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Nematodes surfi ng the waves: long-distance dispersal of soil-borne microfauna via sea swept rhizomes

Eduardo de la Peña, Martijn L. Vandegehuchte, Dries Bonte and Maurice Moens

E. de la Pe ñ a (eduardo.delapena@ugent.be), M. L. Vandegehuchte and D. Bonte, Dept of Biology, Faculty of Sciences, Ghent Univ., K.L. Ledeganckstraat 35, BE-9000 Ghent, Belgium. – M. Moens, Inst. for Agriculture and Fisheries Research (ILVO), Burg. Van Gansberghelaan 96, BE-9220 Merelbeke, Belgium. MM also at: Dept of Crop Protection, Faculty of Bioscience Engineering, Ghent Univ., Coupure Links 653, BE-9000 Ghent, Belgium.

Dispersal mechanisms of soil-borne microfauna have hitherto received little attention. Understanding dispersal mecha-nisms of these species is important to unravel their basic life history traits, biogeography, exchange of individuals between populations, and local adaptation. Soil-borne nematodes and root-feeding nematodes in particular occupy a key position in soil-food webs and can be determinants for plant growth and vegetation structure and succession. However, their dispersal abilities have been scarcely addressed, predominantly focusing on species of agricultural importance. Still, root-feeding nematodes are usually considered as being extremely limited and bound to the rhizosphere of plants. We investigated a mechanism for long distance dispersal of root-feeding nematodes associated to two widespread coastal dune grasses. Th e nematodes are known to be crucial for the functioning of these grasses. We experimentally tested the hypothesis that root-feeding nematodes are able to move across long distances inside rhizome fragments that are dispersed by seawater. We also tested the survival capacities of the host plants in seawater. Our study demonstrates that root-feeding nematodes and plants are able to survive immersion in seawater, providing a mechanism for long distance dispersal of root feeding nematodes together with their host plant. Drifting rhizome fragments enable the exchange of plant material and animals between dune systems. Th ese results provide new insights to understand the ecology of dune vegetation, the interaction with soil-borne organ-isms and more importantly, re-set the scale of spatial dynamics of a group of organisms considered extremely constrained in its dispersal capacities.

Dispersal, defi ned as the movement of individuals from a source location (birth or breeding site) to another where they might establish (Bullock et al. 2002), has important conse-quences for gene fl ow, the genetic cohesion of species, the global persistence of species in the face of local extinction and the evolution of species life-history traits (Ronce 2007, Kokko and L ó pez-Sepulcre 2006). Probably all organisms show some mechanisms to disperse within their close by neighbourhood, thereby aff ecting local demography (Ronce 2007). Long distance dispersal (i.e. low frequency dispersal events in the tail of the dispersal kernel; Nathan 2006) by unusual dispersal means is, however, a rare phenomenon, with per defi nition only a limited number of propagules involved (Higgins and Richardson 1999). Understanding these rare events is therefore crucial to improve our under-standing of global patterns in species distribution and diver-sity (Nathan 2006).

Assisted dispersal events enable plants and animals to dis-perse over very large distances attached to other organism.

Th e vast collection of literature on the dispersal of seeds and fruits by, endo- and epizoochory could serve as an example (Nathan et al. 2008). However, the dispersal (direct or indi-rect) of less conspicuous groups like soil-borne nematodes, have been scarcely addressed in the literature (Bohonak and Jenkins 2003, Foissner 2006). Th is is rather surprising given their often widespread distribution and their importance in a wide array of ecosystem processes.

Although nematodes occupy a key position in terrestrial food webs and ecosystem functioning, their spatial dynamics are often unknown or regarded as extremely local (Ettema and Wardle 2002). During the last decade a considerable body of literature has highlighted the importance of soil biota, including plant-parasitic nematodes, in understanding the structuring, functioning and dynamics of plant commu-nities (Van der Putten 2003). Movement and distribution patterns in the rhizosphere have been relatively well studied for soil nematodes (reviewed by Robinson 2003). However, long distance dispersal has been the subject of very limitedresearch; mainly focused on wind- or water- mediated dispersal of root-feeding nematodes that have life-stages with morphological adaptations for adverse conditions (White 1953, Cadet and Albergel 1999, Villenave et al. 2003,

Oikos 120: 1649–1656, 2011 doi: 10.1111/j.1600-0706.2011.19540.x

© 2011 Th e Authors. Oikos © 2011 Nordic Society Oikos Subject Editor: Wim van der Putten. Accepted 4 March 2011

Th e review and decision to publish this paper has been taken by the above noted SE. Th e decision by the handling SE is shared by a second SE and the EiC.

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Plantard and Porte 2004, Nkem et al. 2006) or animal para-sites (Campos-Herrera et al. 2006, Krishnan et al. 2010).

One case highlighting the importance of soil-borne nematodes for plant community dynamics is the relation between these organisms and dune grasses along the Atlantic European coast (Van der Putten et al. 1993, Van der Stoel et al. 2002). In the absence of sand deposition, as in stabi-lized dunes, a complex of diff erent soil-borne organisms comprising mainly root-feeding nematodes, arbuscular mycorrhizal fungi and pathogenic fungi accumulates in the rhizosphere of dune plants which gradually leads to the die-out of plant stands making way for later successional species (Van der Putten et al. 1993, Vandegehuchte et al. 2010). Species of root-feeding nematodes occurring in dunes are often highly specialised, having a very narrow host range i.e. a few grass species occurring in fore-dunes (Karssen et al. 1998a, b, 2000, de la Pe ñ a et al. 2006, 2007). Migra-tion of nematodes in the soil occurs on a seasonal basis and is restricted to a few centimetres up to about a metre per year (Van der Stoel et al. 2002). However, rhizomes of dune plants get fragmented and swept away by the action of waves (Harris and Davy 1986a, Aptekar and Marcel 2000, Konlechner and Hilton 2009). Th erefore, rhizome fragments might serve as dispersal vectors for the associated fauna thus leading to long-distance dispersal of relatively sessile organisms.

Experimental evidence has shown the lack of local adap-tation between root-feeding nematodes and Ammophila arenaria , a dominant grass in foredunes. Still, studies have suggested a frequent allele exchange among populations of nematodes and host plants, thus assuming high disper-sal rates (Gandon and Michalakis 2002, de la Pe ñ a et al. 2009, Vandegehuchte et al. 2011). Long distance dispersal of nematodes might be an explanation for the observed pat-tern in plant – root herbivore interactions in coastal dunes. Nevertheless, there is no information about such events and the survival of nematodes or other fauna when embedded in rhizomes that are exposed to seawater. To investigate this possible mechanism of long distance dispersal, we experi-mentally tested the survival capacities of rhizomes of two common dune grasses A. arenaria and Elymus farctus, and of the root-feeding nematodes within these rhizomes, after immersion in seawater. Moreover, we assessed whether rhi-zome fragments recently stranded along the shore contained living nematodes and whether those fragment were able to resprout after passage through seawater. Th us, we experi-mentally tested the hypothesis that root-feeding nematodesare able to move across long distances inside rhizome fragments that are dispersed by seawater.

Material and methods

Studied species

Coastal dunes are characterized by primary succession, where plant species abundance and the replacement of plant species are related to the decrease in sand accretion and the plant-species specifi c pathogenic eff ects of the soil community (Van der Putten 2003). Elymus farctus occupies embryonic dunes and is replaced in yellow dunes by A. arenaria. Both species occur naturally in fore-dunes

along the European coastline (Tutin et al. 1980). Both species fl ower and produce abundant seeds and in some cases establishment and recruitment may occur through germination of these seeds. However, the main mechanism of spread is clonal growth (Laing 1958, Huiskes 1979, Harris and Davy 1986 b). Both species form rhizome buds from which shoots and roots are able to emerge after burial thus serving as propagules after catastrophic events (e.g. sea-storms) (Harris and Davy 1986a).

Resprouting capacities of dune rhizomes collected in the fi eld and infestation by nematodes

In March 2004, we visited the Het Zwin nature reserve in Belgium. Th e reserve is situated at the farthest north side of the Belgian coast and lies at the border with the Netherlands (51 ° 22 ’ 0.70 ” N, 3 ° 21 ’ 26.08 ” E). It consists of a salt marsh area (formerly a sea inlet that connected the coast-line with inland polders) fl anked by two dune areas domi-nated by A. arenaria . After a week of stormy weather (with strong off shore-inlandward winds) we collected recently stranded plant fragments along the beach parallel to the dune complex (Fig. 1A – D). In the laboratory the collected plant material was sorted out and only rhizome fragments with some shoots, allowing identifi cation, were kept for further analysis (Fig. 1E). A total of 403 fragments were selected for further analysis. From the 403 rhizomes, 289 fragments were identifi ed as A. arenaria and 96 fragments as Elymus farctus. Eighteen fragments were identifi ed as other grasses occur-ring at inner dune areas or salt marshes, i.e. Elymus athericus, Carex arenaria, Lolium spp., or were unidentifi able. Fromeach fragment roots were excised with a scalpel, weighed and transferred to a petri dish (5 cm ø ) fi lled with 15 ml of distilled water. Petri dishes were stored in an incubator (dark and 18 ° C). Th en, nematodes were counted in each petri dish daily during 10 days. When nematodes were detected, we transferred them to a glass slide and inspected them by microscope. Identifi cation of mature specimens was based on the morphology and morphometrics of dune-associated populations (de la Pe ñ a et al. 2007).

In order to assess the resprouting capacities of plant fragments stranded along the beach, rhizome fragments (5 – 12 cm) containing buds and from which nematodes had been extracted were planted in a tray fi lled with sterile dune sand (i.e. autoclaved for 1 h at 120 ° C and 1 atm.). Dur-ing two months we monitored on a daily basis the rhizome fragments and checked whether shoots or roots had emerged from the rhizomes.

Resprouting capacities of A. arenaria and E. farctus after passage through seawater

Ammophila arenaria rhizomes were collected from the dunes of ‘ Het Zwin ’ , about half way up the seaward dune face. Th e sand was removed until A. arenaria stems became visible. At the uppermost well-rooted node, 15 cm 3 of sand was col-lected including the upper rhizome buds of the plant tus-sock. We collected material from at least four diff erent plant stands separated 25 m from each other. Th e same procedurewas followed to obtain E. farctus rhizomes, but in this case we collected material from embryonic dunes and

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because E. farctus usually grows horizontally, we dug out the main runner and cut out fragments containing rhizome buds. Seawater was collected from the lower beach and fi ltered through a sieve of 40 μ m to remove algae and other particles suspended in the water.

In the laboratory the rhizome fragments were manually separated from the bulk soil and the rhizomes containing buds were chopped into fragments of 10 – 20 cm. For each species, we submerged the rhizome fragments in a plastic container fi lled with seawater. Water temperature ranged between 15 – 19 ° C during the whole course of the experiment. In order to avoid bacterial growth we renewed the stock with fresh seawater (kept at 4 ° C) at weekly intervals. At 0, 1, 3, 5, 10, 12, 15, 20, 30, 40, 50 and 60 days, 25 fragments were collected, washed with distilled water and placed in a tray with sterilized sand. Th e number of resprouting fragments was counted. Th is protocol was followed for a period of one month after the removal from the seawater tank.

Survival of root-feeding nematodes in roots incubated in seawater

Roots connected to rhizome buds were excised and chopped into fragments of approx. 1 cm and transferred to petri dishes with 20 ml fi ltered seawater. Root fragments were left in water for 0, 1, 3, 5, 10 and 15 days. After each incubation

period the root fragments were transferred to a similar petri dish containing distilled water in which nematodes were extracted from the fragments and counted. Th e petri dishes in which root fragments were incubated were also checked to assure the absence of root-feeding nematodes in the water. We had eight replicates per species and incubation time.

Survival of three Pratylenchus species associated with dune grasses

Species of the genus Pratylenchus (Nematoda: Pratylenchi-dae) are a key group of the nematode complex associated with the plants in coastal dunes (Van der Stoel et al. 2002, de la Pe ñ a et al. 2007, 2008). Th ey are endoparasites, mean-ing that they invade, feed and multiply within the root of the host plants causing necrotic lesions and eventually reducing plant growth (Moens and Perry 2009). We com-pared three species of root-feeding nematodes associated with A. arenaria i.e. P. brzeskii , P. dunensis and P. penetrans and examined their survival in roots after seawater immer-sion . Cultures of the three species used for the experiment were maintained in 15-l pots and contained between six and eight tussocks of A. arenaria (de la Pe ñ a et al. 2009) . For the experiment we uprooted plants from each culture and manually fragmented the root system into pieces of 5 cm. Rhizome fragments with roots were incubated in seawater

Figure 1. (A) Ammophila arenaria and Elymus fractus rhizomes deposited with sea debris along the shoreline; (B) Sea debris with rhizome fragments; (C – E) Detail of rhizome fragments collected along the shoreline; (F – G) Detail of root-feeding nematodes in Ammophila roots.

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seawater immersion than E. farctus. Resprouting of A. arenaria was observed even after 50 days in seawater with mean percentages of resprouting near 10%, while for E. farctus the percentage of resprouting dropped to percent-ages lower than 10% after only six days of submergence. For A. arenaria the percentage of survival did not change between 0 and 20 days in seawater (mean percentage of shooting frag-ments approx 60%). Only from 30 days onwards a signifi -cant reduction in the resprouting percentage was detected (Fig. 2).

Survival of root-feeding nematodes in roots incubated in seawater

We were able to extract living nematodes from the frag-ments collected from dune plant stands and submerged in seawater during diff erent lengths of time. In both grass spe-cies the number of nematodes extracted from root fragments decreased with the number of days in seawater (Table 1). Th e overall survival capacity of the nematodes did not dif-fer between the two plant species (Table 1). Nevertheless, the interaction between species and time in seawater was signifi cant indicating that the eff ect of the time in seawa-ter on survival depended on the plant species. For example, while in A. arenaria nematodes survived even after a stay of 20 days in seawater, in E. farctus nematodes could only be extracted from the roots that were incubated up to seven days (Fig. 3).

Survival of three Pratylenchus species associated to dune grasses

Root-feeding nematodes of the genus Pratylenchus were able to survive seawater passage, although the number of nema-todes extracted from roots decreased with the number of days in seawater. Th ere were diff erences in survival accord-ing to the nematode species, duration of incubation of root-fragments in seawater and the interaction of both factors (Table 1). Pratylenchus brzeskii was the most sea-tolerant species followed by P. dunensis and P. penetrans (Fig. 4) . While for P. brzeskii and P. dunensis survival of nematodeswas observed after 10 days of incubation in seawater (Fig. 4), survival of P. penetrans decreased dramatically after only one day of submersion, which is a reduction of 49%. No survival was observed for this species after 5 and 10 days of incubation of root fragments in seawater (Fig. 4).

Discussion

Th e results obtained from both the experiments and the analysis of rhizome fragments collected from the shore line demonstrate that root-feeding nematodes are able to sur-vive seawater immersion inside rhizomes. As these rhizomes are also able to resprout following submersion, dispersal by seawater may provide a mechanism for long distance disper-sal of both dune grasses and root-feeding nematodes.

Th e limiting factor in the movement of root-feeding nematodes is the rapid depletion of energy resources due to the lack of storage tissues. Th erefore, as obligate biotrophs, they rely on plants as a constant source of carbohydrates for

for 0 (control), 1, 3, 5 and 10 days. After each incubation period, we transferred the root fragments to distilled water and extracted root-feeding nematodes. We did not test the survival of nematodes directly in seawater, because pre-liminary experiments have shown that they were unable to survive the osmotic pressure.

Data analysis

Data on resprouting capacities of rhizome fragments were analyzed with a generalised linear mixed model (GLIM-MIX) using the Proc Glimmix in SAS for a binomial dis-tribution and correcting for under-dispersion. Resprouting probability was the dependent factor, whereas the fac-tors ‘ species ’ , ‘ time in seawater ’ and their interaction were modeled as independent variables. For the nematode counts in the two experiments studying nematode survival in sea-water, we used a generalised linear mixed model with a Poisson distribution with the same predictors. Diff erences between the three nematode species were assessed by post hoc Tukey test.

Results

Resprouting capacities of dune rhizomes after seawater passage and infestation by nematodes

Root-feeding nematodes (Fig. 1F – G) were extracted from 94 rhizome fragments of Ammophila arenaria (28% of all rhizomes) and from 10 rhizome fragments of Elymus farctus (22% of all rhizomes). Th e mean density of nema-todes in infested rhizomes was 19 � 4.67 nematodes g −1 and 3.82 � 1.66 nematodes g −1 for A. arenaria and E. farctus , respectively. A total of 367 nematodes were extracted from roots and their morphology revealed that the two most com-mon genera were Meloidogyne and Pratylenchus , present in 39% and 48% of the colonized fragments, respectively. Other genera were encountered less frequently: Heterodera (3%), Rotylenchus (2%) and Tylenchorhynchus (1.7%). Only second-stage juveniles (J2) of Meloidogyne could be retrieved and therefore identifi cation up to species level was not pos-sible. For Pratylenchus we identifi ed only adult female speci-mens. Th e most abundant species was P. brzeskii, followed by P. dunensis . No other Pratylenchus species were observed.

Only 22 A. arenaria rhizome fragments out of the 327 were able to resprout (i.e. 6.7%). From these 22 fragments we extracted root-feeding nematodes from the attached roots. None of the E. farctus fragments resprouted after two months in the experimental trays.

Resprouting capacities of A. arenaria and E. farctus after seawater passage

Th e survival of rhizomes in seawater was measured as the capacity of a rhizome fragment to resprout and to reroot after incubation in seawater. As expected, the number of days the fragment had spent in seawater had a signifi cant eff ect on the resprouting capacities of both species (Table 1). How-ever, the resprouting capacity as a function of the time spent in seawater, diff ered between the compared species (Table 1,Fig. 2). Ammophila arenaria had a greater resistance to

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While the resprouting percentage of fragments collected along the shore was near 7%, fragments directly collected from living plants stands and subsequently submerged in water showed 60% resprouting. Th ere are several possible explanations for the diff erent results between these two experiments: (1) even though we went one week after a cat-astrophic event (i.e. sea storm), a number of the collected fragments could have been there for several days and even weeks, therefore decreasing the mean resprouting percent-age; (2) the fragments used in the second experiment were only subject to seawater submersion but did not experience any other stress (e.g. mechanical stress caused by waves, friction against beach sand, exposure to sun and desicca-tion). Th erefore, the resprouting percentages are much higher than those of the fragments collected from the beach; (3) we might have biased the outcome of the fi rst experiment by selecting fragments with shoots and only one rhizome node (Fig. 1D). In A. breviligulata and E. farctus multi-node fragments have a greater chance of establishment and emer-gence than those on a single node (Maun 1984, Harris and Davy 1986b).

We did not examine the success of establishment of the fragments under fi eld conditions, but based on the results

establishment and multiplication (Wright and Perry 2006). Th e rationale behind the present study is that establishment of nematode populations after rhizome dispersal would only be possible if the rhizome is able to reshoot, thereby con-tinuing to provide carbohydrates to the nematodes. Th ere-fore, we studied the survival of the both nematodes and their host plants. Th e survival after passage through seawater of Ammophila arenaria, and in lesser extent Elymus farctus , was greater than that of nematodes, suggesting that: (1) a signifi -cant fraction of rhizome fragments that are washed up at a suitable site will establish free of their root herbivores and (2) those fragments containing living nematodes will provide upon establishment a direct source of nutrients to the nema-todes they harbor. It may also be possible that nematodes can migrate from recently stranded rhizome fragments into estab-lished plant roots. For that process, the success will depend on the time that the nematodes have been in seawater and the quantity of body resources available (Robinson 2003). Both these processes illustrate a dispersal mechanism previ-ously undescribed for root-feeding nematodes and – to our knowledge – other groups of soil micro- and mesofauna.

An important question that rises from our study is the likelihood of rhizome establishment in under fi eld conditions.

Table 1. Statistics of generalized linear mixed models in which the effect of species, time in seawater and the interaction term on several variables (resprouting of rhizomes, nematodes extracted/rhizome fragment and nematodes g −1 ) are compared .

Factor DF (numerator, denominator) F-value p

Resprouting of rhizomesSpecies (i.e. E. farctus and A. arenaria ) 1,671 0.001 0.9681Time in seawater 11,671 4.05 0.0001 Species � Time in seawater 10,671 2.01 0.0296

Nematodes extracted of rhizome fragmentSpecies (i.e. E. farctus and A. arenaria ) 1,128 0.001 0.97Time in seawater 8,128 12.11 0.0001 Species � Time in seawater 8,128 3.35 0.0016

Comparison of survival of three species of root-feeding nematodes

Species (i.e. P. brzeskii, P. dunensis, P. penetrans ) 3,79 588.07 0.0001 Time in seawater 4,79 632.77 0.0001 Species � Time in seawater 8,79 127.64 0.0001

Time in seawater (days)0 10 20 30 40 50 60

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Figure 2. Resprouting rates (mean � SE) of rhizome fragments of Ammophila arenaria ( • ) and Elymus farctus ( ) after diff erent numbers of days in seawater.

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izom

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Figure 3. Comparison of nematodes extracted (mean � SE) from rhizome fragment of Ammophila arenaria ( • ) and Elymus farctus ( ) according to time in seawater of the fragments (i.e. from 0 to 30 days in seawater).

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conditions. Alternatively, species may express specifi c responses (i.e. A. breviligulata vs A. arenaria ).

Th e survival of rhizome fragments after seawater immer-sion is not only relevant for the survival of nematodes inside the rhizomes, but is also to understand processes occurring at larger scales, for example the exchange of genetic material and individuals between populations of hosts and parasites. Regional studies on A. arenaria in Europe have already sug-gested that the exchange of plant material across regions occurs frequently but so far there was no direct evidence of the mechanisms of exchange. Analysis of ISSR microsatel-lite markers of diff erent European populations revealed that Dutch A. arenaria stands were more closely related to popu-lations located in south England than Flemish populations fl anking the Dutch populations (Rodr í guez-Echeverr í a et al. 2008); a fact suggesting gene fl ow between the popula-tions situated at both sides of the English Channel. Taking into account that along-shore drift in the southern area of the North Sea can reach constant speeds of 0.2 – 0.6 m s −1 (Koster 2005), there is a likelihood that rhizomes travel distances greater than 100 km in less than 10 days. Seed dispersal and establishment of A. arenaria from seeds is very rare in the fi eld (Huiskes 1977). Given the high resprout-ing rates after seawater immersion, rhizome dispersal seems to be a likely mechanism of exchange of individuals (i.e. propagules) between populations. To our knowledge there is no information available on the genetic structure of the European populations of E. farctus . Based on our results, we would predict that the dispersal range propagules and assisted dispersal of nematodes, should be considerably less than that of A. arenaria .

Herbivory (both below- and aboveground) may impose strong selection pressures in natural populations of these dune plants (de la Pe ñ a et al. 2008, Vandegehuchte et al. 2010). However, as seen in other systems, the forces exerted by herbivores on plant populations, and vice versa, may vary spatially and result in a geographic structuring of plant – herbivore interactions. Low dispersal and divergent selection pressures across sites will often result in genetic diff erentia-tion and eventually in local adaptation between the herbi-vores and the host-plant (Kingsolver et al. 2002, Hendry 2002). However, where this has been studied for A. arenaria (de la Pe ñ a et al. 2009, Vandegehuchte et al. 2011) the out-come of the interactions was strongly idiosyncratic with no diff erences in the performance of root-herbivores between host populations separated by long distances or in some cases performing better on allopatric plant populations. Our results suggest that amongst other factors the dispersal distance of nematodes may be greater when also including assisted long-distance dispersal, resulting in gene exchange between populations and, consequently, maladaptation in plant-herbivore associations (Gandon and Michalakis 2002, de la Pe ñ a et al. 2009).

Th e spatial distribution of soil fauna is assumed to be so extremely restricted that only rhizosphere-bound migra-tions seem to be important. However, our results provide a mechanism for long-distance dispersal and the interchange of genetic material of plants and nematode populations that can be applicable to other taxa and natural systems. For example, the roots of dune plants are also colonized by a high diversity of microorganisms, including pathogens,

obtain by Maun (1984) with Ammophila breviligulata , estab-lishment after one growth season occurren in 30% (Maun 1984, Maun 2009). Th erefore , when putting together the outcome of our two studies, we may expect in fi eld condi-tions relatively high percentages of establishment per season(i.e. higher than 6.7% as in our fi eld experiment, but probably less than 60% as in the second experiment).

Th e experiments of rhizome survival revealed that the two grass species diff er in their capacity to resprout after immersion in seawater. Interestingly, E. farctus being the fi rst species in the plant succession in coastal dune, was less able to resprout after seawater immersion than A. arenaria , which appears later in succession. Elymus farctus experienced a drastic decrease in resprouting capacity after only 3 days in seawater. In contrast, A. arenaria was able to resprout even after 60 days in seawater. Th ese results contradict our expectations since E. farctus is considered more salt-tolerant than A. arenaria (Laing 1955, Huiskes 1979). Diff erential resistance to seawater passage might account for diff erent re-colonization capacity after a catastrophic event. In suchevents, A. arenaria might re-colonize more easily than E. farctus .

In a previous study the North American pioneer dune grass Leymus mollis showed diff erent resprouting capaci-ties after long-term seawater submergence (Aptekar and Rejm á nek 2000). Th e percent bud viability (equivalent to what we called resprouting) dropped to 8.5% after only 13 days of incubation in seawater. On the other hand, the resprouting capacity of rhizomes of A. arenaria in New Zealand depended on seawater temperature, with a maxi-mal survival at winter seawater temperatures (Konlechner and Hilton 2009). Instead, our experiment was conducted at a fi xed temperature, with the growth chamber tempera-ture conditions set at 15 – 17 ° C, mimicking the conditions of seawater during late spring. Nonetheless, the percentages of resprouting are comparable to those of Konlechner and Hilton (2009) with a high viability of rhizomes after long term exposure to seawater. Th e disparity among our results and those of Apetekar and Rejm á nek may point at poten-tial eff ects of rhizome age, seawater composition, because the three experiments were conducted under diff erent

0 1 day 3-days 5-days 10-days

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Figure 4. Number of nematodes extracted (mean � SE) from Ammophila arenaria roots according to number of days of the roots in seawater and nematode species. Diff erent letters indicate signifi -cant diff erences between nematode species after post hoc Tukey-test (p � 0.05).

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fungal endophytes and mycorrhizal fungi (Koske et al. 1996, Hol et al. 2007, Rodr í guez-Echeverr í a et al. 2009). Th ese microbes may benefi t from assisted dispersal in the same way as root-feeding nematodes. Our results show that assisted dispersal of root-feeding nematodes may also depend on host species-specifi c susceptibility to adverse environmental conditions during transport.

Acknowledgements – Eduardo de la Pe ñ a is granted with a postdoc-toral fellowship by the Founds for Scientifi c Research-Flanders, Belgium (FWO). Th e authors thank Lieven Devreese and Robbert Scheppers for their assistance with some of the preliminary experi-ments of this project. We also thank Dr. Sergei Spiridonov for helping out in the collection of rhizome material in the fi eld and Prof. Roland Perry for correcting the manuscript and revising the English usage, and Prof. Wim van der Putten for providing valuable comments on a previous version of this manuscript.

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