Band 49, Heft 5_6

Sediment loading and siltation affect seagrasses directly by burying shoots, and indirectly by attenuating light through resuspension. In the Indo-Pacific tropical region, Cymodocea serrulata is considered to be one of the species most tolerant to burial and light attenuation, because it is able to form its canopy in a higher position above the bottom, due to the presence of a long vertical rhizome. To test whether C. serrulata survives in shaded environments by forming a tall canopy (through elongation of the vertical rhizome), we compared variability of this species in vertical rhizome length among sites with different levels of siltation and sedimentation with morphological variability in C. rotundata , which is less tolerant to the stresses. Monitoring of environmental conditions, seagrass collection and morphological measurements of seagrasses were carried out in a meadow in Japan and three meadows in Thailand. C. serrulata and C. rotundata showed different types of among-site variation. To produce a tall canopy, C. serrulata elongated vertical rhizomes, whereas C. rotundata extended blades. Upward elongation of the vertical rhizome or blade was not observed in either species in sites with heavier siltation, suggesting upward elongation is not related to low light stress or sedimental burial. As a conspicuous morphological trait, roots on vertical rhizomes of C. serrulata were abundant at sites where sediment deposition and erosion occur frequently. Having abundant roots on vertical rhizomes is considered to be adaptive for stabilizing unsettled sediment.

To identify the optimal nitrogen and phosphorus concentrations for development and differentiation of vegetative cells of Porphyra yezoensis cultured on solidified agar medium, vegetative cells were isolated with marine gastropod enzymes from mature thalli and varied concentrations of nitrogen and phosphorus were added to the culture media. Our results showed that: (1) nitrate is a good nitrogen source in solid culture even at a concentration as high as 18.80 mM. In contrast, ammonium is not, because few vegetative cells developed into germlings over 14 days in medium containing this ion; (2) at 0.47 mM and higher nitrogen concentrations, isolated cells produced large numbers of normal leafy thalli within 14 days, while at lower concentrations of 0.23 mM–0.47  mM, regenerated cells showed obvious nitrogen deficiency after 20 days in culture; (3) optimum phosphorus concentrations were between 0.07 mM–0.14 mM and produced >52% normal vegetative cells within 14 days. Most germlings were abnormal and showed no further development when the phosphorus concentration was below 0.03 mM or over 0.30 mM, and (4) the differentiation of germlings was significantly correlated to the ratio of N and P (a ratio of 5:1 produced best results).

Although herbivory by fishes is recognized as a fundamental process structuring coral reef communities, the susceptibility of macroalgae and potential effects of this process at Rocas Atoll are unknown. Macroalgal consumption was evaluated through field transplant assays onto reef sites to investigate the differential susceptibility of species to herbivory by fishes. Of 13 species examined, the red macroalga Digenea simplex was highly preferred and probably constitutes the most important food resource to herbivorous fishes in almost all parts of the atoll. The green macroalga Bryopsis plumosa and the brown algae Dictyopteris jamaicensis , Dictyota crispata , D . ciliolata , D . cervicornis , D . menstrualis , D . mertensii , D. pfaffii and D . pinnatifida were less preferred, probably due to chemical defenses, since all are known to produce secondary metabolites, many of which are broad spectrum feeding deterrents against herbivores including reef fishes. Finally, the algae Sargassum polyceratium and Gelidiella acerosa were less preferred, probably because of both morphological features and chemical defenses. This differential susceptibility of the seaweeds to herbivory associated with different spatial herbivory pressure may be responsible for the variable distribution and abundance of seaweeds around the Rocas Atoll reef.

The distributions of three submersed macrophytes Najas marina , Myriophyllum spicatum and Zostera noltii were surveyed in the inner coastal lagoons of the German Baltic Sea. Growth and photosynthetic performances of these species were investigated in crossed laboratory experiments (salinity × irradiance). Z. noltii showed the potential to grow in mesohaline lagoons, while N. marina seemed to be restricted to oligohaline areas. The potential distribution area of M. spicatum overlapped with those of the other two species. The laboratory results for N. marina and M. spicatum correspond with the recent distribution of the two species in the investigation area. However, neither historical nor recent distribution records of Z. noltii match the potential of this species to grow at 3 and 8 psu. Potential depth limits were calculated for the three species at their collection sites. Accordingly, N. marina had the potential to grow at 0.9–1.1 m depth in the Saaler Bodden, M. spicatum at 1.1–2.1 m depth in the Großer Jasmunder Bodden and Z. noltii down to 2.5–5.1 m depth in the Salzhaff. N. marina grows maximally at 1.0 m in the Saaler Bodden, M. spicatum at 2.0 m in the Großer Jasmunder Bodden and Z. noltii at 3.0 m in the Salzhaff. Thus, recent and modelled historical values of all three species do not differ, and recent distribution limits do not indicate an eutrophication-induced decrease in depth distributions of these species at their respective collection sites.

In areas of Tasmania and New Zealand, the introduced Japanese alga, Undaria pinnatifida , grows attached to unstable substrata such as small cobbles and shells. Observations suggest these algae may disperse by saltation (i.e., a series of jumps) while attached to their substratum. A predictive model was developed to estimate the dispersal rate of U. pinnatifida by this mechanism, depending on drag force (as a function of lamina surface area and water velocity), the buoyant weight of the attached substratum and the depth at which the alga was located. The model was parameterised based on empirical measurements of 89 tagged thalli in the field, and estimates of drag on different sized laminae determined from experiments in a large flume tank. Empirical observations and model predictions suggest that under typical conditions at moderately exposed sites, dispersal via this mechanism is likely to be of the order of at least 10 1 –10 2  m per year. When compared to the likely dispersal of spores from the parent (≈10 1  m per year), and the likely dispersal of fertile drift thalli (≈10 3 –10 4  m per year), we suggest that algae on unstable substrata may provide a mechanism for intermediate dispersal over moderate distances, providing sufficient spore density to ensure high fertilisation success. This is likely to enhance the rate of spread of U. pinnatifida in circumstances where habitat suitable for establishment is available.

Sublittoral populations of Zonaria tournefortii off the Canary Islands were studied to monitor vegetative and reproductive phenology. Sterile individuals, sporophytes and gametophytes coexisted throughout the year, but significant differences were detected between seasons and life-history stages for most parameters tested. Density was maximal in autumn (3513 individuals m -2 ). Twenty-five percent of all individuals were fertile, with sporophytes at much higher densities than gametophytes in all seasons. The highest average individual thallus length occurred in winter, but the highest frequencies of long specimens and the greatest size inequalities were in summer. Fertile thalli were significantly longer than sterile thalli, however, sporophytes and gametophytes had similar average lengths. Populations exceeded 100% overall cover in all seasons (up to 552% in autumn), with highest cover occupied by fertile individuals. Overall biomass, produced mainly by sporophytes, was maximal in summer-autumn (981 g DW m -2 ). Results suggest that this perennial species exhibits a somewhat exceptional phenological pattern amongst dictyotaleans. Its phenology was regulated in most years by elevated water turbulence in early winter, which causes breakage or loss of whole individuals. Blade regeneration and recruitment from the remaining damaged individuals began in winter-spring, and continued until autumn. Individual growth, hierarchical structuring and maturation of populations occurred mainly in spring-summer. Optimal reproductive potential per individual occurred during summer, although the reproductive potential per unit area was similar among seasons.

Our understanding of diatom reproduction is limited, especially for marine planktonic centric diatoms. This project aimed to elucidate sexual behavior and gametogenesis in the mediophycean diatom, Thalassiosira angulata , and to determine the fine structure of its reproductive cells. A monoclonal culture of T. angulata was successfully induced and enumerated, and all stages of auxosporulation documented. Spermatogenesis was absent in all inductions. Nuclear behavior of the auxospore mother cell (AMC) during auxosporulation indicated that reproduction was autogamic. In our clone, reproduction began with a differentiating mitosis and subsequent pervalvar expansion of the AMC, a meiocyte. The smaller cell often continued normal vegetative growth. In the AMC, meiosis I was acytokinetic and resulted in one viable haploid and one pycnotic nucleus. Meiosis II resulted in the production of two viable haploid nuclei, which we postulate later fused to produce the zygotic nucleus. Cell expansion was observed in very young AMCs both prior to and following autosyngamy. The initial cell valves were produced after at least two acytokinetic and pycnotic mitotic divisions, after which the cell was capable of undergoing its first normal mitosis involving production of two sibling hypovalves.

Peyssonnelia imbricata sp. nov. occurs in shallow protected coral reef, mangrove and deep-reef habitats in southwestern Puerto Rico, Caribbean Sea. The species is characterized by its small imbricate blades which may form extensive colonies, vegetative production of new blades from the dorsal surface of parent blades, polyflabellate arrangement of hypothallial cells and its Peyssonnelia atropurpurea -type anatomy.

We present a taxonomical revision of Cocconeis distans and C. debesi based on historical collections from Hustedt, and live material sampled from leaves of Posidonia oceanica . Our observations clarify the specific morphology of the two taxa, which have been confused by past and modern authors. In addition, we demonstrate that the valve structure of C. debesi deviates from that of the genus Cocconeis , but agrees with that of the genus Amphicocconeis . Therefore, we propose a new nomenclatural combination, Amphicocconeis debesi . The study also provides additional information on the geographical distribution of these taxa.

The breakdown of dead twigs of Avicennia officinalis and Rhizophora mucronata was studied in the Udyavara mangrove on the southwestern coast of India. Samples were exposed in litter bags for up to 18 months at low tide levels. Water temperature, pH, oxygen and salinity were determined throughout the study period. No clear seasonal trends were observed except for salinity, which fluctuated between 1.1 psu (freshwater inflow due to monsoon season) and 34.1 psu (dry season). Twig decay was slow during the first 10–12 months, and accelerated during the remaining six months. Changes in phenolics content of the twigs were best described by an exponential loss function. Nitrogen increased in both substrates, peaked around 120 days, and then declined. Phosphorus levels declined rapidly in the first two months, and then gradually increased.