Chemical defenses and the susceptibility of tropical marine brown algae to herbivores

Summary I assayed phenolic and tannin concentrations in a number of species of temperate and tropical brown algae of the genera Sargassum and Turbinaria. Tropical species in both genera contained consistently low levels of phenolics and tannins (species means ranged between 0 and 1.6% [measured as % dry weight of the thallus]). Levels of phenolics in temperate species of Sargassum were variable and consistently much higher than in tropical species (species means ranged between 3 and 12% by dry weight). This pattern of latitudinal variation in phenolic levels in Sargassum conflicts with previous predictions for latitudinal variation in the chemical defenses of marine organisms. The low levels of phenolics present in the tropical species that I analyzed may also explain recent results (Hay 1984; Lewis 1985) demonstrating that tropical Sargassum and Turbinaria are often preferentially consumed by herbivorous fishes and echinoids.     

Differential thermal tolerance between algae and corals may trigger the proliferation of algae in coral reefs

Marine heatwaves can lead to rapid changes in entire communities, including in the case of shallow coral reefs the potential overgrowth of algae. Here we tested experimentally the differential thermal tolerance between algae and coral species from the Red Sea through the measurement of thermal performance curves and the assessment of thermal limits. Differences across functional groups (algae vs. corals) were apparent for two key thermal performance metrics. First, two reef‐associated algae species (Halimeda tuna and Turbinaria ornata) had higher lethal thermal limits than two coral species (Pocillopora verrucosa and Stylophora pistillata) conferring those species of algae with a clear advantage during heatwaves by surpassing the thermal threshold of coral survival. Second, the coral species had generally greater deactivation energies for net and gross primary production rates compared to the algae species, indicating greater thermal sensitivity in corals once the optimum temperature is exceeded. Our field surveys in the Red Sea reefs before and after the marine heatwave of 2015 show a change in benthic cover mainly in the southern reefs, where there was a decrease in coral cover and a concomitant increase in algae abundance, mainly turf algae. Our laboratory and field observations indicate that a proliferation of algae might be expected on Red Sea coral reefs with future ocean warming.     

Molecular and biochemical studies of virus infections in two filamentous brown algae

The filamentous marine brown algae Ectocarpus siliculosus and Feldmannia simplex are infected by host specific DNA-viruses. Under Percoll isolation, Ectocarpus siliculosus-virus (EsV)-particles maintained their infective potential.The EsV has a circular genome of dsDNA with a size of 320 kb. A restriction map has been established. The gene of a major coat protein (gp1) was detected in a genomic library and partly sequenced. Using gpl- sequences for polymerase chain reaction (PCR) amplification analysis, EsV specific sequences could be detected in various symptom-free, clonal cultures of Ectocarpus. The PCR was also used to follow the passage of the virus genome during the meiosis of hosts. A monospecific antibody against recombinant gpl was used for immunostaining and infection experiments.The Feldmannia simplex-virus (FlexV-1) has a circular genome with a size of 220 kb and a 43% G+C content. FsV-DNA contains methylated bases. 5-methylcytosine (5 mC) makes up 12% of the total cytosines.     

Gibberellins as endogenous growth regulators in green and brown algae

Summary Gibberellin-like activity was detected in extracts from Enteromorpha prolifera (Chlorophyta) and Ecklonia radiata (Phaeophyta). Material eluted from chromatograms of extracts and active in dwarf maize also brought about growth responses in the species from which the extract had been made. The responses were similar to those brought about by gibberellic acid. It is concluded that gibberellins are normally involved in regulating growth in green and brown algae.     

Contrasting population makeup of two intertidal gastropod species that differ in dispersal opportunities

This study of population structure of two intertidal monodontine species: Diloma nigerrima and Diloma subrostrata, revealed the level of genetic connectedness among populations. Despite their markedly different geographic ranges (D. nigerrima is a geographically widespread species, inhabiting both Chile and New Zealand, including its subantarctic islands, whereas D. subrostrata is endemic to New Zealand), both species are believed to possess short-lived lecithotrophic larval stages. Polymorphic DNA microsatellite sequences were used to reveal the level of genetic connectedness among populations, thus inferring the two species' relative effective dispersal abilities. For each species F statistics, AMOVA values and the strength of the relationship between geographic and genetic distance were calculated. We observed a higher within-species level of genetic variation (Φ_ST = 0.099 vs. Φ_ST = 0.016) and a higher proportion of variance (11.15% vs. 0%) among populations of D. nigerrima than of D. subrostrata. A larger fraction of significant F_ST values was observed among D. nigerrima population pairs (65%) than among D. subrostrata population pairs (33%). Significant correlation between genetic and geographic distance was observed for D. nigerrima but not for D. subrostrata, but this relationship was not consistent among pairwise D. nigerrima population comparisons and PCA analysis confirms that, for each species, population structure does not follow a consistent pattern of increasing with geographic distance. The lack of population structure among D. subrostrata populations is probably due to its ubiquitous distribution, meaning little opportunity exists for genetic structure. D. nigerrima, by contrast has a patchier distribution, which allows for greater opportunities for genetic differentiation to occur. We argue that, despite the probable short larval stage in this species, the lack of geographical pattern in the genetic structure found in D. nigerrima is best explained by occasional dispersal over relatively short distances around the coast of New Zealand, over longer distances from New Zealand to the subantarctic islands and even across the Pacific Ocean from New Zealand to Chile.     

Local and geographical distributions for a tropical tree genus,Scaphium (Sterculiaceae) in the Far East

Tropical rain forests have an amazingly large number of closely related, sympatric species. How the sympatric species coexist is central to understanding the maintenance of high biodiversity in tropical rain forests. We compared local and geographical distributions among trees in Scaphium (Sterculiaceae), a tropical canopy tree genus. Scaphium is endemic to the Far Eastern tropics and comprises six species. Scaphium scaphigerum is distributed in drier regions than the other species' geographical distribution ranges. Scaphium longiflorum is distributed swamp forests, whereas the others were distributed in lowland and hill tropical rain forests on undulating land. Scaphium borneense, S. longipetiolatum, and S. macropodum co-occurred in a 52-ha plot in Lambir, Sarawak and clearly showed an allopathic pattern of distribution related to elevation in it. In the plot, the elevational difference was correlated with soil variation. Consequently, the difference in edaphic condition promoted the habitat segregation of the species. Thus these five Scaphium species have divergent habitats at various spatial scales and coexist because they reduce direct competition by habitat niche differentiation. Although the non-equilibrium hypothesis for the coexistence of Scaphium species cannot be rejected categorically due to the lack of enough information about S. linearicarpum, the equilibrium force may play the predominant role which permits their coexistence.     

Geographic variation in thermal tolerance of a widespread minnow Notropis lutrensis of the North American mid-west

Eighteen populations of Notropis lutrensis , representing all of the major river systems occupied by this species across an 1100km north-south span of its range, showed no significant differences in mean critical thermal maximum (CTM), although general north-south differences in climatic temperatures exist. There were no clinal trends in CTMs, and CTMs were unrelated to stream size or any other discernible feature of the local habitats. Previous research has shown that N. lutrensis can adapt rapidly to an artificially altered thermal regime. The present results suggest that even though a species may have potentially 'labile' thermal physiology, it may nonetheless be 'static' or evolutionarily conservative in such characters when natural populations are examined empirically.     

Genetic traces of recent long-distance dispersal in a predominantly self-recruiting coral

Background

Understanding of the magnitude and direction of the exchange of individuals among geographically separated subpopulations that comprise a metapopulation (connectivity) can lead to an improved ability to forecast how fast coral reef organisms are likely to recover from disturbance events that cause extensive mortality. Reef corals that brood their larvae internally and release mature larvae are believed to show little exchange of larvae over ecological times scales and are therefore expected to recover extremely slowly from large-scale perturbations.

Methodology/Principal Findings

Using analysis of ten DNA microsatellite loci, we show that although Great Barrier Reef (GBR) populations of the brooding coral, Seriatopora hystrix, are mostly self-seeded and some populations are highly isolated, a considerable amount of sexual larvae (up to ∼4%) has been exchanged among several reefs 10 s to 100 s km apart over the past few generations. Our results further indicate that S. hystrix is capable of producing asexual propagules with similar long-distance dispersal abilities (∼1.4% of the sampled colonies had a multilocus genotype that also occurred at another sampling location), which may aid in recovery from environmental disturbances.

Conclusions/Significance

Patterns of connectivity in this and probably other GBR corals are complex and need to be resolved in greater detail through genetic characterisation of different cohorts and linkage of genetic data with fine-scale hydrodynamic models.     

Fruit characters as a basis of fruit choice and seed dispersal in a tropical forest vertebrate community

Summary Interactions between a large community of vertebrate frugivore-granivores (including 7 species of large canopy birds, 19 species of rodents, 7 species of ruminants, and 6 species of monkeys), and 122 fruit species they consume, were studied for a year in a tropical rainforest in Gabon.The results show how morphological characters of fruits are involved in the choice and partitioning of the available fruit spectrum among consumer taxa. Despite an outstanding lack of specificity between fruit and consumer species, consideration of simple morphological traits of fruits reveals broad character syndromes associated with different consumer taxa. Competition between distantly related taxa that feed at the same height is far more important than has been previously supposed. The results also suggest how fruit characters could have evolved under consumer pressure as a result of consumer roles as dispersers or seed predators. Our analyses of dispersal syndromes show that fruit species partitioning occurs more between mammal taxa than between mammals and birds. There is thus a bird-monkey syndrome and a ruminant-rodent-elephant syndrome. The bird-monkey syndrome includes fruit species on which there is no pre-dispersal seed predation. These fruits (berries and drupes) are brightly colored, have a succulent pulp or arillate seeds, and no protective seed cover. The ruminant-rodent-elephant syndrome includes species for which there is pre-dispersal predation. These fruits (all drupes) are large, dull-colored, and have a dry fibrous flesh and well-protected seeds.     

Projected changes in distributions of Australian tropical savanna birds under climate change using three dispersal scenarios

Identifying the species most vulnerable to extinction as a result of climate change is a necessary first step in mitigating biodiversity decline. Species distribution modeling (SDM) is a commonly used tool to assess potential climate change impacts on distributions of species. We use SDMs to predict geographic ranges for 243 birds of Australian tropical savannas, and to project changes in species richness and ranges under a future climate scenario between 1990 and 2080. Realistic predictions require recognition of the variability in species capacity to track climatically suitable environments. Here we assess the effect of dispersal on model results by using three approaches: full dispersal, no dispersal and a partial-dispersal scenario permitting species to track climate change at a rate of 30 km per decade. As expected, the projected distributions and richness patterns are highly sensitive to the dispersal scenario. Projected future range sizes decreased for 66% of species if full dispersal was assumed, but for 89% of species when no dispersal was assumed. However, realistic future predictions should not assume a single dispersal scenario for all species and as such, we assigned each species to the most appropriate dispersal category based on individual mobility and habitat specificity; this permitted the best estimates of where species will be in the future. Under this "realistic" dispersal scenario, projected ranges sizes decreased for 67% of species but showed that migratory and tropical-endemic birds are predicted to benefit from climate change with increasing distributional area. Richness hotspots of tropical savanna birds are expected to move, increasing in southern savannas and southward along the east coast of Australia, but decreasing in the arid zone. Understanding the complexity of effects of climate change on species' range sizes by incorporating dispersal capacities is a crucial step toward developing adaptation policies for the conservation of vulnerable species.     

Sex, isolation and fidelity: unbiased long-distance dispersal in a terrestrial amphibian

Amphibians in general are considered poor dispersers and thus their dispersal curve should be dominated by short movements. Additionally, as male toads do not compete for females and sexual selection is by female choice, dispersal should be male-biased. Furthermore, since adults are site-loyal and polygynous, juveniles should move farther and faster than adults. We tested the hypotheses that dispersal would be limited and both sex- and age-biased in a population of Fowler's toads Bufo fowleri at Lake Erie, Ontario, Canada. Based on a mark-recapture study of 2816 toads, 1326 recaptured at least once, we found that although the toads did show high site fidelity, the dispersal curve was highly skewed with a significant "tail" where the maximum distance moved by an adult was 34 km. Dispersal was neither sex-biased nor age-biased despite clear theoretical predictions that dispersal should be biased towards males and juveniles. We conclude that the resource competition hypothesis of sex-biased dispersal does not predict dispersal tendencies as readily for amphibians as for mammals and birds. Toad dispersal only appears to be juvenile-biased because the juveniles are more abundant than the adults, not because they are the more active dispersers.     

Contrasting reproductive traits in two species of mangrove‐dwelling littorinid snails in a seasonal tropical habitat

Previous studies on the reproductive biology of littorinid snails have focused on rocky shore species, investigating how these gastropods can achieve maximal reproductive success, as well as on processes of sexual selection. This study documented differences in the reproductive traits of two mangrove‐dwelling littorinids, Littoraria ardouiniana and L. melanostoma, in Hong Kong. Reproductive activity of both species was most intense during the summer months. Mating pairs of the two species generally occurred in the tree canopies. Few false mating pairs (same sex or heterospecific pairs: <10%) were recorded, and members of both species showed size‐assortative mating. Littoraria ardouiniana had a shorter reproductive season but a higher intensity of mating and higher seasonal fecundity, than did L. melanostoma. Members of both species showed bi‐lunar periodicities of egg or larval release, synchronized with spring tides. Fecundity showed a strong positive relationship with body size in L. ardouiniana, but not in L. melanostoma. Females of L. ardouiniana released entire broods of larvae in a single brief event, whereas females of L. melanostoma released fewer eggs over 1–8 d. Release of larvae in L. ardouiniana involved a series of short bursts and was much faster than the trickle release of eggs in L. melanostoma. The contrasting reproductive traits in these two species represent different strategies to optimize reproductive success in mangrove habitats.     

Extreme long-distance dispersal of the lowland tropical rainforest tree Ceiba pentandra L. (Malvaceae) in Africa and the Neotropics

Many tropical tree species occupy continental expanses of rainforest and flank dispersal barriers such as oceans and mountains. The role of long-distance dispersal in establishing the range of such species is poorly understood. In this study, we test vicariance hypotheses for range disjunctions in the rainforest tree Ceiba pentandra, which is naturally widespread across equatorial Africa and the Neotropics. Approximate molecular clocks were applied to nuclear ribosomal [ITS (internal transcribed spacer)] and chloroplast (psbB-psbF) spacer DNA sampled from 12 Neotropical and five West African populations. The ITS (N=5) and psbB-psbF (N=2) haplotypes exhibited few nucleotide differences, and ITS and psbB-psbF haplotypes were shared by populations on both continents. The low levels of nucleotide divergence falsify vicariance explanations for transatlantic and cross-Andean range disjunctions. The study shows how extreme long-distance dispersal, via wind or marine currents, creates taxonomic similarities in the plant communities of Africa and the Neotropics.     

Vicariance and dispersal effects on phylogeographic structure and speciation in a widespread estuarine invertebrate

Vicariance and dispersal can strongly influence population genetic structure and allopatric speciation, but their importance in the origin of marine biodiversity is unresolved. In transitional estuarine environments, habitat discreteness and dispersal barriers could enhance divergence and provide insight to evolutionary mechanisms underlying marine and freshwater biodiversity. We examined this by assessing phylogeographic structure in the widespread amphipod Gammarus tigrinus across 13 estuaries spanning its northwest Atlantic range from Quebec to Florida. Mitochondrial cytochrome c oxidase I and nuclear internal transcribed spacer 1 phylogenies supported deep genetic structure consistent with Pliocene separation and cryptic northern and southern species. This break occurred across the Virginian-Carolinian coastal biogeographic zone, where an oceanographic discontinuity may restrict gene flow. Ten estuarine populations of the northern species occurred in four distinct clades, supportive of Pleistocene separation. Glaciation effects on genetic structure of estuarine populations are largely unknown, but analysis of molecular variance (AMOVA) supported a phylogeographic break among clades in formerly glaciated versus nonglaciated areas across Cape Cod, Massachusetts. This finding was concordant with patterns in other coastal species, though there was no significant relationship between latitude and genetic diversity. This supports Pleistocene vicariance events and divergence of clades in different northern glacial refugia. AMOVA results and private haplotypes in most populations support an allopatric distribution across estuaries. Clade mixture zones are consistent with historical colonization and human-mediated transfer. An isolation-by-distance model of divergence was detected after we excluded a suspected invasive haplotype in the St. Lawrence estuary. The occurrence of cryptic species and divergent population structure support limited dispersal, dispersed habitat distribution, and historical factors as important determinants of estuarine speciation and diversification.