Ecology and systematics of mangrove crabs of the genus <i>Perisesarma</i> (Crustacea: Brachyura: Sesarmidae) from East Africa

East Africa has a reduced mangrove crab species richness when compared to Asian mangroves. To date, only one species of Perisesarma de Man, 1895 has been reported in East Africa, despite more than 30 years of mangrove research in this region. Based on morphology, colour, mtDNA and behaviour, we describe a new species of Perisesarma from Kenya, P. samawati sp. nov. Surprisingly, when comparing molecular data from other species within this genus,P. samawati sp. nov. and the sympatric P. guttatum (A. Milne Edwards, 1869) are not sister species. Some aspects of the ecology of P. guttatum and P. samawati sp. nov. are compared and the differences discussed. Additionally, we compare P. samawati sp. nov. with the ecological literature of a possible sister species P. eumolpe de Man, 1895 from Malaysian mangroves. Our findings suggest that the new species is an ecologically important species in East African mangroves

Geochemistry of marine bivalve shells: the potential for paleoenvironmental reconstruction

Bivalve shells offer a great potential as environmental proxies, since they have a wide geographical range and are well represented in the fossil record since the Cretaceous. Nevertheless, they are much less studied than corals and foraminifera and are largely limited to isotopic studies. This is probably due to the fact that the literature has been contradictory regarding the faithfulness of elemental proxies in bivalves. The general aim of this dissertation is to increase our knowledge of proxies in bivalve carbonate. More specifically, d18O, d13C, Sr/Ca, Mg/Ca, U/Ca, Ba/Ca, and Pb/Ca were investigated in both aragonite and calcite bivalve shells and their potential as environmental proxies were evaluated.The most well studied proxy of sea surface temperature (SST) in bivalve carbonate is d18O, and it is well known that in addition to SST, the d18O of the water dictates the d18O value of the shell.This study cIearly demonstrates that unknown d18O of the water can cause severe errors when calculating SST from estuarine bivalve shells; with the example presented here providing calculated SSTs 1.7 to 6.4 °C warmer than measured. Therefore, a salinity independent or salinity proxy would greatly benefit SST reconstructions. In estuaries, shell d13C has long been regarded as a potential salinity indicator. However, more recent works have demonstrated that the incorporation of light carbon from metabolic CO2 interferes with the environmental signal. This study confirms that the amount of metabolic CO2 increases in internal fluids with age, resulting in the strong ontogenic decrease in d13C values of bivalve shells. However, this is not always the case, with Saxidomus giganteus shells showing no discernable decrease over ~10 years growth. On the other hand, this study also demonstrates that the percent metabolic CO2 (%M) incorporated into bivalve shells can be large -up to 35 % in some individuals of Mercenaria mercenaria. An attempt was made to remove this metabolic influence using the relationsbip between %M and shell biometrics; however the inter- and intra-site variability was too large. This was also the case for the relatively short-lived bivalve Mytilus edulis, where the %M varied between 0 and 10%. Within the studied estuary (Schelde) the shells were close to equilibrium, but at the seaward site, where wave action is stronger, the shells contained -10 %M and the absolute d13C values were indistinguishable from specimens within the estuary, despite a salinity difference of 4. Therefore, interpreting d13C values in bivalve carbonate should be done with caution. In addition to d13C, Ba/Ca ratios were investigated as a salinity proxy as well. In the calcite shells of M. edulis a strong linear relationship between shell 'background' Ba/Ca and water Ba/Ca was found in both the laboratory and field. Although each estuary wiII have different relationships between salinity and water Ba/Ca, shell Ba/Ca can be used as an indicator ofsalinity within one estuary. Similar patterns of relatively stable background levels interrupted with sharp episodic peaks were also found in the aragonite shells of S. giganteus, and appear nearly ubiquitous to all bivalves. However, there was an ontogenic decrease in S. giganteus background Ba/Ca ratios, ilIustrating that these proxies can be species specific. Previous hypotheses regarding the cause of the peaks include ingestion of Ba rich phytoplankton or barite. This study ilIustrates that there is no direct relationship between Chl a and Ba/Ca peaks in S. giganteus shells, but they still may be related to blooms of specific species of phytoplankton.The ratios of Sr/Ca and Mg/Ca were investigated as salinity independent SST proxies. Ratios of Sr/Ca were found to be highly correlated to growth rate in S. giganteus, but not in M. mercenaria, contradictory to an earlier study on M. mercenaria. Although growth rates and temperature are often correlated, there was only a poor correlation between Sr/Ca and SST in S. giganteus (maximum R² = 0.27). Similarly, Mg/Ca and U/Ca ratios in S. giganteus were not correlated to SST, with U/Ca exhibiting a strong ontogenic trend.Finally, the use of bivalve shells as recorders of pollution was also assessed. There was both large inter- and intra-specimen variability in Pb/Ca ratios of M. mercenaria shells, but when enough shells were averaged, the typical anthropogenic Pb profile from 1949 to 2003 was evident.Overall, this study demonstrates the difficulties inherent to utilizing bivalve shells as recorders of their environment. It is clear that factors determining proxy incorporation are strongly species specific and that a mechanistic understanding is needed before we can progress further in this line of research. However, this study also ilIustrates that there is indeed environmental information that can be extracted from bivalve shells. Furthermore, the physiological influence on many of the studied proxies may prove to be useful as proxies of bivalve physiology, which in turn could provide information about bivalve paleo-ecology

Evaluating Freshwater Mussel Shell δ13C Values as a Proxy for Dissolved Inorganic Carbon δ13C Values in a Temperate River

The stable isotope ratio of dissolved inorganic carbon (δ13CDIC) in rivers reflects the dominant vegetation type in the surrounding watershed, rates of chemical weathering, atmospheric CO2 fluxes, and the relative rates of photosynthesis and respiration. Reconstructing past δ13CDIC values may reveal changes in these characteristics before watersheds experience land-use change and/or climate change. This study uses freshwater mussels, Elliptio complanata, and coeval environmental data to assess how high-resolution changes in the oxygen and carbon isotope ratios of shell carbonate (δ18Oshell and δ13Cshell, respectively) can be used as proxies of paleotemperature and paleo-DIC in rivers. To test our hypotheses, we analyzed time-series δ18Oshell and δ13Cshell values, δ13C values in bivalve tissues, and environmental data collected fortnightly from the Neuse River, North Carolina. Shell microsamples milled every 150 μm along the maximum growth axis represent an average of 12 ± 5 days (n = 524; excluding periods of growth cessation of &gt;30 days), which is similar to the environmental data resolution (∼14 days). Serially sampled δ18Oshell and δ13Cshell values did not capture the full range of environmental conditions due to growth cessation during winter shutdown and extreme weather events. Low water temperature and elevated turbidity appear to be significant drivers of growth cessation. Spatial and temporal differences in the amount of metabolic carbon incorporated in the shell (∼0%–44%) likely occur due to variable mussel metabolic rates within and among study sites. Though high-resolution δ13Cshell values did not reflect variations in δ13CDIC values, average δ13Cshell values were indicative of average δ13CDIC values in the Neuse River

Assessing δ15N values in the carbonate-bound organic matrix and periostracum of bivalve shells as environmental archives

Though previous studies demonstrate the utility of nitrogen and carbon stable isotope ratios (δ15N and δ13C, respectively) in bivalve soft tissues as biogeochemical proxies, it is necessary to develop alternative proxies for environmental reconstructions when soft tissues are unavailable, such as with fossils or in museum-archived specimens. This study assesses the reliability of the δ15N values of carbonate-bound organic matrix (δ15NCBOM) and periostracum (δ15Nperiostracum) in bivalve shells as recorders of the δ15N values of particulate nitrogen (δ15NPN) by comparing the δ15NCBOM and δ15Nperiostracum values of live-collected freshwater mussels (Elliptio complanata) and estuarine clams (Rangia cuneata) to the δ15N values of particulate nitrogen (δ15NPN) in the water column. The δ15NCBOM and δ15Nperiostracum values in both species were within the range of the δ15NPN values that have been corrected for trophic-level enrichment. Thus, our findings illustrate that δ15NCBOM and δ15Nperiostracum values reliably record δ15NPN values in rivers and estuaries. The significant positive correlation between δ15NCBOM and δ15Nperiostracum values in both species indicates that they may be used in a similar manner to record δ15NPN values. The δ15N values in E. complanata muscle, mantle, and gill tissues were enriched by about +3.4‰ compared to δ15NPN from the water column, which suggests that they are primary consumers that reflect baseline trophic levels. On the other hand, δ15N values in the soft tissues of R. cuneata have trophic-level enrichment consistent with both primary and secondary consumption. Therefore, variations in the δ15N values of tissues in R. cuneata may be related to trophic-level shifts and/or changes in N sources. Differences between the δ15N values of soft tissue, CBOM, and periostracum in E. complanata and R. cuneata can be attributed to asynchronous growth, metabolic rate, and organic molecule composition. The δ15NCBOM values vary along a freshwater-estuarine gradient because of land-use change and differences in the trophic level of the compared species. The δ15NCBOM values between neighboring sites reflect influences from biosolid application and treated wastewater discharge. While δ15NCBOM values did not differentiate between sites dominated by urban and forested land-cover, δ15NCBOM values were highest at the site with the highest agricultural land-use. These results demonstrate the potential of δ15NCBOM values in bivalve shells to record long-term changes in watershed land use

Osmoregulatory ability of <i>Chiromantes ortmanni</i> (Crosnier, 1965) subjected to dilute and hypersaline seawater

The short-term osmoregulatory ability and salinity tolerance of Chiromantes ortmanni was studied in the laboratory. C. ortmanni is a non-burrowing, high shore mangrove crab often subjected to extreme salinity fluctuations. They were found to be exceptional osmoregulators in salinities ranging from 0‰ to 65‰ after three days. They could also osmoregulate in 80‰ for at least 24 hours, but 96‰ proved to be lethal after just 12 hours. Osmotic capacity suggests that they are best suited for salinities ranging from about 16‰ to 48‰. As they are frequently found in salinities higher than this, it is hypothesized that they tolerate the higher energetic costs of large osmotic gradients as a trade-off for other benefits such as reduced predation and food competition

Advances in mollusc sclerochronology and sclerochemistry: tools for understanding climate and environment

This Special Issue of Geo-Marine Letters compiles papers on marine, estuarine and freshwater mollusc shells as recorders of environmental and climatic conditions. Considering that many past studies have differentiated geochemical investigations from sclerochronological investigations, we propose that the sub-discipline term “sclerochemistry” be used when geochemical investigations are undertaken. This issue starts with two review papers that discuss the importance of physiology or vital effects on both sclerochronology and sclerochemistry. Several sclerochemical calibration studies on modern specimens of both bivalves and gastropods are presented (including δ18O, Mg/Ca, Sr/Ca and Ba/Ca), which illustrate the usefulness and difficulties associated with using these proxies. Studies on fossil mollusc shells are also provided, with one study that uses Pliocene scallop shells to understand past ocean circulation and another that addresses the problem of diagenesis. Finally, a sclerochronological study of crystal prism width across the shell is presented. This Special Issue demonstrates that many elemental and isotopic proxies contained in mollusc shells are complex. In spite of these complexities, environmental and climatic conditions can be extracted for use in palaeoclimatic and palaeoenvironmental research

Determination of ´δ18O of water and δ13C of dissolved inorganic carbon using a simple modification of an elemental analyzer-isotope ratio mass spectrometer an evaluation

no abstract