Numerous small sedentary herbivores (mesograzers such as amphipods, small crabs, and gastropods) are resistant to seaweed secondary metabolites that deter larger, more mobile herbivorous fishes. In addition, specialist mesograzers experience reduced predation from fishes when living on seaweeds that produce these compounds. In this study we tested the hypothesis that generalist, as opposed to specialist, mesograzers can also benefit from reduced predation when they occupy chemically defended plants. Secondly, we assessed the hypothesis that low herbivore mobility, unconfounded by herbivore size or specialized feeding, selects for tolerance of seaweed chemical defenses, by comparing responses to the chemically defended brown seaweed Dictyota menstrualis of three sympatric, generalist amphipods that differ in mobility (Ampithoe longimana, Ampithoe valida, and Gammarus mucronatus). Response to Dictyota's chemical defenses varied as much among these three amphipods as among the phylogenetically distant fishes and mesograzers studied previously and supported the hypothesis that less mobile herbivores should be most tolerant of plant chemical defenses. In laboratory experiments, A. longimana moved little, preferentially consumed Dictyota over other seaweeds, and was unaffected by all Dictyota secondary metabolites tested. In contrast, G. mucronatus was active, it did not feed on Dictyota, and two of three Dictyota secondary metabolites deterred its grazing. Distribution of amphipods in the field suggested that these feeding patterns affected amphipod risk of predation. A. longimana reached its highest abundance on Dictyota, which is unpalatable to omnivorous fish predators, during the season when fish are most abundant. At the same time, the highly active G. mucronatus decreased to near extinction. Like G. mucronatus, A. valida was detterred by two Dictyota secondary metabolites, did not eat Dictyota, and disappeared when fishes were abundant. Experiments confirmed that A. longimana was less vulnerable to fish predation when occupying a chemically defended seaweed than when occupying a palatable seaweed. This decreased predation resulted primarily from a decreased frequency of encounter with predators when amphipods were on chemically defended plants. When we experimentally equalized encounter rates between omnivorous pinfish (Lagodon rhomboides) and the seaweed Dictyota menstrualis and Ulva curvata (unpalatable and palatable, respectively, to pinfish) in the laboratory, amphipods occupying these two plants were eaten at similar rates. In contrast, when live amphipods were affixed to Ulva and Dictyota and deployed in the field, amphipods survived only on Dictyota. Heavy fish grazing on Ulva in the latter experiment suggests that poor survival of amphipods on Ulva may have resulted from greater detection and/or incidental ingestion of amphipods on this plant, due to frequent visitation by fishes. Infrequent visitation of Dictyota by foraging fish also may explain A. longimana's persistence through the summer on this chemically defended seaweed while the two Ulva—associated amphipods declined precipitously. These results (1) confirm that association with chemically defended plants can reduce predation on generalist, as well as specialist, herbivores and (2) suggest that preferential feeding on chemically defended plants is most likely for sedentary mesograzers because low mobility enhances the ability to exploit chemically defended seaweeds as refuges from fish predation.
Part I: Processes Influencing Pattern in Benthic Communities Physical Processes that Generate Pattern in Marine Communities Geological History of the Living Shore Biota Genetic Structure in the Sea: From Populations to Communities Natural Disturbance and the Dynamics of Marine Benthic Communities The Ecology and Evolution of Marine Consumer-Prey Interactions The Larval Ecology of Marine Communities Supply Side Ecology: The Nature and Consequences of Variations in Recruitment of Intertidal Organisms Habitat Modification and Facilitation in Benthic Marine Communities Part II: Community Types Soft Sediment Communities Salt Marsh Communities Seagrass Community Ecology Rocky Subtidal Communities Deep-Sea Communities The Ecology of Coral Reefs Mangrove Communities Part III: Marine Community Conservation Issues Human Alterations of Marine Communities Caveat Studium Conservation and Management of Marine Communities The Ecology of Marine Protected Areas
When different growth phases (exponential or stationary) or forms (solitary cells or colonies) of a single clone of Phaeocystis globosa were fed to three copepods, grazing, measured indirectly by fecal-pellet production, on different types of P. globosa differed by nearly two orders of magnitude, with differences on this clone sometimes exceeding differences between different phytoplankton species. The copepods Pseudodiaptomus pelagicus and Eucalanus pileatus fed more on colonies than solitary cells, with P. pelagicus also feeding more on exponential than on stationary cultures. Feeding by Acartia tonsa was complex and dramatically more variable. A. tonsa consumed 16–92 times more P. globosa when feeding on stationary-phase colonies than on any other P. globosa cell type. It fed five times more on stationary-phase colonies of P. globosa than on the palatable Rhodomonas baltica, but fed more on R. baltica than on other stages of P. globosa. Diet effects on copepod fitness were not related to amounts of foods consumed. Survivorship of A. tonsa and E. pileatus did not differ on any of the P. globosa cell types, but survivorship of P. pelagicus was suppressed on colonies (which they consumed more of) versus solitary cells. A. tonsa consumed 30 times more stationary-phase colonies than exponential-phase solitary cells, but produced two times more eggs on the lesser consumed food. Dramatic consumption of stationary-phase colonies may occur because this is a low-quality food and A. tonsa attempts to compensate by consuming more. The limited consumption of other types is suggestive of chemical defenses that may be compromised when colonies enter stationary phase.
A table listing the biomass of each herbivorous fish species observed inside and outside of no-take marine reserves at three locations in Fiji.
ABSTRACT In the laboratory, we documented large variation in the morphology, toxicity, and maximum population growth rates for 32 Microcystis aeruginosa strains isolated from 12 lakes. Growth rates and mean colony sizes varied significantly across strains and were positively correlated. However, growth rates were unrelated to toxin production.
A table showing calories for palatable and unpalatable worms and worm body parts.
Species-specific enemies may promote prey coexistence through negative distance- and density-dependent survival of juveniles near conspecific adults. We tested this mechanism by transplanting juvenile-sized fragments of the brooding corals Pocillopora damicornis and Seriatopora hystrix 3, 12, 24 and 182 cm up- and down-current of conspecific adults and monitoring their survival and condition over time. We also characterized the spatial distribution of P. damicornis and S. hystrix within replicate plots on three Fijian reef flats and measured the distribution of small colonies within 2 m of larger colonies of each species. Juvenile-sized transplants exhibited no differences in survivorship as a function of distance from adult P. damicornis or S. hystrix . Additionally, both P. damicornis and S. hystrix were aggregated rather than overdispersed on natural reefs. However, a pattern of juveniles being aggregated near adults while larger (and probably older) colonies were not suggests that greater mortality near large adults could occur over longer periods of time or that size-dependent mortality was occurring. While we found minimal evidence of greater mortality of small colonies near adult conspecifics in our transplant experiments, we did document hot-spots of species-specific corallivory. We detected spatially localized and temporally persistent predation on P. damicornis by the territorial triggerfish Balistapus undulatus . This patchy predation did not occur for S. hystrix . This variable selective regime in an otherwise more uniform environment could be one mechanism maintaining diversity of corals on Indo-Pacific reefs.
Investigators working in both terrestrial and marine systems hypothesize that prey defenses are better developed in tropical than in temperate habitats because tropical consumers are more active and diverse. To investigate the hypothesis that chemical defenses are more potent in tropical than in temperate seaweeds, we conducted 38 feeding assays involving seven genera of algae. We offered tropical and temperate sea urchins the choice of a temperate alga from North Carolina or a closely related tropical alga from the Bahamas. Plants were freeze—dried, ground to a fine powder, embedded in agar, and presented in congeneric pairs (North Carolina species vs. Bahamian species), or same—species pairs, to the temperate urchin Arbacia punctulata and the tropical urchin Lytechinus variegatus. For the genera we tested (the red alga Amphiroa, the green alga Udotea, and the brown algae Dictyopteris, Dictyota, Lobophora, Padina, and Sargassum), we documented a general pattern in which the mean amount of North Carolina seaweeds eaten was approximately twice that of their Bahamian congeners. Dictyota was an exception to the overall pattern, in that mean consumption of temperate and tropical species was equivalent. In addition to showing a latitudinal difference in seaweed palatability, we found that some species exhibited significant, between—population variation in herbivore resistance on a local scale. The most notable example was Dictyota menstrualis from North Carolina. When collected from a shallow inshore site, this species was significantly more palatable than tropical species of Dictyota in four of six contrasts. When collected from a deeper offshore site, it was significantly less palatable than the tropical species in four of six contrasts. Thus, for this species, the between—population variance in palatability within a geographic region (North Carolina) bracketed the variance in palatability that we found among species and between regions. Feeding assays with both temperate and tropical urchins produced similar results, indicating that choices were made based on fundamental characteristics of the algae rather than on greater familiarity with local prey species. To see if chemical defenses could explain the differences in herbivore resistance that we noted using freeze—dried algae, we tested the effects of lipid—soluble and water—soluble extracts from each alga on urchin feeding by incorporating extracts into a standard palatable food. We also measured protein content and percent ash—free dry mass of the seaweeds in order to correlate urchin feeding preferences with these measures of food quality. Lipophilic chemical extracts explained urchin preferences in 9 of 15 (60%) North Carolina—Bahamas contrasts. Water—soluble extracts, protein content, and percent ash—free dry mass generally did not account for feeding patterns seen in the original assays. Our results support the general hypothesis that tropical seaweeds have stronger chemical defenses than temperate seaweeds. For some genera, however, smaller—scale local variability in herbivore resistance within a species can be as striking as overall latitudinal differences.
