Amphipod Grazing Research Articles

Nutritional qualities partially predict diet of amphipod grazers: implications for modern coastal biofilms in microbialite pools

Mobile metazoans in the Phanerozoic transformed shallow coastal habitats covered in biofilms into bioturbated mixed grounds. Benthic biofilms during this time rarely preserved into the fossil record as microbialites, and more specifically their layered form, stromatolites. Living microbialites are rare for this reason, in addition to other factors such as unfavourable modern geochemical conditions. Counterintuitively, macroinvertebrate grazers and laminated stromatolites coexist along the Nelson Mandela Bay coastline, South Africa. Here we aimed to determine if nutritional qualities of algae in these microbialite pools are linked to the diet of a dominant amphipod grazer, Melita zeylanica Stebbing, 1904. We assessed this in 3 microbialite pools using stable isotopes coupled to seasonal shifts in abundant food resources of microbialite biofilms and exposed (bleached and rimstone) or submerged (pool) macroalgae, Ulva spp. Amphipod diet was proportional to the quantity available (biomass) and nutritional properties of available algal food (energy, protein, total phosphorus and C:N ratios). Macroalgae in all states were the most nutritious, and the diet of M. zeylanica was dominated by submerged macroalgae. Although microbialite material had the worst nutritional profile, it was consumed in larger quantities compared to exposed bleached and rimstone macroalgae. Therefore, factors unrelated to nutritional profiles, such as predation, allelopathy or exposure risks, likely drive food choices. High levels of grazing on nutritious pool macroalgae likely releases most of the grazing pressure on the microbialite biofilm, therefore partially explaining why the microbialites can persist in this environment instead of being bioturbated through foraging.

A trophic cascade facilitates native habitat providers within assemblages of multiple invasive marine species

AbstractAnthropogenically influenced habitats are becoming increasingly impacted by multiple invasive species that can regionally control main ecological processes and functions. Management requires predictions of invasive species’ effects, but multiple invasions may cause unpredictable outcomes, for example, synergistic impacts or interactions among trophic levels causing trophic cascades. The Baltic Sea is prone to species invasions that increasingly occur at multiple trophic levels. We used a laboratory experiment to test whether multiple invasive predators may cause interactive effects in the Baltic Sea. In aquaria, we simulated a coastal community that has developed dominated by invasive species—the decapod predators Palaemon elegans and Rhithropanopeus harrisii, the amphipod grazer Gammarus tigrinus, and the filter‐feeder Dreissena polymorpha. The orthogonal experiment determined separate and interactive effects from predators on the lower trophic level invasive species and native algae including the foundation species Fucus radicans. The main result was that while both predators significantly impacted the community, in no cases were their effects interactive. Another important result was that P. elegans appeared to cause a trophic cascade, as in its presence F. radicans and its epiphytes had the greatest biomass while G. tigrinus grazers were heavily reduced. G. tigrinus were negatively correlated with F. radicans biomass, so variable amphipod grazing may be the link allowing an indirect effect of P. elegans on F. radicans. R. harrisii also reduced G. tigrinus abundance but did not affect F. radicans biomass. Both predators significantly affected the lengths of F. radicans plant stems that were comprised of new vegetative growth—P. elegans reduced the lengths of new vegetative tissue while R. harrisii increased it. Thus, the presence of P. elegans induced shorter and denser F. radicans bushes compared to taller and thinner bushes observed in other treatments. Lastly, R. harrisii caused significant declines in D. polymorpha numbers. Overall, ecological management needs to consider positive effects from predator invasion which may reduce grazing pressure on low trophic level foundation species. A trophic cascade involving two invasive species highlights the importance of multi‐species approaches to management of Baltic Sea invasive species and in other regions prone to biological invasions.

Every Rule Has an Exception: a Cheater in the Community-Wide Mutualism in Antarctic Seaweed Forests

Dense macroalgal forests on the Western Antarctic Peninsula serve important ecological roles both in terms of considerable biomass for primary production as well as in being ecosystem engineers. Their function within the Antarctic ecosystem has been described as a crucial member of a community-wide mutualism which benefits macroalgal species and dense assemblages of associated amphipod grazers. However, there is a cheater within the system that can feed on one of the most highly chemically defended macroalgal hosts. The amphipod Paradexamine fissicauda has been found to readily consume the finely branched red macroalga Plocamium cartilagineum. This amphipod grazer not only feeds on its host, but also appears to sequester its host's chemical defenses for its own utilization. This review summarizes what we know about both of these exceptions to the community-wide mutualism.

Habitat fragmentation has some impacts on aspects of ecosystem functioning in a sub-tropical seagrass bed

Habitat fragmentation impacts ecosystem functioning in many ways, including reducing the availability of suitable habitat for animals and altering resource dynamics. Fragmentation in seagrass ecosystems caused by propeller scarring is a major source of habitat loss, but little is known about how scars impact ecosystem functioning. Propeller scars were simulated in seagrass beds of Abaco, Bahamas, to explore potential impacts. To determine if plant-herbivore interactions were altered by fragmentation, amphipod grazers were excluded from half the experimental plots, and epiphyte biomass and community composition were compared between grazer control and exclusion plots. We found a shift from light limitation to phosphorus limitation at seagrass patch edges. Fragmentation did not impact top-down control on epiphyte biomass or community composition, despite reduced amphipod density in fragmented habitats. Seagrass and amphipod responses to propeller scarring suggest that severely scarred seagrass beds could be subject to changes in internal nutrient stores and amphipod distribution.

Reactive oxygen species as a marine grazing defense: H2O2 and wounded Ascoseira mirabilis both inhibit feeding by an amphipod grazer

Marine macroalgae can release reactive oxygen species (ROS) upon wounding and grazing. Here we address the potential role of ROS in herbivore defense. We performed feeding assays in the presence of varying concentrations of hydrogen peroxide (H2O2), a common type of ROS. H2O2 inhibited feeding by a marine amphipod grazer, Gondogeneia antarctica, over broad levels of concentration, and its potency was strongly dependent on its rate of decay in natural seawater. Because it is possible that the inhibitory levels of H2O2 are encountered in the vicinity of a sympatric macroalgal wound, we suggest that H2O2 has the potential to act as a direct anti-grazing defense in marine ecosystems. Since some sympatric macroalgae release a burst of non-H2O2 ROS, we also performed experiments to evaluate the role of these naturally-produced ROS on sympatric grazers. The presence of wounded Ascoseira mirabilis (which releases a burst of non-H2O2 ROS after wounding) during a feeding assay inhibited feeding of G. antarctica compared to the presence of intact A. mirabilis. These data are consistent with a role for ROS as a direct anti-herbivore defense in nature. However, the data are also consistent with hypotheses that involve other putative activated anti-grazing defenses.

Epifaunal community structure and ammonium uptake compared for the invasive algae, Gracilaria salicornia and Acanthophora specifera, and the native alga, Padina thivyi

Macroalgae play an important role in coastal ecosystems by taking up nutrients from the water column and providing habitats and food for epifauna. Invasion of alien macroalgae can displace native macroalgae and cause habitat modification. This may result in changes in the capacity of the community to remove nutrients and the structure of epifaunal assemblages in the area. In Hawaii, the alien invasive macroalgae, Gracilaria salicornia and Acanthophora specifera have been the focus of removal projects in recent years, but little has been studied about their ecological impacts and functions. We investigated potential impacts of these invasive algae by comparing their capacity for ammonium uptake and their epifaunal community structure to those of the native alga Padina thivyi. The capacities of G. salicornia and P. thivyi to remove ammonium as individuals (per gram nitrogen in algae) were comparable. However, ammonium uptake as a community was, on average, higher in the invasive-dominated canopies than in the native-dominated canopies, probably due to a higher density (biomass per sediment surface area) of G. salicornia than P. thivyi. Invasive canopies harbored higher abundances and species richness of epifauna (per sediment surface area) than native canopies. Most of the numerically dominant epifauna in the present study were detritivores and were associated with G. salicornia. Amphipod grazers were the only numerically dominant taxon whose abundance was positively correlated with the biomass of P. thivyi. A. specifera had no apparent effects on epifaunal distributions. This study showed facilitative effects of G. salicornia, but the difference in the epifaunal community structure (i.e. detritivores in the invasive canopies vs. herbivores in the native canopies) should be treated as a potential ecological concern and calls for further investigations of how nutrients taken up by these algal species are cycled in the food web.

Community Interactions Modify the Effects of Pharmaceutical Exposure: A Microcosm Study on Responses to Propranolol in Baltic Sea Coastal Organisms

This study investigated the uptake and effects of a common human pharmaceutical, propranolol, on the structure and function of a coastal Baltic Sea model community consisting of macroalga (Ceramium tenuicorne), mussels (Mytilus edulis trossulus), amphipods (Gammarus spp.), water and sediment. The most sensitive species, the mussel, was affected to the same extent as in previous single species studies, while the effects on the amphipod and alga were smaller or even positive compared to experiments performed in less complex test systems. The observed cascade of beneficial effects was a result of inter-specific species interactions that buffered for more severe effects. The poor condition of the mussel led to a feeding shift from alga to mussel by the amphipods. The better food quality, due to the dietary shift, counteracted the effects of the exposure. Less amphipod grazing, together with increased levels of nutrients in the water was favourable for the alga, despite the negative effects of propranolol. This microcosm study showed effects on organisms on different organizational levels as well as interactions among the different components resulting in indirect exposure effects of both functional and structural nature. The combination of both direct and indirect effects would not have been detected using simpler single- or even two-species study designs. The observed structural changes would in the natural environment have a long-term influence on ecosystem function, especially in a low-biodiversity ecosystem like the Baltic Sea.

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

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

Non-consumptive predator effects indirectly influence marine plant biomass and palatability

Summary 1. Predators can reduce herbivory and increase plant biomass by consuming herbivores, lowering individual herbivore feeding rates, or both. We tested whether the presence of predators increases plant quality by non-consumptively reducing grazing pressure and thereby weakening the strength of the induced response in plant chemical defences. 2. We performed a 42-day outdoor mesocosm experiment in which the herbivorous amphipod Ampithoe longimana was cultured on the brown seaweed Sargassum filipendula in the presence and absence of olfactory cues of its principal fish predator, the pinfish Lagodon rhomboides. The presence of fish cues reduced per capita rates of amphipod grazing by nearly 50%. Over the span of the mesocosm experiment, this per capita reduction in feeding rate yielded at least a 40% lower growth rate of amphipod populations (i.e. r reduced from 1.01 to 0.61). The lower rates of amphipod grazing (overall or per capita) correlated with higher algal biomass. 3. We pursued a series of laboratory-based feeding choice assays with naive amphipods to determine tissue palatability and the plant traits that mediate feeding choices. Tissue from tanks without grazers was more palatable than tissue from tanks with grazers, a pattern of induced plant defences that has been documented previously. Surprisingly, however, plant tissue from tanks with grazers and fish cues was more palatable than tissue from tanks with grazers but without fish cues. All changes in algal palatability were mediated by polar, but not lipophilic metabolites. These results suggest that the non-consumptive effects of fish predators increases the food quality of Sargassum by weakening the strength of its induced chemical defences. 4.Synthesis. The smell of predators has the potential to regulate herbivore populations and affect the ecological dynamics of plant biomass and chemical defences.

Omnivory and grazer functional composition moderate cascading trophic effects in experimental Fucus vesiculosus habitats

We tested the relative strength of direct versus indirect effects of an aquatic omnivore depending on the functional composition of grazers by manipulating the presence of gastropod and amphipod grazers and omnivorous shrimp in outdoor mesocosms. By selectively preying upon amphipods and reducing their abundance by 70–80%, omnivorous shrimp favoured the dominance of gastropods. While gastropods were the main microalgal grazers, amphipods controlled macroalgal biomass in the experiment. However, strong predation on the amphipod by the shrimp had no significant indirect effects on macroalgal biomass, indicating that when amphipod abundances declined, complementary feeding by the omnivore on macroalgae may have suppressed a trophic cascade. Accordingly, in the absence of amphipods, the shrimp grazed significantly on green algae and thereby suppressed the diversity of the macroalgal community. Our experiment demonstrates direct consumer effects by an omnivore on both the grazer and producer trophic levels in an aquatic food web, regulated by prey availability.Electronic supplementary materialThe online version of this article (doi:10.1007/s00227-010-1602-6) contains supplementary material, which is available to authorized users.

IMPACTS OF MESOGRAZERS ON EPIPHYTE AND ENDOPHYTE GROWTH ASSOCIATED WITH CHEMICALLY DEFENDED MACROALGE FROM THE WESTERN ANTARCTIC PENINSULA: A MESOCOSM EXPERIMENT(1).

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

Density- and trait-mediated effects of fish predators on amphipod grazers: potential indirect benefits for the giant kelp Macrocystis pyrifera

Indirect effects of predators on primary producers may arise when predators suppress herbivore abundance and/or alter the behaviour patterns of herbivores in ways that reduce grazing pressure. Most studies highlight the role of predators in reducing herbivore abundance (i.e. density- mediated interactions), while behavioural effects (i.e. trait-mediated interactions) induced by preda- tors are less commonly considered and are often assumed to be of secondary importance. We used a mesocosm experiment to evaluate the consumptive and behavioural effects of 2 species of predatory fishes on amphipod grazers that feed and nest on the giant kelp Macrocystis pyrifera. Only one, Noto- labrus celidotus, of the 2 predator species reduced grazer abundance. Although the second predator species, N. fucicola, did not affect grazer abundance directly, it significantly reduced the grazing effi- ciency of amphipods on blades of M. pyrifera. Our study illustrates how density- and trait-mediated interactions reduce grazer efficiency and provide potential indirect benefits to primary producers. To our knowledge, this is the first experimental evidence of a potential trophic cascade in kelp beds gen- erated by consumptive and nonconsumptive effects. When considered alongside results of other studies that highlight positive effects of macroalgae on reef fish demographic rates, our results pro- vide compelling evidence that mutualistic relationships may exist between kelp and associated predatory fishes.

UV-radiation versus grazing pressure: long-term floating of kelp rafts (Macrocystis pyrifera) is facilitated by efficient photoacclimation but undermined by grazing losses

Large quantities of floating macroalgae are traveling in coastal waters of the SE Pacific and in other temperate climate zones. While afloat, these algae are potentially exposed to full solar radiation, including UVA and UVB, which can have profound effects on their physiological and growth performance. Latitudinal variations in UV-radiation (UVR) are hypothesized to affect floating algae differently with higher impacts at low latitudes than at high latitudes. In addition, UVR together with grazing might accelerate the demise of floating kelps. This hypothesis was tested with outdoor laboratory experiments in which sporophytes of the giant kelp Macrocystis pyrifera (L.) C. Agardh were exposed to a combination of different UVR regimes (PAR only, PAR + UV) and grazing at three sites along the Chilean coast (20°S, 30°S, and 40°S). A latitudinal trend in irradiance was detected with increasing values from 40°S to 20°S. Surprisingly, floating M. pyrifera responded with a high acclimation potential within this latitudinal UVR gradient. At 20°S, floating kelps were slightly sensitive to UVR, which was reflected in reduced blade growth. At 30°S, physiological responses were hardly affected by the prevailing irradiance but sporophyte growth and thus persistence mainly depended on the presence or absence of amphipod grazers. At high latitudes, grazing had only minor impacts on algal biomass and blade growth, and kelps thrived well under all tested environmental conditions. Overall, our results reveal that floating M. pyrifera was only slightly affected by UVR and that sporophytes can efficiently acclimate over a latitudinal UVR gradient that spans from 20°S to 40°S. Given this high acclimation potential, we suggest that these (and possibly other) positively buoyant algae are important dispersal agents over a wide range of temperate latitude conditions.

Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 406:33-45 (2010) - DOI: https://doi.org/10.3354/meps08533 Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows Lesley P. Baggett1,2,*, Kenneth L. Heck Jr.1,2, Thomas A. Frankovich3, Anna R. Armitage3,4, James W. Fourqurean3 1Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA 2Department of Marine Science, University of South Alabama, Mobile, Alabama 36688, USA 3Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International University, 3000 NE 151st Street, North Miami, Florida 33181, USA 4Department of Marine Biology, Texas A&M University at Galveston, PO Box 1675, Galveston, Texas 77553, USA *Email: lbaggett@disl.org ABSTRACT: We tested the relative importance of top-down and bottom-up effects by experimentally evaluating the combined and separate effects of nutrient availability and grazer species composition on epiphyte communities and seagrass condition in Florida Bay. Although we succeeded in substantially enriching our experimental cylinders, as indicated by elevated nitrogen concentrations in epiphytes and seagrass leaves, we did not observe any major increases in epiphyte biomass or major loss of Thalassia testudinum by algal overgrowth. Additionally, we did not detect any strong grazer effects and found very few significant nutrient-grazer interactions. While this might suggest that there was no important differential response to nutrients by individual grazer species or by various combinations of grazers, our results were complicated by the lack of significant differences between control and grazer treatments, and as such, these results are best explained by the presence of unwanted amphipod grazers (mean = 471 ind. m–2) in the control cylinders. Our estimates of grazing rates and epiphyte productivities indicate that amphipods in the control cylinders could have lowered epiphyte biomass to the same level that the experimental grazers did, thus effectively transforming the control treatments into grazer treatments. If so, our experiments suggest that the effects of invertebrate grazing (and those of amphipods alone) were stronger than the effects of nutrient enrichment on epiphytic algae, and that it does not require a large density of grazers to control epiphyte biomass even when nutrient loading rates are substantially elevated. KEY WORDS: Nutrient enrichment · Grazing · Seagrass · Epiphytes · Gastropods · Caridean shrimp · Hermit crab Full text in pdf format PreviousNextCite this article as: Baggett LP, Heck KL Jr, Frankovich TA, Armitage AR, Fourqurean JW (2010) Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows. Mar Ecol Prog Ser 406:33-45. https://doi.org/10.3354/meps08533 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 406. Online publication date: May 10, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

EFFECT OF TEMPERATURE AND GRAZING ON GROWTH AND REPRODUCTION OF FLOATING MACROCYSTIS SPP. (PHAEOPHYCEAE) ALONG A LATITUDINAL GRADIENT(1).

Macroalgal rafts frequently occur floating in coastal waters of temperate regions of the world's oceans. These rafts are considered important dispersal vehicles for associated organisms with direct development. However, environmental factors may limit the floating potential of kelp and thereby the dispersal of associated organisms. To examine the effect of water temperature and grazing on growth, reproductive output, and survival of floating Macrocystis spp., experiments were conducted in outdoor tanks during austral summer 2006/2007 at three sites along the Chilean Pacific coast (20° S, 30° S, 40° S). At each site, Macrocystis spp. was maintained individually at three different water temperatures (ambient, ambient - 4°C, ambient + 4°C) and in the presence or absence of the amphipod grazer Peramphithoe femorata for 14 d. High water temperatures (>20°C) provoked rapid degradation of Macrocystis spp. rafts. At moderate temperatures (15°C-20°C), algal survival depended on the presence of associated grazers. In the absence of grazers, algal rafts gained in biomass while grazing caused considerable losses of algal biomass. Algal survival was the highest under cooler conditions (<15°C), where raft degradation was slow and grazer-induced biomass losses were compensated by continuing algal growth. Our results indicate that floating kelp rafts can survive for long time periods at the sea surface, but survival depends on the interaction between temperature and grazing. We suggest that these processes limiting the survival of kelp rafts in warmer temperatures may act as a dispersal barrier for kelp and its associated passengers.

Consequences of light reduction for anti-herbivore defense and bioactivity against mussels in four seaweed species from northern-central Chile

It is widely assumed that the production of secondary metabolites against grazing and fouling is costly for seaweeds in terms of metabolic energy and should therefore be reduced under conditions of resource limitation. Here we tested the hypothesis that anti-herbivore defenses and bioactivity against mussels in 4 brown seaweeds from northern-central Chile will be reduced when light is limited. In a 2 wk experiment, seaweeds were kept under different low-light conditions (~76 to 99% reduction of ambient sunlight) and grazing situations. Subsequently, we tested their anti- herbivore defense against a common amphipod grazer in feeding assays with living algal tissue and reconstituted food pellets. A standard test employing the production of byssus threads by mussels was furthermore used as an indicator for deterrents in crude algal extracts. All investigated seaweeds showed decreased growth under the stepwise light reduction. Lessonia nigrescens exhibited reduced defense ability under severe low-light conditions when living tissue was offered to the amphipod, probably caused by changes in the tissue structure or in nutritional traits. In Dictyota kunthii, L. tra- beculata and Macrocystis integrifolia this effect was absent. None of the investigated seaweeds showed a clear effect of light reduction on chemically mediated defenses against the mesograzer and there was no effect of light limitation on the bioactivity against mussels. Thus, against general assumptions, chemical defense in the investigated seaweeds does not appear to be reduced under severe resource limitation. Results suggest that seaweeds may use different strategies of energy allo- cation to cope with low-light conditions.

Factors influencing temporal variation of a Sargassum filipendula (Phaeophyta: Fucales) bed in a subtropical shore

In the present study, we evaluate the influence of biotic and abiotic factors on temporal fluctuations of Sargassum filipendula in a subtropical shore. Monthly algal samples, abiotic components, amphipod grazer density, and epiphyte biomass were obtained from a Sargassum bed in south-eastern Brazil. Density of S. filipendula fronds decreased during the sampling period, whereas dry mass was more constant, although with a noticeable reduction in the warmer months. Hypnea musciformis was the most frequent epiphyte on S. filipendula, occurring in all sampling periods, although with significant temporal variation. Sargassum filipendula density and dry mass were both influenced by epiphyte dry mass, temperature, and amphipod grazers. Sargassum filipendula biomass negatively influenced total epiphyte biomass, whereas H. musciformis biomass was positively influenced by phosphate, nitrite, and S. filipendula density and negatively influenced by S. filipendula dry mass and amphipod grazer abundance. Algal temporal fluctuations can be related to local abiotic and biotic factors, but the variation observed for S. filipendula and its epiphytes suggest that these factors have quite distinct effects for these algae.

Species-specific defense strategies of vegetative versus reproductive blades of the Pacific kelps Lessonia nigrescens and Macrocystis integrifolia

Chemical defense is assumed to be costly and therefore algae should allocate defense investments in a way to reduce costs and optimize their overall fitness. Thus, lifetime expectation of particular tissues and their contribution to the fitness of the alga may affect defense allocation. Two brown algae common to the SE Pacific coasts, Lessonia nigrescens Bory and Macrocystis integrifolia Bory, feature important ontogenetic differences in the development of reproductive structures; in L. nigrescens blade tissues pass from a vegetative stage to a reproductive stage, while in M. integrifolia reproductive and vegetative functions are spatially separated on different blades. We hypothesized that vegetative blades of L. nigrescens with important future functions are more (or equally) defended than reproductive blades, whereas in M. integrifolia defense should be mainly allocated to reproductive blades (sporophylls), which are considered to make a higher contribution to fitness. Herein, within-plant variation in susceptibility of reproductive and vegetative tissues to herbivory and in allocation of phlorotannins (phenolics) and N-compounds was compared. The results show that phlorotannin and N-concentrations were higher in reproductive blade tissues for both investigated algae. However, preferences by amphipod grazers (Parhyalella penai) for either tissue type differed between the two algal species. Fresh reproductive tissue of L. nigrescens was more consumed than vegetative tissue, while the reverse was found in M. integrifolia, thus confirming the original hypothesis. This suggests that future fitness function might indeed be a useful predictor of anti-herbivore defense in large, perennial kelps. Results from feeding assays with artificial pellets that were made with air-dried material and extract-treated Ulva powder indicated that defenses in live algae are probably not based on chemicals that can be extracted or remain intact after air-drying and grinding up algal tissues. Instead, anti-herbivore defense against amphipod mesograzers seems to depend on structural traits of living algae.

Survival strategies in Polysiphonia adamsiae and P. strictissima (Rhodophyta, Rhodomelaceae) subjected to sediment deposition and grazing pressure

Adaptive strategies of Polysiphonia adamsiae and P. strictissima living in conditions subject to sand deposition and amphipod grazing were investigated by in situ field observations and experimental culture in southern New Zealand between September 2001 and July 2003. The findings from this study showed that for both species, branches regenerated from prostrate axes and fragments grew 7 to 10 times faster than sporelings under the same culture conditions. Also, prostrate axes exhibited extensive creeping growth and produced dense upright branches. In contrast, sporelings exclusively grew upright until they reached c. 1–2 cm. These findings indicate that vegetative propagules have a much higher probability of forming algal turf than sporelings. In addition, prostrate axes survived darkness longer than sporelings, implying that algal thalli might be protected from amphipod grazing by sediments. This study shows that vegetative modes of reproduction predominated over sporic modes in both species. Vegetative reproduction might be advantageous for thalli to survive amphipod grazing under turf‐trapped sand.

INDUCTION AND REDUCTION OF ANTI‐HERBIVORE DEFENSES IN BROWN AND RED MACROALGAE OFF THE KENYAN COAST1

Herbivory is particularly intense in tropical benthic communities, suggesting preference of constitutive, rather than inducible, anti‐herbivory defense. The objective of the study was to examine whether anti‐herbivore defenses in the red alga Hypnea pannosa J. Agardh and the brown algae Sargassum asperifolium Hering and G. Martens ex J. Agardh and Cystoseira myrica (S.G. Gmelin) C. Agardh could be induced and subsequently reduced in response to grazing by the amphipod Cymadusa filosa Savigny. During a 14‐day treatment phase, algae were exposed to amphipod grazing or were left ungrazed (control). Subsequently, one subset of algae was used in feeding assays, whereas another was cultivated for additional 14 days without consumers (recovery phase). At the end of each phase, bioassays were conducted to detect defensive traits in terms of differences in consumption rates of grazed and control pieces of live algae and agar‐based food containing nonpolar algal extracts. Consumption of grazed live S. asperifolium and H. pannosa specimens was lower than of control algae. Furthermore, nonpolar extracts of grazed S. asperifolium and C. myrica were less preferred than those from control algae. Defensive responses were exclusively detected after the treatment phase, although strong preference of ungrazed H. pannosa and C. myrica over grazed conspecifics continued throughout the recovery phase. These findings suggest that phenotypic plasticity in anti‐herbivory defense of marine macroalgae 1) might be more common than previously shown, 2) could be switched on and off within 2 weeks, and 3) can be found in nonpolar algal extracts.