Posidonia Species Research Articles

The trajectory in catalytic evolution of Rubisco in Posidonia seagrass species differs from terrestrial plants.

The CO2-fixing enzyme Ribulose bisphosphate carboxylase-oxygenase (Rubisco) links the inorganic and organic phases of the global carbon cycle. In aquatic systems, the catalytic adaptation of algae Rubiscos has been more expansive and followed an evolutionary pathway that appears distinct to terrestrial plant Rubisco. Here, we extend this survey to differing seagrass species of the genus Posidonia to reveal how their disjunctive geographical distribution and diverged phylogeny, along with their CO2 concentrating mechanisms (CCMs) effectiveness, have impacted their Rubisco kinetic properties. The Rubisco from Posidonia species showed lower carboxylation efficiencies and lower sensitivity to O2 inhibition than those measured for terrestrial C3 and C4-plant Rubiscos. Compared with the Australian Posidonia species, Rubisco from the Mediterranean Posidonia oceanica had 1.5-2-fold lower carboxylation and oxygenation efficiencies, coinciding with effective CCMs and five Rubisco large subunit amino acid substitutions. Among the Australian Posidonia species, CCM effectiveness was higher in Posidonia sinuosa and lower in the deep-living Posidonia angustifolia, likely related to the 20%-35% lower Rubisco carboxylation efficiency in P. sinuosa and the two-fold higher Rubisco content in P. angustifolia. Our results suggest that the catalytic evolution of Posidonia Rubisco has been impacted by the low CO2 availability and gas exchange properties of marine environments, but with contrasting Rubisco kinetics according to the time of diversification among the species. As a result, the relationships between maximum carboxylation rate and CO2- and O2-affinities of Posidonia Rubiscos follow an alternative path to that characteristic of terrestrial angiosperm Rubiscos.

First report of hybridization in the seagrass genus Posidonia (Posidoniaceae)

Hybridization is common among plants and has been crucial in the evolution of many aquatic plant groups. However, hybrid individuals are often difficult to identify, particularly in the marine environment. We aim to confirm a hybrid origin between two seagrass species for a morphologically-intermediate plant observed in shallow subtidal environments near Perth, Western Australia. Both morphological measures and DNA markers demonstrated a clear case for hybridization among two sympatric Posidonia species from different species complexes: Posidonia australis and P. coriacea. All five samples with the intermediate morphology had the same multilocus genotype (MLG), providing evidence that it resulted from a single hybridization event, followed by local clonal (rhizome) expansion. This result highlights weak reproductive barriers within the genus Posidonia, a trait common among other seagrass genera with sympatric ranges, apparently overlapping morphologies and low phylogenetic divergence.

Radically different lignin composition in Posidonia species may link to differences in organic carbon sequestration capacity

There is considerable variability in the ability of seagrass ecosystems to sequester organic carbon (Corg) in their sediments, which act as natural carbon sinks contributing to climate change mitigation. In this work, we studied the chemistry of two Posidonia seagrass species aiming to elucidate whether differences in chemical composition might explain differences in their Corg sequestration capacity. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and Thermally assisted Hydrolysis and Methylation (THM) GC-MS data showed a remarkable difference in phenolic compound patterns between P. oceanica and P. australis bulk plants and individual organs (leaves, sheaths, roots and rhizomes). The lignin of P. australis generates a series of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) products that are typical of herbaceous plants, whereas P. oceanica is particularly rich in p-hydroxybenzoic acid (pBA) derivatives. The structural characteristics of the lignins were further investigated by two-dimensional Nuclear Magnetic Resonance (2D-NMR) spectroscopy and Derivatization Followed by Reductive Cleavage (DFRC), focusing on sheath tissues. The analyses confirmed important differences in the lignin content (19.8% in P. australis and 29.5% in P. oceanica) and composition between the two species; intriguingly, the cell-walls of P. oceanica sheaths were highly enriched in pBA, a component that was completely absent in P. australis. 2D-NMR and DFRC data further revealed that pBA was esterified to the lignin, acylating the γ-OH of the lignin side-chain. Interestingly, P. oceanica lignin presented an extremely high degree of p-hydroxybenzoylation in both guaiacyl (73%) and syringyl (61%) lignin units; the highest p-hydroxybenzoylation degree reported in plant lignins to date. It is tempting to conclude that the higher Corg storage capacity of P. oceanica ecosystems might be related to the higher abundance of pBA-rich lignin and its recalcitrant nature.

Rocking the Boat: Damage to Eelgrass by Swinging Boat Moorings.

Seagrass meadows commonly reside in shallow sheltered embayments typical of the locations that provide an attractive option for mooring boats. Given the potential for boat moorings to result in disturbance to the seabed due to repeated physical impact, these moorings may present a significant threat to seagrass meadows. The seagrass Zostera marina (known as eelgrass) is extensive across the northern hemisphere, forming critical fisheries habitat and creating efficient long-term stores of carbon in sediments. Although boat moorings have been documented to impact seagrasses, studies to date have been conducted on the slow growing Posidonia species’ rather than the fast growing and rapidly reproducing Z. marina that may have a higher capacity to resist and recover from repeated disturbance. In the present study we examine swinging chain boat moorings in seagrass meadows across a range of sites in the United Kingdom to determine whether such moorings have a negative impact on the seagrass Zostera marina at the local and meadow scale. We provide conclusive evidence from multiple sites that Z. marina is damaged by swinging chain moorings leading to a loss of at least 6 ha of United Kingdom seagrass. Each swinging chain mooring was found to result in the loss of 122 m2 of seagrass. Loss is restricted to the area surrounding the mooring and the impact does not appear to translate to a meadow scale. This loss of United Kingdom seagrass from boat moorings is small but significant at a local scale. This is because it fragments existing meadows and ultimately reduces their resilience to other stressors. Boat moorings are prevalent in seagrass globally and it is likely this impairs their ecosystem functioning. Given the extensive ecosystem service value of seagrasses in terms of factors such as carbon storage and fish habitat such loss is of cause for concern. This indicates the need for the widespread use of seagrass friendly mooring systems in and around seagrass.

Linking Seed Photosynthesis and Evolution of the Australian and Mediterranean Seagrass Genus Posidonia.

Recent findings have shown that photosynthesis in the skin of the seed of Posidonia oceanica enhances seedling growth. The seagrass genus Posidonia is found only in two distant parts of the world, the Mediterranean Sea and southern Australia. This fact led us to question whether the acquisition of this novel mechanism in the evolution of this seagrass was a pre-adaptation prior to geological isolation of the Mediterranean from Tethys Sea in the Eocene. Photosynthetic activity in seeds of Australian species of Posidonia is still unknown. This study shows oxygen production and respiration rates, and maximum PSII photochemical efficiency (Fv : Fm) in seeds of two Australian Posidonia species (P. australis and P. sinuosa), and compares these with previous results for P. oceanica. Results showed relatively high oxygen production and respiratory rates in all three species but with significant differences among them, suggesting the existence of an adaptive mechanism to compensate for the relatively high oxygen demands of the seeds. In all cases maximal photochemical efficiency of photosystem II rates reached similar values. The existence of photosynthetic activity in the seeds of all three species implicates that it was an ability probably acquired from a common ancestor during the Late Eocene, when this adaptive strategy could have helped Posidonia species to survive in nutrient-poor temperate seas. This study sheds new light on some aspects of the evolution of marine plants and represents an important contribution to global knowledge of the paleogeographic patterns of seagrass distribution.

Flowers and inflorescences of the seagrass Posidonia (Posidoniaceae, Alismatales)

The predominantly aquatic order Alismatales displays a highly variable flower groundplan associated with a diverse range of developmental patterns. We present the first detailed description of flower anatomy and development in Posidonia, the sole genus of the seagrass family Posidoniaceae. Existing accounts provide conflicting interpretations of floral and inflorescence structure, so this investigation is important in clarifying morphological evolution within this early-divergent monocot order. • We investigated two species of Posidonia using light microscopy and scanning electron microscopy. Our observations are interpreted in the framework of a recent molecular phylogeny. • Partial inflorescences are bracteate spikes, which are arranged into a botryoid or a panicle. The flowers are perianthless. The gynoecium is monomerous with the ventral carpel side oriented abaxially. The carpel contains a single pendent bitegmic ovule with a nucellus and long chalaza, both extending along the carpel wall. The ovule develops an integumentary outgrowth. Each flower is supplied by a vascular bundle, whereas the flower-subtending bracts are nonvascularized. • Our data support a racemose interpretation for the partial inflorescence of Posidonia and the presence of flower-subtending bracts. In common with some other Alismatales, Posidonia has simultaneous development of the flower and its subtending bract and loss of the bract vascular supply accompanied by innervation of the flower by a single vascular strand. The unusual carpel orientation could be an evolutionary reduction of a formerly tricarpellate gynoecium. The ovule of Posidonia is campylotropous and unusual within Alismatales in possessing an integumentary outgrowth.

Season and sediment nutrient additions affect root architecture in the temperate seagrasses Posidonia australis and P. sinuosa

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 446:23-30 (2012) - DOI: https://doi.org/10.3354/meps09483 Season and sediment nutrient additions affect root architecture in the temperate seagrasses Posidonia australis and P. sinuosa Renae K. Hovey*, Marion L. Cambridge, Gary A. Kendrick School of Plant Biology and The UWA Oceans Institute, The University of Western Australia, Crawley, WA 6009, Australia *Email: renae.hovey@uwa.edu.au ABSTRACT: We examined the root systems of 2 temperate seagrasses, Posidonia australis and P. sinuosa, testing the effects of nutrients and season on root architecture. Transplants of each species were grown in pots containing a standardised sand medium and a localised supply of nitrogen (N), phosphorus (P), or N and P combined. P. australis and P. sinuosa showed similar root architecture (primary roots with numerous second-order laterals and few higher-order laterals), but P. australis produced a larger root system with longer laterals making up the bulk of root length. Seagrass with combined N and P produced a higher lateral root density (330 ± 18 [SE] branches m−1 root compared to controls with 250 ± 22), and a lower topological index (TI: 0.72 ± 0.03 compared to controls with 0.84 ± 0.02). Despite this response in root architecture, there was no significant change in traits associated with absorptive capacity. Separate nutrient additions had little effect on roots. Root architecture was more complex in summer for both species, with more higher-order laterals (TI: 0.73 ± 0.02) resulting in a more dichotomous pattern compared to winter (TI: 0.86 ± 0.01). P. australis had twice the root length and higher specific root length in summer than winter, mostly due to increases in lateral root number and length. This study demonstrated that P. australis and P. sinuosa display architectural and morphological plasticity with season and to a lesser extent nutrient addition. The complex root architecture in the slow-growing temperate Posidonia species may reflect hydrodynamic exposure and the importance for anchorage in wave-swept environments. KEY WORDS: Posidonia . Nitrogen . Phosphorus . Root morphology . Seagrass . Topological index Full text in pdf format PreviousNextCite this article as: Hovey RK, Cambridge ML, Kendrick GA (2012) Season and sediment nutrient additions affect root architecture in the temperate seagrasses Posidonia australis and P. sinuosa . Mar Ecol Prog Ser 446:23-30. https://doi.org/10.3354/meps09483 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 446. Online publication date: February 02, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

115. 北上山地南部井内統 (?) 産のPosidonia 1新種

115. 北上山地南部井内統 (?) 産のPosidonia 1新種

How does burrowing by the isopod Limnoria agrostisa (Crustacea: Limnoriidae) affect the leaf canopy of the southern Australian seagrass Amphibolis griffithii?

In south-western Australia, the isopod Limnoria agrostisa commonly burrows into leaf clusters and immature shoots of Amphibolis griffithii. The isopod also burrows into the sheath and rhizomes of Posidonia species. In A. griffithii, the isopod consumes new tissue within the sheath, damaging or destroying the meristem. This results in malformation of new leaves or destruction of whole leaf clusters with the potential to reduce the photosynthetic area of a shoot. The isopod has been found in all but one meadow of A. griffithii examined over 1,000 km of the Western Australian coastline. It was present throughout the year and showed little variation in abundance. Young were produced year round, but were more abundant in the summer months. Females, 3.5 mm in length or larger, produced 2–5 young that were brooded within the leaf cluster or base of an immature shoot. Within a meadow, 40–70% of shoots and 10–20% of leaf clusters were damaged by isopods. Seasonal trends were not consistent, but damage appeared to be higher in summer when isopod abundance was higher. Approximately 40% of clusters were destroyed by isopod damage. Isopods attack shoots of all ages, but damage was often located on apical clusters. There was no evidence that isopod damage initiated branching or leaf cluster formation. Estimations of clusters damaged or destroyed may be conservative, as only those clusters remaining on a shoot could be counted, and cluster loss could not be quantified. Examination of defoliated stems on upright shoots and horizontal rhizomes indicated that many were destroyed by isopods. The effect of L. agrostisa although substantial appears to be a feature of healthy seagrasses throughout southern Australia.

13C and 15N translocation within and among shoots in two Posidonia species from Western Australia

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 361:69-82 (2008) - DOI: https://doi.org/10.3354/meps07405 13C and 15N translocation within and among shoots in two Posidonia species from Western Australia Patricia Prado1,*, Catherine J. Collier2, Paul S. Lavery3 1Centre d’Estudis Avançats de Blanes, CSIC, Ctra. Acceso Cala St. Francesc 14, 17300 Blanes, Girona, Spain 2School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia 3School of Natural Sciences, Edith Cowan University, 100 Joondalup Drive, Joondalup, Perth, Western Australia 6027, Australia *Email: patricia@ceab.csic.es ABSTRACT: Translocation of 13C and 15N was investigated at the spatial scales of within-shoot (i.e. the seagrass clonal unit including leaves and associated vertical rhizome) and among-shoots in a mixed meadow of Posidonia sinuosa and P. australis. Incubation with 13C and 15N was conducted in either the oldest leaf of a shoot (i.e. within-shoot scale) or in the first shoot on the 4th or 5th branch of the main axis (i.e. among-shoots scale) and collected several times within a 1 mo period. We tested the following hypotheses: (1) developmental features in P. australis such as thicker and more open vascular system, higher primary production but lower leaf lifespan cause higher translocation in this species than in P. sinuosa, (2) translocation of 15N and 13C are largely influenced by source–sink organ relationships resulting in higher partitioning of C to rhizomes, whereas N is preferentially moved away to leaves, and (3) 15N and 13C transport towards the apical region is more dominant in P. australis than in P. sinuosa. As predicted, higher isotope content was found at both spatial scales in P. australis but differences were related to enhanced incorporation during incubation in this species. When both spatial scales were compared, both species showed higher 15N translocation to young leaves within the same shoot, whereas in the among-shoots experiment most of the material remained within the leaves of the incubated shoot. In contrast, translocation of 13C occurred mainly to rhizomes and tended to be higher at the among-shoots scale, particularly in P. sinuosa. No directionality was detected for either P. australis or P. sinuosa, possibly as a result of the low rates of N translocation at the among-shoots scale and the morphology of the vascular system allowing the integration of neighbouring plant parts for C requirements. Unlike for Western Australian species, the available literature on P. oceanica indicates patterns of among-shoots N distribution that are similar to those of C, which suggests that species are adapted to distinctive ambients. KEY WORDS: Posidonia spp. · 15N and 13C translocation · Stable isotopes · Plant scale · Vascular architecture · Source–sink relationships · Western Australia Full text in pdf format PreviousNextCite this article as: Prado P, Collier CJ, Lavery PS (2008) 13C and 15N translocation within and among shoots in two Posidonia species from Western Australia. Mar Ecol Prog Ser 361:69-82. https://doi.org/10.3354/meps07405Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 361. Online publication date: June 09, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Investigator Group Expedition 2006: Seagrasses of the Investigator Groupregion: Posidonia Meadow Condition in a pristine Offshore Marine Environment

As part of the ‘Investigator Expedition’, a subtidal survey of seagrasses was conducted in the Investigator Group region of South Australia (S.A.) during May 2006. Seagrasses were collected and identified from Pearson, Flinders, and Topgallant Islands, and from Waterloo Bay on the adjacent mainland. Twelve species of seagrasses have now been documented in the region (including Ward I.), with six species of Posidonia, two species of Amphibolis, two species of Heterozostera, and Halophila australis. The previously undescribed seagrasses of Flinders I. were found to be particularly diverse and widespread. Measures of Posidonia angustifolia/sinuosa condition were taken at 7 m depth from eight sites across four locations (Pearson I., northern Flinders I., eastern Flinders I., Waterloo Bay). A comparison of P. angustifolia/sinuosa condition at these sites with other studies in S.A. indicates that the Investigator Group sites are healthy, and that a P. angustifolia/sinuosa meadow can be suspected of being unhealthy or degraded if aboveground biomass is <100 g DW.m−2, leaf density is <1000 m−2, average leaf length is <200 mm, and/or epiphyte load is >1 g DW epiphytes.g DW seagrass−1. Five species of gastropod were found associated with P. angustifolia/sinuosa across the eight sites. Whilst epiphyte biomass/load was relatively low in the region, a significant positive relationship existed between epiphyte biomass and gastropod density. Flowering in P. angustifolia/sinuosa was noted at some of the sites, particularly on Flinders I.

Global seagrass distribution and diversity: A bioregional model

Seagrasses, marine flowering plants, are widely distributed along temperate and tropical coastlines of the world. Seagrasses have key ecological roles in coastal ecosystems and can form extensive meadows supporting high biodiversity. The global species diversity of seagrasses is low (< 60 species), but species can have ranges that extend for thousands of kilometers of coastline. Seagrass bioregions are defined here, based on species assemblages, species distributional ranges, and tropical and temperate influences. Six global bioregions are presented: four temperate and two tropical. The temperate bioregions include the Temperate North Atlantic, the Temperate North Pacific, the Mediterranean, and the Temperate Southern Oceans. The Temperate North Atlantic has low seagrass diversity, the major species being Zostera marina, typically occurring in estuaries and lagoons. The Temperate North Pacific has high seagrass diversity with Zostera spp. in estuaries and lagoons as well as Phyllospadix spp. in the surf zone. The Mediterranean region has clear water with vast meadows of moderate diversity of both temperate and tropical seagrasses, dominated by deep-growing Posidonia oceanica. The Temperate Southern Oceans bioregion includes the temperate southern coastlines of Australia, Africa and South America. Extensive meadows of low-to-high diversity temperate seagrasses are found in this bioregion, dominated by various species of Posidonia and Zostera. The tropical bioregions are the Tropical Atlantic and the Tropical Indo-Pacific, both supporting mega-herbivore grazers, including sea turtles and sirenia. The Tropical Atlantic bioregion has clear water with a high diversity of seagrasses on reefs and shallow banks, dominated by Thalassia testudinum. The vast Tropical Indo-Pacific has the highest seagrass diversity in the world, with as many as 14 species growing together on reef flats although seagrasses also occur in very deep waters. The global distribution of seagrass genera is remarkably consistent north and south of the equator; the northern and southern hemispheres share ten seagrass genera and only have one unique genus each. Some genera are much more speciose than others, with the genus Halophila having the most seagrass species. There are roughly the same number of temperate and tropical seagrass genera as well as species. The most widely distributed seagrass is Ruppia maritima, which occurs in tropical and temperate zones in a wide variety of habitats. Seagrass bioregions at the scale of ocean basins are identified based on species distributions which are supported by genetic patterns of diversity. Seagrass bioregions provide a useful framework for interpreting ecological, physiological and genetic results collected in specific locations or from particular species.

Does transported seagrass provide an important trophic link in unvegetated, nearshore areas?

The contribution of detritus from seagrass and other primary producers to faunal production in unvegetated nearshore areas was examined primarily using stable isotopes. Fish, macroinvertebrates, meiofauna and primary producers (seagrasses, macroalgae, seston and benthic microalgae) were sampled from sites in south-western Australia. All samples were analysed for δ 13C and δ 15N values and fish gut contents were determined. δ 13C values for seagrasses in the region were high compared to other macrophytes, ranging from 49.9 to −8.2‰ compared to −19.8 to −12.6‰ for macroalgae. The δ 15N values ranged between 4.0 and 7.7‰ for the red, brown and green algae, and between 3.2 and 5.9‰ for seagrasses. Seston and benthic microalgae samples had a mean δ 13C value of −12.8 and −14.0‰, respectively, and their δ 15N values were comparable to the macroalgae. All invertebrate fauna had mean δ 13C values considerably lower than seagrasses. However, individual samples harpacticoid copepods and polychaetes had a value as high as −11.7‰. δ 15N values for consumers were higher than those of the primary producers, except for copepods and amphipods. The δ 13C values for fish had a relatively small range, between −16.6 and −13.1‰, and the δ 15N values of fish were elevated compared to the invertebrates and primary producers, ranging mostly between 10.0 and 12.6‰. Mixing model analysis based on δ 13C values indicated that seagrass ranked low as a likely carbon source for all invertebrates other than harpacticoid copepods at a single site and some samples of polychaetes. The δ 13C values for fish were similar to those of a combination of harpacticoid and calanoid copepods, amphipods and polychaetes. The consumption of harpacticoid copepods by some fish species indicates that Amphibolis and Posidonia species in south-western Australia can contribute to the food web of unvegetated nearshore areas as detritus, but brown algae is likely to make a greater contribution. At least for the time of year that was sampled, the flow of detrital seagrass material into the foodweb may be mediated by specific detrivores, in this case harpactacoid copepods, rather than by all detritivores.

Benthic fossil assemblages and depositional facies of the Middle Triassic (Anisian) Yuqing Member of the Qingyan Formation, southern China

Abstract The Middle Triassic (Anisian) Yuqing Member of the Qingyan Formation crops out in Guizhou Province, southwestern China, and consists of storm-dominated inner and outer shelf deposits. It contains Tethyan benthic fossils that are grouped into two fossil assemblages. The allochthonous Mentzelia multi-costata-Mentzelia mentzeli assemblage is characterized by brachiopods in the inner shelf facies. The parautochthonous and allochthonous Posidonia wengensis assemblages, composed mainly of epifaunal bivalves, dominate in the inner and outer shelf facies, although some species of Posidonia generally are characteristic of deep-sea environments or low-oxygen conditions.

Patterns in the abundance and size-distribution of syngnathid fishes among habitats in a seagrass-dominated marine environment

Syngnathid fishes were sampled using a 1 m wide beam trawl during the day and night in each season from summer 1996/1997 to summer 1997/1998 in five habitat types in an approximately 90 km 2 area located on the lower west coast of Australia. The seagrasses Amphibolis griffithii, Posidonia sinuosa and Posidonia coriacea and shallow unvegetated sand were all in depths of 4–9 m, while deep habitat, which comprised mainly bare sand with isolated patches of the seagrasses Heterozostera tasmanica and Halophila ovalis, occurred at depths of 12–16 m. While A. griffithii and P. sinuosa each formed dense monospecific meadows, P. coriacea occurred in sparse clumps surrounded by areas of bare sand and patches of H. tasmanica. While catches of spotted pipefish Stigmatopora argus occurred mainly in P. sinuosa and P. coriacea, individuals of this species that exceeded ca. 55 mm in snout–vent length were far more abundant in the former habitat whereas smaller fish occurred mostly in the latter. Densities of S. argus were similar in P. sinuosa and P. coriacea, but differed between seasons, and a season/habitat interaction was present. In contrast, wide-bodied pipefish Stigmatopora nigra were collected mainly in P. coriacea and deep habitat and, although the densities were greater in P. coriacea than in deep habitat, the size-distributions of this species in these habitats were similar. Notably, S. nigra was never collected in P. sinuosa. Although less abundant than the above pipefish, the long-snouted pipefish Vanacampus poecilolaemus was collected almost exclusively in P. sinuosa. Few syngnathids were collected from shallow unvegetated sand or from A. griffithii, which differed markedly in plant structure from both Posidonia species. It is suggested that these syngnathid species occupy habitats that best enable them to remain inconspicuous to predators. Both S. argus and S. nigra have green or brown colouration and mimic strap-like seagrass leaves which they grasp with prehensile tails, whereas V. poecilolaemus, which is dark coloured and lacks a prehensile tail, most likely lies among the rich detrital material that accumulates beneath the dense P. sinuosa canopy. The movement of larger-sized S. argus from the narrow-leaved P. coriacea to the relatively broad-leaved P. sinuosa may enable the adults of this species to be better camouflaged than if they had remained in the former habitat. As they grow to only approximately half the size of S. argus, S. nigra of all sizes could remain concealed in narrow-leaved seagrasses like P. coriacea and H. tasmanica. The presence of more S. nigra in P. coriacea compared with deep habitat probably reflected the greater expanse of seagrass canopy available in the former habitat, while the inability of either Stigmatopora species to mimic the short leaves of A. griffithii could help to explain their scarcity in this seagrass. The almost total absence of S. nigra and small S. argus from P. sinuosa could be due to their inability to grasp the relatively broad leaves of this seagrass or the presence in this habitat of predators that selectively predate these smaller pipefish. That very few small-sized S. argus were present in P. coriacea during winter 1997, despite the fact that males of both Stigmatopora species carried broods of young in all seasons, suggests that the reproductive output of this species and/or the survival of recruits varied during the study period.

Changes in seagrass coverage in Cockburn Sound, Western Australia between 1967 and 1999

Changes in seagrass coverage in Cockburn Sound from 1967 to 1999 were assessed from aerial photographs using modern mapping methods with the aim of accurately determining the magnitude of change in hectares of seagrasses between 1967 and 1999 and to set up a baseline for future monitoring of seagrass loss in Cockburn Sound. Firstly, coverage and assemblages of seagrasses in Cockburn Sound were mapped using the best available aerial photographs from 1999, rectified to a common geodesic base with comprehensive groundtruth information, and with a semi-automated mapping algorithm. Then the same technique was used to map historical seagrass coverage in Cockburn Sound from aerial photographs taken in 1967, 1972, 1981 and 1994. The seagrass coverage in Cockburn Sound has declined by 77% since 1967. Between 1967 and 1972, 1587 ha of seagrass, were lost from Cockburn Sound, mostly from shallow subtidal banks on the eastern and southern shores. By 1981, a further 602 ha had been lost. Since 1981, further seagrass losses (79 ha) have been restricted to a shallowing of the depth limit of seagrasses, localised losses associated with port maintenance and a sea urchin outbreak on inshore northern Garden Island. There has been no recovery of seagrasses on the eastern shelf of Cockburn Sound after nutrient loads were reduced in the 1980s, suggesting that this shallow shelf environment has been altered to an environment not suited for large-scale recolonisation by Posidonia species.

Mechanical seagrass transplantation in Western Australia

An underwater seagrass harvesting and planting machine (‘ECOSUB1’) was designed with the specific parameters that a large seagrass ‘sod’ (0.25 m 2 in area and 0.5 m deep) could be extracted and planted with minimal disturbance to the leaves, roots and rhizomes contained within it. A prototype was developed and tested by the end of September 1996 and transplantation commenced in November 1996. Over 1500 sods have been planted up to March 1999. Survival appears dependent on the species being transplanted; Posidonia species have shown good survival to date, with Posidonia sinuosa and Posidonia coriacea plants showing 76.8 and 75.8% survival, respectively, 2 years after transplantation. Amphibolis griffithii plants have not done as well (44.3%), although this appears to be largely dependent on planting technique and winter storm damage to the plants at the time of transplanting.

Isopod miners in the leaves of two Western Australian Posidonia species

Isopods of the genus Lynseia have been found burrowing (leaf mining) in meadow forming seagrasses ( Posidonia) from Western Australia. These small (1.5–1.8 mm) isopods burrow beneath the epidermis consuming the mesophyll, forming a linear mine of similar width to the isopod along the leaf lamina. At Rottnest Island in January 1991, 75% of Posidonia australis Hook. f. and 66% of Posidonia sinuosa Cambridge and Kuo leaves were burrowed by isopods. Isopods were found in leaves of all sizes, but number of burrows, number of isopods and the length of burrows increased in older leaves. In P. australis, there were 3.15 ± 0.1 burrows and 2.6 ± 0.1 isopods per burrowed leaf, in P. sinuosa there were 2.9 ± 0.1 burrows and 2.48 ± 0.2 isopods per burrowed leaf. Isopods were mobile, vacating older burrows and forming new ones. Male and female isopods were found in pairs within burrows and young isopods were then brooded within the burrows, forming new branches from the parent burrow. Within monospecific stands of P. australis and P. sinuosa there were 2950 and 2280 isopods m −2 respectively. Mean length of individual burrows was 22 mm in P. australis and 28 mm in P. sinuosa. Total burrow length per leaf (mean ± SE) was 69.6 ± 3.6 mm in P. australis and 81.7 ± 4.7 mm in P. sinuosa. The consumption of leaf tissues and the resulting lines of weakness along burrow lines may have negative effects on the seagrass. Burrows facilitate entry of water, bacteria and fungi to the blade, leading to localised tissue death. Epiphytes may proliferate along burrow lines, owing to surface roughness or solute loss associated with the burrow. Burrows also split and increase fragmentation of leaves in storm events. Loss of photosynthetic area due to epiphytic growth and fragmentation of the leaves may reduce the growth of Posidonia. These potential effects of lynseid seagrass leaf miners may be important if the seagrass meadow was already under stress.

Effects of light and temperature on the photosynthesis of temperate meadow-forming seagrasses in Western Australia

A technique is described for measuring the metabolic response of seagrasses to different light intensities and water temperatures in the laboratory. The observed depth distribution of the seagrass species can be related to their light requirements and metabolic rates. The photosynthetic efficiency and maximum photosynthetic rates of Amphibolis griffithii (Black) den Hartog were significantly higher than those of Posidonia sinuosa Cambridge & Kuo and Posidonia australis Hook.f. The optimum temperature range for net photosynthesis of Posidonia sinuosa was 18–23°C and photoinhibition occurred at relatively low temperature and high light intensity. Maximum gross photosynthetic rates of P. sinuosa in an attenuated light regime simulating that experienced by plants in the field were higher than in a uniform light field. The respiration rates of the leaves were four to seven times higher than those of the root/rhizome component in the Posidonia species. There was no evidence for physiological adaptation to low light environments, using P. sinuosa from different depths and with different epiphyte loadings.

Ultrastructural Differentiation of the Tanniniferous Cells in the Marine Phanerogam Posidonia oceanica (L.) Delile

The mediterranean seagrass Posidonia oceanica (L.) Delile differs from the Australian species of Posidonia in the occurrence of special tanniniferous cells which are located in the first superficial cell layers of mesophyll and sometimes among the sieve elements of the vascular system. Ultrastructural observations of the basal leaf growing region have made it possible to study the development pattern of these secretory cells. Different processes must be involved in their differentiation, some of them related to the cell and its vacuolar system and others more especially to the secretion of tannins. Important lytic processes occur in small grouped and undifferentiated cells and induce the formation of highly vacuolated cells dispersed among less differentiated cells