John E. Fauth

Oxidative stress and seasonal coral bleaching.

During the past two decades, coral reefs have experienced extensive degradation worldwide. One etiology for this global degradation is a syndrome known as coral bleaching. Mass coral bleaching events are correlated with increased sea-surface temperatures, however, the cellular mechanism underlying this phenomenon is uncertain. To determine if oxidative stress plays a mechanistic role in the process of sea-surface temperature-related coral bleaching, we examined corals along a depth transect in the Florida Keys over a single season that was characterized by unusually high sea-surface temperatures. We observed strong positive correlations between accumulation of oxidative damage products and bleaching in corals over a year of sampling. High levels of antioxidant enzymes and small heat-shock proteins were negatively correlated with levels of oxidative damage products. Corals that experienced oxidative stress had higher chaperonin levels and protein turnover activity. Our results indicate that coral bleaching is tightly coupled to the antioxidant and cellular stress capacity of the symbiotic coral, supporting the mechanistic model that coral bleaching (zooxanthellae loss) may be a final strategy to defend corals from oxidative stress.

Experimental Aquatic Food Webs: Interactions between Two Predators and Two Prey

We performed four replicates of all 16 combinations of the presence and absence of two predators (larvae of the dragonfly Anax junius and adult salamanders Notophthalmus viridescens) and two anuran prey (Bufo americanus and Rana palustris larvae) in an array of artificial temporary ponds. The two species of anurans were introduced at densities high enough to cause density-dependent reductions in survival and body size at metamorphosis and to increase larval period. The two species were in competition when raised together. Notophthalmus reduced the density of Bufo and caused the survivors to metamorphose early and at a small size. Anax caused an even greater reduction in survival but not as strong an acceleration of metamorphosis. Newts also reduced the density of Rana, but survivors benefited by growing rapidly. The effects of Anax were even stronger; the only Rana tadpoles that were able to metamorphose in the 2 mo of the experiment were from ponds in which Anax reduced densities enough to permit rapid growth of the surviving tadpoles.

A molecular biomarker system for assessing the health of coral (Montastraea faveolata) during heat stress.

Abstract: Using a novel molecular biomaker system (MBS), we assessed the physiological status of coral (Montastraea faveolata) challenged by heat stress by assaying specific cellular and molecular parameters. This technology is particularly relevant for corals because heat stress is thought to be an essential component of coral bleaching. This phenomenon is widely believed to be responsible for coral mortality worldwide, particularly during 1997–1998. Specific parameters of coral cellular physiology were assayed using the MBS that are indicative of a nonstressed or stressed condition. The MBS distinguished the separate and combined effects of heat and light on the 2 coral symbionts, a scleractinian coral and a dinoflagellate algae (zooxanthellae). This technology aids in the accurate diagnosis of coral condition because each parameter is physiologically well understood. Finally, the MBS technology is relatively inexpensive, easy to implement, and precise, and it can be quickly adapted to a high-throughout robotic system for mass sample analysis.

Assessing the health of grass shrimp (Palaeomonetes pugio) exposed to natural and anthropogenic stressors: a molecular biomarker system.

Abstract: We developed a molecular biomarker system (MBS) to assess the physiological status of Palaomenetes pugio (grass shrimp) challenged with exposure to heat stress, cadmium, atrazine, and the water-accommodating fraction of either diesel fuel or bunker fuel No. 2. The MBS assayed 9 specific cellular parameters of shrimp that are indicative of a nonstressed or stressed condition: heat-shock protein 60, heat-shock protein 70, αB-crystallin homologue, lipid peroxide, total glutathione level, ubiquitin, mitochondrial manganese superoxide dismutase, metallothionein, and cytochrome P-450 2E homologue. Using these 9 parameters, the MBS can distinguish between the responses to each stressor, and to the nonstressed control conditions. The MBS was able to determine the structural integrity of the cell as defined by protein turnover, protein chaperoning, and lipid composition via lipid peroxide levels, and the status of key metabolic processes such as cytoskeletal integrity and glutathione redox potential. This technology aids in the accurate diagnosis of the health of shrimp because the physiological significance of changes of each parameter is well known. This technology is particularly relevant for environmental monitoring because grass shrimp are used as key indicator species in many estuarine ecosystems. Finally, this system is easy to implement, precise, and can be quickly adapted to an automated high-throughput system for mass sample analysis.

Elevational Patterns of Species Richness, Evenness, and Abundance of the Costa Rican Leaf-Litter Herpetofauna

The abundance, species richness, and evenness of the Costa Rican leaf-litter herpetofauna was estimated during the late wet season of 1985 by quantitative sampling of replicate plots at ten sites encompassing an elevation range of 3 to 1670 m. Species richness was positively correlated with leaf-litter depth, and negatively correlated with elevation. Herpetofaunal density also tended to increase with litter depth and decline with elevation. A strong positive correlation existed between species richness and herpetofaunal density. Evenness was highly variable and independent of both leaf-litter depth and elevation. Analysis of a subset of the data, representing an elevational transect from Tortuguero to the Braulio Carrillo National Park Extension, yielded similar results. Tropical leaf-litter reptiles and amphibians appear to be both more diverse and more abundant at lower elevations. Sites with deep leaf litter generally sustain dense and diverse reptile and amphibian populations. Local herpetofaunas typically consist of a few very common species along with a large number of comparatively rare species. REPTILES AND AMPHIBIANS are a major constituent of the fauna inhabiting tropical forest litter. Studies of tropical leaf-litter herpetofaunas have emphasized patterns of abundance and distribution among geographic regions (Lloyd et al. 1968; Heyer & Berven 1973; Scott 1976, 1982; Inger 1980a, b; Heatwole 1982), vegetational zones (Brown & Alcala 1961, Heatwole & Sexton 1966, Heyer 1967), and elevational gradients (Brown & Alcala 1961, Scott 1976) as well as seasonal changes (Lieberman 1982, 1986). It has become generally accepted that leaf-litter reptiles and amphibians are more abundant in the New World Tropics than in Southeast Asia (Scott 1976, Inger 1980b, Duellman & Trueb 1986) although the exact reason remains a subject of debate (May 1980). Similarly, the general consensus is that the abundance of tropical leaf-litter herpetofaunas increases with increasing elevation, while species richness and equitability both decline (Brown & Alcala 1961, Scott 1976, Heatwole 1982, Duellman & Trueb 1986). Scott (1976) attributed this pattern to greater overall forest productivity at intermediate elevations, coupled with increased densities of the most common species at higher elevations. Before attempting to determine the mechanisms responsible for observed patterns of tropical leaf-litter herpetofaunal abundance and diversity, we must document the phenomenon conclusively. Previous quantitative studies have sampled few sites, have been unable to provide replicated samples (Brown & Alcala 1961), or may have confounded site and year effects by sampling different sites in different years (Scott 1976). We surveyed the Costa Rican leaf-litter herpetofauna by quantitatively sampling replicate plots at ten sites, ranging from 3 to 1670 m in elevation, during the late wet season of July-September 1985. Our sampling strategy permitted statistical analyses to determine the relationship between litter depth and elevation, and leaf-litter herpetofaunal richness, evenness and abundance. The results of our study contradict several of the generally accepted patterns of tropical leaf-litter herpetofaunal abundance and diversity.

Interactions Between the Salamander Siren Intermedia and the Keystone Predator Notophthalmus Viridescens

Is the ability to function as a keystone predator a property of a species or an emergent property of the community? We addressed this question in a temporary-pond community where the broken-striped newt Notophthalmus viridescens dorsalis is known to act as a keystone predator on larval anurans. We independently manipulated the initial density of adult Notophthalmus (two or four) and the presence or absence of one adult Siren intermedia in a set of 20 artificial ponds to determine if this additional predator affected the ability of Notophthalmus to function as a keystone predator. Each pond received a diverse assemblage of larval anuran prey: a spring-breeding assemblage of 275 Rana utricularia, 100 Pseudacris crucifer and 25 Bufo americanus tadpoles, and a summer assemblage of 200 Hyla chrysoscelis and 150 Gastro- phryne carolinensis tadpoles. Two additional ponds received neither predator, to assay the outcome of competition among the tadpoles. The effect of the salamander Siren on Notophthalmus survival and fecundity depended on the density of Notophthalmus. At low newt density Siren had no effect on survival, but at high newt density competition reduced the survival and growth rates of Notophthalmus. The density of Notophthalmus and the presence of Siren interacted to determine the fe- cundity of Notophthalmus. At low newt density Siren reduced Notophthalmus reproductive success by preying on larvae. At high newt density Siren indirectly enhanced reproductive success by reducing survival of adults, thus releasing larvae from intraspecific competition and cannibalism. Notophthalmus density had no effect on adult Siren survival or growth rate in this experiment, but other evidence indicates that competition with Notophthalmus reduces the growth rates of Siren. Notophthalmus acted as a keystone predator on the assemblage of spring-breeding anurans; it increased the number of metamorphs of the weak competitor Pseudacris crucifer by releasing them from interspecific competition. Siren preyed on tadpoles in a non- selective manner. An additive model was sufficient to describe the effects of Siren and initial density of Notophthalmus on the structure of the tadpole assemblage. In our system of experimental ponds, the strong interaction between Siren and Notoph- thalmus density did not extend its effects to lower trophic levels, and the direct and indirect effects of Siren did not alter the role of Notophthalmus as a keystone predator.

Local variation in the genetic basis of paedomorphosis in the salamander Ambystoma talpoideum

The hypothesis that local isolated populations differed in the genetic basis for life‐history traits was tested in the salamander Ambystoma talpoideum. Genetic basis was defined as the specific genetic architecture (additive and nonadditive) that contributes, along with maternal and environmental factors, to the phenotype. All crosses within and between three populations were made to produce nine F1 populations. Nine within‐population crosses produced the F2 generation. This design does not permit an estimation of the exact nature of the genetic basis (e.g., additive, nonadditive) for any trait within populations. However, hybrid dissimilarity in the F2 generation was taken as evidence of a different genetic basis for a trait in each population. The genetic basis of life‐history pathway (metamorphosis vs. paedomorphosis) and per capita fecundity differed between two populations. The genetic basis of life‐history pathway, per capita fecundity, survival, and growth rate was similar between the remaining sets of populations. This study and related ones (Semlitsch and Wilbur, 1989; Semlitsch et al., 1990) suggest that a heterochronic shift that causes rapid morphological evolution between metamorphosis and paedomorphosis (a macroevolutionary pattern) can evolve independently and does not require a macromutation or other nonmicroevolutionary mechanisms.

A molecular biomarker system for assessing the health of gastropods (Ilyanassa obsoleta) exposed to natural and anthropogenic stressors

We developed a Molecular Biomarker System (MBS) to assess the physiological status of mud snails (Ilyanassa obsoleta) challenged by exposure to high temperature, cadmium, atrazine, endosulfan and the water-accommodating fraction of bunker fuel #2. The MBS is used to assay specific cellular parameters of the gastropod cell that are indicative of a non-stressed or stressed condition. The MBS distinguished among responses to each stressor and to non-stressed control conditions. For example, the biomarkers metallothionein and cytochrome P450 2E1 homologue distinguished between metal and non-metal stresses. MBS data from this study corroborate toxicological studies of organismal responses to endosulfan, atrazine, fuel and cadmium stresses. The MBS technology aids in the accurate diagnosis of the snails health condition because the physiological significance of the changes of each biomarker is well known. This technology is particularly relevant for environmental monitoring because gastropods are used as key indicator species in many estuarine, marine, freshwater and terrestrial ecosystems. Finally, the Molecular Biomarker System technology is relatively inexpensive, easy to implement, precise and can be quickly adapted to an automated, high-throughput system for large sample analysis.

Interactive Effects of Predators and Early Larval Dynamics of the Treefrog Hyla Chrysocelis

Ecologists need to determine the frequency and nature of higher order interactions and to predict when such interactions will be important to understand community dynamics. I experimentally tested for higher order interactions in a freshwater food web by independently manipulating the presence and absence of two predators (the salamander Notophthalmus viridescens and the crayfish Cambarus bartonii) and their tadpole prey (Hyla chrysoscelis) in a series of artificial ponds. By destructively sampling selected ponds early in the larval period (days 5 and 6), I hoped to predict whether the predators would have an interactive effect on tadpole survival and growth on day 30. Early destructive samples adequately predicted subsequent influences of interactions on the survival, but not on the size of Hyla. Early samples predicted that the predators would interact to determine tadpole survival and size. The two predators did not act additively to determine tadpole abundance: more survived in the presence of both predators than was predicted. This higher order interaction was most likely caused by physical interference between the predators and predator avoidance by the tadpoles. Early samples correctly predicted higher order interactions would be seen on tadpole growth, but completely failed to predict their direction because the biological interactions changed over time. Early in their larval period, tadpoles behaviorally avoided predators, which caused slower growth, but later, tadpoles in predator treatments grew faster due to diminished intraspecific competition. These results suggest that higher order interactions may be important in nature. Short—term experiments may prove useful in predicting when higher order interactions will occur, but detailed models may be required to predict their outcome accurately.

Heat-stress and light-stress induce different cellular pathologies in the symbiotic dinoflagellate during coral bleaching

Coral bleaching is a significant contributor to the worldwide degradation of coral reefs and is indicative of the termination of symbiosis between the coral host and its symbiotic algae (dinoflagellate; Symbiodinium sp. complex), usually by expulsion or xenophagy (symbiophagy) of its dinoflagellates. Herein, we provide evidence that during the earliest stages of environmentally induced bleaching, heat stress and light stress generate distinctly different pathomorphological changes in the chloroplasts, while a combined heat- and light-stress exposure induces both pathomorphologies; suggesting that these stressors act on the dinoflagellate by different mechanisms. Within the first 48 hours of a heat stress (32°C) under low-light conditions, heat stress induced decomposition of thylakoid structures before observation of extensive oxidative damage; thus it is the disorganization of the thylakoids that creates the conditions allowing photo-oxidative-stress. Conversely, during the first 48 hours of a light stress (2007 µmoles m−2 s−1 PAR) at 25°C, condensation or fusion of multiple thylakoid lamellae occurred coincidently with levels of oxidative damage products, implying that photo-oxidative stress causes the structural membrane damage within the chloroplasts. Exposure to combined heat- and light-stresses induced both pathomorphologies, confirming that these stressors acted on the dinoflagellate via different mechanisms. Within 72 hours of exposure to heat and/or light stresses, homeostatic processes (e.g., heat-shock protein and anti-oxidant enzyme response) were evident in the remaining intact dinoflagellates, regardless of the initiating stressor. Understanding the sequence of events during bleaching when triggered by different environmental stressors is important for predicting both severity and consequences of coral bleaching.