sm83n4-4892

INTRODUCTION Top

Herbivores play a functional role in benthic marine ecosystems by channelling primary production to higher trophic levels (Poore et al. 2012Poore A.G.B., Campbell A.H., Coleman R.A., et al. 2012. Global patterns in the impact of marine herbivores on benthic primary producers. Ecol. Lett. 15: 912-922., Hillebrand 2009Hillebrand H. 2009. Meta-analysis of grazer control of periphyton biomass across aquatic ecosystems. J. Phycol. 45: 798-806., Gruner et al. 2008Gruner D.S., Smith J.E., Seabloom E.W., et al. 2008. A cross-system synthesis of consumer and nutrient resource control on producer biomass. Ecol. Lett. 11: 740-755.). Current seagrass herbivores are dominated by waterfowl, fish, urchins and small invertebrates, which have replaced large vertebrate herbivores (e.g. dugongs and manatees, turtles) (Thayer et al. 1984Thayer G.W., Bjorndal K.A., Ogden J.C., et al. 1984. Role of larger herbivores in seagrass community. Estuaries 7: 351-376., Heck and Valentine 2006Heck K.L., Valentine J.F. 2006. Plant-herbivore interactions in seagrass meadows. J. Exp. Mar. Bio. Ecol. 330: 420-436., Valentine and Duffy 2006Valentine J.F., Duffy J.E. 2006. The central role of grazing in seagrass ecology. In: Larkum A.W.D. et al. (eds) Seagrasses: Biology, Ecology and Conservation. Springer, Dordrecht, pp. 463-501.).

Invertebrate abundance associated with seagrass meadows may be three times greater than that of other highly productive ecosystems such as coral reefs (Nakamura and Sano 2005Nakamura Y., Sano M. 2005. Comparison of invertebrate abundance in a seagrass bed and adjacent coral and sand areas at Amitori Bay, Iriomote Island, Japan. Fish. Sci. 71: 543-550.). The invertebrate communities associated with seagrass meadows have a crucial importance in the cycling of carbon, controlling epiphyte biomass (Jaschinski et al. 2009Jaschinski S., Aberle N., Gohse-Reimann S., et al. 2009. Grazer diversity effects in an eelgrass-epiphyte-microphytobenthos system. Oecologia 159: 607-615., Jernakoff and Nielsen 1997Jernakoff P., Nielsen J. 1997. The relative importance of amphipod and gastropod grazers in Posidonia sinuosa meadows. Aquat. Bot. 56: 183-202.), sustaining higher trophic levels (Edgar and Shaw 1995aEdgar G.J., Shaw C. 1995a. The production and trophic ecology of shallow-water fish assemblages in southern Australia II. Diets of fishes and trophic relationships between fishes and benthos at Western Port, Victoria. J. Exp. Mar. Bio. Ecol. 194: 83-106.) and enabling, for example, the achievement of higher fish densities compared with adjacent environments (Edgar and Shaw 1995bEdgar G.J., Shaw C. 1995b. The production and trophic ecology of shallow-water fish assemblages in southern Australia 3. General relationships between sediments, seagrasses, invertebrates and fishes. J. Exp. Mar. Bio. Ecol. 194: 107-131).

Crustaceans are one of the most abundant invertebrate taxonomic group in epifaunal seagrass communities and, among crustaceans, amphipods are one of the dominant groups (Barnes 2017Barnes R.S.K. 2017. Patterns of benthic invertebrate biodiversity in intertidal seagrass in Moreton Bay, Queensland. Reg. Stud. Mar. Sci. 15: 17-25., Sturaro et al. 2015Sturaro N., Lepoint G., Vermeulen S., et al. 2015. Multiscale variability of amphipod assemblages in Posidonia oceanica meadows. J. Sea Res. 95: 258-271., Moore and Hovel 2010Moore E.., Hovel K. 2010. Relative influence of habitat complexity and proximity to patch edges on seagrass epifaunal communities. Oikos 119: 1299-1311., Sanchez-Jerez et al. 1999Sanchez-Jerez P., Barberá-Cebrián C., Ramos Esplá A. 1999. Comparison of the epifauna spatial distribution in Posidonia oceanica, Cymodocea nodosa and unvegetated bottoms: Importance of meadow edges. Acta Oecologica 20: 391-405.). The amphipods associated with seagrass systems are considered to be dominated by detritus or/and epiphyte feeders (Valentine and Duffy 2006Valentine J.F., Duffy J.E. 2006. The central role of grazing in seagrass ecology. In: Larkum A.W.D. et al. (eds) Seagrasses: Biology, Ecology and Conservation. Springer, Dordrecht, pp. 463-501.). Apart from sea urchins, the direct consumption of seagrass leaves by invertebrates is considered accidental and is generally associated with grazing on epiphytes (but see Rueda et al. 2009Rueda J.L., Salas C., Urra J., et al. 2009. Herbivory on Zostera marina by the gastropod Smaragdia viridis. Aquat. Bot. 90: 253-260.). Invertebrate herbivores’ preference for epiphytic algae rather than seagrass (Michel et al. 2014Michel L., Dauby P., Gobert S., et al. 2014. Dominant amphipods of Posidonia oceanica seagrass meadows display considerable trophic diversity. Mar. Ecol. 36: 969-981.) is frequently explained by the presence of chemical defence compounds in seagrass tissues and/or their lower nutrient content compared with macroalgae (Cruz-Rivera and Hay 2000Cruz-Rivera E., Hay M.E. 2000. Can quantity replace quality ? Food choice, compensatory feeding, and fitness of marine mesograzers. Ecology 81: 201-219., 2003Cruz-Rivera E., Hay M.E. 2003. Prey nutritional quality interacts with chemical defenses to affect consumer feeding and fitness. Ecol. Monogr. 73: 483-506.). Several amphipod species, particularly from the family Ampithoidae and the genus Gammarus (e.g. Gammarus mucronatus, G. locusta, G. oceanicus and Ampithoe longimana) show a specific level of tolerance to algal (e.g. Dyctiota, Gracilaria and Ulva) chemical defences (Andersson et al. 2009Andersson S., Persson M., Moksnes P.-O., et al. 2009. The role of the amphipod Gammarus locusta as a grazer on macroalgae in Swedish seagrass meadows. Mar. Biol. 156: 969-981., Duffy and Hay 1994Duffy J.E., Hay M.E. 1994. Herbivore resistance to seaweed chemical defense: the roles of mobility and predation risk. Ecology 75: 1304-1319.).

The dominant Mediterranean seagrass species, Posidonia oceanica, flowers irregularly, both spatially and temporally, and consequently seeds represent an eventual and ephemeral resource for herbivores (Díaz-Almela et al. 2006Díaz-Almela E., Marbà N., Alvarez E., et al. 2006. Patterns of seagrass (Posidonia oceanica) flowering in the Western Mediterranean. Mar. Biol. 148: 723-742.). Nutritionally, free sugars and starch are the main carbohydrates stored in P. oceanica seeds and represent between 2% and 10% (free sugars) and 4% and 30% (starch) of seed dry weight (DW) (Hernán et al. 2017Hernán G., Ortega M.J., Gándara A.M., et al. 2017. Future warmer seas: Increased stress and susceptibility to grazing in seedlings of a marine habitat-forming species. Glob. Chang. Biol. 23: 4530-4543., Celdrán and Marín 2013Celdrán D., Marín A. 2013. Seed photosynthesis enhances Posidonia oceanica seedling growth. Ecosphere 4: 1-11.). Regarding nutrient content, P. oceanica seeds exceed both adult and seedling leaves (Balestri et al. 2009Balestri E., Gobert S., Lepoint G., et al. 2009. Seed nutrient content and nutritional status of Posidonia oceanica seedlings in the northwestern Mediterranean Sea. Mar. Ecol. Prog. Ser. 388: 99-109.), but despite their comparatively low nutritional value, leaves represent an abundant and permanent potential trophic resource for the invertebrate community. P. oceanica leaves have a lower nutrient content (as % of DW) and a higher C/N ratio than leaf epiphytes or algae (Prado et al. 2010Prado P., Alcoverro T., Romero J. 2010. Influence of nutrients in the feeding ecology of seagrass (Posidonia oceanica L.) consumers: A stable isotopes approach. Mar. Biol. 157: 715-724., Lepoint et al. 2007Lepoint G., Jacquemart J., Bouquegneau J.M., et al. 2007. Field measurements of inorganic nitrogen uptake by epiflora components of the seagrass Posidonia oceanica (Monocotyledons, Posidoniaceae). J. Phycol. 43: 208-218.).

Vergés et al. (2007Vergés A., Becerro M.A., Alcoverro T., et al. 2007. Variation in multiple traits of vegetative and reproductive seagrass tissues influences plant-herbivore interactions. Oecologia 151: 675-686., 2011)Vergés A., Alcoverro T., Romero J. 2011. Plant defences and the role of epibiosis in mediating within-plant feeding choices of seagrass consumers. Oecologia 166: 381-390. studied the macroherbivore (Paracentrotus lividus) feeding preferences for different P. oceanica tissues and found that the inflorescences were preferred to leaves. The authors found no differences in the concentration of chemical defence compounds or in the nutritional value of different parts of the plant and suggested that this preference was driven by plant structural traits. Similar drivers (e.g. structural traits and nutrient content) could also affect amphipod preference to consume epiphytes rather than Posidonia leaves or litter fragments (i.e. Apherusa chiereghinii, Aora spinicornis and Gammarus aequicauda) or rizhomes (Dexamine spiniventris) (Michel et al. 2014Michel L., Dauby P., Gobert S., et al. 2014. Dominant amphipods of Posidonia oceanica seagrass meadows display considerable trophic diversity. Mar. Ecol. 36: 969-981.). There is little published evidence of direct consumption of P. oceanica tissues by amphipod crustaceans or by other herbivores (but see Guidetti 2000Guidetti P. 2000. Invertebrate borers in the Mediterranean sea grass Posidonia oceanica: Biological impact and ecological implications. J. Mar. Biol. Assoc. UK 80: 725-730., Peirano et al. 2001Guidetti P. 2000. Invertebrate borers in the Mediterranean sea grass Posidonia oceanica: Biological impact and ecological implications. J. Mar. Biol. Assoc. UK 80: 725-730.).

Here we assess whether P. oceanica seeds and leaves represent a trophic resource for two amphipod species commonly found in Mediterranean seagrass meadows and whether these amphipods show any feeding preference for leaves or seeds. Consumption and food choice experiments were performed in microcosms with the amphipods Ampithoe ramondi Audouin, 1826 and Gammarella fucicola Leach, 1814, two species commonly found in P. oceanica meadows (Bellan-Santini et al. 1982Bellan-Santini D., Karaman G., Krapp-Schickel G., et al. 1982. The Amphipoda of the Mediterranean. Mem. Inst. Oceanogr.(Monaco) 13: 1-364.). A. ramondi and G. fucicola show a broad distribution across the Mediterranean, Atlantic, Red Sea and Indian Ocean. Both species are described as mainly algae and detritus feeders (Michel et al. 2015Michel L.N., Dauby P., Dupont A., et al. 2015. Selective top-down control of epiphytic biomass by amphipods from Posidonia oceanica meadows: implications for ecosystem functioning. Belg. J. Zool. 145: 83-93., Zakhama-Sraieb et al. 2011Zakhama-Sraieb R., Sghaier Y.R., Charfi-Cheikhrouha F. 2011. Community structure of amphipods on shallow Posidonia oceanica meadows off Tunisian coasts. Helgol. Mar. Res. 65: 203-209., Lepoint et al. 2006Lepoint G., Cox A.-S.S., Dauby P., et al. 2006. Food sources of two detritivore amphipods associated with the seagrass Posidonia oceanica leaf litter. Mar. Biol. Res. 2: 355-365.). First, we performed consumption tests to determine whether A. ramondi and G. fucicola could feed on P. oceanica leaves and seeds. To this end, non-epiphytized leaves and seeds with or without a damaged coat were offered to amphipods. Next, we performed food choice experiments to determine whether amphipods preferred epiphytized versus non-epiphytized leaves and whether they preferred seeds (richer in stored resources) to leaves. We distinguished between seeds with an undamaged coat (“sealed seeds”) and a damaged coat (“open seeds”) to determine whether the seed coat protection was an intrinsic seed trait affecting amphipod food choice. We analysed nitrogen and phosphorus concentration in leaves and seeds and determined the mechanical resistance to puncture (a proxy of resistance to herbivory) of the same organs to enrich the discussion about food choices.

MATERIALS AND METHODSTop

Collection and identification

Drifting, naturally-produced Posidonia oceanica fragments, including leaves, rhizomes and roots and associated fauna, were collected at Alcúdia Bay (39.826292°N 3.177788°E) in June 2014 and housed in the laboratory inside a 4000 L tank (4 m long × 1 m wide × 1 m high) with continuous seawater input (84 L per hour) and recirculation. Tank temperature was kept below 22°C and day/night natural cycle was simulated with daylight fluorescent lights (280.0-0 lux). Light intensity and temperature were recorded using a data logger (Onset Hobo). A second collection of P. oceanica leaves and seedlings for the consumption and feeding choice tests was performed during summer 2015 and housed in a second tank of similar conditions to the one described above.

During summer 2015, amphipods associated with P. oceanica were collected in the first tank, fixed in ethanol 95% and transported to the laboratory for taxonomic identification. They were identified using a stereomicroscope (Leica MZ16 with integrated camera EC3) with the animals submerged in lactic acid. Four species were recorded in the samples: Ampithoe ramondi, Gammarella fucicola, Liljeborgia dellavallei and Microdeutopus stationis. Hereafter, the identifications were done on living amphipods and animal manipulation was reduced to the minimum to avoid stress or damage. Due to the low number of available individuals of L. dellavallei and M. stationis, consumption and preference tests were performed with A. ramondi and G. fucicola only.

Consumption tests

Herbivory on P. oceanica leaves and seeds was tested using containers made of transparent acrylic pipe of 4.8 cm internal diameter and 10 cm length. The top and bottom of each container were closed with a 0.5 mm nylon mesh to allow water exchange with the main tank and prevent amphipods from going out/in. The leaf portions and seeds used in the tests were measured (length and width) before the assays.

A number (between 5 and 12) of similar-sized amphipods of the same species were placed into each container together with one piece of leaf or seed. The consumption test endpoint was established after 6 days but the tests ended when detectable consumption occurred (with a minimum duration of 21 hours). Three feeding materials were offered to the amphipods separately: a portion of the second youngest leaf of a non-epiphytized P. oceanica shoot (gently scraped with a razor blade), a P. oceanica seed cut from a seedling at shoot base (as a proxy of a naturally damaged seed having holes in the seed coat, hereafter “open seed”) and a similar P. oceanica seed with the cut section sealed with 100% bee wax (“sealed seed”), as a proxy of a seed with an undamaged coat. Seed coats are usually covered by a hydrophobic waxy cuticle to prevent water exchange with the environment (Freeman 2008Freeman B.C., Beattie G.A. 2008. An overview of plant defenses against pathogens and herbivores. Plant Path. Microbiol. Publ. 94.), so beeswax was used to innocuously cover the section plane formed after cutting the seed from the seedling and to avoid amphipod access to internal seed tissues through the scar. A total of 43 tests were performed, 20 with A. ramondi (open and sealed seeds n=16; leaves n=4), 23 with G. fucicola (open and sealed seeds n=14; leaves n=9).

Preference tests

The same acrylic containers described above were placed in the tank with one amphipod and one of the following choice options: epiphytized leaf versus non-epiphytized leaf; non-epiphytized leaf versus open seed; and non-epiphytized leaf versus sealed seed. For each combination, between 10 and 20 trials were performed. Trials in which both or none of the offered materials were eaten were excluded. The number of valid replicates analysed for each combination of choice options and species were the following for A. ramondi and G. fucicola, respectively: epiphytized leaf/non-epiphytized leaf, n=10 and n=8; open seed/non-epiphytized leaf, n=10 and n=10; sealed seed/non-epiphytized leaf, n=11 and n=14. Preference tests lasted until the first consumption mark appeared (19-143 hours) or the animal died. Consumption marks were detected with a stereomicroscope (Zeiss Stemi DV4) and the most representative ones were captured using a Leica MZ16 with integrated EC3 camera.

Nutrient concentration analysis

At the end of the preference assays, the leaves and seeds were placed individually in plastic bags and stored frozen at –20°C until processing. In the laboratory, the leaves and seeds were dried out (60°C, 48 h) and ground to powder with a stainless steel ball mill (MM200 RETSCH, Haan, Germany). An aliquot of the ground material was used to determine total nitrogen content using a Heraeus CHN-o-rapid elemental analyser and phosphorous content following the protocol described by Fourqurean et al. (1992)Fourqurean J.W., Zieman J.C., Powell G.V.N. 1992. Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment. Mar. Biol. 114: 57-65. with certified standard beech leaves (CRM No. 100). Nitrogen and phosphorous content in leaves and seeds are expressed as the % of DW.

Tissue mechanical property tests

During the summer of 2016, eight P. oceanica seeds and shoots were collected to perform mechanical resistance tests. We tested second the youngest leaves in shoots, seeds with intact coat and seeds without coat (emulating open seeds in the treatments). To avoid differences in thickness between tested leaves, basal and apical portions of each leaf were not used. Seed slices 2 mm thick were used in tests. A Zwick Z100 mechanical testing machine was employed to perform punching tests, which measure the force (N mm^–2) required to punch a hole through the leaf lamina, a proxy of mechanical resistance to herbivory (Ibanez et al. 2013Ibanez S., Lavorel S., Puijalon S., et al. 2013. Herbivory mediated by coupling between biomechanical traits of plants and grasshoppers. Funct. Ecol. 27: 479-489., Aranwela et al. 1999Aranwela N., Sanson G., Read J. 1999. Methods of assessing leaf-fracture properties. New Phytol. 144: 369-393.). The punch and die method was adapted from Onoda et al. (2008)Onoda Y., Schieving F., Anten N.P.R. 2008. Effects of light and nutrient availability on leaf mechanical properties of Plantago major: A conceptual approach. Ann. Bot. 101: 727-736..

Statistical analyses

A chi-squared test was used to assess differences in amphipod feeding frequency depending on the type of food offered (i.e. epiphytized leaf, non-epiphytized leaf, open seed or sealed seed). The null hypothesis assumes independence of amphipod consumption pressure (frequency of bites) from food type. The expected frequencies under the null hypothesis were compared with the observed frequency of bites. Analysis of variance was performed to assess differences in mechanical resistance between leaf, coated seed and uncoated seed. A t-test was performed to evaluate leaf and seed nutritional features. A chi-squared test was done following Sokal and Rohlf (1981)Sokal R.R., Rohlf F.J. 1981. Biometry, W.H. Freeman and Company, New York, 859 pp.. One-way ANOVA and a t-test were performed with the Statistica 7.1 data analysis software system, StatSoft Inc.

RESULTSTop

Ampithoe ramondi and Gammarella fucicola were able to feed on Posidonia oceanica leaves and seeds (either open or sealed). All the leaves offered to A. ramondi were attacked (100%), whereas 80% and 60% of open and sealed seeds were bitten, respectively. G. fucicola bit all the open seeds offered and 60% of the sealed seeds; it fed on 67% of the leaves offered. A. ramondi and G. fucicola started scraping the seed coat and bored through, forming irregular holes. The marks on the leaves displayed a dogtooth pattern (Fig. 1) on the leaf margin. Visual differences between marks produced by the two species were unnoticeable using a stereomicroscope (Zeiss Stemi DV4).

Full size image

Fig. 1. – Dogtooth bite pattern on leaves and irregular holes on seeds produced by Gammarella fucicola and Ampithoe ramondi. Scale bars show 1.0 cm and 0.5 cm for leaf (A, B) and seed (C, D) photos respectively. A specimen of A. ramondi is also shown in photo A.

Feeding choice test

The two species showed a similar pattern with respect to feeding preferences. Open seeds were preferred over non-epiphytized leaves, but this choice reversed when sealed seeds were offered. Apparently, both species preferred epiphytized leaves over non-epiphytized leaves, although the chi-squared statistic was not significant at this point, probably because of the low number of replicates (Fig. 2).

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Fig. 2. – Frequency of bites of Gammarella fucicola and Ampithoe ramondi on leaves, open seeds and sealed seeds. Chi-squared statistic and statistical significance is shown: ** p<0.01, *** p<0.001.

Mechanical and nutritional traits

The leaves showed a lower mechanical resistance to herbivory (mean±SE: 0.94±0.086 N mm^–2) than open seeds (1.88±0.171 N mm^–2), which were pierced more easily than seeds with a coat (2.21±0.168 N mm^–2) (ANOVA: F=51.7016; p<0.0001) (Fig. 3). Nitrogen content was higher (t-value=3.6078, p<0.01) in seeds (1.86±0.055% N) than in leaves (1.14±0.066% N). There were no differences (t-value=1.6468, p>0.5) in phosphorus content between leaves and seeds) (Fig. 3).

Full size image

Fig. 3. – Mechanical resistance and nutrient content of leaves, open seeds and seed with coat. Error bars represent standard error. Punch strength (N mm^–2) for leaves, open seeds and seeds with coat. Nutrient content (% DW) of seeds and leaves. Differences between groups are indicated by different letters.

DISCUSSIONTop

We identified two potential consumers of Posidonia oceanica leaves and seeds in the field: the gammarid amphipods Ampithoe ramondi and Gammarella fucicola. Both species preferred nutritionally poorer leaves to the richer seed tissue when the seed coat was intact. However, this choice pattern reversed when the seed coat was damaged, suggesting that the coat protects the seed against invertebrate herbivory. Seed protection against herbivory assures carbon and nutrient supply, which are essential for seedling survival and the success of recruitment. Plants can prevent seed herbivory through chemical (i.e. secondary metabolites) (e.g. Rhoades and Cates 1976Rhoades D.F., Cates R.G. 1976. Toward a general theory of plant antiherbivore chemistry. In: Wallace J.W., Mansell R.L. (eds) Biochemical Interaction Between Plants and Insects. Recent Advances in Phytochemistry book series vol. 10. Springer, Boston, pp. 168-213., Veldman et al. 2007Veldman J.W., Greg Murray K., Hull A.L., et al. 2007. Chemical defense and the persistence of pioneer plant seeds in the soil of a tropical cloud forest. Biotropica 39: 87-93.) or structural defences (e.g. coat strength) (Davis et al. 2008Davis A.S., Schutte B.J., Iannuzzi J., et al. 2008. Chemical and physical defense of weed seeds in relation to soil seedbank persistence. Weed Sci. 56: 676-684., Rodgerson 1998Rodgerson L. 1998. Mechanical defense in seeds adapted for ant dispersal. Ecology 79: 1669-1677.). The coat, as the outermost protective tissue of seeds, is the first line of defence against pathogens and herbivores (Freeman 2008Freeman B.C., Beattie G.A. 2008. An overview of plant defenses against pathogens and herbivores. Plant Path. Microbiol. Publ. 94.). Our results suggest that the mechanical defence associated with the presence of a coat on P. oceanica seeds effectively discourages A. ramondi and G. fucicola herbivory. Apart from this study, the understanding of P. oceanica seed mechanical defence is still poor. Seed coat chemical defences have not been evaluated in this work, but they might also drive herbivore preference. In turn, P. oceanica leaves display the strongest mechanical defences known among seagrasses; they show a substantially higher proportion of fibre than terrestrial herbaceous plants (De los Santos et al. 2016De los Santos C.B., Onoda Y., Vergara J.J., et al. 2016. A comprehensive analysis of mechanical and morphological traits in temperate and tropical seagrass species. Mar. Ecol. Prog. Ser. 551: 81-94., Onoda et al. 2011Onoda Y., Westoby M., Adler P.B., et al. 2011. Global patterns of leaf mechanical properties. Ecol. Lett. 14: 301-312.), which seems to deter macroherbivores (Vergés et al. 2007Vergés A., Becerro M.A., Alcoverro T., et al. 2007. Variation in multiple traits of vegetative and reproductive seagrass tissues influences plant-herbivore interactions. Oecologia 151: 675-686., 2011Vergés A., Alcoverro T., Romero J. 2011. Plant defences and the role of epibiosis in mediating within-plant feeding choices of seagrass consumers. Oecologia 166: 381-390.). P. oceanica seed phenolic content (about 6% of seed DW) (Hernán et al. 2016Hernán G., Ramajo L., Basso L., et al. 2016. Seagrass (Posidonia oceanica) seedlings in a high-CO₂ world: from physiology to herbivory. Sci. Rep. 6: 38017., 2017Hernán G., Ortega M.J., Gándara A.M., et al. 2017. Future warmer seas: Increased stress and susceptibility to grazing in seedlings of a marine habitat-forming species. Glob. Chang. Biol. 23: 4530-4543.) exceeds the phenolic concentration found for seagrass species with higher seed production (e.g. Reynolds et al. 2012Reynolds L.K., Carr L.A., Boyer K.E. 2012. A non-native amphipod consumes eelgrass inflorescences in San Francisco Bay. Mar. Ecol. Prog. Ser. 451: 107-118.). Given the high amount of seed structural reserve storage, P. oceanica has a high theoretical reproductive effort capacity (Cabaço and Santos 2012Cabaço S., Santos R. 2012. Seagrass reproductive effort as an ecological indicator of disturbance. Ecol. Indic. 23: 116-122.). P. oceanica seed production is low compared with other seagrass species (Díaz-Almela et al. 2006Díaz-Almela E., Marbà N., Alvarez E., et al. 2006. Patterns of seagrass (Posidonia oceanica) flowering in the Western Mediterranean. Mar. Biol. 148: 723-742., Conacher et al. 1994bConacher C.A., Poiner I.R., O’Donohue M. 1994b. Morphology, flowering and seed production of Zostera capricorni Aschers. in subtropical Australia. Aquat. Bot. 49: 33-46., Silberhorn et al. 1983Silberhorn G.M., Orth R.J., Moore K.A. 1983. Anthesis and seed production in Zostera marina L. (eelgrass) from the Chesepeak Bay. Aquat. Bot. 15: 133-144.), and a strong chemical defence would be essential for seed protection, seedling recruitment and, thus, for the maintenance and persistence of the meadows (Kendrick et al. 2012Kendrick G.A., Waycott M., Carruthers T.J.B., et al. 2012. The central role of dispersal in the maintenance and persistence of seagrass populations. Bioscience 62: 56-65.). Since P. oceanica seeds have no dormancy, a positive effect of seed scarring by amphipods is not expected (Conacher et al. 1994aConacher C.A., Poiner I.R., Butler J., et al. 1994a. Germination, storage and viability testing of seeds of Zostera capricorni Aschers. from a tropical bay in Australia. Aquat. Bot. 49: 47-58., Loques et al. 1990Loques F., Caye G., Meinesz A. 1990. Germination in the marine phanerogam Zostera noltii Hornemann at Golfe Juan, French Mediterranean. Aquat. Bot. 38: 249-260.). Our results show that a seed coat deters the attack by small invertebrates, likely enhancing seedling survival.

Previous studies have assessed the influence of nutritional quality of algae and the presence of chemical defence compounds on the feeding choice and ingestion rate of marine invertebrate herbivores (Vergés et al. 2007Vergés A., Becerro M.A., Alcoverro T., et al. 2007. Variation in multiple traits of vegetative and reproductive seagrass tissues influences plant-herbivore interactions. Oecologia 151: 675-686., 2011Vergés A., Alcoverro T., Romero J. 2011. Plant defences and the role of epibiosis in mediating within-plant feeding choices of seagrass consumers. Oecologia 166: 381-390., Cruz-Rivera and Hay 2000Cruz-Rivera E., Hay M.E. 2000. Can quantity replace quality ? Food choice, compensatory feeding, and fitness of marine mesograzers. Ecology 81: 201-219., Duffy and Hay 1994Duffy J.E., Hay M.E. 1994. Herbivore resistance to seaweed chemical defense: the roles of mobility and predation risk. Ecology 75: 1304-1319.), but similar studies on seagrass are still scarce. In addition, plant tissue toughness has been widely recognized as the main constrictor of invertebrate herbivory in terrestrial systems, well above plant nitrogen content (Caldwell et al. 2016Caldwell E., Read J., Sanson G.D. 2016. Which leaf mechanical traits correlate with insect herbivory among feeding guilds? Ann. Bot. 117: 349-361., Ibanez et al. 2013Ibanez S., Lavorel S., Puijalon S., et al. 2013. Herbivory mediated by coupling between biomechanical traits of plants and grasshoppers. Funct. Ecol. 27: 479-489.). The importance of mechanical characteristics of seagrass compared with its nutritional quality in determining food choice in small marine invertebrates had not been recognized until now. The importance of mechanical traits and fibre content in the food choice of large marine invertebrates (i.e. sea urchins) had been previously acknowledged in algae (Cruz-Rivera and Friedlander 2011Cruz-Rivera E., Friedlander M. 2011. Feeding preferences of mesograzers on aquacultured Gracilaria and sympatric algae. Aquaculture 322-323: 218-222.) and seagrasses (Jiménez-Ramos et al. 2017Jiménez-Ramos R., Egea L.G., Ortega M.J., et al. 2017. Global and local disturbances interact to modify seagrass palatability. PLoS ONE 12: e0183256.), and our results suggest that similar food choice mechanisms may also operate for amphipods.

Posidonia oceanica leaves and seeds are a complementary food source for certain species, especially in healthy meadows where the amphipod community is richer and denser (Zakhama-Sraieb et al. 2006Zakhama-Sraieb R., Sghaier Y.-R., Charfi-Cheikhrouha F. 2006. Is amphipod diversity related to the quality of Posidonia oceanica beds? Biol. Mar. Mediterr. 13: 174-180.). Our work shows that small amphipods (e.g. A. ramondi and G. fucicola) may use P. oceanica epiphytized leaves as a trophic resource and eventually benefit from seeds, especially when the coat protection is damaged. Seed availability and their higher nutritional value compared with leaves (Hernán et al. 2016Hernán G., Ramajo L., Basso L., et al. 2016. Seagrass (Posidonia oceanica) seedlings in a high-CO₂ world: from physiology to herbivory. Sci. Rep. 6: 38017., 2017Hernán G., Ortega M.J., Gándara A.M., et al. 2017. Future warmer seas: Increased stress and susceptibility to grazing in seedlings of a marine habitat-forming species. Glob. Chang. Biol. 23: 4530-4543.) would also drive the amphipod food preference for seeds. The preference of A. ramondi and G. fucicola for epiphytized leaves rather than non-epiphytized leaves is in accordance with the algae and detritus feeding behaviour considered for both species elsewhere (Michel et al. 2014Michel L., Dauby P., Gobert S., et al. 2014. Dominant amphipods of Posidonia oceanica seagrass meadows display considerable trophic diversity. Mar. Ecol. 36: 969-981., Navarro-Barranco et al. 2013Navarro-Barranco C., Tierno-de-Figueroa J.M., Guerra-García J.M., et al. 2013. Feeding habits of amphipods (Crustacea: Malacostraca) from shallow soft bottom communities: Comparison between marine caves and open habitats. J. Sea Res. 78: 1-7., Lepoint et al. 2006Lepoint G., Cox A.-S.S., Dauby P., et al. 2006. Food sources of two detritivore amphipods associated with the seagrass Posidonia oceanica leaf litter. Mar. Biol. Res. 2: 355-365.). However, the relative importance of the different food sources in their diet varies among studies; even crustacean rests have been found in the gut content of G. fucicola (Michel et al. 2014Michel L., Dauby P., Gobert S., et al. 2014. Dominant amphipods of Posidonia oceanica seagrass meadows display considerable trophic diversity. Mar. Ecol. 36: 969-981.), suggesting an opportunist and generalist feeding behaviour. Changes in available trophic resources, nutritional quality, quantity, and palatability will have stronger effects on the food choice and consumption rate of opportunistic consumers than on specialists.

The role of small herbivores as drivers of ecological processes in Mediterranean meadows, such as in seed-based seagrass recruitment or the percentage of seagrass organic matter transferred to higher trophic levels, remains elusive; mesocosm or tethering field study approaches should be performed to address it. Results of studies on Western Australian (Orth et al. 2002Orth R.J., Heck K.L., Tunbridge D.J. 2002. Predation on seeds of the seagrass Posidonia australis in Western Australia. Mar. Ecol. Prog. Ser. 244: 81-88.) and North Pacific (Nakaoka 2002Nakaoka M. 2002. Predation on seeds of seagrasses Zostera marina and Zostera caulescens by a tanaid crustacean Zeuxo sp. Aquat. Bot. 72: 99-106.) meadows suggest that seed ingestion by small invertebrates may be a significant factor in seed-based recruitment failure (percentage of damaged seeds: 34%-53% in Posidonia australis, 14% in Zostera marina and 27% in Zostera caulescens). A field assessment of the importance of amphipod herbivore pressure on P. oceanica tissues remains to be carried out.

ACKNOWLEDGEMENTSTop

This work was possible thanks to the collaboration of the Cabrera Archipelago National Park. Funds were provided by Red Eléctrica de España in the framework of the project “Use of P. oceanica seedlings and fragments for the restoration of areas affected by Red Eléctrica de España activity”. Red Eléctrica de España was not involved in the study design, collection, analysis, interpretation of data or the writing of the manuscript.

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