Metamorphosis of mangrove crab megalopae, Ucides cordatus (Ocypodidae): Effects of interspecific versus intraspecific settlement cues

It has recently been shown that metamorphosis of Ucides cordatus megalopae is triggered by substrata from the mangrove forest habitat, and, in particular, adult conspecific odours. Here we demonstrate that the gender of the odour-emitting crabs is insignificant for the metamorphic response in this species. We further investigate whether other estuarine crabs (Goniopsis cruentata, Uca spp., and Callinectes danae) also induce settlement and metamorphosis of U. cordatus megalopae. This is of special interest for population recovery in areas hit by lethargic crab disease (LCD), a fungus that selectively kills U. cordatus but not co-occurring species. Ucides megalopae were reared in four treatments with interspecific-conditioned seawater and tested against the effects of conspecific-conditioned seawater (positive control) and pure seawater (negative control). All megalopae in the positive control metamorphosed successfully, while only one (2%) moulted in the negative control, with a delay of 10 days compared with the latest metamorphosis in the former treatment. In seawater conditioned with U. maracoani and C. danae, which occur on sediment banks and in tidal creeks respectively, all larvae died before reaching the juvenile stage. In the treatments with odours of species that share the same mangrove forest microhabitat as U. cordatus, i.e. G. cruentata and a group of five fiddler crab species (mixed-odour treatment), 20 and 10% respectively of the megalopae moulted with a delay of up to 11 days. No specimens metamorphosed after day 39, but megalopae lived up to 93 days. Since only the conspecific- and coexisting-species treatments stimulated development, we hypothesize that Ucides megalopae are able to precisely identify species-habitat-specific settlement cues. This will be investigated in more detail in future studies, which will also test the effects of the odours of the five forest fiddler crab species separately. The impact of the interspecific odour treatments was much smaller than that of the conspecific odours, nevertheless elevated moulting rates of up to 18% relative to seawater may still significantly accelerate the repopulation of U. cordatus in areas lacking conspecifics, e.g. after massive crab mortalities or at first colonization.     

Allelopathic effects of fruits of the Brazilian pepper Schinus terebinthifolius on growth, leaf production and biomass of seedlings of the red mangrove Rhizophora mangle and the black mangrove Avicennia germinans

Exotic plant species can negatively affect native flora and fauna by changing the diversity and productivity of the system and altering successional processes. The novel weapons hypothesis has been suggested as one mechanism for invasion and spread of exotic plant species. It states that exotic species with “new weapons”, such as allelopathic chemicals, may have greater impacts on species in the invaded range without co-evolved defense strategies against the exotic chemicals. In Florida, one successful exotic species, Schinus terebinthifolius (Brazilian pepper), has invaded nearly every habitat throughout the state. Past studies have documented allelopathic properties of S. terebinthifolius negatively impacting the growth of native terrestrial flora. Here we document the effect of S. terebinthifolius for the first time on mangrove systems. We examined growth and survival of Rhizophora mangle (red mangrove) and Avicennia germinans (black mangrove) seedlings when exposed to a range of densities of intact and crushed S. terebinthifolius fruits (0, 25, 50) at two different salinities (15, 30 ppt). We also documented the natural density of S. terebinthifolius fruits beneath the tree canopy after fruit production in mangrove habitat. Growth and biomass were significantly reduced in A. germinans when exposed to the highest density of intact S. terebinthifolius fruits growing in 30 ppt saltwater. Trials with intact fruits had no significant effects on R. mangle; however, crushed fruits significantly decreased growth, and leaf production. The ecological importance of the different effects of crushed and intact fruits is supported by field observations which found equal or greater numbers of crushed fruits compared to intact fruits beneath the tree canopy of female S. terebinthifolius. In addition, abiotic factors, such as salinity, may interact with allelopathic chemicals in natural systems and needs to be addressed when planning future allelopathic studies.     

Inter- and intra-specific variation in the facial colours of <Emphasis Type="Italic">Perisesarma eumolpe</Emphasis> and <Emphasis Type="Italic">Perisesarma indiarum</Emphasis> (Crustacea: Brachyura: Sesarmidae)

The colour of the characteristic facial bands in the mangrove crabs Perisesarma eumolpe (De Man) and Perisesarma indiarum (Tweedie) were investigated in relation to species, sex and size. Quantitative red/blue/green (RBG) values extracted from close-up digital photographs of 27 P. indiarum and 48 P. eumolpe specimens revealed significant differences between species and sexes. Males of both species possessed facial bands characterized by a greater intensity of blue, whereas green was more pronounced in female facial bands. It is postulated that these colour differences play a role in intra-specific sexual recognition. There existed a significant positive relationship between size and the intensity of blue in the facial bands of females of both species, suggesting these traits function as an indicator of their maturity (and possibly body condition). In large males, facial band colours contrast strongly against the surrounding mudflat and may play an important role in conspecific signalling during territorial disputes or contests over access to females. Handling editor: K. Martens     

Influence of species richness and environmental context on early survival of replanted mangroves at Gazi bay, Kenya

Mangrove reforestation projects often suffer from low sapling survival, especially after transplanting saplings from nurseries to reforestation areas. This may be due to the sediment conditions at the target site, the planting strategy or failure to re-establish ecosystem processes. We examined experimentally the influence of environmental context, species richness and identity, sapling height and position on sapling survival and environmental variables linked to ecosystem functioning at deforested sites in Gazi Bay, Kenya. At site 1, a high shore location, 32 plots (36 m²) were planted with 8 treatments: all possible combinations of Avicennia marina, Bruguiera gymnorrhiza, and Ceriops tagal and an unplanted control (total: 3390 saplings; 4 plots/treatment). At site 2, a low shore location, the influence of sapling height, sapling position and sediment depth were tested by planting with 697 Sonneratia alba in a single monospecific plot (341 m²). After ∼2 years, there were significant differences in survival among the three species at site 1 with Bruguiera gymnorrhiza recording the lowest survival rate (29%). Survival was correlated with salinity (a strong effect) and height above chart datum (a weaker effect) at site 1. Sapling position did not significantly affect survival at either site. There was thus no evidence that early survival of transplanted saplings is influenced by the species mix in which they are grown, or by their position in the plot. Rather the tolerance of individual species to salinity was the key to their survival at the high tidal site. Species mix also had no significant effects on environmental variables in the plots. The former presence of a species at a site does not guarantee it will succeed there again if environmental degradation has exceeded species’ tolerance. Handling editor: T. P. Crowe     

紫红笛鲷幼鱼对集群密度和投饵定额的摄食反应

研究了紫红笛鲷(Lutjanus argentimaculatus)幼鱼在不同集群密度和投饵定额下的摄食反应.实验设7个不同的集群组(1、2、3、4、8、12、16尾/组),每个集群组又分成7个不同的投饵定额组(18.4×10-227.6×10-2、36.8×10-2、46.0 × 10-2、55.2×10-2、64.4×10-2、73.6×10-2g/组),通过测定摄食量观测幼鱼的摄食反应.在定额投饵的情况下,紫红笛鲷每尾幼鱼的一次平均摄食量随群体密度的增大呈先增加、超过一定密度后下降的趋势.1尾组与其他组差异极显著(P＜0.01),其组平均一次摄食量最低.当集群密度不变时,紫红笛鲷幼鱼一次平均摄食量随投饵定额的增大而增加,然后减小.随着集群密度增大,不同密度群体的最大摄食量向投饵定额增加的方向移动;而随着投饵定额增加,不同定额组的最大摄食量相对集中在2～8尾的群体中.结果表明,集群密度、投饵定额以及两者的交互作用对紫红笛鲷幼鱼的一次摄食量有极显著影响,群居的互利性和竞争性作用效果明显.     

Participatory appraisal for potential community-based mangrove management in East Africa

Mangroves are among the most threatened ecosystems in the world and the coastal forests of East Africa are no exception to this trend. Although conservation, rehabilitation and sustainable management plans have been developed in various tropical regions, only a few locally based approaches have been launched along the Indian Ocean coast of Africa. In order to identify possible conditions for sustainable management of mangroves based on socio-economic and ecological considerations, we present a participatory approach designed to evaluate the relationships between mangroves and human activities and the use of multi-criterion analysis to identify management solutions. To achieve this goal, all the subjects involved in mangrove management (local communities, institutions and researchers) took an active part in the process. The research was carried out in three communities relying on mangrove swamps: Kisakasaka village on the island of Zanzibar, Mida Creek in Kenya, both relying on mangrove forests known to be endangered by over-exploitation, and Inhaca Island in Mozambique, where mangroves are more pristine. Families were the hub of the research and the importance of each of their economic activities was assessed. We then examined the methods by which mangroves are exploited by the local community and by other stakeholders working in the study areas. Our results show that the mangroves in Inhaca are exploited only for household needs and the pressure on the forests is still ecologically sustainable. In contrast, there is a well-established demand for mangrove products in Kisakasaka and Mida Creek and the mangroves represent an essential source of income for the families, resulting in an ecologically unsustainable rate of exploitation. Therefore, possible alternatives to the current management practices were identified in the two areas by means of a participatory approach. Multi-criterion analysis was then used to compare and discuss the alternatives in terms of social, economic and ecological criteria.     

Phylogenetic diversity of nitrogen-fixing bacteria in mangrove sediments assessed by PCR-denaturing gradient gel electrophoresis

Culture-independent PCR–denaturing gradient gel electrophoresis (DGGE) was employed to assess the composition of diazotroph species from the sediments of three mangrove ecosystem sites in Sanya, Hainan Island, China. A strategy of removing humic acids prior to DNA extraction was conducted, then total community DNA was extracted using the soil DNA kit successfully for nifH PCR amplification, which simplified the current procedure and resulted in good DGGE profiles. The results revealed a novel nitrogen-fixing bacterial profile and fundamental diazotrophic biodiversity in mangrove sediments, as reflected by the numerous bands present DGGE patterns. Canonical correspondence analysis (CCA) revealed that the sediments organic carbon concentration and available soil potassium accounted for a significant amount of the variability in the nitrogen-fixing bacterial community composition. The predominant DGGE bands were sequenced, yielding 31 different nifH sequences, which were used in phylogenetic reconstructions. Most sequences were from Proteobacteria, e.g. α, γ, β, δ-subdivisions, and characterized by sequences of members of genera Azotobacter, Desulfuromonas, Sphingomonas, Geobacter, Pseudomonas, Bradyrhizobium and Derxia. These results significantly expand our knowledge of the nitrogen-fixing bacterial diversity of the mangrove environment.     

Threats to mangroves from climate change and adaptation options: A review

Mangrove ecosystems are threatened by climate change. We review the state of knowledge of mangrove vulnerability and responses to predicted climate change and consider adaptation options. Based on available evidence, of all the climate change outcomes, relative sea-level rise may be the greatest threat to mangroves. Most mangrove sediment surface elevations are not keeping pace with sea-level rise, although longer term studies from a larger number of regions are needed. Rising sea-level will have the greatest impact on mangroves experiencing net lowering in sediment elevation, where there is limited area for landward migration. The Pacific Islands mangroves have been demonstrated to be at high risk of substantial reductions. There is less certainty over other climate change outcomes and mangrove responses. More research is needed on assessment methods and standard indicators of change in response to effects from climate change, while regional monitoring networks are needed to observe these responses to enable educated adaptation. Adaptation measures can offset anticipated mangrove losses and improve resistance and resilience to climate change. Coastal planning can adapt to facilitate mangrove migration with sea-level rise. Management of activities within the catchment that affect long-term trends in the mangrove sediment elevation, better management of other stressors on mangroves, rehabilitation of degraded mangrove areas, and increases in systems of strategically designed protected area networks that include mangroves and functionally linked ecosystems through representation, replication and refugia, are additional adaptation options.     

Vegetation dynamics in the Bangladesh Sundarbans mangroves: a review of forest inventories

The Sundarbans, the largest contiguous mangrove forest in the world, covers 6,017 km² of the coastal zone of Bangladesh. Heritiera fomes Buch.-Ham., Excoecaria agallocha L. and Ceriops decandra (Griff.) Ding Hou jointly cover 95% of the forest area. In this study, the results of four forest inventories have been analyzed to understand observed vegetation dynamics of the Sundarbans from 1926 to 1997. The diversity of forest types has been gradually reduced over time, but H. fomes and E. agallocha have maintained their dominance over large portions of the forest. H. fomes is spread over 67% of the vegetated area of the forest, concentrated mostly in the northeastern area, and constitutes 75% of the density of trees with >15 cm dbh. The distribution and stem density of H. fomes show negative relationships with that of E. agallocha and C. decandra. In terms of coverage, E. agallocha is the most common species, spread over 74% of the vegetated area of the forest, and constitutes 39% of the density of trees with >2.5 cm dbh. On a longer timeframe (1926–1997), the dominance (coverage and density of larger diameter trees) of H. fomes as well as that of E. agallocha is declining. Even on parameters such as density of trees with >15 cm dbh, the dominance of E. agallocha is declining at a much greater rate than H. fomes. This observation contradicts the successional schemes proposed by different authors. This might indicate that theorizing successional schemes based on short-term observations on vegetation dynamics is not sufficient. The effect of human interference, changes in hydroedaphic condition and species interaction should be taken into consideration during explaining observed vegetation dynamics. Moreover, the need to understand vegetation trajectories at the micro-scale should be emphasized.     

Isolation and characterization of microsatellite loci in the red mangrove Rhizophora mangle (Rhizophoraceae) and its related species

Fourteen microsatellite markers were isolated from the red mangrove Rhizophora mangle (Rhizophoraceae), a widely distributed mangrove plant in the New World and West Africa. The range of expected heterozygosity of these markers was 0.000–0.672 in the two populations of R. mangle. Cross-species testing was examined for five other species of Rhizophora, and Kandelia candel and Bruguiera gymnorrhiza. All 14 markers could be amplified in R. samoensis, thirteen in R. racemosa, and six markers in all other species of Rhizophora. Our findings greatly increase the utility of these markers.