ONTOGENETIC CHANGES IN BUOYANCY,BREAKING STRENGTH,EXTENSIBILITY, AND REPRODUCTIVE INVESTMENT IN A DRIFTING MACROALGA TURBINARIA ORNATA (PHAEOPHYTA)1

Turbinaria ornata (Turner) J. Agardh is a tropical alga that disperses by detached, reproductively mature floating fronds. Material properties (breaking stress, breaking extension), buoyancy, and the proportion of reproductive tissue per frond were measured for juvenile, adult, and old fronds of T. ornata. Correlations between these factors indicate that as fronds age and become more reproductively mature, the tissue in their stipes (where they break) becomes weaker, more brittle, and the overall buoyancy of the frond increases. Measurement of drag force experienced by fronds from each ontogenetic stage allowed calculation of the environmental stress factor (ESF), which indicates the likelihood of detachment of a frond in the flow environment of its habitat. The ESF for fronds of each ontogenetic stage predicted that reproductively mature fronds (adult and old) break more readily than immature (juvenile) fronds. Increased proportions of reproductively mature fronds in floating rafts following storms compared with the proportion of mature fronds attached to the substratum support the ESF predictions. This combination of ontogenetic changes in material properties, buoyancy, and reproductive maturity in combination with the life history of T. ornata may contribute to the dispersal of this alga throughout French Polynesia.     

The role of buoyancy in mitigating reduced light in macroalgal aggregations

Many organisms live in aggregations. For marine organisms attached to the substratum, the presence of neighbors may reduce both the water flow and light encountered by an individual within a group. We used the marine macroalga Turbinaria ornata to investigate how water velocity and photon flux density were affected by aggregations under natural flow and light conditions in the field, and to explore how the ability to stand upright due to buoyancy impacted light interception by individuals in aggregations. We found that the flow experienced by thalli in the middle of a group in a backreef habitat was only about half the speed of the water flow they encountered after their neighbors were removed. This suggests that aggregated algae are subjected to lower hydrodynamic forces than are solitary thalli, but may also experience reduced rates of transport of dissolved nutrients and gases. Light sensors placed along the length of thalli positioned within and outside of clumps of T. ornata showed that the tops of buoyant upright thalli experienced similar light levels to solitary thalli, but that neighbors shaded the lower portions of thalli in aggregations. Individuals that were rendered non-buoyant (by filling their airbladders with water) could not support their own weight and those in aggregations experienced lower light at all points along their lengths than did upright buoyant individuals. Using in situ determinations of the rate of photosynthesis of T. ornata as a function of irradiance, we converted our field measurements of light interception to mg carbon fixed over the course of a day for whole fronds. Such estimates indicated that carbon fixation was higher for solitary buoyant and non-buoyant T. ornata than for buoyant individuals in aggregations, all of which were greater than for non-buoyant thalli in those clumps. Our results indicate that living in aggregations reduces the productivity of T. ornata, but this effect is mitigated by the buoyancy of thalli.     

Variability in the Effects of Macroalgae on the Survival and Growth of Corals: The Consumer Connection

Shifts in dominance from corals to macroalgae are occurring in many coral reefs worldwide. Macroalgal canopies, while competing for space with coral colonies, may also form a barrier to herbivorous and corallivorous fish, offering protection to corals. Thus, corals could either suffer from enhanced competition with canopy-forming and understorey macroalgae or benefit from predator exclusion. Here, we tested the hypothesis that the effects of the brown, canopy-forming macroalga, Turbinaria ornata, on the survival and growth of corals can vary according to its cover, to the presence or absence of herbivorous and corallivorous fish and to the morphological types of corals. Over a period of 66 days, two coral species differing in growth form, Acropora pulchra and Porites rus, were exposed to three different covers of T. ornata (absent versus medium versus high), in the presence or absence of fish. Irrespective of the cover of T. ornata, fish exclusion reduced mortality rates of A. pulchra. Following fish exclusion, a high cover of T. ornata depressed the growth of this branched coral, whilst it had no effect when fish species were present. P. rus suffered no damage from corallivorous fish, but its growth was decreased by high covers of T. ornata, irrespective of the presence or absence of fish. These results show that negative effects of T. ornata on some coral species are subordinate to those of fish predation and are, therefore, likely to manifest only on reefs severely depleted of predators. In contrast, space dominance by T. ornata may decrease the growth of other coral species regardless of predation intensity. In general, this study shows that susceptibility to predation may determine the severity of the effects of canopy-forming macroalgae on coral growth.     

Spatial genetic analysis of two polyploid macrophytes reveals high connectivity in a modified wetland

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Ontogenetic Variation in Blade Toughness May Contribute to the Spread of Turbinaria ornata Across the South Pacific

Coral reefs are shifting from coral to algal-dominated ecosystems worldwide. Recently, Turbinaria ornata, a marine alga native to coral reefs of the South Pacific, has spread in both range and habitat usage. Given dense stands of T. ornata can function as an alternative stable state on coral reefs, it is imperative to understand the factors that underlie its success. We tested the hypothesis that T. ornata demonstrates ontogenetic variation in allocation to anti-herbivore defense, specifically that blade toughness varied nonlinearly with thallus size. We quantified the relationship between T. ornata blade toughness and thallus size for individual thalli within algal stands (N = 345) on seven fringing reefs along the north shore of Moorea, French Polynesia. We found that blade toughness was greatest at intermediate sizes that typically form canopies, with overall reduced toughness in both smaller individuals that refuge within the understory and older reproductive individuals that ultimately detach and form floating rafts. We posit this variation in blade toughness reduces herbivory on the thalli that are most exposed to herbivores and may facilitate reproduction in dispersing stages, both of which may aid the proliferation of T. ornata.     

Characterization of thallus mechanical and physiological traits of tropical fucoids: A preliminary study

Thallus mechanical strength, as well as production of secondary metabolites as defensive compounds, of tropical macroalgae is often essential for protection from herbivory. In tropical macroalgae, thallus mechanical strength is negatively correlated with productivity — a trade‐off between productivity and thallus toughness. The tropical fucoids, Turbinaria ornata which has defensive morphological traits against herbivory and Sargassum ilicifolium which is expanding its distribution in Japanese waters, were examined to determine thallus traits related to mechanical strength and productivity and their pair‐wise relationships were also examined. These traits that are directly or indirectly related to the trade‐off between productivity and thallus toughness were compared to data for various other temperate macroalgae by regression analyses. We found two strong positive correlations between thallus mechanical strength and thallus mass or thallus thickness, confirming that higher levels of mechanical strength for tropical fucoids is associated with higher biomass or thallus thickness. Also, negative correlations between thallus toughness and productivity were found indicating structural and physiological trade‐offs. However, the tropical fucoids exhibited relatively high productivity regardless of their higher level of thallus toughness. These traits of the tropical fucoids slightly deviate from the typical conservative strategy with higher thallus toughness and lower productivity as a trade‐off between productivity and thallus toughness.     

Density-dependent individual growth of marble trout (<Emphasis Type="Italic">Salmo marmoratus</Emphasis>) in the Soca and Idrijca river basins,Slovenia

The aim of this field study was to investigate effects of estimated fish- and sea urchin herbivory on the reproductive potential of four species of macroalgae; Halimeda macroloba (Decasine), H. renschii (Hauck), Turbinaria ornata (Turner) and Padina boergesenii (Allender et Kraft). Fish and sea urchin herbivory were calculated based on reported consumption rates for their biomass estimates. We hypothesized that reduced herbivory would increase algal size and the reproductive potential, which may promote algal recruitment and be one of the driving mechanisms behind algal shifts and persistent algae-dominated reefs. Algae were investigated in field sites where the estimated fish- and or sea urchin herbivory differed. Our results suggest that algal fecundity of T. ornata and P. boergesenii are positively correlated to their size. Fecundity of T. ornata was higher and individuals grew larger in areas where estimated fish herbivory was lower. The two species of Halimeda grew larger and had higher fecundity in areas where estimated sea urchin herbivory was lower. P. boergesenii responded ambiguously to patterns in herbivory. Due to species-specific responses to different herbivores, it is difficult to generalize about effects of overfishing on algal fecundity. Handling editor: S. Wellekens     

Isolation of turbinaric acid as a chemomarker of Turbinaria conoides (J. Agardh) Kützing from South Pacific Islands

Several species of the genus Turbinaria coexist along the coasts of islands in the Indian and Pacific Oceans. Among these brown algae, Turbinaria ornata and T. conoides are sister species that are difficult to differentiate using exclusively morphological characters. Based on in vivo nuclear magnetic resonance and chromatographic techniques, i.e., liquid and gas chromatography‐mass spectrometry analysis, combined with phylogenetic data, we successfully identified turbinaric acid in T. conoides samples from several Indian and Pacific Ocean islands. This nonvariable discriminant molecule was only identified in T. conoides specimens, but not in the two allied species T. ornata and T. decurrens. Results are discussed with regard to turbinaric acid as an interesting chemomarker isolated from T. conoides and the rapid discrimination of Turbinaria specimens using chemical assays.     

Molecular phylogeny of Central and South American slider turtles: implications for biogeography and systematics (Testudines: Emydidae: Trachemys)

We analyse phylogeny, systematics and biogeography of slider turtles (Trachemys spp.) using sequence data of four mitochondrial genes (3242 bp) and five nuclear loci (3396 bp) of most South American and southern Central American taxa and representatives of northern Central American, West Indian and North American slider species (16 species and subspecies) and allied North American species (genera Chrysemys, Deirochelys, Graptemys, Malaclemys, Pseudemys). By applying maximum likelihood, relaxed molecular clock and ancestral range analyses, we provide evidence for two successive colonizations of South America by slider turtles. In addition, we show that the current species delineation of Central and South American slider turtles is incorrect. Our data suggest that Trachemys grayi is a distinct polytypic species that embraces, besides the nominotypical subspecies, T. g. emolli and T. g. panamensis. Trachemys ornata is also polytypic with the subspecies T. o. ornata, T. o. callirostris, T. o. cataspila, T. o. chichiriviche and T. o. venusta. Moreover, T. adiutrix should be regarded as a subspecies of T. dorbigni. All studied Trachemys species are inferred to have originated in the Late Miocene to Early Pliocene. The ancestor of the two subspecies of T. dorbigni colonized South America most probably prior to the establishment of the land bridge connecting Central and South America, whereas the two South American subspecies of T. ornata represent a younger independent immigration wave from Central America.     

WATER MOTION, MARINE MACROALGAL PHYSIOLOGY, AND PRODUCTION

Water motion is a key determinant of marine macroalgal production, influencing directly or indirectly physiological rates and community structure. Our understanding of how marine macroalgae interact with their hydrodynamic environment has increased substantially over the past 20 years, due to the application of tools such as flow visualization to aquatic vegetation, and in situ measurements of seawater velocity and turbulence. This review considers how the hydrodynamic environment in which macroalgae grow influences their ability to acquire essential resources and how macroalgae might respond physiologically to fluctuations in their hydrodynamic regime with a focus on: (1) the biochemical processes occurring within the diffusion boundary layer (DBL) that might reduce rates of macroalgal production; (2) time scales over which measurements of velocity and DBL processes should be made, discussing the likelihood of in situ mass transfer limitation; (3) if and how macroalgal morphology influences resource acquisition in slow flows; and (4) ecobiomechanics and how hydrodynamic drag might influence resource acquisition and allocation. Finally, the concept that macroalgal production is enhanced in wave-exposed versus sheltered habitats is discussed.     

Evaluating contemporary pollen dispersal in two common grassland species Ranunculus bulbosus L. (Ranunculaceae) and Trifolium montanum L. (Fabaceae) using an experimental approach

Pollen flow is a key biological process that connects plant populations, preventing genetic impoverishment and inbreeding. Pollen‐mediated long‐distance dispersal (LDD) events are especially important for plant species in increasingly fragmented landscapes. Patterns of pollen dispersal were directly estimated and dispersal kernels modelled in an experimental population of Ranunculus bulbosus and Trifolium montanum to determine the potential for LDD. Eight and 11 microsatellite markers were used for R. bulbosus and T. montanum, respectively, to run a likelihood‐based paternity analysis on randomly chosen offspring (N_total = 180 per species) from five maternal plants. High rates of selfing were found in R. bulbosus (average 45.7%), while no selfing was observed in T. montanum. The majority (60%) of mating events occurred at very short distances: the median of the observed dispersal distances was 0.8 m in both species, and the average distances were 15.9 and 10.3 m in R. bulbosus and T. montanum, respectively. Modelling the pollen dispersal kernel with four different distribution functions (exponential‐power, geometric, 2Dt and Weibull) indicated that the best fit for both species was given by a Weibull function. Yet, the tail of the T. montanum pollen dispersal kernel was thinner than in R. bulbosus, suggesting that the probability for LDD is higher in the latter species. Even though the majority of pollen dispersal occurred across short distances, the detection of several mating events up to 362 m (R. bulbosus) and 324 m (T. montanum) suggests that pollen flow may be sufficient to ensure population connectivity in these herb species across fragmented grasslands in Swiss agricultural landscapes.     

Kelp versus coralline: cellular basis for mechanical strength in the wave‐swept seaweed Calliarthron (Corallinaceae,Rhodophyta)1

Previous biomechanical studies of wave‐swept macroalgae have revealed a trade‐off in growth strategies to resist breakage in the intertidal zone: growing in girth versus growing strong tissues. Brown macroalgae, such as kelps, grow thick stipes but have weak tissues, while red macroalgae grow slender thalli but have much stronger tissues. For example, genicular tissue in the articulated coralline Calliarthron cheilosporioides Manza is more than an order of magnitude stronger than some kelp tissues, but genicula rarely exceed 1 mm in diameter. The great tissue strength of Calliarthron genicula results, at least in part, from a lifelong strengthening process. Here, a histological analysis is presented to explore the cellular basis for mechanical strengthening in Calliarthron genicula. Genicula are composed of thousands of fiber‐like cells, whose cell walls thicken over time. Thickening of constitutive cell walls likely explains why older genicula have stronger tissues: a mature geniculum may be >50% cell wall. However, the material strength of genicular cell wall is similar to the strength of cell wall from a freshwater green alga, suggesting that it may be the quantity—not the quality—of cell wall material that gives genicular tissue its strength. Apparent differences in tissue strength across algal taxa may be a consequence of tissue construction rather than material composition.     

Finite element modeling of shell shape in the freshwater turtle Pseudemys concinna reveals a trade‐off between mechanical strength and hydrodynamic efficiency

Aquatic species can experience different selective pressures on morphology in different flow regimes. Species inhabiting lotic regimes often adapt to these conditions by evolving low‐drag (i.e., streamlined) morphologies that reduce the likelihood of dislodgment or displacement. However, hydrodynamic factors are not the only selective pressures influencing organismal morphology and shapes well suited to flow conditions may compromise performance in other roles. We investigated the possibility of morphological trade‐offs in the turtle Pseudemys concinna. Individuals living in lotic environments have flatter, more streamlined shells than those living in lentic environments; however, this flatter shape may also make the shells less capable of resisting predator‐induced loads. We tested the idea that “lotic” shell shapes are weaker than “lentic” shell shapes, concomitantly examining effects of sex. Geometric morphometric data were used to transform an existing finite element shell model into a series of models corresponding to the shapes of individual turtles. Models were assigned identical material properties and loaded under identical conditions, and the stresses produced by a series of eight loads were extracted to describe the strength of the shells. “Lotic” shell shapes produced significantly higher stresses than “lentic” shell shapes, indicating that the former is weaker than the latter. Females had significantly stronger shell shapes than males, although these differences were less consistent than differences between flow regimes. We conclude that, despite the potential for many‐to‐one mapping of shell shape onto strength, P. concinna experiences a trade‐off in shell shape between hydrodynamic and mechanical performance. This trade‐off may be evident in many other turtle species or any other aquatic species that also depend on a shell for defense. However, evolution of body size may provide an avenue of escape from this trade‐off in some cases, as changes in size can drastically affect mechanical performance while having little effect on hydrodynamic performance. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Macroalgal spore dysfunction: ocean acidification delays and weakens adhesion

Early life stages of marine organisms are predicted to be vulnerable to ocean acidification. For macroalgae, reproduction and population persistence rely on spores to settle, adhere and continue the algal life cycle, yet the effect of ocean acidification on this critical life stage has been largely overlooked. We explicitly tested the biomechanical impact of reduced pH on early spore adhesion. We developed a shear flume to examine the effect of reduced pH on spore attachment time and strength in two intertidal rhodophyte macroalgae, one calcified (Corallina vancouveriensis) and one noncalcified (Polyostea robusta). Reduced pH delayed spore attachment of both species by 40%–52% and weakened attachment strength in C. vancouveriensis, causing spores to dislodge at lower flow‐induced shear forces, but had no effect on the attachment strength of P. robusta. Results are consistent with our prediction that reduced pH disrupts proper curing and gel formation of spore adhesives (anionic polysaccharides and glycoproteins) via protonation and cation displacement, although experimental verification is needed. Our results demonstrate that ocean acidification negatively, and differentially, impacts spore adhesion in two macroalgae. If results hold in field conditions, reduced ocean pH has the potential to impact macroalgal communities via spore dysfunction, regardless of the physiological tolerance of mature thalli.     

Breakage by hydrodynamic shear of the bonds between cohered ends of lambda-DNA molecules

The rate of breakage by hydrodynamic shear of the cohered ends of λ-DNA molecules has been observed for the circular monomers, joined half molecules, and joined quarter molecules, in a capillary apparatus with known flow parameters. The rate constant for breakage has been measured as a function of shear stress, temperature, ionic strength, and molecular length. There is a large temperature coefficient, with an activation energy of 120 ± 20 kcal./mole. The values of d ln k/dG, where k is the rate constant for breaking and G is shear gradient, in aqueous solution at 25°C. are about 3.8 ± 0.3 × 10^?4 see. The shear stresses needed for breakage of joined quarter molecules and of circular monomers, respectively, are about equal, and about half that needed for breakage of joined half molecules. The rate of breakage at a given shear stress increases with decreasing ionic strength, approximately as [Na⁺]^?1.6. Self-protection effects are not observed for opening of circular monomers at a DNA concentration of 5 μg./ml. but are observed for breakage of joined half molecules at concentrations down to 0.5 μg./ml. The large temperature coefficient which is approximately equal to that of the thermal dissociation of the cohered ends is interpreted to mean that shear breakage is a mechanically assisted thermal reaction in which the thermal fluctuations provide most of the free energy of activation for breakage. A detailed model for this interpretation is presented. The self-protection effect implies that those molecules which break are not average molecules but exceptional ones which, due to some fluctuation, are more fully extended in the flow field.     

Population structure of a widespread bat (Tadarida brasiliensis) in an island system

Dispersal is a driving factor in the creation and maintenance of biodiversity, yet little is known about the effects of habitat variation and geography on dispersal and population connectivity in most mammalian groups. Bats of the family Molossidae are fast‐flying mammals thought to have potentially high dispersal ability, and recent studies have indicated gene flow across hundreds of kilometers in continental North American populations of the Brazilian free‐tailed bat, Tadarida brasiliensis. We examined the population genetics, phylogeography, and morphology of this species in Florida and across islands of The Bahamas, which are part of an island archipelago in the West Indies. Previous studies indicate that bats in the family Phyllostomidae, which are possibly less mobile than members of the family Molossidae, exhibit population structuring across The Bahamas. We hypothesized that T. brasiliensis would show high population connectivity throughout the islands and that T. brasiliensis would show higher connectivity than two species of phyllostomid bats that have been previously examined in The Bahamas. Contrary to our predictions, T. brasiliensis shows high population structure between two groups of islands in The Bahamas, similar to the structure exhibited by one species of phyllostomid bat. Phylogenetic and morphological analyses suggest that this structure may be the result of ancient divergence between two populations of T. brasiliensis that subsequently came into contact in The Bahamas. Our findings additionally suggest that there may be cryptic species within T. brasiliensis in The Bahamas and the West Indies more broadly.     

Dispersal of diapausing Tetranychus urticae and T. kanzawai

The spider mites Tetranychus urticae Koch and Tetranychus kanzawai Kishida (Acari: Tetranychidae) overwinter mostly as mated adult diapausing females. Their overwintering survival depends in part on their dispersal towards suitable habitats. We investigated the dispersal behaviour of diapausing females of T. urticae and T. kanzawai with respect to factors known to affect the dispersal of non‐diapausing mites: light, population density, gravity, and humidity. In general, diapausing females of T. urticae showed a stronger tendency to disperse than did those of T. kanzawai under all test conditions. High population density promoted the dispersal of diapausing T. urticae, but not of T. kanzawai. Dispersal of diapausing females of both species was not significantly affected by gravity, humidity, or whether feeding damage was caused by conspecifics or heterospecifics. On plants, more T. urticae than T. kanzawai moved downward. We propose that dispersal after the onset of diapause may be an important life‐history strategy in T. urticae, but not in T. kanzawai.     

Effects of territorial damselfish on an algal-dominated coastal coral reef

Territorial damselfish are important herbivores on coral reefs because they can occupy a large proportion of the substratum and modify the benthic community to promote the cover of food algae. However, on coastal coral reefs damselfish occupy habitats that are often dominated by unpalatable macroalgae. The aim of this study was to examine whether damselfish can maintain distinctive algal assemblages on a coastal reef that is seasonally dominated by Sargassum (Magnetic Island, Great Barrier Reef). Here, three abundant species (Pomacentrus tripunctatus, P. wardi and Stegastes apicalis) occupied up to 60% of the reef substrata. All three species promoted the abundance of food algae in their territories. The magnitudes of the effects varied among reef zones, but patterns were relatively stable over time. Damselfish appear to readily co-exist with large unpalatable macroalgae as they can use it as a substratum for promoting the growth of palatable epiphytes. Damselfish territories represent patches of increased epiphyte load on macroalgae, decreased sediment cover, and enhanced cover of palatable algal turf.     

Hydrodynamic shear breakage of DNA

The rate of breakage of duplex DNA molecules by laminar flow through a capillary has been studied. For λb2b5c DNA (molecular wt., M = 25 × 10⁶) the point at which breakage occurs is normally distributed around the center of the molecule with a standard deviation of 12.5% of the molecular length. At constant shear stress or shear rate, the breakage rate is independent of ionic strength. Thus, shear induced local denaturation is not a rate limiting, preliminary step in breakage. In experiments at constant temperature with varying solvent viscosity (controlled by added sucrose) the breakage rate is a function of shear rate, not of shear stress. The rate of opening of hydrogenbonded circles into linear molecules by hydrodynamic shear is also shown to be a function of shear rate and not of shear stress. The breakage rate at constant shear rate is not greatly dependent on temperature. The shear rate required to achieve breakage is inversely proportional to M^1,2. The breakage rate constant, k varies as a very high power of the shear rate; at 25°C, d In k/d In Gm ～ 15; at 10°C, d In k/d In Gm ～ 26, where Gm is the maximum shear rate at the capillary wall. The unexpected result that breakage rate is mainly dependent on shear rate, not shear stress, supports a model in which the DNA molecule is distorted with a driving force which depends on the hydrodynamic shear stress, ηG, but the rate limiting step is segment diffusion into a highly extended configuration. The characteristic time to achieve this configuration is proportional to solvent viscosity, η, hence the breakage rate is dependent on ηG/η or G, the shear rate.     

Spatial genetic structure of aquatic bryophytes in a connected lake system

Using genetic markers, we investigated the genetic structure of three clonal aquatic moss species, Calliergon megalophyllum Mikut., Fontinalis antipyretica Hedw. and F. hypnoides Hartm. on two scales: among populations in a connected lake system (large‐scale spatial genetic structure) and among individuals within populations (fine‐scale spatial genetic structure). Mean genetic diversities per population were 0.138, 0.247 and 0.271, respectively, and total diversities equalled 0.223, 0.385 and 0.421, respectively. Relative differentiation levels (F_ST values of 0.173, 0.280 and 0.142, respectively) were significant but showed that there is a moderate amount of gene flow taking place within the lake system connected with narrow streams. Bayesian STRUCTURE analysis provided some indication that the direction of water flow influences population genetic structuring in the studied aquatic mosses. We propose that dispersal leading to gene flow in C. megalophyllum, F. antipyretica and F. hypnoides takes place both along water via connecting streams and by animal vectors, such as waterfowl. Nevertheless, the slight genetic structuring pattern along the direction of water flow suggests that dispersal of shoots or their fragments along water is a means of dispersal in these mosses. The absence of sexual reproduction and spores may have caused the observed spatial genetic structure within populations, including aggregations of similar genotypes (clones or closely related genotypes) at short distances in populations otherwise showing an isolation by distance effect. Regardless of the results pointing to the dominance of vegetative propagation, it is impossible to completely rule out the potential role of rare long‐distance spore dispersal from areas where the species are fertile.