A table showing percentage consumption of northern vs. southern congeners for the larger freeze-dried data set.
Tropical reefs are commonly transitioning from coral to macroalgal dominance, but the role of macroalgae in coral decline remains inadequately understood. A growing body of research suggests that algae may harm corals via disruptions to the homeostasis of the coral holobiont, including resident microbial communities, but the processes that mediate these potential microbial effects and the spatial scales at which they operate are uncertain. Resolving the relative importance and context dependencies of microbially-mediated algal-coral competition is critical for understanding and predicting coral dynamics as reefs further degrade. In this review, we examine the current state of knowledge surrounding algal impacts on corals via disruption of their microbiomes, with a particular focus on the mechanisms hypothesized to mediate microbial effects, the scales at which they are thought to operate, and the evidence from laboratory- and field-based studies for their existence and ecological relevance in the wild. Lastly, we highlight challenges for further advancing the field.
The Structurally similar diterpenoid alcohols pachydictyol—A and dictyol—E are produced by the brown seaweed Dictyota dichotoma. This seaweed and several related species that also produce these compounds are known to be relatively low preference foods for tropical fishes and urchins. We evaluated the effect of various concentrations of these compounds on feeding by the three common types of herbivores that co—occur with Dictyota in coastal North Carolina. Fish (Diplodus holbrooki), sea urchins (Arbacia punctulata), and a mixed species group of gammarid amphipods were offered pieces of the palatable seaweed Gracilaria tikvahiae coated with either (1) dictyol—E or pachydictyol—A dissolved in diethyl ether or (2) diethyl ether alone. Dictyol—E significantly reduced consumption by fish and urchins at concentrations of 0.5 and 1.0% of algal dry mass, but had no effect on amphipod grazing. Pachydictyol—A significantly reduced fish grazing at the relatively high concentrations of 1.0 and 1.3% of plant dry mass; at 0.5% it tended to decrease grazing, but the effect was not significant (P = .07). Pachydictyol—A had no effect on urchin grazing and significantly increased amphipod grazing. When Pachydictyol—A was fed to fish as 1.0% of food dry mass, their growth rate was reduced by a significant 48%. In feeding preference experiments with several seaweeds, Dictyota ranks low for fish and urchins but high for amphipods. This is consistent with the hypothesis that the secondary metabolites produced by Dictyota play a major role in determining its susceptibility to herbivores. The ability of amphipods to circumvent the chemical defenses (Dictyota, and the fact that the two species of algae most readily consumed by amphipods (Codium and Dictyota) were the two species least readily consumed by fish, suggest that predation and herbivory by fishes may be major factors selecting for amphipods that can live on, and eat, seaweeds that are unpalatable to fishes. Amphipods that fed on Dictyota did not appear to sequester the Dictyota metabolites; when exposed to fish predation, Dictyota—fed amphipods were eaten as readily as amphipods that had fed on an alga with no defensive chemistry. Tubicolous amphipods and other small marine herbivores that may spend significant portions of their lives on only a few plants my be under very different evolutionary constraints than the larger, more mobile herbivores that commonly moved between many plants. Several characteristics of these smaller, less mobile, and much less studied, marine herbivores suggest that they may be ecologically similar to terrestrial insects and may play a large, but presently unappreciated, role in structuring marine plant communities.
This community profile provides an introduction to the ecology of the communities living on and around rubble structures in the South Atlantic Bight (Cape Hatteras to Cape Canaveral). The most prominent rubble structures in the bight are jetties built at the entrances to major harbors. After an initial discussion of the various kinds of rubble structures and physical factors that affect the organisms associated with them, the major portion of the text is devoted to the ecology of rubble structure habitats. Community composition, distribution, seasonality, and the recruitment patterns of the major groups of organisms are described. The major physical and biological factors affecting the organization of intertidal, sunlit subtidal, and shaded subtidal communities are presented and the potential effects of complex interactions in structuring these communities are evaluated. The profile concludes with a general review of the effects of rubble structures on nearshore sediment dynamics and shoreline evolution. 295 refs., 33 figs., 4 tabs.
Strips of the seagrass Thalassia testudinum were used in a field bioassay to assess herbivory on 11 coral reefs scattered throughout the Caribbean. Patterns of herbivory on overfished reefs in Haiti and the United States Virgin Islands were compared to patterns on seven less fished reefs (United States Virgin Islands, Panama, Honduras, Belize, and the Bahamas). On the overfished reefs, the rate of Thalassia removal increased significantly with depth, urchin densities were high, and urchin grazing was equal to, or greater than, fish grazing in shallow (<10 m deep) habitats. On reefs subject to little fishing pressure, the rate of Thalassia removal decreased with depth, urchin densities were low, and herbivorous fishes were responsible for almost all Thalassia removal. Previous studies assessing the importance of urchin grazing in the Caribbean have been conducted on overfished reefs where urchin densities were unusually high and the density of grazing fish unusually low. It is doubtful that the intensity of urchin—algal and urchin—coral interactions observed on these heavily fished reefs occurs on reefs unaffected by humans.
The susceptibility of 82 species of tropical seaweeds to grazing by herbivorous fishes was assessed on 8 different coral reefs in the Florida Keys, USA.Most species were simultaneously assayed for the presence or absence of unusual secondary rnetabolites and recorded as having either calcified or uncalcified thalli.Both production of secondary rnetabolites and of a calcified thallus were associated with low susceptibility to herbivory.However, the relative importance of calcification versus chemical deterrents cannot be assessed for the calcified species since almost all these also contained secondary metabolites.Eighty-five % of calcified species, but only 39 % of uncalcified species, produced secondary metabohtes.Secondary metabolites were produced by 71 % of the species least suscephble to herbivory ( t 2 5 % eaten) but by only 20 O/ O of the species most susceptible to herbivory (>75 % eaten).Calcified thalh were produced by 50 % of the lowest preference species but by only 9 O/O of the highest preference species.Thus, several common reef seaweeds appear to resist herbivory by relying primarily on chemical deterrents (genera Dictyota, Dilophus, Stypopodium, Lobophora, Avrainvrllea, and some Caulerpa species) but many appear to combine both chemical and morphological defenses (genera Penicillus, Halimeda, Rhipocephalus, Udotea, Amphiroa, and Galaxaura).We suggest that multiple defenses will be common among seaweeds on coral reefs since herbivore diversity is high and it is unlikely that any single defense will be effective against the many types of herbivores that encounter these plants.Thls may account, in part, for the diversity of secondary metabolites produced by some tropical seaweeds.
In North Carolina, the decorator crab Libinia dubia camouflages almost exclusively with the chemically noxious alga Dictyota menstrualis. By placing this alga on its carapace, the crab behaviorally sequesters the defensive chemicals of the plant and gains protection from omnivorous consumers. However, Dictyota is absent north of North Carolina, whereas Libinia occurs as far north as New England. Crabs from three northern locations where Dictyota is absent (Rhode Island, Connecticut, and New Jersey) camouflaged to match their environment, rather than selectively accumulating any one species. When D. menstrualis was offered to crabs from northern sites, they did not distinguish between it and other seaweeds for camouflage, whereas crabs from Alabama and two locations in North Carolina used D. menstrualis almost exclusively. In addition, in winter and spring, when Dictyota was seasonally absent in North Carolina, Libinia selectively camouflaged with the sun sponge Hymeniacidon heliophila, which was chemically unpalatable to local fishes. Thus, southern crabs were consistent specialists on chemically defended species for camouflage, while northern crabs were more generalized. The geographic shift in crab behavior away from specialization coincides with a reported decrease in both total predation pressure and the frequency of omnivorous consumers. These shifts in the nature and intensity of predation pressure may favor different camouflage strategies (generalist vs. specialist), contributing to the observed geographic differences in camouflage behavior.
� Abstract Mutualisms occur when interactions between species produce reciprocal benefits. However, the outcome of these interactions frequently shifts from positive, to neutral, to negative, depending on the environmental and community context, and indirect effects commonly produce unexpected mutualisms that have communitywide consequences. The dynamic, and context dependent, nature of mutualisms can transform consumers, competitors, and parasites into mutualists, even while they consume, compete with, or parasitize their partner species. These dynamic, and often diffuse, mutualisms strongly affect community organization and ecosystem processes, but the historic focus on pairwise interactions decoupled from their more complex community context has obscured their importance. In aquatic systems, mutualisms commonly support ecosystem-defining foundation species, underlie energy and nutrient dynamics within and between ecosystems, and provide mechanisms by which species can rapidly adjust to ecological variance. Mutualism is as important as competition, predation, and physical disturbance in determining community structure, and its impact needs to be adequately incorporated into community theory.
Embryos of the shrimp Palaemon macrodactylus are remarkably resistant to infection by the fungus Lagenidium callinectes , a recognized pathogen of many crustaceans. An Alteromonas sp. bacterial strain consistently isolated from the surface of the embryos, produces 2,3-indolinedione (isatin), a compound that inhibits the pathogenic fungus. If exposed to the fungus, bacteria-free embryos quickly die, whereas similar embryos reinoculated with the bacteria or treated only with 2,3-indolinedione live well. The commensal Alteromonas sp. bacteria protect shrimp embryos from fungal infection by producing and liberating the antifungal metabolite 2,3-indolinedione.
Chemical, structural, and morphological defenses are often adjusted to spatial or temporal patterns of attack Herbivory on seaweeds can be intense, with nearly 100% of production being consumed in some habitats (Carpenter 1986).Seaweeds minimize damage from herbivores by any of three strategies (Lubchenco and Gaines 1981).They can escape in space or time so they do not co-occur with important herbivores or are not detected when they do co-occur.They can deter feeding by herbivores that encounter and recognize the plant.And they can minimize the decrease in fitness that results from herbivore attack.Herbivory's profound effects on the organization of seaweed communities has been extensively reviewed (Carpenter 1986, Dayton 1975, Hay 1985, Lewis 1986, Lubchenco and Gaines 1981).In this article, we describe seaweed characteristics that diminish losses to herbivores.Seaweeds are consumed by a diverse assemblage of herbivores that includes fishes, urchins, gastropods, crabs, and numerous smaller herbivores such as amphipods, isopods, and polychaetes (collectively termed mesograzers).In tropical habitats, fish and urchins have the greatest effect on seaweed community struc-
biodiversity on the genetic, species, and ecosystem levels.TREMENDOUS ATTENTION has been focused on the loss oK and threats to, terrestrial biodiversity.Until very recently (Norse, 1993), much less attention was devoted to marine biodiversity despite the fact that marine systems are larger, older, have a huge impact on global climate, and support nearly twice as many phyla of animals as do terrestrial systems (see Table 2-2 in Norse, 1993).Our reduced attention to marine biodiversity reflects our relative ignorance of marine systems rather than their lack of importance to humans or to the ecosystem functions on which virtually all terrestrial life depends.Humans have spent thousands of years in intimate association with terrestrial biota, but only a few decades using SCUBA and submersibles to explore the world's oceans.In this article we discuss how chemically mediated interactions affect mafine biodiversity and consider the insights that can be gained from understanding the mechanisms involved in these interactions.Biodiversity is often used incorrectly as a synonym of species diversity; however, biodiversity also encompasses genetic and ecosystem diversity (Norse, 1993).The factors that produce and maintain patterns of biodiversity at these different levels are complex and poorly understood.In this review, we argue that secondary metabolites produced by a variety of marine organisms as defenses against consumers, pathogens, and competitors {Hay and Steinberg, 1992: Paul, 1992) can have important complex or indirect efl'ects that alter marine biodiversity on the genetic, species, and ecosystem levels.The role of biogenic structural complexity in affecting species diversity has been extensively studied and is broadly appreciated in both marine and terrestrial communities (Ehrlich and Roughgarden,
There are many examples of macroalgae inducing defence in response to small invertebrate herbivores like amphipods, isopods, and gastropods but few cases of induction in response to vertebrate macrograzers like herbivorous fishes. This may be because larger grazers rapidly consume large quantities of seaweed before induction can occur, thus selecting for constitutive rather than induced defences. Alternatively, the pattern could occur because induction due to feeding by macrograzers is less commonly investigated. In Fiji, field assays with the brown macroalga <i>Sargassum polycystum</i> demonstrated that thalli growing in marine protected areas (MPAs) with abundant herbivorous fishes were significantly less palatable than those growing in adjacent fished areas (non-MPAs) with few herbivorous fishes. This significant preference occurred in 11 of 13 trials over 5 time periods and across 3 pairs of MPAs and spatially associated non-MPAs. This preference was not positively associated with algal nitrogen content or with the toughness of algal fronds. When <i>S. polycystum</i> ramets were taken from the non-MPA and half were partially grazed by fishes while the other half were protected from grazing, new growth from the controls was strongly preferred to new growth from the previously grazed ramets although these fronds originated from the same holdfast. This suggests that <i>S. polycystum</i> upregulates defences (probably chemical) in response to grazing by herbivorous fishes. This is one of the few published examples of induction of macroalgal defence in response to feeding by large, mobile grazers. It is unclear whether induced defences against fishes are rare or just under-investigated.
