eelgrass or macroalgae. This detrital consumption, however, was not spread evenly over leaves shed from different eelgrass clones. Palatability and consumption varied because of genotype specific differences in leaf texture, secondary metabolites (phenolics), and nutritional quality (nitrogen). Further, detritus derived from some eelgrass genotypes was palatable to all grazers, while detritus from other genotypes was preferentially consumed by only one grazer species. Under monospecific grazer assemblages, plant genetic identity but not diversity influenced detritus consumption. However, more realistic, diverse mesoconsumer communities combined with high plant‐detrital genotypic diversity resulted in greater consumption and grazer survival. These results provide a mechanism for field observations of increased mesograzer density and diversity in genetically diverse seagrass assemblages and offer a potential explanation for variation in results of resource diversity– detrital processing experiments in the literature, which often exclude macroinvertebrate taxa. More broadly, our findings support the emerging principle that biodiversity effects are strongest when diversity in both consumer and resource taxa are present.     

LACK OF DEFENSE OR PHLOROTANNIN INDUCTION BY UV RADIATION OR MESOGRAZERS IN DESMARESTIA ANCEPS AND D. MENZIESII (PHAEOPHYCEAE)1

Phlorotannins are polyphenoloic metabolites occurring only in the Phaeophyceae that have numerous putative primary roles (e.g. cell‐wall construction and storage) as well as secondary metabolic roles, which include herbivore feeding deterrence and protection from UV radiation. The proposed role of phlorotannins in the defense against UV radiation is of particular importance in the Antarctic due to depletion of the stratospheric ozone layer in that area. Several studies of brown algae have found evidence of an induction response (the production of defensive metabolites, including phlorotannins) after grazing by various mesograzers, after simulated grazing/wounding, and after exposure to increases in UV radiation. This study aimed to determine if phlorotannin production or other defenses in two dominant, endemic Antarctic species (Desmarestia menziesii Montagne and Desmarestia anceps J. Agardh) could be induced by an increase in exposure to UV radiation or by natural and artificial grazing. An in situ experiment failed to detect any effect of UV radiation on phlorotannin concentrations in either species or on subsequent palatability in feeding bioassays. A laboratory‐based experiment did not detect any effect of mesoherbivore grazing or simulated grazing (wounding) on palatability or the concentration of phlorotannins in D. menziesii. Instead, phlorotannin concentrations increased in all treatments in both experiments, consistent with an increase in overall resource availability due to an increase in available PAR compared with the in situ irradiance at the algal collection sites.     

GRAZING ON GIANT KELP MICROSCOPIC PHASES AND THE RECRUITMENT SUCCESS OF ANNUAL POPULATIONS OF MACROCYSTIS PYRIFERA (LAMINARIALES,PHAEOPHYTA) IN SOUTHERN CHILE1

The giant kelp Macrocystis pyrifera (L.) C. Agardh is widely distributed in the Northern Hemisphere and Southern Hemisphere, yet it exhibits distinct population dynamics at local to regional spatial scales. Giant kelp populations are typically perennial with the potential for year‐round reproduction and recruitment. In southern Chile, however, annual giant kelp populations exist and often persist entirely on secondary substrata (e.g., shells of the slipper limpet Crepipatella fecunda [Gastropoda, Calyptraeidae]) that can cover up to 90% of the rocky bottom. In these populations, the macroscopic sporophyte phase disappears annually during winter and early spring, leaving a 3–4 month period in which a persistent microscopic phase remains to support the subsequent year’s recruitment. We tested the effects of a suite of grazers on the recruitment success of this critical microscopic phase at two sites in southern Chile. Field experiments indicated that the snail Tegula atra negatively impacted M. pyrifera sporophyte recruitment, but that recruitment was highest in the presence of sessile female limpets, C. fecunda. Conversely, small male C. fecunda (biofilm grazers) did not regulate kelp recruitment. Laboratory observations showed that C. fecunda males only grazed on microscopic kelp gametophytes and small (<250 μm) sporophytes, rejecting larger sporophytes, whereas T. atra grazed on all the kelp stages. Recruitment to the C. fecunda treatments far exceeded that to bare rock in the absence of grazers but was not due to the physical presence of C. fecunda shells. We concluded that the key to M. pyrifera recruitment success in southern Chile is its capacity to colonize secondary substrates provided by the slipper limpet C. fecunda.     

High acclimation potential in floating Macrocystis pyrifera to abiotic conditions even under grazing pressure – a field study

The persistence of floating seaweeds, which depends on abiotic conditions but also herbivory, had previously been mostly tested in outdoor mesocosm experiments. In order to investigate if the obtained mesocosm results of high seaweed persistence under natural environmental conditions and under grazing pressure can be extrapolated to field situations, we conducted in situ experiments. During two summers (2007 and 2008), Macrocystis pyrifera was tethered (for 14 d) to lines in the presence and absence of the amphipod Peramphithoe femorata at three sites (Iquique, Coquimbo, Calfuco). We hypothesized that grazing damage and seaweed persistence vary among sites due to different abiotic factors. By incubating the sporophytes in mesh bags, we were either able to isolate (grazing) or exclude (control) amphipods. To test for a mesh bag artifact, a set of sporophytes was incubated without mesh bags (natural). Mesh bags used to exclude herbivores influenced sporophyte growth and physiological performance. The chlorophyll a (Chl a) content depended largely on grazers and grazed sporophytes grew less than natural and control sporophytes within the two summers. A decrease in Chl a content was found for the sites with the highest prevailing irradiances and temperatures, suggesting an efficient acclimation to these sea surface conditions. Our field‐based results of sporophyte acclimation ability even under grazing pressure widely align with previous mesocosm results. We conclude that M. pyrifera and other temperate floating seaweeds can function as long‐distance dispersal vectors even with hitchhiking mesoherbivores.     

A MULTISPECIES LABORATORY ASSESSMENT OF RAPID SPOROPHYTE RECRUITMENT FROM DELAYED KELP GAMETOPHYTES1

Recent work suggests that the ability to delay reproduction as resistant haploid gametophytes may be important for seaweeds that experience unpredictable disturbances or seasonal periods of poor conditions that result in adult sporophyte absence. Further, delayed gametophytes of some kelp species (order Laminariales) may produce sporophytes more rapidly than if they had never experienced a delay, conferring a competitive advantage when conditions improve or after disturbance events. Here, it was determined that the gametophytes of the canopy‐forming kelp Macrocystis pyrifera (L.) C. Agardh could delay reproduction in a one‐ to two‐cell state (<50 μm) for at least 7 months when grown under nutrient‐limiting conditions. These stages retained reproductive viability and produced sporophytes within 5 d once nutrients were increased. This finding suggests that gametophytes could potentially promote recovery of M. pyrifera populations after extended periods of sporophyte absence. In addition, the time required for sporophyte production between gametophytes of the four most conspicuous kelp species in Southern California that had delayed reproduction and gametophytes that had not was compared. For these four kelp species, a delay of at least 30 d conferred a 40%–76% reduction in the time required for sporophyte production once nutrients were received. Fecundity did not decrease with delay duration, suggesting there is no apparent cost of delayed development for kelps as has been observed in other organisms. Thus, delayed development may be a viable strategy for surviving and initially dominating in environments with variable quality.     

Tropical herbivores provide resilience to a climate‐mediated phase shift on temperate reefs

Climate‐mediated changes to biotic interactions have the potential to fundamentally alter global ecosystems. However, the capacity for novel interactions to drive or maintain transitions in ecosystem states remains unresolved. We examined temperate reefs that recently underwent complete seaweed canopy loss and tested whether a concurrent increase in tropical herbivores could be maintaining the current canopy‐free state. Turf‐grazing herbivorous fishes increased in biomass and diversity, and displayed feeding rates comparable to global coral reefs. Canopy‐browsing herbivores displayed high (~ 10 000 g 100 m^?2) and stable biomass between 2006 and 2013. Tropical browsers had the highest abundance in 2013 and displayed feeding rates approximately three times higher than previously observed on coral reefs. These observations suggest that tropical herbivores are maintaining previously kelp‐dominated temperate reefs in an alternate canopy‐free state by grazing turfs and preventing kelp reestablishment. This remarkable ecosystem highlights the sensitivity of biotic interactions and ecosystem stability to warming and extreme disturbance events.     

Prevailing sea surface temperatures inhibit summer reproduction of the kelp Laminaria digitata at Helgoland (North Sea)

The impact of abiotic factors on kelp sporophyte reproduction has rarely been investigated. Laminaria digitata (Hudson) J.V. Lamouroux is one of the few summer fertile Laminaria species worldwide and reproduction is subjected to relatively high water temperatures. We investigated the impact of prevailing summer temperatures (~18°C in August) on the induction of sporangia, meiospore release, and germination at the island of Helgoland (North Sea). At Helgoland, fertile sporophytes are found between April and December with a maximum in late summer. While released meiospore numbers were constant between June and October, germination rates decreased significantly in summer. Short‐term exposure of mature sori to 17°C–22°C induced a significantly higher meiospore release indicating enhancement of sporulation by elevated temperatures. Induction of sporangia on vegetative blade disks was not possible at 20°C, and fertility was only 20% at 18°C–19°C, but it was 100% in cool temperatures of 1°C–10°C. It was shown for the first time in a kelp species that “sporogenesis” is the life‐cycle process with the narrowest temperature window compared to growth or survival of the sporophyte or reproduction, growth, and survival of the gametophyte. We incorporated several parameters (induction time, fertile area, and relative fertility) into a “Reproductive efficiency index.” This indicates that sporogenesis of L. digitata is a cold‐adapted process with an optimum at (5)–10°C. The results show that the population at Helgoland is at its reproduction limit despite the existence of other geographically more southerly located populations.     

Intertidal mesograzers in field microcosms: linking laboratory feeding rates to community dynamics

Mesograzers (herbivores <2.5 cm) are both diverse and abundant, but their relative effects on intertidal communities have rarely been quantified. Here I examine the effects of crustacean and polychaete mesograzers on two intertidal resources, the red alga Odonthalia floccosa Esp. (Falkenb.) and the epiphytic diatom Isthmia nervosa Kütz. The mesograzers were hermit crabs (Pagurus hirsutiusculus (Dana) and P. granosimanus (Benedict)), amphipods (Hyale frequens Stout and H. pugettensis (Dana)), isopod (Idotea wosnesenskii (Brandt)), juvenile kelp crab (Pugettia producta (Randall)), and polychaete worm (Platynereis bicanaliculata (Baird)). Feeding rates on Isthmia, measured in the laboratory for different consumer species and size classes, scaled allometrically with body mass. Consumption rates were 2-23% of body mass daily on a fresh weight basis. However, feeding rates on Odonthalia did not scale, suggesting that size will not always indicate per capita effect. Mesograzer densities were measured on Tatoosh Island, Washington, USA. The mesograzer predicted to have the largest total effect (P. hirsutiusculus), based on densityxfeeding rate, was neither the most abundant nor the most voracious. The validity of these sorts of predictions depends on how well feeding rates measured in the laboratory approximate per capita effects under field conditions. Predictions were compared to observed effects in field microcosms. Given known numbers of mesograzers, predictions were made about the amount of Isthmia biomass that should disappear over 2 weeks from microcosms (9x9x12 cm) anchored in tidepools. Average per capita effects in field microcosms were correlated with laboratory feeding rates, but, for three species with significant feeding on Isthmia, effects were lower than feeding rates predicted. Feeding trials may overestimate community impact because they fail to account for alternative food, search times, resource productivity and stimulation of growth, or interference from other consumers. Nevertheless, densities of mesograzers can reach sufficiently high levels so that even feeble per capita effects combine to alter biomass of epiphytes and perhaps other small algae.