Aquatic Botany, 46 (1993) 293-299 0304-3770/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

293

Sexual reproduction and seed banks of Cymodocea nodosa (Ucria) Ascherson meadows

on the southeast Mediterranean coast of Spain

J. Terrados 1 Departamento de Biologia Animal y Ecologia, Universidad de Murcia, Campus Universitario de

Espinardo, 30100 Murcia, Spain

(Accepted 10 August 1993 )

Abstract

Flowering, fruiting and seed germination of the seagrass Cymodocea nodosa (Ucria) Ascherson are recurrent in the Mar Menor coastal lagoon and other places on the southeast Mediterranean coast of Spain. Flowers showed a clumped pattern of spatial distribution (P< 0.01 ) and their density values ranged between 4.7 and 44.8 flowers m -2. NO spatial association was found between male and female flowers (P< 0.01 ) at the spatial scale considered (2500 cm2). Seed banks ranged between 0 and 220.3 seeds m -z, and the seeds showed also a dumped spatial distribution (P<0.01). The vegetative de- velopment of some low-density meadows originating from seeds and the effect of water movement on the fate of seed production are discussed.

Introduction

The seagrass Cymodocea nodosa (Ucria) Ascherson is widely distributed throughout the Mediterranean Sea and the North-Atlantic coast of Africa (den Hartog, 1970). Although flowering and fruiting of this species is common in some places (Lipkin, 1977; Caye and Meinesz, 1985; Buia and Mazzella, 1991 ), regular germination of seeds has only been recorded at meadows around the island of Ischia, Italy (Buia and Mazzella, 1991 ). Thus, it appears that successful sexual reproduction of C. nodosa is scarce in many places of the Mediterranean.

Cymodocea nodosa forms extensive meadows in the shallow Mar Menor (depth less than 2 m), a hypersaline coastal lagoon on the southeast Mediter- ranean coast of Spain (Terrados and Ros, 1992). Several field observations (Terrados, 1991 ) suggest that sexual reproduction of C. nodosa is regular in this lagoon. This seagrass also forms extensive meadows outside the Mar

lpresent address: Department of Biology, San Diego State University, San Diego, CA 92182- 0057, USA.

294 J. Terrados / Aquatic Botany 46 (1993) 293-299

Menor (Calvin et al., 1989), but nothing is known about the occurrence of sexual reproduction in them. This study addresses the following questions about the sexual reproduction of C. nodosa: does sexual reproduction occur in the C. nodosa meadows on the southeast Mediterranean coast of Spain?; if so, are there differences in the magnitude of the seed banks of the main C. nodosa meadows along this coast?; does the proximity between male and fe- male flowers in a meadow have an effect on plant seed production?; can new C. nodosa meadows be originated from seeds?

Materials and methods

All the large C. nodosa meadows present on the coast of Murcia (Calvin et al., 1989; Terrados, 1991 ) (Fig. 1 ) were sampled during the flowering season of 1992. Seeds were collected randomly from the meadows by SCUBA divers using PVC cylinders of 8.5 cm diameter and 15 cm height. The number of sample units per meadow varied between 54 and 90. Samples were sieved in situ through a 2 ram-mesh. After counting, the seeds were buried again in the sediment. Fifty-two seeds randomly collected at Isla del Ciervo meadow were tested for viability using the 2,3,5-triphenil tetrazolium chloride vital-stain

:i.~:~!~i.+:~.i . . . . . '~7-'.~. ~-,~-Z' -~.-r~ . . . . . , r ' ~ -~

~ A ¢ . . . ¢ - - ~ t ~ : . " ~ " _ _ _ +~., + . ~ . ~ ' ¢ ~ 1 4 0 5 10 15 2 0 KM ~ .~,Z'~ ~ ~ ~ • '

Fig. 1. Location of the C. nodosa meadows studied on the southeast Mediterranean coast of Spain: 1, Barraca Quemada ( 3 m depth) ; 2, Encafiizada de la Torre (0.5 m depth); 3, Veneziola (0.5 m depth) ; 4, Pudrimel (6 m depth) ; 5, Gahin (0.5 m depth) ; 6, Isla del Ciervo (1 m depth); 7, Cala Reona (12 m depth) ; 8, Azohia (3 m depth) ; 9, Isla Plana (F, 3 m depth; P, 8 m depth) ; 10, Bolnuevo (6 m depth) ; 11, Percheles (8 m depth) ; 12, Calnegre (6 m depth) ; 13, Cope (7 m depth) ; 14, Isla del Fraile (5 m depth) .

J. Terrados /Aquatic Botany 46 (1993) 293-299 295

test (Harrison, 1990). The density and the spatial distribution pattern of male and female flowers were estimated from countings using 2500 cm 2 squares randomly located inside the meadow. The number of sample units per meadow varied between 47 and 62. The density and the spatial distribution pattern of seedlings around well-established meadows were estimated from countings using 1 m 2 squares located at random. The number of sample units per meadow varied between 41 and 68. The number of shoots and rhizome branchings were recorded for each seedling, and its age was estimated accord- ing to Caye and Meinesz ( 1985 ).

Significant differences of seed, flower and seedling density between mead- ows were tested using Student's t-test (texp) and a one way analysis of vari- ance (ANOVA, F) or the non-parametric tests of Mann-Whitney (U) and Kruskal-Wallis (W) (Sokal and Rohlf, 1979). The spatial distribution of seeds, flowers and seedlings was first established by calculating the variance- to-mean sample ratio (index of dispersion, ID). Random and clumped pat- terns were detected by performing goodness-of-fit chi-square tests to Poisson and Negative Binomial probability distributions (Sokal and Rohlf, 1979). Spatial association between male and female flowers was established by con- structing 2 × 2 contingency tables and calculating a chi-square test statistic (X2t) (Ludwig and Reynolds, 1988 ).

Results

Except for only one sampling station (Cope), seeds were found in all the meadows studied (Table 1 ). Seed banks varied between 2.7 and 220.3 seeds m-2; the higher values were recorded from meadows inside the Mar Menor lagoon (Isla del Ciervo, Veneziola). The seeds showed a clumped pattern of spatial distribution (Table 1 ). The viability of seeds at Isla del Ciervo meadow was 100%.

Male and female flowers were observed at all the sampling stations, with the exception of the Cope meadow. Isla del Ciervo and Encafiizada de la Torre meadows (located inside the Mar Menor) had higher flower densities ( W= 73.3120, P= 0.0000) than the Percheles meadow (outside Mar Menor) (Table 2). Male flowers were more abundant than females in Encafiizada de la Torre (U=7.696, P=0.0000) and Percheles (U=2.411, P=0.0159) meadows, but not at Isla del Ciervo (U= 1.709, P=0.0875). No spatial as- sociation was found between male and female flowers, and their pattern of spatial distribution was clumped (Table 2).

Although seedlings were found in all the C. nodosa meadows with seed banks, only the meadows inside the Mar Menor had seedlings in their sur- roundings. The density of seedlings was similar (U= 1.079, P=0.2807) at Veneziola and Gal~ln I sampling stations (Table 3), and higher ( W= 23.0251, P= 0.0000) than at Gal~tn II. Seedlings were distributed at random at Gal~in

296 J. Terrados / Aquatic Botany 46 (1993.) 293-299

Table 1 Density of seeds in the sediment of the C. nodosa meadows studied

Sampling station n Seeds per core Seeds m -2 ID X 2 d.f. P

Mar Menor Isla del Ciervo 60 1.25 (0.26) 220.3 3.16 1.1489 1 0.2838 Veneziola 60 1.10 (0.22) 193.8 2.69 0.9691 1 0.3269 Gahin 65 0.01 (0.01) 2.7 0.98 n.d. n.d. n.d.

Mediterranean Isla del Fraile 60 0.53 (0.21) 93.9 4.78 0.1773 1 0.6737 Bolnuevo 90 0.42 (0.12) 74.4 2.95 0.4610 1 0.4971 Percheles 80 0.40 (0.15) 70.5 4.79 0.4752 1 0.4906 Azohia 60 0.28 (0.08) 49.9 1.42 0.0082 1 0.9779 Calnegre 60 0.27 (0.16) 46.9 5.48 0.1261 1 0.7225 Pudrimel 73 0.12 (0.05) 21.7 1.54 0.3419 1 0.5587 Isla Plana F 60 0.07 (0.03) 11.7 0.93 n.d. n.d. n.d. Isla Plana P 60 0.05 (0.03) 8.8 0.95 n.d. n.d. n.d. Barraca Quemada 70 0.04 (0.04) 7.5 2.96 n.d. n.d. n.d. Cala Reona 54 0.04 (0.02) 6.5 0.96 n.d. n.d. n.d. Cope 60 0.00 (0.00) 0.0 n.d. n.d. n.d. n.d.

n, number of sampling units (sediment cores) taken; seeds per core, mean number of seeds present in a sampling unit (standard error in brackets); ID, variance-to-mean sample ratio; X 2, chi-square statistic of the goodness-of-fit test to Negative Binomial probability distribution; d.f., degrees of free- dom; P, significance level; n.d., not determined because of the small number of seeds found.

Table 2 Density and spatial distribution of flowers in some C. nodosa meadows

Sampling n Flowers per quadrat Flowers m -2 ID Z 2 d.f. P station

Percheles Male 62 0.9 (0.2) 3.9 3.26 0.7008 1 0.4025 Female 62 0.2 (0.1) 0.8 1.31 0.0241 1 0.8767

Z2t 1.7592 1 0.1847 Isla del Ciervo Male 47 3.3 (0.7) 13.3 6.88 0.5192 1 0.4712 Female 47 1.3 (0.3) 5.1 3.59 2.0464 2 0.3594

Z2t 0.0040 1 0.9495 Encafiizada de la Torre Male 57 9.7 (1.0) 39.1 6.25 4.3987 4 0.3547 Female 57 1.4 (0.2) 5.7 1.95 2.9626 2 0.2273

Z2t 0.0156 1 0.9006

n, number of sampling units (2500 cm 2 quadrats) taken; flowers per quadrat, mean number of flowers present in a sampling unit (standard error in brackets); ID, variance-to-mean sample ratio; X 2, chi- square statistic oftbe goodness-of-fit test to Negative Binomial probability distribution; d.f., degrees of freedom; P, significance level. Z2t, chi-square statistic of the spatial association test between male and female flowers.

J. Terrados / Aquatic Botany 46 (1993.) 293-299 297

Table 3 Density and spatial distribution of C. nodosa seedlings in some of the meadows studied

Sampling n Seedlings m - 2 ID Z 2 d.f. P station

Galen I 53 2.9 (0.4) 3.63 7.2527 3 0.0643 Gahin II 68 0.9 (0.1) 1.35 3.5787 1 0.0585 Veneziola 41 7.0 (1.4) 12.31 0.9505 1 0.3296

n, number of sampling units (SU) ( 1 m 2 quadrats), taken; seedlings m - 2, mean number of seedlings present (standard error in brackets ); ID, variance-to-mean sample ratio; Z 2, chi-square statistic of the goodness-of-fit test to Negative Binomial probability distribution; d.f., degrees of freedom; P, signifi- cance level.

Table 4 Phenological features of C. nodosa seedlings

Sampling n Shoots per Rhizome Seedling age, station seedling branchings years

per seedling

Gahin I 153 2.14 (0.06) 0.85 (0.04) 0.05 (0.03) Gahin II 65 1.66 (0.10) 0.77 (0.08) 2.06 (0.24) Veneziola 291 2.27 (0.06) 1.14 (0.04) 0.00 (0.00)

n, number of seedlings studied; standard error is given in brackets.

II (Z2=3.7156, d.f.=2, P=0.1560), but showed a clumped pattern at Vene- ziola and Gal~in I meadows (Table 3 ). The number of shoots per seedling was smaller ( W= 23.0795, P= 0.0000) in Gal~tn II than in Gahin I and Veneziola sampling stations (Table 4), where similar values were recorded ( U= 0.347, P=0.7284). The number of rhizome branchings per seedling was higher (W=25.4168, P=0.0000) in Veneziola than in Gahin stations (Table 4), where the values were similar ( U= 1.053, P= 0.2922 ). Seedlings at Veneziola and Gahin I had just grown, while those at Gahtn II were 2 years old (Table 4).

Discussion

The presence of seeds in the sediment of a C. nodosa meadow may be con- sidered as an indicator of successful pollination and fruiting of the plants in that place. The presence of seedlings in almost all the meadows studied sug- gests that C nodosa plants can reproduce sexually in them. Based on flower density values and seed-bank sizes, sexual reproduction may be more impor- tant in the C. nodosa meadows inside the Mar Menor than in those on the Mediterranean coast. The seed-bank sizes obtained are similar to those found by Buia and MazzeUa ( 1991 ) in Ischia (Italy), and Caye and Meinesz (1985) on the French Mediterranean coast.

2 9 8 ~. Terrados ~Aquatic Botany 46 (1993) 293-299

The values for flower density found in this study are lower than those pre- viously published (Caye and Meinesz, 1985; Buia and Mazzella, 1991 ). However, direct comparisons cannot be made because the values mentioned by these authors only refer to patches of the meadow where flowers were pres- ent, while those obtained in this study refer to the whole meadow surface. As C. nodosa flowers develop in several shoots of the same plant, their pattern of spatial distribution must be clumped. The results obtained in this study sup- port this hypothesis. Buia and Mazzella ( 1991 ) also found that flowers had a clumped spatial distribution. Caye and Meinesz (1985 ) suggested that the seed production of a C. nodosa meadow may depend on the proximity be- tween male and female flowers. The lack of spatial association between male and female flowers in meadows with high seed production (Table 2) suggests that the separation between them does not affect the pollination success at the spatial scale considered in this study (separation distance, less than 0.5 m).

As seeds of C. nodosa are denser than seawater and they are formed at the base of the shoots buried in the sediment, it has been supposed that sexual reproduction contributes mainly to the maintenance of C. nodosa meadows already established (Buia and Mazzella, 1991 ), rather than to the formation of new ones. The presence of seedlings in the surroundings of all the C. nodosa meadows studied in the Mar Menor and the finding of low-density meadows originating from seeds at G a l ~ and Veneziola sampling stations, do not sup- port this hypothesis. The partition of seed production between the parent meadow and the surroundings appears to be controlled by water movement: when water movement is high enough to resuspend the upper layer of meadow sediment the seeds can be transported to other places and, therefore, the for- mation of new meadows is possible. The shallowness of the meadows studied (depth, less than 1 m) facilitates the resuspension of sediment and seeds, and the transport of the latter by water. The small quantity of seeds present in the sediment of Gal~in main meadow patch (Table 1 ), which was uprooted in the winter of 1991-1992, and the presence of low-density meadows originating from seeds in adjacent areas, could be explained by this process.

The relatively similar vegetative development of seedlings at Galdn and Veneziola sampling stations, in spite of their differences in age (Table 4), suggests that successful germination of seeds and seedling rhizome branching do not have to result in the formation of a C. nodosa meadow. Duarte and Sand-Jensen (1990) have shown that the mortality of C. nodosa patches in Alfacs Bay (northeast Spain) was high at their first stages of development and that patch growth was positively correlated with patch size. These obser- vations support the existence of positive feedbacks in seagrass growth (McRoy and Lloyd, 1981 ) and suggest that the survival of seedlings may be higher within an established meadow than on bare substrate and that, therefore, the

J. Terrados /Aquatic Botany 46 (1993) 293-299 299

contribution of within-meadow seedlings to maintain a patch may be also higher than the capacity of isolated seedlings to form new C. nodosa patches.

Acknowledgements

Thanks to J.M. Ruiz-Fern~indez for his help with the diving work. The sug- gestions of the reviewers greatly improved an earlier version of the manu- script. This research was funded by an Ayuda a Graduados Universitarios para la Ampliacion de Estudios from the Comunidad Aut6noma de la Regi6n de Murcia.

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