BACKGROUND
several theories have been advanced to explain the chemical pathways and tissue differentiation strategies that have evolved to reduce the effect of competition between different individuals of different species.
common theories proposed to explain defense strategies in plants are: optimal defense theory   <cit> ; the growth-differentiation balance hypothesis   <cit> ; the resource availability theory   <cit>  and the plant apparency theory   <cit> .
odt predicts that plants should have the highest defense levels in parts that have the highest value in terms of fitness.
gdbh predicts that defense allocation will be a result of trade-offs between growth  and defense ; as long as all environmental factors are favorable for growth, growth processes predominate over differentiation  <cit> .
according to rat plants with abundant resources invest in growth rather than defense.
finally pat is based on the observation that both types of strategy  occur in plants but that they differ in cost.
odt arises from cost assumptions identified by pat, that is that defenses are costly in terms of fitness.
a further consequence is that environmentally stressed plants should be less well defended against herbivores, and therefore more palatable, than unstressed plants, as they have fewer resources available for defense  <cit> .
clearly, odt-pat assumptions  may seem incongruent with gdb-rat assumptions .
patterns of plant defense and resource allocation as a function of stress, disturbance and herbivore pressure have given rise to a considerable body of literature, especially in the terrestrial realm .
however, marine models have been relatively poorly investigated  <cit> .
interaction between the green alga c. taxifolia  c. agardh introduced into the mediterranean sea  <cit>  and the endemic seagrass p. oceanica  delile offers the opportunity to investigate the reliability, incongruence and/or complementarity of the theories comparing defense, growth and competition.
in addition, both species produce defense compounds  in such a way that interactions can be isolated and investigated.
here, we investigate defense strategies at the molecular level by evaluating the production of defense compounds  in c. taxifolia) and the influence of this production on growth over an annual growth cycle.
to this end, we identified an interaction gradient, i.e.
isolated populations and co-occurring populations and examined the effect of interaction on the two species.
the purpose of this paper is to determine whether the fitness of either plant is compromised in the presence of the other; and if fitness is indeed compromised, whether a pattern of defense may be identified.
RESULTS
leaf and frond length
p. oceanica shoots exhibited seasonal variation in the mean number  and mean length of leaves .
the length of both adult  and intermediate leaves  decreased significantly when the level of interaction with c. taxifolia increased .
as a result, the biomass of p. oceanica shoots decreased from l <dig>  through l <dig> ; for instance in may  <dig>  biomass is respectively,  <dig>  ±  <dig> ,  <dig>  ±  <dig>  and  <dig>  ±  <dig>  mg dry weight, for l <dig>  l <dig> and l <dig> .
similarly, the mean frond length of c. taxifolia changed seasonally  but in the opposite direction from p. oceanica : the length significantly increased with the level of interaction .
leaf renewal and primary production of posidonia oceanica
the number of p. oceanica leaves formed during a one-year period increased with the level of interaction  while the mean life-span of leaves decreased significantly .
the increase in the number of leaves produced during the study period generated an increase in the net primary production dedicated to leaf blades and sheaths , with an 82% increase observed between l <dig>  and l <dig>  .
tannin cells in posidonia oceanica leaves
the density of tannin cells varied significantly along p. oceanica adult leaf, with a peak in the central part of the leave .
a significant increase in the density of tannin cells in blades was apparent with increasing levels of interaction with c. taxifolia .
for example, at  <dig> mm above the base of the leaf, the mean density was  <dig>  ±  <dig>  cells cm- <dig> ,  <dig>  ±  <dig>   and  <dig>  ±  <dig>  .
phenolic compounds of posidonia oceanica leaves
five major phenolic compounds were identified; 4-hydroxybenzoic acid, 4-coumaric acid, trans-cinnamic acid, caffeic acid and a mixture  of at least two compounds, one of which is ferulic acid.
among minor phenolic compounds, the methyl 12-acetoxyricinoleate  presented changes with the level of interaction .
no clear seasonal trend was evident over time in the total phenolic content of p. oceanica leaves .
there is a weak but not significant  increase in mean total phenolic content between l <dig>  and l <dig> .
conversely, p <dig> and p <dig> exhibited a significant increase with the level of interaction with c. taxifolia .
caulerpenyne  content in caulerpa taxifolia fronds
cyn content of c. taxifolia fronds varied seasonally , with a maximum in summer and a minimum in spring .
whatever the season, cyn content varied as a function of the level of interaction with p. oceanica ; in july, for instance, cyn values were  <dig>  ±  <dig>  mg cyn g wet weight- <dig> ,  <dig>  ±  <dig>   and  <dig>  ±  <dig>  .
discussion
caulerpa taxifolia strategy
when in competition with p. oceanica, c. taxifolia exhibitsincreased frond length  and decreased cyn content .
this may be influenced by the low levels of irradiance observed beneath the p. oceanica canopy .
increased growth is often linked to light availability.
a similar competition type  was also observed in another invasive species, sargassum muticum  fensholt  <cit> .
clearly, the response of c. taxifolia to competition is to invest in growth rather than defense.
however, it is worth noting that increased frond length does not necessarily imply an increase in primary production, because longer fronds may be slender.
though terpenes should be considered as rather low cost defense metabolites  <cit> , they do appear to be too costly for c. taxifolia, since the plant reaction is to lower cyn concentration.
in general, terpene production, mainly cyn, defends c. taxifolia against herbivory  <cit>  but is also essential for the wound closure of the cells  <cit> .
for example c. taxifolia is avoided by herbivorous sea-urchins  and fish   <cit> .
according to  <cit> , c. taxifolia is less palatable to sea-urchins than p. oceanica in summer, when the terpene content is maximum, whereas the reverse occurs in winter.
being more palatable when co-existing with p. oceanica, c. taxifolia couldactually be grazed more frequently.
however, no conspicuous herbivore bites were observed at any time during field work.
conversely when compared with winter and spring values, the level of chemical defense in c. taxifolia remains relatively high in summer .
we confirmed observations by  <cit>  and  <cit>  with the finding that the annual cycle of cyn content exhibits dramatic changes between high summer and autumn values and relatively low winter and spring concentration .
this cycle is coupled with the growing season of c. taxifolia  <cit> .
our finding that the actively growing summer fronds of c. taxifolia could be more strongly chemically defended than decaying winter fronds illustrates odt  <cit> .
posidonia oceanica strategy
in contrast with c. taxifolia, the length of p. oceanica leaves decreases when in competition with c. taxifolia .
however, the leaf turn-over is faster, resulting in a reduction of leaf longevity and an increase in the number of leaves produced per year.
the primary production of p. oceanica is therefore enhanced by the presence of c. taxifolia .
<cit>  also observed the reduction of leaf longevity; however leaf length was either increased  or reduced .
as far as the production of defense metabolites is concerned, the p. oceanica strategy also differs from that of c. taxifolia.
firstly, the overall concentration of phenolic compounds does not decline.
secondly, some phenolic compounds  display an increase.
third, the density of tannin cells, which are specialized in the synthesis of the phenolic compounds  <cit>  increases .
while terpenes are recognized mostly in anti-herbivore interactions, phenolic compounds are supposed to be involved in defense against pathogens and allelopathy by reducing the growth of competing plants  <cit> .
nevertheless, c. taxifolia appears not affected by the increase of these phenolic compounds.
though superficial interpretation of our results could lead to the conclusion than p. oceanica invests in defense rather than growth , it appears that it in fact invests both in defense and growth .
another explanation for this energy imbalance may relate to the structure of the p. oceanica meadow and the production of below ground rhizomes.
these rhizomes constitute a storage organ for nutrients and polysaccharids  <cit> , as well as a route for the transfer of photosynthate between shoots  <cit> .
however, the translocation hypothesis would only work for site l <dig>  with photosynthates possibly provided by shoots located in the inner meadow.
as far as site l <dig> is concerned, the surface area of the meadow colonized by c. taxifolia is much larger than the maximum distance of translocation , which could invalidate the hypothesis.
CONCLUSIONS
like many invasive species, c. taxifolia is able to quickly colonize open areas and synthesize defense metabolites, namely terpene compounds, that are cheap to produce but with high turnover rates  <cit> .
conversely, p. oceanica grows very slowly  <cit> , is a late successional species  <cit>  and synthesizes defense phenolic compounds, that are costly to produce but more economical over time.
interference between these two species determines the reaction of both , which demonstrates that they compete for space and/or resources.
c. taxifolia invests in growth rather than in chemical defense, more or less matching the assumptions of odt and/or pat.
it is worth noting that the terpenes this plant produces are efficient against herbivores, but probably inefficient against competition with other plants, due to low water-solubility and rapid degradation in sea water  <cit> .
though investing in growth, c. taxifolia fronds are unable to grow taller than p. oceanica leaves .
this suggests that, at this study site, c. taxifola cannot successfully compete with p. oceanica for light.
in contrast, p. oceanica apparently invests in defense rather than growth, as predicted by the gdb hypothesis  <cit> .
however, we cannot rule out the possibility that p. oceanica may be successful in combining growth and defense.

 several theories have been advanced to explain the chemical pathways and tissue differentiation strategies that have evolved to reduce the effect of competition between different individuals of different species. common theories proposed to explain defense strategies in plants are: optimal defense theory; the growth-differentiation balance hypothesis; the resource availability theory and the plant apparency theory. odt predicts that plants should have the highest defense levels in parts that have the highest value in terms of fitness. gdbh predicts that defense allocation will be a result of trade-offs between growth and defense; as long as all environmental factors are favorable for growth, growth processes predominate over differentiation. according to rat plants with abundant resources invest in growth rather than defense. finally pat is based on the observation that both types of strategy occur in plants but that they differ in cost. odt arises from cost assumptions identified by pat, that is that defenses are costly in terms of fitness. a further consequence is that environmentally stressed plants should be less well defended against herbivores, and therefore more palatable, than unstressed plants, as they have fewer resources available for defense. clearly, odt-pat assumptions may seem incongruent with gdb-rat assumptions. patterns of plant defense and resource allocation as a function of stress, disturbance and herbivore pressure have given rise to a considerable body of literature, especially in the terrestrial realm. however, marine models have been relatively poorly investigated. interaction between the green alga c. taxifolia c. agardh introduced into the mediterranean sea and the endemic seagrass p. oceanica delile offers the opportunity to investigate the reliability, incongruence and/or complementarity of the theories comparing defense, growth and competition. in addition, both species produce defense compounds in such a way that interactions can be isolated and investigated. here, we investigate defense strategies at the molecular level by evaluating the production of defense compounds in c. taxifolia) and the influence of this production on growth over an annual growth cycle. to this end, we identified an interaction gradient, i.e. isolated populations and co-occurring populations and examined the effect of interaction on the two species. the purpose of this paper is to determine whether the fitness of either plant is compromised in the presence of the other; and if fitness is indeed compromised, whether a pattern of defense may be identified.  leaf and frond length p. oceanica shoots exhibited seasonal variation in the mean number and mean length of leaves. the length of both adult and intermediate leaves decreased significantly when the level of interaction with c. taxifolia increased. as a result, the biomass of p. oceanica shoots decreased from l through l; for instance in may biomass is respectively, ±, ± and ± mg dry weight, for l l and l. similarly, the mean frond length of c. taxifolia changed seasonally but in the opposite direction from p. oceanica: the length significantly increased with the level of interaction. leaf renewal and primary production of posidonia oceanica the number of p. oceanica leaves formed during a one-year period increased with the level of interaction while the mean life-span of leaves decreased significantly. the increase in the number of leaves produced during the study period generated an increase in the net primary production dedicated to leaf blades and sheaths, with an 82% increase observed between l and l. tannin cells in posidonia oceanica leaves the density of tannin cells varied significantly along p. oceanica adult leaf, with a peak in the central part of the leave. a significant increase in the density of tannin cells in blades was apparent with increasing levels of interaction with c. taxifolia. for example, at mm above the base of the leaf, the mean density was ± cells cm-, ± and ±. phenolic compounds of posidonia oceanica leaves five major phenolic compounds were identified; 4-hydroxybenzoic acid, 4-coumaric acid, trans-cinnamic acid, caffeic acid and a mixture of at least two compounds, one of which is ferulic acid. among minor phenolic compounds, the methyl 12-acetoxyricinoleate presented changes with the level of interaction. no clear seasonal trend was evident over time in the total phenolic content of p. oceanica leaves. there is a weak but not significant increase in mean total phenolic content between l and l. conversely, p and p exhibited a significant increase with the level of interaction with c. taxifolia. caulerpenyne content in caulerpa taxifolia fronds cyn content of c. taxifolia fronds varied seasonally, with a maximum in summer and a minimum in spring. whatever the season, cyn content varied as a function of the level of interaction with p. oceanica; in july, for instance, cyn values were ± mg cyn g wet weight-, ± and ±. discussion caulerpa taxifolia strategy when in competition with p. oceanica, c. taxifolia exhibitsincreased frond length and decreased cyn content. this may be influenced by the low levels of irradiance observed beneath the p. oceanica canopy. increased growth is often linked to light availability. a similar competition type was also observed in another invasive species, sargassum muticum fensholt. clearly, the response of c. taxifolia to competition is to invest in growth rather than defense. however, it is worth noting that increased frond length does not necessarily imply an increase in primary production, because longer fronds may be slender. though terpenes should be considered as rather low cost defense metabolites, they do appear to be too costly for c. taxifolia, since the plant reaction is to lower cyn concentration. in general, terpene production, mainly cyn, defends c. taxifolia against herbivory but is also essential for the wound closure of the cells. for example c. taxifolia is avoided by herbivorous sea-urchins and fish. according to, c. taxifolia is less palatable to sea-urchins than p. oceanica in summer, when the terpene content is maximum, whereas the reverse occurs in winter. being more palatable when co-existing with p. oceanica, c. taxifolia couldactually be grazed more frequently. however, no conspicuous herbivore bites were observed at any time during field work. conversely when compared with winter and spring values, the level of chemical defense in c. taxifolia remains relatively high in summer. we confirmed observations by and with the finding that the annual cycle of cyn content exhibits dramatic changes between high summer and autumn values and relatively low winter and spring concentration. this cycle is coupled with the growing season of c. taxifolia. our finding that the actively growing summer fronds of c. taxifolia could be more strongly chemically defended than decaying winter fronds illustrates odt. posidonia oceanica strategy in contrast with c. taxifolia, the length of p. oceanica leaves decreases when in competition with c. taxifolia. however, the leaf turn-over is faster, resulting in a reduction of leaf longevity and an increase in the number of leaves produced per year. the primary production of p. oceanica is therefore enhanced by the presence of c. taxifolia. also observed the reduction of leaf longevity; however leaf length was either increased or reduced. as far as the production of defense metabolites is concerned, the p. oceanica strategy also differs from that of c. taxifolia. firstly, the overall concentration of phenolic compounds does not decline. secondly, some phenolic compounds display an increase. third, the density of tannin cells, which are specialized in the synthesis of the phenolic compounds increases. while terpenes are recognized mostly in anti-herbivore interactions, phenolic compounds are supposed to be involved in defense against pathogens and allelopathy by reducing the growth of competing plants. nevertheless, c. taxifolia appears not affected by the increase of these phenolic compounds. though superficial interpretation of our results could lead to the conclusion than p. oceanica invests in defense rather than growth, it appears that it in fact invests both in defense and growth. another explanation for this energy imbalance may relate to the structure of the p. oceanica meadow and the production of below ground rhizomes. these rhizomes constitute a storage organ for nutrients and polysaccharids, as well as a route for the transfer of photosynthate between shoots. however, the translocation hypothesis would only work for site l with photosynthates possibly provided by shoots located in the inner meadow. as far as site l is concerned, the surface area of the meadow colonized by c. taxifolia is much larger than the maximum distance of translocation, which could invalidate the hypothesis.  like many invasive species, c. taxifolia is able to quickly colonize open areas and synthesize defense metabolites, namely terpene compounds, that are cheap to produce but with high turnover rates. conversely, p. oceanica grows very slowly, is a late successional species and synthesizes defense phenolic compounds, that are costly to produce but more economical over time. interference between these two species determines the reaction of both, which demonstrates that they compete for space and/or resources. c. taxifolia invests in growth rather than in chemical defense, more or less matching the assumptions of odt and/or pat. it is worth noting that the terpenes this plant produces are efficient against herbivores, but probably inefficient against competition with other plants, due to low water-solubility and rapid degradation in sea water. though investing in growth, c. taxifolia fronds are unable to grow taller than p. oceanica leaves. this suggests that, at this study site, c. taxifola cannot successfully compete with p. oceanica for light. in contrast, p. oceanica apparently invests in defense rather than growth, as predicted by the gdb hypothesis. however, we cannot rule out the possibility that p. oceanica may be successful in combining growth and defense. 
the primary production is therefore enhanced by the presence of c. taxifolia. in contrast, the length of p. oceanica leaves decreases when in competition with c. taxifolia. we have accordingly investigated defense metabolite content and growth year-round, on the basis of an interaction gradient. when in competition with p. oceanica, c. taxifolia exhibits increased frond length and decreased caulerpenyne – cyn content. c. taxifolia invests in growth rather than in chemical defense, more or less matching the assumptions of the odt and/or pat theories. however, on the basis of closer scrutiny of our results, the possibility that p. oceanica is successful in finding a compromise between more growth and more defense cannot be ruled out. in contrast, p. oceanica apparently invests in defense rather than growth, as predicted by the gdb hypothesis. interference between these two species determines the reaction of both, confirming that they compete for space and/or resources. while the overall concentration of phenolic compounds does not decline, there is an increase in some phenolic compounds and the density of tannin cells. however, the turnover is faster, resulting in a reduction of leaf longevity and an increase on the number of leaves produced per year.
 plant defense strategy is usually a result of trade-offs between growth and differentiation. interaction between the introduced green alga caulerpa taxifolia and the endemic seagrass posidonia oceanica in the mediterranean sea offers the opportunity to investigate the plausibility of these theories. we have accordingly investigated defense metabolite content and growth year-round, on the basis of an interaction gradient.  when in competition with p. oceanica, c. taxifolia exhibits increased frond length and decreased caulerpenyne – cyn content. in contrast, the length of p. oceanica leaves decreases when in competition with c. taxifolia. however, the turnover is faster, resulting in a reduction of leaf longevity and an increase on the number of leaves produced per year. the primary production is therefore enhanced by the presence of c. taxifolia. while the overall concentration of phenolic compounds does not decline, there is an increase in some phenolic compounds and the density of tannin cells.  interference between these two species determines the reaction of both, confirming that they compete for space and/or resources. c. taxifolia invests in growth rather than in chemical defense, more or less matching the assumptions of the odt and/or pat theories. in contrast, p. oceanica apparently invests in defense rather than growth, as predicted by the gdb hypothesis. however, on the basis of closer scrutiny of our results, the possibility that p. oceanica is successful in finding a compromise between more growth and more defense cannot be ruled out. 