Pathology survey of the short-neck clam Ruditapes philippinarum occurring on sandy tidal flats along the coast of Ariake Bay, Kyushu, Japan

Occurrence and seasonal variation of Perkinsus sp. Infection in wild mollusk populations from coastal waters of Qingdao, northern China

Perkinsosis has been recognized as one of the major threats to natural and farmed bivalve populations, many of which are of commercial as well as environmental significance. Three Perkinsus species have been identified in China, and the Manila clam (Ruditapes philippinarum) was the most frequently infected species in northern China. Although the occurrence and seasonal variation of Perkinsus spp. have previously been examined, the pathological characteristics of these infections in wild Manila clams and sympatric species in China have seldom been reported. In the present study, the prevalence and intensity of Perkinsus infection in wild populations of Manila clams and 10 sympatric species from three sites were investigated by Ray's fluid thioglycolate medium (RFTM) assay seasonally across a single year. Perkinsus infection was only identified in Manila clams, with a high prevalence (274/284 = 96.48 %) and low intensity (89.8 % with a Mackin value ≤ 2, suggesting generally low-intensity infections) throughout the year. Heavily infected clams were mainly identified in Tianheng in January, which displayed no macroscopic signs of disease. An overview of the whole visceral mass section showed that the trophozoites mostly aggregated in gills and connective tissue of the digestive tract, to a lesser extent in the mantle and foot, and even less frequently in adductor muscle and connective tissues of the gonad. PCR and ITS-5.8S rRNA sequencing of 93 representative RFTM-positive samples revealed a 99.69 to 100 % DNA sequence identity to Perkinsus olseni. Unexpectedly, significantly higher infection intensities were usually identified in January and April when the Condition Index (CI) was relatively high. We propose that factors associated with the anthropogenic harvesting pressure and irregular disturbances should be responsible for the uncommon seasonal infection dynamics of perkinsosis observed in the present study.

Molecular characterization of Urosporidium tapetis sp. nov., a haplosporidian hyperparasite infecting metacercariae of Parvatrema duboisi (Dollfus 1923), a trematode parasite of Manila clam Ruditapes philippinarum on the west coast of Korea

Recently, a putative new hyperparasitic haplosporidian in the genus Urosporidium was identified from metacercariae of the trematode Parvatrema duboisi infecting Manila clam Ruditapes philippinarum on the west coast of Korea. In this study, we applied small subunit ribosomal DNA (SSU rDNA) sequences as a marker to substantiate the phylogenetic relationship of the unidentified Urosporidium within the Order Haplosporida. In our phylogenetic analysis, the 1890 bp of SSU rDNA sequences obtained were closely related to a haplosporidian parasite forming a sister clade to Urosporidium group, although the gene sequences were only 89.22-89.70% similar to Urosporidium spp. Such molecular phylogenetic distance within the genus suggested that the unidentified Urosporidium is a new member of the genus. Accordingly, we report the unidentified haplosporidian hyperparasite as Urosporidium tapetis sp. nov.

Scanning electron microscopic observation of the in vitro cultured protozoan, Perkinsus olseni, isolated from the Manila clam, Ruditapes philippinarum

Background

Perkinsosis is a major disease affecting the commercially important marine mollusk Ruditapes philippinarum (Manila clam) in Asian waters. In this study, we investigated the morphological characteristics of Perkinsus olseni, the causative agent of perkinsosis, cultured under laboratory conditions at different stages of its life cycle using a scanning electron microscope (SEM).

Results

The prezoosporangia formed after induction with Ray’s fluid thioglycollate medium (RFTM) developed into zoosporangia. During this process, a discharge tube formed a porous sponge-like structure that detached before the zoospores were released; thus, this organelle operated as a bung. Liberated zoospores gradually transformed into immature trophozoites, during which detachment of the anterior flagella occurred, but the loss of the posterior flagella was not clearly observed in the present study. Mature trophozoites underwent schizogony by cleaving the cell forming some merozoites in schizonts, which were released by the rupturing of the cellular membrane of the schizont within a few days.

Conclusions

Our morphological and ultrastructural studies contribute new information on the life cycle and propagation of P. olseni.

Effects of marine harmful algal blooms on bivalve cellular immunity and infectious diseases: A review

Bivalves were long thought to be "symptomless carriers" of marine microalgal toxins to human seafood consumers. In the past three decades, science has come to recognize that harmful algae and their toxins can be harmful to grazers, including bivalves. Indeed, studies have shown conclusively that some microalgal toxins function as active grazing deterrents. When responding to marine Harmful Algal Bloom (HAB) events, bivalves can reject toxic cells to minimize toxin and bioactive extracellular compound (BEC) exposure, or ingest and digest cells, incorporating nutritional components and toxins. Several studies have reported modulation of bivalve hemocyte variables in response to HAB exposure. Hemocytes are specialized cells involved in many functions in bivalves, particularly in immunological defense mechanisms. Hemocytes protect tissues by engulfing or encapsulating living pathogens and repair tissue damage caused by injury, poisoning, and infections through inflammatory processes. The effects of HAB exposure observed on bivalve cellular immune variables have raised the question of possible effects on susceptibility to infectious disease. As science has described a previously unrecognized diversity in microalgal bioactive substances, and also found a growing list of infectious diseases in bivalves, episodic reports of interactions between harmful algae and disease in bivalves have been published. Only recently, studies directed to understand the physiological and metabolic bases of these interactions have been undertaken. This review compiles evidence from studies of harmful algal effects upon bivalve shellfish that establishes a framework for recent efforts to understand how harmful algae can alter infectious disease, and particularly the fundamental role of cellular immunity, in modulating these interactions. Experimental studies reviewed here indicate that HABs can modulate bivalve-pathogen interactions in various ways, either by increasing bivalve susceptibility to disease or conversely by lessening infection proliferation or transmission. Alteration of immune defense and global physiological distress caused by HAB exposure have been the most frequent reasons identified for these effects on disease. Only few studies, however, have addressed these effects so far and a general pattern cannot be established. Other mechanisms are likely involved but are under-studied thus far and will need more attention in the future. In particular, the inhibition of bivalve filtration by HABs and direct interaction between HABs and infectious agents in the seawater likely interfere with pathogen transmission. The study of these interactions in the field and at the population level also are needed to establish the ecological and economical significance of the effects of HABs upon bivalve diseases. A more thorough understanding of these interactions will assist in development of more effective management of bivalve shellfisheries and aquaculture in oceans subjected to increasing HAB and disease pressures.

Ultrastructural analysis of Apicomplexa-Like parasites in two conch species Laevistrombus canarium and canarium urceus from Johor Straits, Malaysia

The tropical conch, Laevistrombus canarium (Linnaeus, 1758) and Canarium urceus (Linneaus, 1758) are ecologically and economically important shellfish species in Malaysia and neighboring region. Their populations, however are currently declining and this histopathological study investigates the aspect of parasitism and diseases that may affect their well-being. Conch samples were randomly collected from their natural habitat and histological sections (4-5 µm) of various organs and tissues were examined under light microscope. This was followed by ultrastructure analysis on infected tissues using transmission electron microscope (TEM). Based on the histological analysis, large numbers of gamonts, sporocysts and trophozoites of Apicomplexa-like parasites were observed in the vacuolated cells and pyramidal crypt cells of the digestive tubules, and in the digestive ducts. Furthermore, coccidian and oocysts-like Pseudoklossia sp. stages were also observed in the cells of the kidney. Apart from that, spores with cyst-like structure were observed in the digestive gland and kidney. Although the parasites were present in most of the organs analyzed, there was no obvious symptom, inflammatory response or mortality incurred on both species, which implies the possibility of a non-virulent relationship like commensalisms or mutualism. However, more investigations, including molecular studies, are needed to confirm the parasite identification and dynamics, and to further evaluate the nature of relationship between Apicomplexa parasites and their host.

Duration rather than frequency of hypoxia causes mass mortality in ark shells (Anadara kagoshimensis)

Hypoxia is associated with mass mortality in estuaries, but a direct causal relationship has not been proven to date. This study aimed to demonstrate this relationship and to evaluate how the duration of hypoxia affects the survival of ark shells (Anadara kagoshimensis) using mathematical modeling. The dissolved oxygen concentration was monitored at two stations in the innermost area of Ariake Bay, Japan, to calculate the duration of hypoxia. This was then included in a mathematical model to simulate the population density with sequential computation. The population density decreased with prolonged hypoxia, reaching a value close to the observed population density, indicating that hypoxia is the main cause of mass mortality in ark shells. Furthermore, the ark shell population disappeared in 8days with constant hypoxia but persisted when hypoxia was alternated with normoxia every 6 h. Therefore, mass mortality is caused by the duration rather than the frequency of hypoxia.

Parasites and pathological conditions in the edible oyster, Crassostrea madrasensis (Preston), from the east and west coasts of India

A detailed pathological survey was carried out on the commercially important edible oyster, Crassostrea madrasensis (Preston), from two distinct coastal/brackish water ecosystems of south India. Samples were collected twice a year during wet and dry seasons from 2009 to 2012. Bacterial colonies in the form of prokaryotic inclusions, protozoans (Perkinsus beihaiensis, Nematopsis sp. and ciliates Sphenophrya sp. and Stegotricha sp.), metazoans (trematodes, turbellaria, cestodes and crustaceans) and shell parasites (Polydora spp. and Cliona spp.) along with various pathological conditions (digestive tubule atrophy, ceroid bodies, haemocytic infiltration, tissue necrosis and neoplastic disorders) were observed in C. madrasensis collected from two sites. Intensity, spatial and seasonal variations in infection prevalence and pathological effects on the host were studied. The protozoan parasite, P. beihaiensis; shell parasite, Polydora spp. and pathological condition, digestive gland atrophy were most prevalent in occurrence. High-intensity infections with P. beihaiensis, larval trematodes and Polydora spp. were found to cause significant impact on host physiology. All other parasites were observed with low mean prevalence and intensity. Karapad in Tuticorin bay, the site reported with marked pollution levels, exhibited higher number of parasitic taxa and high mean prevalence and intensity for pathological conditions.

Is metal contamination responsible for increasing aneuploidy levels in the Manila clam Ruditapes philippinarum?

The present study assessed the metal genotoxicity potential at chromosome-level in the bivalve Ruditapes philippinarum collected along different areas of the Tagus estuary. Higher levels of aneuploidy on gill cells were detected at the most sediment contaminated area both in May (31.7%) and October (36.0%) when compared to a less contaminated area over the same periods (20.3% and 29.0% respectively). Interestingly, metal bioaccumulation in gills was higher in the specimens collected at the least contaminated area with the exception of Pb. Indeed, the multivariate analysis revealed a stronger relation between aneuploidy and sediment contamination than between aneuploidy and the bioaccumulation of the metals. The temporal and spatial inconsistency found for the bioaccumulation of metals in R. philippinarum and the positive correlation between sediment contamination and aneuploidy at the most contaminated area suggest that these chromosome-level effects might be due to chronic metal contamination occurring in the Tagus estuary, rather than a direct result of the temporal variation of bioavailable contaminants. The vertical transmission phenomenon of bivalve aneuploidy levels may then be perpetuating those levels on clams from the most contaminated area. The present results shed light about the effect of metal toxicity at the chromosome-level in species inhabiting chronic contaminated areas and highlight the use of aneuploidy as an effective tool to identify persistent contamination in worldwide transitional waters.

First report of Urosporidium sp., a haplosporidian hyperparasite infecting digenean trematode Parvatrema duboisi in Manila clam, Ruditapes philippinarum on the west coast of Korea

In this study, we first report on the occurrence of Urosporidium sp., a haplosporidian hyperparasite infecting the trematode, Parvatrema duboisi, which parasitizes Manila clams, Ruditapes philippinarum on the west and south coasts of Korea. The larval P. duboisi infected by the sporocyst stage of Urosporidium sp. demonstrated numerous small yellowish spores in their tissues. The heavily infected metacercariae exhibited degenerate bodies and the larvae were often motionless. Clams heavily infected by the metacercariae of P. duboisi also displayed abnormal golden spots on the mantle tissue. In histology, different life stages of Urosporidium sp. could be identified, including the uni-nucleate, plasmodial, sporogonic stages and the acid fast mature spores released from the cyst. In scanning electron microscopy (SEM), the mature spore exhibited a semi-circular rim around the apical end and the orifice was covered internally with a flap. Loop-like filaments ornamentation was also identified from Urosporidium sp. in SEM, suggesting that Urosporidium sp. found in this study is a new member in the genus. Prevalence of Urosporidium sp.-infected trematodes in this study ranged from 2.5% to 24.0% in April 2010 and the infection was observed from 8 sampling sites out of the 26 sites surveyed on the west and south coasts.

Metal and metallothionein distribution in different tissues of the Mediterranean clam Venerupis philippinarum during copper treatment and detoxification

Filter feeding animals can accumulate large amount of contaminants in their body through particles filtered from seawater. In particular, copper is interesting since it plays important roles as co-factor of numerous proteins but its toxicity is well established, also because it can readily generate free radicals or oxidize cellular components through their redox activity. Its availability is tightly regulated within cells: it is immediately transferred to metallothionein (MT) that in turn provides efficient and specific mechanisms for its intracellular storage and transport. The aim of this study was to evaluate the acute effect of sublethal copper concentrations in Venerupis philippinarum, by studying the kinetics of copper, zinc (for its interactions at the sites of intake or elimination with the accumulation of other essential and not essential trace metals) and metallothionein accumulation under laboratory conditions. The time-course of metal accumulation/elimination is similar in digestive gland and gills and importantly it is dose-dependent. Both copper and zinc increase slowly within cells, reaching a maximum concentration at the end of the exposure period. During the detoxification period, the metal levels in digestive gland and gills rapidly decrease, with different kinetics in the two tissues. Positive correlations between metallothionein accumulation and copper or zinc concentrations have been verified in both treated groups. The obtained data demonstrated the involvement of MTs in detoxification strategies after a recovery period in clean seawater.

Marine invasions and parasite escape: updates and new perspectives

Marine invasions have risen over time with enhanced globalization, and so has the introduction of non-native hosts and their parasites. An important and well-supported paradigm of invasion biology is the significant loss of parasites that hosts enjoy in introduced regions compared to native regions (i.e. parasite escape), yet less is known about the factors that influence parasite escape in marine systems. Here, we compile an up-to-date review of marine parasite invasions and test several hypotheses related to host invasion pathway that we suspected could influence parasite escape across the 31 host-parasite systems included in our investigation. In general, we continued to show significant support for parasite escape; however, escape varied among parasite taxa, with most taxa demonstrating moderate levels of escape and a few showing complete or no escape. Moreover, we revealed several important factors related to host taxa, geography, time, and vector of introduction that influenced parasite escape, and in some cases demonstrated significant interactions, revealing the complexity of the invasion pathway in filtering parasites from native to introduced regions. In some (but not all) cases, there was also evidence of invasive host advantages due to parasite escape, but more evidence is required to demonstrate clear support for the enemy release hypothesis. In general, our study revealed the need for further research across systems, especially in understudied regions of the world.

Development of real-time PCR assays for discrimination and quantification of two Perkinsus spp. in the Manila clam Ruditapes philippinarum

The Manila clam Ruditapes philippinarum is infected with 2 Perkinsus species, Perkinsus olseni and P. honshuensis, in Japan. The latter was described as a new species in Mie Prefecture, Japan, in 2006. Ray's Fluid Thioglycollate Medium (RFTM) assay has been most commonly used to quantify Perkinsus infection. However, this assay cannot discriminate between species that resemble one another morphologically. We developed real-time PCR assays for the specific quantification of P. olseni and P. honshuensis. DNA was extracted using Chelex resin. Cultured P. olseni and P. honshuensis cells were counted and spiked into uninfected clam gill tissue prior to DNA extraction to generate standard curves, which allowed quantification based on the PCR cycle threshold values. We compared the RFTM assay with both real-time PCR assays by quantifying Perkinsus spp. in gill tissue samples from the same individual clams obtained from various localities in Japan. Infection intensities estimated by both assays were significantly correlated (r2 = 0.70). Our results suggest that the prevalence and infection intensity of P. honshuensis are much lower than for P. olseni in Manila clams.

Experimental challenges of wild Manila clams with Perkinsus species isolated from naturally infected wild Manila clams

Manila clams, Ruditapes philippinarum, are widely harvested in the coastal waters in Japan. However, there have been significant decreases in the populations of Manila clams since the 1980s. It is thought that infection with the protozoan Perkinsus species has contributed to these decreases. A previous study demonstrated that high infection levels of a pure strain of Perkinsus olseni (ATCC PRA-181) were lethal to hatchery-raised small Manila clams, however, the pathogenicity of wild strain Perkinsus species to wild Manila clam is unclear. To address this, we challenged large (30-40 mm in shell length) and small (3-15 mm in shell length) wild Manila clams with Perkinsus species isolated from naturally infected wild Manila clams. We report high mortalities among the small clams, but not among the large ones. This is the first report to confirm the pathogenicity of wild isolate of Perkinsus species to wild Manila clams.

Identification and expression profile of a new cytochrome P450 isoform (CYP414A1) in the hepatopancreas of Venerupis (Ruditapes) philippinarum exposed to benzo[a]pyrene, cadmium and copper

With the objective to identify promising molecular biomarkers for marine pollution monitoring, a new cytochrome P450 gene was identified from Venerupis (Ruditapes) philippinarum and classified as a member of a new subfamily, CYP414A1. Phylogenetic analysis showed that CYP414A1 was closely related to members of the CYP2 family. CYP414A1 mRNA expression was significantly induced by 50 μg/L B[a]P at 96 h, while no significant change was found in 5 μg/L B[a]P-exposed samples. For heavy metals exposure, the expression of CYP414A1 was significantly up-regulated by Cd but sharply depressed by Cu exposure. These results suggested that CYP414A1 responded to various xenobiotics stresses, and could be used as a candidate biomarker of heavy metals and B[a]P.

Benzo(a)pyrene-induced metabolic responses in Manila clam Ruditapes philippinarum by proton nuclear magnetic resonance ((1)H NMR) based metabolomics

Benzo(a)pyrene is an important polycyclic aromatic hydrocarbon (PAH) which causes carcinogenic, teratogenic and mutagenic effects in various species and the level of contamination of this toxic agent in the marine environment is of great concern. In this study, metabolic responses induced by two doses (0.02 and 0.2μM) of BaP were characterized in the gill tissues of Manila clam Ruditapes philippinarum after exposure for 24, 48 and 96h. The high dose (0.2μM) of BaP induced the disturbances in energy metabolism and osmotic regulation based on the metabolic biomarkers such as succinate, alanine, glucose, glycogen, branched chain amino acids, betaine, taurine, homarine, and dimethylamine in clam gills after 24h of exposure. In addition, hormesis induced by BaP was found in clams exposed to both doses of BaP. Overall, our results demonstrated the applicability of metabolomics for the elucidation of toxicological effects of marine environmental contaminants in a selected bioindicator species such as the Manila clam.

Metabolic responses in gills of Manila clam Ruditapes philippinarum exposed to copper using NMR-based metabolomics

Copper is an important heavy metal contaminant with high ecological risk in the Bohai Sea. In this study, the metabolic responses in the bioindicator, Manila clam (Ruditapes philippinarum), to the environmentally relevant copper exposures were characterized using NMR-based metabolomics. The significant metabolic changes corresponding to copper exposures were related to osmolytes, intermediates of the Krebs cycle and amino acids, such as the increase in homarine, branched chain amino acids and decrease in succinate, alanine and dimethylamine in the copper-exposed clam gills during 96 h exposure period. Overall, Cu may lead to the disturbances in osmotic regulation and energy metabolism in clams during 96 h experimental period. These results demonstrate that NMR-based metabolomics is applicable for the discovery of metabolic biomarkers which could be used to elucidate the toxicological mechanisms of marine heavy metal contaminants.

Toxicological responses to acute mercury exposure for three species of Manila clam Ruditapes philippinarum by NMR-based metabolomics

The Manila clam (Ruditapes philippinarum) has been considered a good sentinel species for metal pollution monitoring in estuarine tidal flats. Along the Bohai coast of China, there are dominantly distributed three species of clams (White, Liangdao Red and Zebra in Yantai population) endowed with distinct tolerances to environmental stressors. In this study, adductor muscle samples were collected from both control and acute mercury exposed White, Liangdao Red and Zebra clams, and the extracts were analyzed by NMR-based metabolomics to compare the metabolic profiles and responses to the acute mercury exposure to determine the most sensitive clam species capable of acting as abioindicator for heavy metal pollution monitoring. The major abundant metabolites in the White clam sample were branched-chain amino acids (leucine, isoleucine and valine), lactate, arginine, aspartate, acetylcholine, homarine and ATP/ADP, while the metabolite profile of Zebra clam sample comprised high levels of glutamine, acetoacetate, betaine, taurine and one unidentified metabolite. For the Liangdao Red clam sample, the metabolite profile relatively exhibited high amount of branched-chain amino acids, arginine, glutamate, succinate, acetylcholine, homarine and two unassigned metabolites. After 48h exposure of 20μgL(-1) Hg(2+), the metabolic profiles showed significant differences between three clam species, which included increased lactate, succinate, taurine, acetylcholine, betaine and homarine and decreased alanine, arginine, glutamine, glutamate, acetoacetate, glycine and ATP/ADP in White clam samples, and elevated succinate, taurine and acetylcholine, and declined glutamine, glycine, and aspartate in Liangdao Red clam samples, while the increased branched-chain amino acids, lactate, succinate, acetylcholine and homarine, and reduced alanine, acetoacetate, glycine and taurine were observed in the Zebra clam samples. Overall, our findings showed that White clams could be a preferable bioindicator for the metal pollution monitoring based on the more sensitive metabolic changes in the adductor muscle compared with other two (Liangdao Red and Zebra) clam species.

Differential toxicological effects induced by mercury in gills from three pedigrees of Manila clam Ruditapes philippinarum by NMR-based metabolomics

Mercury is a hazardous pollutant in the Bohai marine environments due to its high toxicity to the marine organisms and subsequent ecological risk. Manila clam Ruditapes philippinarum is one of important sentinel organisms in 'Mussel Watch Program' launched in China and therefore used as a bioindicator in marine and coastal ecotoxicology. There are dominantly distributed three pedigrees of clam (White, Liangdao Red and Zebra) in Yantai population endowed with different tolerances to environmental stressors. In this study, gill tissues were collected from both untreated and mercury exposed White, Liangdao Red and Zebra clams, and the extracts were analyzed by NMR-based metabolomics to compare the original metabolomes and the toxicological effects induced by mercury exposure in three pedigrees. The major abundant metabolites in White clam sample were branched-chain amino acids, lactate, alanine, arginine, acetoacetate, glutamate, succinate, citrate, malonate and taurine, while the metabolite profile of Liangdao Red clam sample comprises relative high levels of alanine, arginine, glutamate, succinate and glycogen. For Zebra clam sample, the metabolite profile exhibited relatively high amount of aspartate, acetylcholine and homarine. After 48 h exposure of 20 μg l(-1) Hg(2+), the metabolic profiles from all the three pedigrees of clams commonly showed significant increases in alanine, arginine, glutamate, aspartate, α-ketoglutarate, glycine and ATP/ADP, and decreases in citrate, taurine and homarine. The unique metabolic differences between the metabolomes of gill tissues from Hg(2+)-exposed White, Liangdao Red and Zebra clams were found, including elevated acetylcholine and branched-chain amino acids in White clams, and the declined succinate in both White and Liangdao Red samples as well as the declined betaine in Zebra and White clams. Overall, our findings showed the differential toxicological responses to mercury exposure and that White clams could be a preferable bioindicator for the metal pollution monitoring based on the metabolic changes from gill compared with other two (Liangdao Red and Zebra) pedigrees of clams.

Interactive effects of metal contamination and pathogenic organisms on the introduced marine bivalve Ruditapes philippinarum in European populations

In natural environment, marine organisms are concomitantly exposed to pollutants and multiple disease agents resulting in detrimental interactions. The present study evaluated interactive effects of metal contamination (cadmium) and pathogenic organisms (trematode parasites Himasthla elongata and pathogenic bacteria Vibrio tapetis) singularly and in combination on the bivalve Ruditapes philippinarum, an introduced species to Europe, under laboratory controlled conditions. After 7 days, metal bioaccumulation and pathogen load were analyzed as well as metallothionein (MT) response and hemocyte concentrations and activities. Results showed that infection by opportunistic pathogens affects metal accumulation, leading to maximal Cd accumulation in co-infected clams. Among stressors only V. tapetis induced significant effects on immune parameters whereas a particular interaction "trematode-bacteria" was shown on MT responses. Despite low trematode infection in agreement with the resistant status of R. philippinarum to these macroparasites, significant interaction with bacteria and metal occurred. Such results highlight the necessity of taking pathogens into account in ecotoxicological studies.

Bivalve population health: multistress to identify hot spots

This study investigated some stress (metals, parasites) and response (immunity, metallothionein) factors in two cockle and two Manila clam populations. Data from eight seasons were averaged to obtain global baseline values. Stress/response characteristics of each population were compared to population health status that was determined through population dynamics parameters. Four different scenarios were discussed: (1) a lightly stressed cockle population with correct population health but with a risk of deterioration (hot spot); (2) a lightly stressed introduced cockle population threatened of extinction. In this case ecological factors were suspected; (3) a moderately stressed clam population with moderate adaptative response. The population was sustainable but the level of stress should not increase (hotspot); and (4) a stressed clam population and unfavourable ecological conditions preventing clam settlement. This monitoring highlighted that the discrepancy between population health and stress levels could be due to insufficient response by bivalves and/or by unfavourable ecological factors.