Posidonia Oceanica Meadows Research Articles

Multiscale lepidochronological analysis of Posidonia oceanica (L.) Delile rhizome production in a northwestern Mediterranean coastal area

The influence of geographical location and depth on the rhizome primary production of Posidonia oceanica was investigated over different spatial scales: large (the whole Ligurian coast), medium (the individual meadows) and small (the sites of each individual meadow). Six Posidonia oceanica meadows, separated by 30 to 60 km from each other, were sampled in summer 2002 along the 300 km of the Ligurian coast (NW Mediterranean). In each meadow, two sites, 100 to 200 m apart, were chosen and in each site 10 orthotrophic shoots were sampled at three depth zones: shallow (1.5–10 m), intermediate (10–15 m), and deep (15–26 m). The annual primary production of the rhizomes was reconstructed through lepidochronology for three previous years (1999, 2000 and 2001). Three-way analysis of variance showed no significant differences in rhizome production at small scale whereas differences at medium scale were significant. These differences were strictly confined at the shallow depths suggesting that local impacts from the coastline are likely to be responsible for the variability in the rhizome primary production among meadows.

Physiological responses of the seagrass Posidonia oceanica as indicators of fish farm impact

The development of aquaculture along the Mediterranean coastline degrades the marine environment, in particular Posidonia oceanica meadows, which, in extreme cases, show high mortality. Here we studied the effects of organic matter and nutrient input from the effluents of three fish farms, located along the Mediterranean coast, on P. oceanica physiology. For this purpose, we measured physiological variables such as total nitrogen (N) content, free amino acid (FAA) concentration and composition, N stable isotope ratio ( δ 15N), total phosphorus (P) content and total non-structural carbohydrate (TNC) content in plant tissues and epiphytes affected by organic discharges (highly impacted stations: HI, and less impacted stations: LI) and compared these results with those obtained in references sites (control stations: C). For all the descriptors analyzed in P. oceanica epiphytes, the values recorded in the vicinity of cages were, in general, much higher than those in C. Leaves did not respond consistently in any case. Total N content and δ 15N in epiphytes together with the total P content in rhizomes and epiphytes were the physiological descriptors that showed the most consistent responses to fish farm effluents. On the basis of these observations, we conclude that fish farm activities strongly affect the physiological parameters of nearby P. oceanica meadows. We propose that changes in these physiological parameters may be useful indicators of marine environmental degradation in studies that monitor the effects of fish farming.

Intrabasinal conditions promoting the development of a biogenic carbonate sedimentary facies associated with the seagrass Posidonia oceanica

This paper aims to elucidate the spatial relationships between the distribution and growth dynamics of Posidonia oceanica, the sedimentary depositional facies (carbonate vs. siliciclastic), and the hydrodynamic features of the Gulf of Oristano (Western Sardinia, Mediterranean sea), a complex depositional system characterized by multiple sources of sediments and a marked hydrodynamic gradient. Three depositional environments were identified: (i) a poorly vegetated sector characterized by muddy sediments derived from the river input; (ii) a sector colonized by P. oceanica meadows characterized by biogenic carbonate sediments derived from the sediment production associated with the seagrass ecosystem and (iii) a sector colonized by P. oceanica meadows characterized by coarse siliciclastic sediments, possibly relict sediments. The sedimentary depositional environments are heavily influenced by the spatial distribution of the wind wave energy. Biogenic carbonate reefs associated with P. oceanica meadows develop in sheltered areas characterized by low amplitude of waves generated by the main wind regime. In the exposed sectors, characterized by a higher wave height, the meadows colonize relict siliciclastic sediments, without promoting carbonate particle deposition. Posidonia oceanica meadows in sheltered areas, associated with biogenic sedimentary facies, exhibit higher rhizome growth rate values (1.1 and 1.2 cm year −1 vs. 0.7 cm year −1) and a lower percentage of horizontal shoots (1.1% and 4.1% vs. 18%) in comparison to P. oceanica meadows in exposed areas, associated with siliciclastic sedimentary facies. The former tend to develop in a vertical direction, thus contrasting the sediment deposition rate, the latter tend to expand laterally due to the absence of sediment deposition. These results highlight that wave amplitude is the intrabasinal factor which influences the deposition of biogenic sediments and the growth dynamics of P. oceanica meadows.

Impacts of recreational boating on the marine environment of Cap de Creus (Mediterranean Sea)

A study was set up in summer 2004 in the marine reserve of Cape Creus (Mediterranean Sea) to estimate the environmental impacts of this activity. The large numbers of recreational boats (most of which are motor powered) that can be found affect the marine environment. In particular, results show that Posidonia oceanica meadows, which constitute an ecologically and vulnerable habitat, are suffering from mechanical damage caused by anchors. Anchoring was the largest threat on the marine environment of the area; the activities conducted by boaters such as sunbathing, swimming, snorkelling and scuba diving had little or negligible environmental impact. Overall, our results indicate that a primary element for coastal management in busy boating areas must be the anchoring of recreational boats and the specific impacts associated with motor craft, such as speed and the discharge of bilge waters.

Comparing shoot population dynamics methods on Posidonia oceanica meadows

Prediction capacity of three main shoot population dynamics methods (age structure, net shoot recruitment per plagiotropic rhizome and shoot census) have been tested for a period of four years (2002–2006) on a Posidonia oceanica meadow. Accuracy of each method was checked by comparing measured and predicted densities at the end of the study period. Predicted densities came from the evolution of initial densities (measured in 2002) by a basic exponential model of population growth. The exponential model used the different net shoot recruitment rate estimates by each population dynamics method on three depths (upper, medium and lower limit) and three localities at each depth. Predictions performed by shoot census and net shoot recruitment per plagiotropic rhizome methods matched with measured densities at the end of the study period. Conversely, age structure method underestimated shoot densities at each depth, indicating an unreal decrease of shoot population in the meadow.

Long-term effect of beach replenishment on natural recovery of shallow Posidonia oceanica meadows

The recovery capacity of shallow Posidonia oceanica meadows degraded by beach replenishment eighteen years before was assessed in two impacted meadows and compared with other two undisturbed localities. Inside each locality, we selected randomly three sites separated by 500–1000 m. At site level we study the vitality of P. oceanica meadow assessing the vegetative growth, leaf characteristics, and non-structural carbohydrates of the plants. Additionally, at locality level, silt-clay fraction, organic matter, pH and light intensity incident on the sea bottom were measured to evaluate the environmental conditions. Covering of P. oceanica was significantly lower at the impacted localities while amount of dead “matte” was higher. Leaf production of horizontal rhizomes (14.6 ± 1.11 vs 19.47 ± 1.45 leaves y −1), net total rhizomes recruitment (2.33 ± 0.17 vs 4.3 ± 0.33 branches y −1) and starch concentration (43.625 ± 0.67 vs 54.45 ± 0.74 mg per g of rhizome) at impacted meadows were significantly lower than controls. Leaf features, epiphytes biomass, colonization, elongation and horizontal and vertical rhizome production did not show significant differences. Sediments at impacted localities contained higher silt-clay fraction and higher organic matter load while pH was lower. Light intensity on the sea bottom measured at all localities was over the minimum light requirements estimated for P. oceanica. Our results show that the press impact produced by beach replenishment was enduring in the time slowing natural recovery by 45%. This impact may be related with changes in the sediment features.

Fish farming impact on sediments and macrofauna associated with seagrass meadows in the Mediterranean

The fish farming impact on sediment and macrofauna associated with Posidonia oceanica meadows has been studied in three coastal areas (Spain, Italy and Greece) of the Mediterranean Sea. The stations were characterized by coarse sediment with a low percentage of silt/clay and oxic conditions. Chlorophyll a and organic carbon were occasionally enhanced in the vicinity of the cages and nitrogen remained almost constant between the stations. Phosphorus consistently showed a decreasing trend towards the reference station. Macrofaunal biomass peaked at an intermediate distance from the fish cages. Diversity indices were quite high at all stations, implying that the specific sites were not under severe stress due to aquaculture. A tenuous clustering of stations with distance from the farm was encountered, which was mainly attributed to abundance shifts rather than presence/absence of typical species. It is suggested that oxic conditions induced by intense currents and supported by coarse sediments at the study sites allow the existence of diversified communities with high abundance and biomass.

Physiological responses of Posidonia oceanica to experimental nutrient enrichment of the canopy water

To simulate anthropogenic nutrient loading of coastal waters, experimental N and P enrichment was performed on a Posidonia oceanica meadow in the Western Mediterranean Sea (Calvi, Corsica, France) for the duration of an annual cycle. To identify the physiological descriptors of nutrient enrichment, certain parameters were selected for this study: the chlorophyll a and b content, the total water-soluble phenolic compounds present in the leaves, the nitrogen and carbon contents of the leaves and epiphytes. This study was conducted in parallel with a monthly monitoring program, which measured nutrient levels in the water (canopy and sediment water). Neither the total phenol nor the carbon content of the leaves showed any response to nutrient enrichment. The carbon content of epiphytes, the chlorophyll content and the nitrogen content of the leaves increased significantly with nutrient enrichment. The chlorophyll content and the nitrogen content of the leaves appeared to be effective physiological indicators for assessing water quality. These indicators provide information on nutrient availability, but to use them correctly other influences will also have to be taken into account, in particular irradiance.

Iron Additions Reduce Sulfide Intrusion and Reverse Seagrass (Posidonia oceanica) Decline in Carbonate Sediments

We conducted a 2-year in situ experiment to test the capacity of iron additions to reverse the decline experienced by a Posidonia oceanica meadow colonizing carbonate, iron poor sediment. Iron additions improved the sediment conditions that support seagrass growth by decreasing the sediment sulfide concentration and sulfate reduction rates, and decreased sulfide intrusion into the plants. Iron additions for 2 years did not significantly change survivorship of shoots present at the onset of the experiment, but significantly increased shoot recruitment and survivorship of shoots recruited during the experiment. After 2 years, iron additions reversed seagrass decline and yielded positive growth rates of shoots relative to control populations where seagrass continued to decline. This research demonstrates that seagrass decline in carbonate sediments may be reversed by targeting critical processes such are sediment sulfide pools and seagrass nutritional status, controlling the functioning of the ecosystem.

Relationship between the Evolution of the Shoreline and the Posidonia oceanica Meadow Limit in a Sardinian Coastal Zone

RESUME Cet article concerne l'etude de processus cotiers dans le Golfe d'Oristano (Sardaigne occidentale, Italie) ou d'importants changements environnementaux ont ete observes. Cette recherche, utilisant comme principal outil la teledetection, vise a evaluer l'evolution du littoral au cours du temps, sur la periode allant de 1977 a 2000, et a examiner s'il existe une relation entre la tendanc evolutive du trait de cote et celle de la limite superieure de l'herbier a Posidonia oceanica (L.) Delile, principal ecosysteme en place. Les resultats obtenus montrent d'une part que certaines portions cotieres, concentrees sur la partie sableuse du littoral, ont subi des variations significatives et qu'il existe d'autre part une correlation entre l'evolution de la limite superieure de l'herbier et la position de la cote lui faisant face. Principalement d'origines naturelles, cette tendanc evolutive doit egalement etre reliee a l'impact anthropique sur l'herbier de posidonies. Ce dernier exerce une influence capital...

Posidonia oceanica meadows are not declining globally. Analysis of population dynamics in marine protected areas of the Mediterranean Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 336:111-119 (2007) - doi:10.3354/meps336111 Posidonia oceanica meadows are not declining globally. Analysis of population dynamics in marine protected areas of the Mediterranean Sea José M. González-Correa, Just T. Bayle Sempere*, Pablo Sánchez-Jerez, Carlos Valle Department of Marine Sciences and Applied Biology, University of Alicante, PO Box 99, Alicante 03080, Spain *Email: bayle@ua.es ABSTRACT: Many studies have reported a worldwide decline in seagrasses, including Posidonia oceanica, with evidence of regression on a global scale for different reasons. With the aim of assessing the population dynamics of P. oceanica in the Mediterranean Sea, we chose 6 protected and pristine areas, separated by 200 to 3000 km, to determine whether an overall decline is occurring. Within each locality, we randomly selected 2 sites separated by 2 to 4 km. At each site, we sampled seagrass at 2 fixed depths (10 and 20 m). At each sampled depth, 10 principal plagiotropic rhizomes, separated by 1 to 5 m, were collected. We used lepidochronology to study seagrass dynamics, and analysis of variance (ANOVA) and permutational multivariate ANOVA (PERMANOVA) to analyze the data, testing the 3 main factors of locality, site, and depth. Our results showed positive population dynamics of P. oceanica in all localities and for all studied parameters. We observed exceptional vegetative growth around Zembra Island, Tunisia. We did not find statistically different vegetative growth between the northernmost and the warmer localities, showing a high level of plasticity in this species. This study demonstrates that global processes are not causing an overall decline in P. oceanica meadows around the Mediterranean Sea and attributes the decline reported in the literature to cumulative effects derived from different natural and anthropogenic local processes. KEY WORDS: Posidonia oceanica · Population dynamics · Marine protected areas · Vegetative growth · Decline · Impacts · Global processes Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 336. Online publication date: April 27, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Meadows of the seagrass Posidonia oceanica are a significant source of organic matter for adjoining ecosystems

Stable isotopes of carbon and nitrogen were used to assess the relevance of 4 primary carbon sources (the macroalga Cystoseira balearica, decaying blades of the seagrass Posidonia oceanica, seagrass epiphytes and pelagic particulate organic matter (POM)) for consumers inhabiting 3 adjoining subtidal habitats in the Western Mediterranean: seagrass meadows, unvegetated sandy patches and the overlying pelagic ecosystem. MANOVA and ANOVA revealed statistically signifi- cant differences between the δ 13 C signal of decaying blades of P. oceanica (-13.0 ± 0.6 ‰), epiphytes (-19.7 ± 0.4 ‰) and a third group including POM (-23.1 ± 0.3 ‰) and C. balearica (-22.7 ± 0.8 ‰). However, these primary sources did not differ in their δ 15 N signals (average: 1.0‰; range: 0.8 to 1.2 ‰). IsoSource mixing model software was used to calculate the contribution of each primary source to the carbon assimilated by each animal species. Nested ANOVA showed that the 3 trophic webs differed in the average contribution of the decaying blades of P. oceanica to the carbon assimi- lated by animals. The average contribution was 46.0 ± 14.0% for meadow-dwelling species, 18.3 ± 6.0% for sand-dwelling species and 12.8 ± 3.0% for pelagic species, once zooxanthellae-bearing jellyfish were removed from the analysis. However, IsoSource provided solutions for all the pelagic species and for half the sand-dwelling species that do not use decaying blades of P. oceanica as a car- bon source. Conversely, IsoSource identified the decaying blades of P. oceanica as a relevant carbon source in all the solutions calculated for 3 different sand-dwelling species and in all of the meadow- dwelling species. Thus, we conclude that organic detritus from P. oceanica is a relevant carbon source for species inhabiting seagrass meadows and for those sand-dwelling species living close to the meadows, but not for species exploiting deeper and more distant unvegetated patches.

Feed-backs between genetic structure and perturbation-driven decline in seagrass (Posidonia oceanica) meadows

We explored the relationships between perturbation-driven population decline and genetic/genotypic structure in the clonal seagrass Posidonia oceanica, subject to intensive meadow regression around four Mediterranean fish-farms, using seven specific microsatellites. Two meadows were randomly sampled (40 shoots) within 1,600 m2 at each site: the “impacted” station, 5–200 m from fish cages, and the “control” station, around 1,000 m downstream further away (considered a proxy of the pre-impact genetic structure at the site). Clonal richness (R), Simpson genotypic diversity (D*) and clonal sub-range (CR) were highly variable among sites. Nevertheless, the maximum distance at which clonal dispersal was detected, indicated by CR, was higher at impacted stations than at the respective control station (paired t-test: P < 0.05, N = 4). The mean number of alleles (Â) and the presence of rare alleles ( r) decreased at impacted stations (paired t-test: P < 0.05, and P < 0.02, respectively, N = 4). At a given perturbation level (quantified by the organic and nutrient loads), shoot mortality at the impacted stations significantly decreased with CR at control stations (R 2 = 0.86, P < 0.05). Seagrass mortality also increased with  (R 2 = 0.81, P < 0.10), R (R 2 = 0.96, P < 0.05) and D* (R 2 = 0.99, P < 0.01) at the control stations, probably because of the negative correlation between those parameters and CR. Therefore, the effects of clonal size structure on meadow resistance could play an important role on meadow survival. Large genotypes of P. oceanica meadows thus seem to resist better to fish farm-derived impacts than little ones. Clonal integration, foraging advantage or other size-related fitness traits could account for this effect.

The relationship between seagrass ( Posidonia oceanica) decline and sulfide porewater concentration in carbonate sediments

In this study we test the hypothesized negative relationship between seagrass status and porewater hydrogen sulfide (H 2S) levels, through a comparative analysis within a range of seven Posidonia oceanica meadows growing over carbonate sediments in the NW Mediterranean Sea around Mallorca Island. The studied meadows range from meadows growing on sediments with very low sulfide porewater concentrations (4.6 μM) to those growing over higher sulfide conditions (33.5 μM). Organic matter content, sulfate reduction rates and sulfide porewater concentrations in the sediments were determined concurrently with the assessment of demographic plant dynamics (specific mortality and net population growth rates). Sulfide porewater concentration increased with increasing organic matter content in the sediment, while net population growth decreased significantly with low increases of sulfide concentrations. Our results confirm the previously suspected vulnerability of seagrass meadows growing on carbonate sediments to increased sulfide levels. An excess of 10 μmols H 2S L −1 porewater is identified to already conduce P. oceanica meadows to decline, which this study identifies, particularly, as strongly sensitive to sulfides. The results reported here suggest that even moderate increases in organic carbon inputs may lead to enhancement of dissolved sulfides and may be an important factor for seagrass status in these iron-depleted carbonate sediments from the Mediterranean Sea.

Mortality of shoots of Posidonia oceanica following meadow invasion by the red alga Lophocladia lallemandii

Abstract Lophocladia lallemandii, an Indo-Pacific alga that is currently invading several Mediterranean areas, is especially aggressive when colonizing meadows of Posidonia oceanica in Formentera (Balearic Islands). L. lallemandii settles on rhizomes and old leaves situated mainly at meadow edges and in low density Posidonia patches, growing rapidly, producing disc-like holdfasts along the thalli that allow formation of a mat of the red algal filaments intermingled with P. oceanica leaves. This mat can get so thick and dense that new leaves become confined inside the web of red algal filaments. The seagrass leaves display chlorosis and sometimes die. Invasion by L. lallemandii significantly reduces seagrass shoot size, leaf biomass, and percentage of living shoots. A lower content of sucrose in shoots from invaded plots points to a reduction of photosynthetic activity. However, other causes such as enhanced sediment trapping, organic and nutrient enrichment inside the algal carpet, and increased oxygen consumption may jointly or individually contribute to observed seagrass shoot demise.

Production of Cocconeis neothumensis (Bacillariophyceae) biomass in batch cultures and bioreactors for biotechnological applications: light and nutrient requirements

Cocconeis neothumensis, a benthic diatom living as epiphyte on Posidonia oceanica leaves, was hypothesised to synthesize secondary metabolites inducing apoptosis in the androgenic gland of the protandric shrimp Hippolyte inermis. The optimization of C. neothumensis cultures is a primary aim in order to identify and characterise the active compounds produced by this diatom. Light intensity and concentration of nutrients, such as silicates and selenium, are of major importance in determining the growth rate of diatoms and the maximum produced biomass. Thus, we evaluated the growth performances of C. neothumensis cultures at different light intensities, corresponding to those recorded in Posidonia oceanica meadows in April, when the induction of sexual reversal in Hippolyte inermis occurs; the growth performances were also determined in relation to different selenium and silicate availability in the culture medium and the possible interactions between these two micronutrients were evaluated. Two methods of cultivations were compared: batch cultures in Petri dishes and cultures in a bioreactor with a continuous medium flow in the system. C. neothumensis showed a faster growth at low light intensities, although with a good acclimation capacity in the range from 60 to 140 μmol photons m−2 s−1. The presence of selenium in the medium improved both the exponential growth rate and the maximum cell density. The same results were evidenced for a silicate concentration double that in Guillard f/2 medium. On the other hand, a simultaneous increase of silicate concentration and the presence of selenium led to a loss of the positive effects detected with single nutrients. This result suggests either a possible antagonism in selenium and silicate uptake or a negative interaction between these two micronutrients in Cocconeis. The yields of the two tested cultivation methods calculated in terms of diethyl ether extract dry weight per unit of substrate area were highest for the bioreactor.

Impact of the brine from a desalination plant on a shallow seagrass ( Posidonia oceanica) meadow

Although seawater desalination has increased significantly over recent decades, little attention has been paid to the impact of the main by-product (hypersaline water: brine) on ecosystems. In the Mediterranean, potentially the most affected ecosystems are meadows of the endemic seagrass Posidonia oceanica. We studied the effect of brine on a shallow P. oceanica meadow exposed to reverse osmosis brine discharge for more than 6 years. P. oceanica proved to be very sensitive to both eutrophication and high salinities derived from the brine discharge. Affected plants showed high epiphyte load and nitrogen content in the leaves, high frequencies of necrosis marks, low total non-structural carbohydrates and low glutamine synthetase activity, compared to control plants. However, there was no indication of extensive decline of the affected meadow. This is probably due to its very shallow situation, which results in high incident radiation as well as fast dilution and dispersion of the brine plume.

Extensive direct measurements of Posidonia oceanica defoliation confirm the importance of herbivory in temperate seagrass meadows

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 340:63-71 (2007) - doi:10.3354/meps340063 Extensive direct measurements of Posidonia oceanica defoliation confirm the importance of herbivory in temperate seagrass meadows Patricia Prado1,*, Fiona Tomas1,2, Teresa Alcoverro1, Javier Romero3 1Centro de Estudios Avanzados de Blanes, c/ Accés a la Cala St. Francesc 14, 17300 Blanes, Girona, Spain 2Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093-0202, USA 3Departamento de Ecología, Facultad de Biología, Universidad de Barcelona, Avda. Diagonal 645, 08028 Barcelona, Spain *Email: patricia@ceab.csic.es ABSTRACT: The rates of seagrass defoliation exerted by the herbivorous fish Sarpa salpa and by the sea urchin Paracentrotus lividus were evaluated through both direct (tethering experiment) and indirect (bite marks) methods. Sampling was conducted once per season in 10 shallow meadows of Posidonia oceanica (L.) Delile from the continental NW Mediterranean coast covering a spatial scale of >300 km. Results indicated that a large proportion (ca. 57%) of the annual leaf production is lost to herbivory, yet with considerable spatial variation. Patterns of seagrass defoliation showed high temporal variability, with a peak in summer with values that exceeded about 2.5 times those of leaf production and a minimum during the winter period. On average, defoliation exerted by S. salpa accounted for 40% of leaf production (ca. 70% of total annual losses to herbivory), while P. lividus was also responsible for a substantial 17% removal of leaf production. High discrepancies encountered when comparing direct and indirect measurements suggest that the latter are inappropriate to achieve accurate estimates of herbivory pressure. This study evidences that P. oceanica leaf losses to herbivores are not marginal, but a widespread process that occurs at much higher rates than previously estimated through indirect methods (ca. 2%), resetting the paradigm of the negligible importance of herbivory in temperate systems. KEY WORDS: Tethering experiment · Temporal variation · Posidonia oceanica · Seagrass defoliation · Sarpa salpa · Paracentrotus lividus · Bite marks Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 340. Online publication date: June 18, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Observations on reproductive features of three species of Eunicidae (Polychaeta) associated with &lt;i&gt;Posidonia oceanica&lt;/i&gt; seagrass meadows in the Mediterranean Sea

Observations on the reproductive features are provided for three species of Eunicidae ( Lysidice ninetta Audouin and Milne Edwards, Lysidice collaris Grube and Nematonereis unicornis (Grube)). They all occur in Posidonia oceanica meadows as borers in the sheaths. The material examined was collected during several studies on P. oceanica meadows conducted at various times of the year and in different years along the coasts of Sardinia and the Ischia islands (Tyrrhenian Sea), and Otranto (Southern Adriatic Sea). All individuals of the three species were measured (width 3rd chaetiger, diameter of eyes), and checked for presence of gametes. In each of the females recognized, egg diameter was measured. To integrate the observations, living specimens were collected in March 2003 and reared in the laboratory until August 2003 to check for gamete maturation; mature males were fixed for analysis of the structure of the spermatozoa by transmission electron microscopy (TEM). All three species studied are gonochoric. For both species of Lysidice gametes occurred in individuals with a width greater than 0.8 mm. The overall scarcity of mature males and females observed with respect to the total number of specimens collected is probably due to the dimensional constraint exerted by the sheaths. In Lysidice spp. specimens with small developing oocytes were observed in January-February and reached their maximum diameter between June and August (157 µm). Gametes were located mainly in the posterior part of the body, and an enlargement of the eyes (almost doubled in size) characterized mature specimens close to spawning. N. unicornis is a smaller species, and it is less frequent in Posidonia sheaths than Lysidice spp.; individuals with gametes had a width greater than 0.55 mm. A few mature specimens were observed between March and May with a maximum egg diameter of about 155 µm. In this species maturation of the gametes is coupled with a huge enlargement of the eyes (ten-times larger than in immature individuals), while the posterior part of the body is swollen and full of mature gametes, indicating reproduction by schizogamy. Analysis of mature spermatozoa of the three eunicids revealed a similar morphology, which is that of an “ect-aquasperm” type, typical of many free-spawning species with external fertilization. The nucleus is round and globose (2.5 µm diameter) and the acrosome, slightly different among the three species, has the general shape of a truncate sub-cone.

How do dietary diatoms cause the sex reversal of the shrimp Hippolyte inermis Leach (Crustacea, Decapoda)

Hippolyte inermis Leach 1915 is a protandric shrimp largely distributed in Posidonia oceanica meadows and other Mediterranean seagrasses. Previous studies demonstrated several physiological peculiarities, such as absence of female gonadic buds in adult males (the new female gonad is produced starting from few undifferentiated cells), the consequent absence of an ovotestis, 2 yearly periods of reproduction with different population structures (a spring outburst producing both males and primary females, and a fall reproduction producing mainly males), and a process of sex reversal influenced by the diatom food ingested. We performed several laboratory analyses to compare the effects of various species of benthic diatoms, in order to test the effect of different diatoms and provide information on the mechanism of action of the ingested compounds. In addition, we performed molecular tests (TUNEL) and TEM observations, to check the hypothesis that the effect of benthic diatoms may be mediated by a process of apoptosis acting on the male gonad. The results obtained allowed for a ranking of a series of benthic diatoms according to their effects on sex reversal, and a confirmation of the striking effect of Cocconeis sp. diatoms, which are able to trigger the appearance of primary females. We also demonstrated the presence of apoptosis both in the male gonad and in the androgenic glands of postlarvae. The effect is species specific, strictly localized to the male gonad and androgenic gland, and limited to a very short period of time, from the 5th to the 12th day of postlarval development.