Intertidal Environments Research Articles

Effects of intermittent hypoxia on oxidative stress and protein degradation in molluscan mitochondria.

Oxygen fluctuations represent a common stressor in estuarine and intertidal environments and can compromise the mitochondrial integrity and function in marine organisms. We assessed the role of mitochondrial protection mechanisms (ATP-dependent and -independent mitochondrial proteases, and antioxidants) in tolerance to intermittent hypoxia or anoxia in three species of marine bivalves: hypoxia-tolerant hard clams (Mercenaria mercenaria) and oysters (Crassostrea virginica), and a hypoxia-sensitive subtidal scallop (Argopecten irradians). In clams and oysters, mitochondrial tolerance to hypoxia (18 h at 5% O2), anoxia (18 h at 0.1% O2) and subsequent reoxygenation was associated with the ability to maintain the steady-state activity of ATP-dependent and -independent mitochondrial proteases and an anticipatory upregulation of the total antioxidant capacity under the low oxygen conditions. No accumulation of end-products of lipid or protein peroxidation was found during intermittent hypoxia or anoxia in clams and oysters (except for an increase in protein carbonyl concentration after hypoxia-reoxygenation in oysters). In contrast, hypoxia/anoxia and reoxygenation strongly suppressed activity of the ATP-dependent mitochondrial proteases in hypoxia-sensitive scallops. This suppression was associated with accumulation of oxidatively damaged mitochondrial proteins (including carbonylated proteins and proteins conjugated with a lipid peroxidation product malondialdehyde) despite high total antioxidant capacity levels in scallop mitochondria. These findings highlight a key role of mitochondrial proteases in protection against hypoxia-reoxygenation stress and adaptations to frequent oxygen fluctuations in intertidal mollusks.

Responses of photosynthesis and nitrogen assimilation in the green-tide macroalga Ulva prolifera to desiccation

The Yellow Sea in China has experienced annual large-scale green tides since 2007. Ulva prolifera, the dominant causative species, originating from Pyropia yezoensis aquaculture rafts, experiences periodic tidal emersion in the intertidal nursery. It was proposed that physiological adaptations of U. prolifera may enable it to survive the harsh intertidal environment and contribute to subsequent blooms. In this paper, we measured the responses of photosynthesis and nitrogen assimilation of U. prolifera during desiccation of 0–6 h and subsequent rehydration for 0–24 h. The results suggested that both PSII photosynthesis and nitrogen assimilation were significantly reduced at the onset of desiccation. These reductions were reflected by decreases in the maximum quantum yield (Fv/Fm), effective photochemical quantum yield (YII), maximal electron transport rate (ETRmax), light utilization efficiency (α), NO3-N and NH4-N uptake rates, tissue nitrogen concentration (TN), and nitrate reductase activity (NRA). Desiccation temporarily lowered PSII photosynthesis, but rates recovered after algae were submerged for 24 h. Assimilation of nitrogen was also reduced during exposure to air. Although both NO3-N and NH4-N uptake rate recovered within 12 h upon rehydration, TN and NRA were not completed within 24 h. U. prolifera therefore appears well adapted to survive prolonged periods of desiccation, as it experiences growing in Pyropia farms along the Chinese Jiangsu coast. These traits, along with its ability to grow fast, may explain its success and one of the mechanisms behind the formation of the extreme blooms in the Yellow Sea.

Shallow Marine Cretaceous Sequences and Petroleum Geology of the Onshore Portion Rio del Rey Basin, Cameroon, Gulf of Guinea

Extensive thickly folded sedimentary sequences are exposed onshore of the Rio del Rey Basin contrary to previous reports of their rarity. Shales predominate, frequently intercalated by marlstones, sandstones, mudstones, limestones and marls and have been intruded in places by volcanic rocks of the Cameroon Volcanic Line (CVL). An integration of lithostratigraphic, paleontologic, granulometric and petrographic data generated from field and laboratory studies, permitted the identification of eleven (11) lithofacies which were subsequently grouped into three facies associations to facilitate the reconstruction of the paleo-depositional environment. Graphic logs constructed from the different outcrops studied revealed general coarsening upward sequences. Mega body fossils recovered from these sediments include severely altered, ferruginized fish and turtle-like heads, bivalve shells and moulds. Two bivalve taxa were identified: Exogyra ponderosa (Roemer) and members of theVenericardiinae sub-family. Textural and mineralogical details were obtained from granulometric and thin sections (microscopy). Interpretation of these data inferred an open relatively deeper marine, probably middle neritic to near shore transitional intertidal environments into which these sediments previously dated as Lower to Upper Cretaceous were deposited. Elements of a possible petroleum system: source and reservoir rocks, migratory pathways and trapping conditions, have been pointed out and thought to exist in space and time.

THE SEDIMENTOLOGY OF THE LOWER PERMIAN DANDOT FORMATION: A COMPONENT OF THE GONDWANA DEGLACIATION SEQUENCE OF THE SALT RANGE, PAKISTAN

The Dandot Formation is a part of the Lower Permian, dominantly continental, Gondwanan Nilawahan Group in the Salt Range, Pakistan. The formation conformably overlies the glacio-fluvial Tobra Formation and has a sharp conformable contact with the overlying fluvio-continental Warchha Sandstone. Sedimentary analyses show that the Dandot Formation consists of: 1. bioturbated sandstone lithofacies (L1), 2. dark green mudstone/shale lithofacies (L2), 3. flaser bedded sandstone lithofacies (L3), 4. rippled sandstone lithofacies (L4), 5. cross-bedded sandstone lithofacies (L5), and 6. planar sandstone lithofacies (L6). These can be grouped into shoreface, inner shelf, and tidal flat and estuarine facies associations, deposited in shallow marine to intertidal environments. The upper part of the Tobra Formation at the Choa-Khewra road section, where it conformably underlies the Dandot Formation, contains palynomorphs assignable to the earliest Permian 2141B Biozone. In south Oman, the 2141B Biozone is closely associated with the Rahab Shale Member, a widespread shale unit which is considered to represent part of a Permian deglaciation sequence which culminates in the marine beds of the Lower Gharif Member, interpreted as due to post glacial marine transgression. Thus, the Tobra Formation and the overlying marine Dandot Formation may form part of a similar deglaciation sequence.

Post-metamorphic impact of brief hyposaline stress on recently hatched veligers of the gastropod Crepipatella peruviana (Calyptraeidae)

Intertidal environments experience low salinity during heavy rainfall, creating stressful conditions for organisms, including invertebrate pelagic larvae, which can affect post-metamorphic development. Salinity in tide pools was monitored at low tide during a rainfall event in November 2013 at Puerto Montt, southern Chile. Water samples were taken to identify the presence and size of veligers of the gastropod Crepipatella peruviana. In addition, newly hatched veligers were exposed to different salinities [32 (control), 25, 20, and 15] in the laboratory for 6 h and then cultured at the control salinity until metamorphosis. Shell length (SL), oxygen consumption (OCR) and clearance (CR) rates of juveniles, and survival and growth in the first 2 weeks post-metamorphosis were quantified. The salinity of tide pools decreased to ten during heavy rains and yet contained an abundance of living veliger larvae; more than 85 % of the C. peruviana veligers collected had a SL ≤400 µm, which implies that they had only recently hatched. Exposing early veligers to low salinities (15 and 20) decreased size at metamorphosis and early juvenile survival by approximately 10 and 20 %, respectively. Also, the CR was lower at 5 days post-metamorphosis, decreasing by 30 % in individuals that had been exposed to salinity 15 as larvae. However, the OCR (5 and 15 days post-metamorphosis) and CR (15 days post-metamorphosis) were not different from the control treatment with an average value of 1.7 (×10−2 mg O2 h−1 juv−1) and 0.07 (L h−1 juv−1) for OCR and CR at 15 days post-metamorphosis, respectively. Also there was no effect on juvenile growth rate. This study shows that short hyposaline events suffered by early veligers of C. peruviana can cause reduced size at metamorphosis and potentially reduce juvenile survival, factors that could affect the fitness of future adult populations.

The bivalve Placuna (Indoplacuna) miocenica from the Middle Miocene of Siwa Oasis, Western Desert of Egypt: Systematic paleontology, paleoecology, and taphonomic implications

The present study investigated the epifaunal, free lying bivalve Placuna (Indoplacuna) miocenica (Fuchs, 1883) encountered in the Middle Miocene Marmarica Formation of Siwa Oasis, Western Desert, Egypt, in terms of systematic paleontology, paleoecology and taphonomy. Well to moderately preserved shells of this species were collected from three sections. They have been found embedded in sandy, marly and chalky limestones. Although they are extremely thin and fragile, they occurred as complete disarticulated and articulated valves. Specimens of P. (I.) miocenica are highly accumulated in the north Siwa section forming a coquinoid band (30 cm thick). In addition, they are generally distributed sporadically in different stratigraphic levels within the three studied sequences. Taphonomic observations indicated that these shells were affected by encrustation, bioerosion, disarticulation, fragmentation and abrasion. Moreover, valves of this species suffered minor chipping along their fragile margins. The occurrence of the studied species associated with oysters and other benthic faunal assemblages within carbonate sediments revealed shallow, low energy, warm and intertidal environments with periods of relatively agitated conditions.

Photobiology of the zoanthid Zoanthus sociatus in intertidal and subtidal habitats

Intertidal environments are boundaries between marine and terrestrial ecosystems that are subject to rapid fluctuations across tidal cycles. This study investigates, for the first time, the photobiology of symbiotic zoanthids inhabiting different tidal environments: subtidal, intertidal pools and intertidal areas exposed to air during low tide. More specifically, we assessed the photochemical efficiency, Symbiodinium density and photosynthetic pigments profile of Zoanthus sociatus during low tide. Photochemical efficiency was lower and cell density higher in air exposed zoanthids. The profile of photosynthetic pigments also varied significantly among tidal habitats, particularly photoprotective pigments such as dinoxanthin and diadinoxanthin. Differences were also observed for the pigment content per cell, but the proportion of particular pigments (peridinin/chlorophyll-a and diatoxanthin+diadinoxanthin/chlorophyll-a) remained stable. Results suggest that aerial exposure conditions induce reversible downregulation of photochemical processes but no photophysiological impairment or bleaching. These findings provide a baseline for future studies addressing the prevalence of these overlooked cnidarians in environmentally dynamic reef flats.

Transcription, Signaling Receptor Activity, Oxidative Phosphorylation, and Fatty Acid Metabolism Mediate the Presence of Closely Related Species in Distinct Intertidal and Cold-Seep Habitats.

Bathyal cold seeps are isolated extreme deep-sea environments characterized by low species diversity while biomass can be high. The Håkon Mosby mud volcano (Barents Sea, 1,280 m) is a rather stable chemosynthetic driven habitat characterized by prominent surface bacterial mats with high sulfide concentrations and low oxygen levels. Here, the nematode Halomonhystera hermesi thrives in high abundances (11,000 individuals 10 cm−2). Halomonhystera hermesi is a member of the intertidal Halomonhystera disjuncta species complex that includes five cryptic species (GD1-5). GD1-5’s common habitat is characterized by strong environmental fluctuations. Here, we compared the transcriptomes of H. hermesi and GD1, H. hermesi’s closest relative. Genes encoding proteins involved in oxidative phosphorylation are more strongly expressed in H. hermesi than in GD1, and many genes were only observed in H. hermesi while being completely absent in GD1. Both observations could in part be attributed to high sulfide concentrations and low oxygen levels. Additionally, fatty acid elongation was also prominent in H. hermesi confirming the importance of highly unsaturated fatty acids in this species. Significant higher amounts of transcription factors and genes involved in signaling receptor activity were observed in GD1 (many of which were completely absent in H. hermesi), allowing fast signaling and transcriptional reprogramming which can mediate survival in dynamic intertidal environments. GC content was approximately 8% higher in H. hermesi coding unigenes resulting in differential codon usage between both species and a higher proportion of amino acids with GC-rich codons in H. hermesi. In general our results showed that most pathways were active in both environments and that only three genes are under natural selection. This indicates that also plasticity should be taken in consideration in the evolutionary history of Halomonhystera species. Such plasticity, as well as possible preadaptation to low oxygen and high sulfide levels might have played an important role in the establishment of a cold-seep Halomonhystera population.

Indications of parthenogenesis and morphological differentiation in Hawaiian intertidal Fortuynia (Acari, Oribatida) populations

Fortuynia hawaiiensis n. sp. represents a new species occurring on the archipelago of Hawaii. The complete absence of males from all Hawaiian samples points to thelytokous parthenogenetic reproduction in this species. This reproductive mode may have evolved in F. hawaiiensis n. sp. because it reduces costs of producing males, mate finding and sperm transfer in the extreme conditions of intertidal environments and it facilitates the colonization of new islands. Despite the clonal population structure and the slow evolutionary rate of this species, morphometric analyses demonstrate that populations from different Hawaiian Islands already show slight morphological divergence with the specimens from Maui being by trend the smallest.A morphometric comparison with other Fortuynia species revealed that F. hawaiiensis n. sp. and Fortuynia maledivensis, both supposed parthenogens, show less intraspecific variability than the sexually reproducing Fortuynia atlantica. This may be caused by the narrow and generalized genotypes of the two presumably asexual species. Based on morphometric data and on discrete characters, a close relationship of F. hawaiiensis n. sp. with F. maledivensis is indicated and hence both species are assumed to be members of a recently evolved Indo-Pacific parthenogenetic lineage.

Seasonal Differences in Live Foraminiferal Densities: Case Studies from Tropical and Temperate Intertidal Environments

Statistically significant differences (when p < 0.05 using the permutation t-test) among Live Foraminiferal Densities (LFDs) recovered in August, November, March and May were detected at tropical Caroni swamp, Claxton bay (Trinidad), temperate Cowpen marsh and Brancaster marsh (U.K.). The monthly mean LFDs of the foraminiferal metacommunities (all stations), assemblages (groups of stations defined by cluster analysis), and the agglutinated and calcareous specimens within each were compared separately. The LFDs of the Caroni swamp metacommunity did not fluctuate significantly among months, but significantly higher abundances of agglutinated specimens occurred in the upper assemblage in March; and calcareous specimens in the lower assemblage in November. At Claxton bay, monthly LFDs of the metacommunities and assemblages did not vary significantly, but calcareous and agglutinated species within each favoured dry (March and May) and wet (August and November) months respectively. At the temperate marshes, significantly higher LFDs of the metacommunities and assemblages were recorded in warmer months. August blooms of the Cowpen marsh metacommunity was attributed to agglutinated specimens in the upper assemblage, and calcareous specimens in the lower assemblage. May blooms of the Brancaster marsh upper assemblage were attributed to calcareous specimens, but there were no seasonal blooms of the lower assemblage.

The presence of cadmium in the intertidal environments of a moderately impacted coastal lagoon in western Portugal (Óbidos Lagoon)--spatial and seasonal evaluations.

A seasonal environmental monitoring program was carried out (winter 2009 to summer 2010) to evaluate the spatial and seasonal cadmium concentrations in the intertidal environments of the Óbidos Lagoon (Portugal). Also, some environmental parameters were monitored at each sampling station. Both the water and the sediment samples were contaminated, although to different degrees. In general, cadmium contamination appears to be mostly focused on the inner areas of the lagoon, namely, in Barrosa's arm, which receives a small tributary contaminated by agro-industrial activities. Only cadmium concentration in sediment showed to be significantly influenced by seasons. Some environmental parameters presented spatial and temporal heterogeneity which influenced, to some extent, cadmium bioavailability. The results of this study allow a better understanding of the environmental quality of this ecosystem regarding cadmium contamination and may assist in the definition of future coastal management measures specifically targeted to trace metal contamination and pollution monitoring.

Genome-wide expression profiles of Pyropia haitanensis in response to osmotic stress by using deep sequencing technology.

BackgroundPyropia haitanensis is an economically important marine crop grown in harsh intertidal habitats of southern China; it is also an excellent model system for studying mechanisms of stress tolerance. To understand the molecular mechanisms underlying osmotic tolerance and adaptation to intertidal environments, a comprehensive analysis of genome-wide gene expression profiles in response to dehydration and rehydration in Py. haitanensis was undertaken using digital gene expression profile (DGE) approaches combined with de novo transcriptome sequencing.ResultsRNA-sequencing of the pooled RNA samples from different developmental phases and stress treatments was performed, which generated a total of 47.7 million clean reads. These reads were de novo assembled into 28,536 unigenes (≥200 bp), of which 18,217 unigenes (63.83 %) were annotated in at least one reference database. DGE analysis was performed on four treatments (two biological replicates per treatment), which included moderate dehydration, severe dehydration, rehydration, and normal conditions. The number of raw reads per sample ranged from 12.47 to 15.79 million, with an average of 14.69 million reads per sample. After quality filtering, the number of clean reads per sample ranged from 11.83 to 15.04 million. All distinct sequencing reads were annotated using the transcriptome of Py. haitanensis as reference. A total of 1,681 unigenes showed significant differential expression between moderate dehydration and normal conditions, in which 977 genes were upregulated, and 704 genes were downregulated. Between severe dehydration and normal conditions, 1,993 unigenes showed significantly altered expression, which included both upregulated (1,219) and downregulated genes (774). In addition, 1,086 differentially expressed genes were detected between rehydration and normal conditions, of which 720 genes were upregulated and 366 unigenes were downregulated. Most gene expression patterns in response to dehydration differed from that of rehydration, except for the synthesis of unsaturated fatty acids, several transcription factor families, and molecular chaperones that have been collectively implicated in the processes of dehydration and rehydration in Py. haitanensis.ConclusionsTaken together, these data provide a global high-resolution analysis of gene expression changes during osmotic stress that could potentially serve as a key resource for understanding the biology of osmotic acclimation in intertidal red seaweed.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2226-5) contains supplementary material, which is available to authorized users.

On the morphodynamic stability of intertidal environments and the role of vegetation

We describe the coupled biotic and abiotic dynamics in intertidal environments using a point model that includes suspended sediment deposition, wave- and current-driven erosion, biofilm sediment stabilization, and sediment production and stabilization by vegetation. We explore the effects of two widely different types of vegetation: salt-marsh vegetation and mangroves. These two types of vegetation, which colonize distinct geographical areas, are characterized by different biomass productivities and stabilization mechanisms. We show that changing vegetation and biofilm properties result in differing stable states, both in their type and number. The presence of the biofilm exerts a dominant control on the tidal flat (lower intertidal) equilibrium elevation and stability. Vegetation controls the elevation of the marsh platform (i.e., the upper intertidal equilibrium). The two types of vegetation considered lead to similar effects on the stability of the system despite their distinct biophysical interactions.

Nutrient enrichment intensifies hurricane impact in scrub mangrove ecosystems in the Indian River Lagoon, Florida, USA.

Mangroves are an ecological assemblage of trees and shrubs adapted to grow in intertidal environments along tropical, subtropical, and warm temperate coasts. Despite repeated demonstrations of their ecologic and economic value, multiple stressors including nutrient over-enrichment threaten these and other coastal wetlands globally. These ecosystems will be further stressed if tropical storm intensity and frequency increase in response to global climate changes. These stressors will likely interact, but the outcome of that interaction is uncertain. Here, we examined potential interaction between nutrient over-enrichment and the September 2004 hurricanes. Hurricanes Frances and Jeanne made landfall along Florida's Indian River Lagoon and caused extensive damage to a long-term fertilization experiment in a mangrove forest, which previously revealed that productivity was nitrogen (N) limited across the forest and, in particular, that N enrichment dramatically increased growth rates and aboveground biomass of stunted Avicennia germinans trees in the interior scrub zone. During the hurricanes, these trees experienced significant defoliation with three to four times greater reduction in leaf area index (LAI) than control trees. Over the long-term, the +N scrub trees took four years to recover compared to two years for controls. In the adjacent fringe and transition zones, LAI was reduced by > 70%, but with no differences based on zone or fertilization treatment. Despite continued delayed mortality for at least five years after the storms, LAI in the fringe and transition returned to pre-hurricane conditions in two years. Thus, nutrient over-enrichment of the coastal zone will increase the productivity of scrub mangroves, which dominate much of the mangrove landscape in Florida and the Caribbean; however, that benefit is offset by a decrease in their resistance and resilience to hurricane damage that has the potential to destabilize the system.

Modeling Compound Loss from Polydimethylsiloxane Passive Samplers

Volatile losses were measured from polydimethylsiloxane (PDMS) passive samplers during determination of contaminant porewater concentrations in sediments. Volatile losses could occur between the time of retrieval and processing of the passive sampler or in intertidal environments where the passive sampler could potentially be exposed above the water surface at low tide. A model was developed to predict losses of absorbed compounds as a function of sorbent geometry and the Henry’s Law Coefficient and PDMS-water partition coefficient of the compound of interest. The model suggests that thin layers of PDMS typically used to minimize equilibration times in passive sampling (≤30 µm) may not provide quantitative measurement of naphthalenes or other lighter volatile compounds without special efforts to reduce losses. The results suggest that the samplers should be processed rapidly onsite or kept at low temperatures after retrieval to maximize retention of more volatile compounds or designed with thick PDMS layers. The results also suggest that less volatile compounds, including phenanthrene, and higher molecular weight polynuclear aromatic hydrocarbons (PAHs) exhibit minimal evaporative losses with typical sample processing times.

Observations on seaweed attachment to bivalve shells in Peter the Great Bay (East Sea) and their taphonomic implications

Observations in beach, intertidal and upper subtidal environments in Peter the Great Bay (north-western East Sea) have shown that attached algae were found on empty shells of 13 species of epifaunal and infaunal bivalve mollusks. Thirteen algae species were identified on empty dislodged shells but more than 50 species are known to be epibiotic on living bivalves. The dislodgement of shells with attached algae takes place in semi-enclosed, low-energy areas, as well as those which are open and affected by strong wave action, indicating the large scale of this phenomenon. The significance of seaweed transportation of living mollusks and their empty shells in the coastal zone, involving both taphonomic and ecological processes, is stressed. Algae appear to be a taphonomic agent and play a similar role as compared to birds or hermit crabs, but they act passively and contribute to environmental mixing in death assemblages in coastal environments.

Stromatolitic biotic systems in the mid-Triassic of Israel — A product of stress on an epicontinental margin

Phanerozoic microbialites are carbonate-precipitating biotic forms that are often described in modern systems from inter- or supratidal, hypersaline or fresh-water environments, and from beds associated with a variety of ecologically stressed environments, over geologic time. Middle to Late Triassic microbialitic successions from southern Israel were commonly subtidal, and thrived under cryptic ecological stress conditions related to the regional configuration of the Levant margin at that time.The Ladinian and Carnian sequence at the Ramon outcrop contains 18 successions with 28 microbialitic horizons. These microbialites form isolated patches, small bioherms and biostromes interleaved with other calcareous sediments including poorly diverse molluscan accumulations. Three distinct microbialitic facies and two associated molluscan facies were identified. Most were distinctly subtidal. The two associated molluscan biofacies include a subtidal bivalve boundstone forming mounds, and a bedded packstone of minute gastropods.The Ladinian–Carnian microbialites were not climatically forced. Environmental stress was caused by partial restriction of marine circulation due to tectonic movements and biotic accumulation on the outer edge of the Levant margin. Comparison of coeval successions from central and northern Israel and the North Calcareous Alps and Southern Alps in Europe, indicates a Tethyan or global carbonate system of microbial producers separated from the open marine environment by shelf-edge barriers. Therefore, we conclude that the Ramon microbialites represent a response to ecological stress of the carbonate system within the subtidal environment, rather than solely to normal stresses of the intertidal environment.

Diversity and distribution of tide pool ornamental fishes along the Andaman coast (Andaman Sea), India

The island ecosystem of the Andaman and Nicobar group of islands in the Andaman Sea region is well known for its rich biodiversity, particularly with regard to its intertidal environment. It is especially diverse in its fisheries resources, specifically the marine ornamental fishes that inhabit the tide pools. Information on these organisms, however, particularly with regard to the dominant species within the stressful tide pool ecosystem, is almost absent. In this study, we investigate species diversity and distribution patterns of ornamental fishes and their associated flora and fauna in the tide pool environment along the South Andaman coast. During the study period, a total of 100 tide pools of various sizes were investigated, and 35 species belonging to 22 families of ornamental fishes were recorded. In addition, a total of 128 species including mollusks, crustaceans, echinoderms, cnidarians and seaweeds were recorded as major constituents of associated organisms in the tide pool environment. The study suggests that the tide pools are dominated primarily by juveniles of ornamental fish species, with a few ornamental species preferring to spend different stages of their life cycles in tide pool conditions.

Taken to the limit — Is desiccation stress causing precocious encystment of trematode parasites in snails?

When hosts experience environmental stress, the quantity and quality of resources they provide for parasites may be diminished, and host longevity may be decreased. Under stress, parasites may adopt alternative strategies to avoid fitness reductions. Trematode parasites typically have complex life cycles, involving asexual reproduction in a gastropod first intermediate host. A rare phenomenon, briefly mentioned in the literature, and termed ‘precocious encystment’ involves the next stage in the parasites' life cycle (metacercarial cyst) forming within the preceding stage (redia), while still inside the snail. In the trematode Parorchis sp. NZ using rocky shore snails exposed to long periods outside water, we hypothesised that this might be an adaptive strategy against desiccation, preventing parasite emergence from the snail. To test this, we first investigated the effect of prolonged desiccation on the survival of two species of high intertidal snails. Secondly, we measured the reproductive output (cercarial production) of the parasite under wet and dry conditions. Finally, we quantified the influence of desiccation stress on the occurrence of precocious encystment. Snail mortality was higher under dry conditions, indicating stress, and it was somewhat exacerbated for infected snails. Parasite reproductive output differed between wet and dry conditions, with parasites of snails kept in dry conditions producing more cercariae when placed in water. Little variation was observed in the occurrence of precocious encystment, although some subtle patterns emerged. Given the stresses associated with living in high intertidal environments, we discuss precocious encystment as a possible stress response in this trematode parasite.

Mid Holocene Marine Transgression at Eastern Coastal Margin of Bangladesh—Implications for Past Sea Level Change

Sedimentological, palynological data indicate that mangrove community developed under transgressive condition in and around Maheskhali and Kutubdia Island areas during Mid Holocene time (7000 to 5500 years BP) leading to the locally wide spread deposition of organic-rich sediments. During Holocene time global rise and fall of eustatic sea level played an important role not only on the depositional environment but in creating a geomorphic feature on the island. Recurrent occurence of freshwater and mangrove pollen in Maheskhali and Kutubdia Island area indicate that these areas have undergone cyclic marine and non-marine influence. Since the Last Glacial Maximum (about 20,000 years ago), sea level has risen by over 120 m at locations far from present and former ice sheets, as a result of loss of mass from these ice sheets. There was a rapid rise between 15,000 and 6,000 years ago at an average rate of 10 mm/yr. Based on geological data, global average sea level may have risen at an average rate of about 0.5 mm/yr over the last 6000 years and at an average rate of 0.1 to 0.2 mm/yr over the last 3000 years. Vertical land movements are still occurring today as a result of these large transfers of mass from the ice sheets to the ocean. During the last 6000 years, global average sea level variations on time-scales of a few hundred years and longer are likely to have been less than 0.3 to 0.5 m. First transgression was noticed around 6000-5500 cal BP and then a subsequent regression of the bay had been observed from around 5500 cal BP. This was again followed by another small scale transgression episode occurred around 2500―2000 cal BP. So the palynomorph assemblages from the Holocene sediment sample indicate that Maheskhali and Kutubdia Island and its surrounding area was an intertidal environment occupied by mangrove community.