Integrated multi-trophic aquaculture of sandfish Holothuria scabra with hard clam Meretrix taiwanica and milkfish Chanos chanos under environmental stress

Predators often have large effects on community structure, but these effects can be minimized in habitats subjected to intense physical stress. For example, predators exert large effects on rocky intertidal communities on wave-protected shores but are usually absent from wave-swept shores where hydrodynamic forces prevent them from foraging effectively. The physical environment also can affect predation levels when stressors are not severe enough to be physically risky. In these situations, environmental conditions may constrain a predator's ability to locate prey and alleviate predation pressure. Yet, stress models of community structure have rarely considered the implications of such sensory or behavioral stressors, particularly when the sensory abilities of both predators and prey are affected by the same types of environmental conditions. Ecologists may classify certain environmental conditions as refuges if they impede predator foraging, but these conditions may not actually decrease predation levels if they simultaneously increase prey vulnerability to consumers. Using blue crabs (Callinectes sapidus) and hard clams (Mercenaria mercenaria) as a model system, we investigated the relationship between predation intensity and environmental stress in the form of hydrodynamics (i.e., flow velocity and turbulence). Blue crabs and hard clams are less responsive to each other in faster, more turbulent flows, but studies exploring how flow modulates the outcomes of crab-clam interactions in the field are lacking. We manipulated turbulence within field sites and compared predation levels within and between sites that differed in flow velocity and turbulence. Our results suggest that blue crabs are most effective foragers in flows with intermediate velocities and turbulence levels. Although these conditions are not ideal for blue crabs, lab studies indicate that they also compromise the ability of clams to detect and react to approaching crabs and, thereby, increase clam vulnerability to predators. Our results suggest that environmental stresses on perception (sensory stressors) may not cause a steady decay in predation rates when they simultaneously affect the behaviors of both predators and prey. Moreover, the relative contribution of lethal vs. nonlethal predator effects in communities also may be influenced by environmental forces that enhance the predator-avoidance abilities of prey or the foraging efficiency of predators.

Metal stresses affect the population dynamics of disease transmission in aquaculture species

Aquaporins (AQPs) are highly-selective transmembrane water transporters that are involved in the adaptation to environmental challenges. However, the structure, function, and evolution of AQPs in bivalves remain largely unknown. In this study, AQP gene family members were identified in nine bivalve species, and their abundance rangs from 7 to 15. Nine AQPs (MmAQPs) were identified in the genome of hard clam (Mercenaria mercenaria), which is a euryhaline bivalve that has evolved sophisticated osmoregulatory mechanisms and salinity adaptation. Structurally, all MmAQPs contain 6 or 12 transmembrane α-helices, a major intrinsic protein (MIP) domain, and 2 asparagine-proline-alanine (NPA) motifs. MmAQPs were classified into three subfamilies based on phylogenetic analysis: AQP1-like, AQP3-like, and AQP8-like. No AQP11-like subfamily member was identified in the genome of hard clam. Tandem duplication resulted in a lineage-specific expansion in AQP8-like subfamily in hard clams. MmAQP8 genes showed different expression sensitivity to different environmental stressors. The gene expression patterns of three MmAQP8 were similar under heat, hypoxia, and air exposure stress, but differed greatly under salinity stress, indicating that tandem duplication events may accelerate the functional divergence of AQP8 genes in hard clams. AQP3-like members may have undergone gene loss during evolution, resulting in weakened glycerol and urea penetration in hard clams. Three orthologs of MmAQPs were detected in the genomes of Cyclina sinensis and Archivesica marissinica through synteny analyses. Tissue expression profiles showed that MmAQP genes were highly expressed in the foot and hepatopancreas. Under environmental stress, the expression levels of most of the MmAQP genes changed significantly to maintain metabolic homeostasis. Several MmAQP genes were downregulated to reduce water permeability under salinity and air exposure stress. Several MmAQP genes were significantly upregulated to promote the transmembrane transport of ammonia and reactive oxygen species and activate anti-apoptotic responses to resist stress. This study provides a comprehensive understanding of the AQP gene family in hard clams, and lays a foundation for further studies to explore the functions of AQPs in bivalves.

Ocean acidification due to increasing atmospheric CO2 concentrations results in a decrease in seawater pH and shifts in the carbonate chemistry that can negatively affect marine organisms. Marine bivalves such as the hard-shell clam, Mercenaria mercenaria, serve as ecosystem engineers in estuaries and coastal zones of the western Atlantic and, as for many marine calcifiers, are sensitive to the impacts of ocean acidification. In estuaries, the effects of ocean acidification can be exacerbated by low buffering capacity of brackish waters, acidic inputs from freshwaters and land, and/or the negative effects of salinity on the physiology of organisms. We determined the interactive effects of 21 weeks of exposure to different levels of CO2 (~395, 800 and 1500 μatm corresponding to pH of 8.2, 8.1 and 7.7, respectively) and salinity (32 versus 16) on biomineralization, shell properties and energy metabolism of juvenile hard-shell clams. Low salinity had profound effects on survival, energy metabolism and biomineralization of hard-shell clams and modulated their responses to elevated PCO2. Negative effects of low salinity in juvenile clams were mostly due to the strongly elevated basal energy demand, indicating energy deficiency, that led to reduced growth, elevated mortality and impaired shell maintenance (evidenced by the extensive damage to the periostracum). The effects of elevated PCO2 on physiology and biomineralization of hard-shell clams were more complex. Elevated PCO2 (~800-1500 μatm) had no significant effects on standard metabolic rates (indicative of the basal energy demand), but affected growth and shell mechanical properties in juvenile clams. Moderate hypercapnia (~800 μatm PCO2) increased shell and tissue growth and reduced mortality of juvenile clams in high salinity exposures; however, these effects were abolished under the low salinity conditions or at high PCO2 (~1500 μatm). Mechanical properties of the shell (measured as microhardness and fracture toughness of the shells) were negatively affected by elevated CO2 alone or in combination with low salinity, which may have important implications for protection against predators or environmental stressors. Our data indicate that environmental salinity can strongly modulate responses to ocean acidification in hard-shell clams and thus should be taken into account when predicting the effects of ocean acidification on estuarine bivalves.

Heat shock proteins (HSPs) are molecular chaperones that help organisms cope with stressful conditions. In this study, a novel small HSP (sHSP) gene was identified from the hard clam Meretrix meretrix (designated as Mm-HSP20). The full length of the cDNA is 1222 bp, con- sisting of a 5'-terminal untranslated region (5'UTR) of 103 bp, a 3'UTR of 565 bp, and a 522 bp open reading frame encoding a polypeptide of 173 amino acids. Sequence comparison showed that Mm-HSP20 had a moderate degree of homology to the sHSP of other organisms. A sHSP feature domain, an alpha-crystallin domain, and a V/IXI/V motif in the C-terminal extension were identified in the Mm-HSP20 amino acid sequence, indicating that Mm-HSP20 is a new member of the sHSP family. The Mm-HSP20 transcript was constitutively expressed in 6 different test tissues, with the highest expression level detected in the digestive gland by fluorescent quantitative real- time PCR. The temporal expression of Mm-HSP20 mRNA in the digestive gland and hemocytes was evaluated after exposure to cadmium (Cd) (40 µg l �1 CdCl2) and benzo(a)pyrene (50 µg l �1 B(a)P) individually, and in combination (40 µg l �1 CdCl2 +5 0 µg l �1 B(a)P). Mm-HSP20 expression level increased significantly after Cd and B(a)P exposure in both tissue types. Conversely, signifi- cant Mm-HSP20 transcript repression was detected in the 24 h B(a)P-treated digestive gland sam- ples. Considerable up-regulation of Mm-HSP20 mRNA level was observed after multiple expo- sures. These results indicate that Mm-HSP20 plays a role in mechanisms involved in coping with environmental stress in hard clams. Mm-HSP20 may be suitable for use as a biomarker for heavy metal and B(a)P contamination.

Immunological assays of hemocytes in the Northern Quahog Mercenaria mercenaria

Metabolic response of Mercenaria mercenaria under heat and hypoxia stress by widely targeted metabolomic approach

Effect of heat and hypoxia stress on mitochondrion and energy metabolism in the gill of hard clam

Mechanisms of heat and hypoxia defense in hard clam: Insights from transcriptome analysis

Microplastic ingestion by the sandfish Holothuria scabra in Lampung and Sumbawa, Indonesia

The dominant coastal bivalve in Vietnam, hard clams Meretrix spp., collected from the South Key Economic Zone (SKEZ), the Mekong River Delta, and the Central Coastal Zone (CCZ) were analyzed for 21 trace elements. Comparison of the results from the three regions indicated that levels of most of the trace elements, especially As, Mo, Sn, and Pb, were highest in the samples collected from the CCZ, whereas most of the trace elements were found to be present at relatively low levels in samples from the SKEZ. The high concentrations of these trace elements in the CCZ, a sparsely populated region with less human activity than the other two regions, were believed to have originated from industrial waste produced in a shipyard. Although the trace element concentrations in the bivalves were within safe levels for human-consumption criteria reported by the United State Food and Drug Administration and the European Commission, estimation of cancer risk based on As concentration indicates that the hard clams from the CCZ pose a high risk to consumers. Thus, the industrial waste produced in the less densely populated region might increase the health risk to consumers via the contamination of bivalves commonly used as food.

One hundred one individuals of Protothaca staminea were live-collected from Argyle Lagoon (sand/mud substrate) and Argyle Creek (gravel/sand substrate), San Juan Island, Washington and examined for trace-producing parasite infestation. Eighty-six percent of individuals contained at least one parasite-induced trace. Trematode-induced pits and blisters were identified on 62% of individual clams. Spionid-induced mudblisters and u-shaped borings were identified on 50% and 29% of individuals, respectively. Trematode and spionid parasites were not selective between the left and right valve when infesting the host. Epifaunal clams from Argyle Creek were significantly smaller than their infaunal counterparts from Argyle Lagoon. This size discrepancy between environments may be related to the reduction of growth rates triggered by environmental stress or parasitism, increased susceptibility to durophagous predators, differences in hydrodynamics, or the comparison of different cohorts. Spionid mudblister-infested clams from Argyle Creek are significantly smaller than noninfested clams from the same environment. This suggests that substrate-induced epifaunality and parasite-induced shell weakening reduced the bivalves’ defenses against durophagous predators. These results suggest that parasites may negatively affect the survival of infested bivalves. The frequent occurrence of trematode and spionid trace-producing parasites in modern bivalve populations suggests that these traces are common in the fossil record, making the systems amenable to study in deep time.

The purpose of this article is to provide a risk-based predictive model to assess the impact of false mussel Mytilopsis sallei invasions on hard clam Meretrix lusoria farms in the southwestern region of Taiwan. The actual spread of invasive false mussel was predicted by using analytical models based on advection-diffusion and gravity models. The proportion of hard clam colonized and infestation by false mussel were used to characterize risk estimates. A mortality model was parameterized to assess hard clam mortality risk characterized by false mussel density and infestation intensity. The published data were reanalyzed to parameterize a predictive threshold model described by a cumulative Weibull distribution function that can be used to estimate the exceeding thresholds of proportion of hard clam colonized and infestation. Results indicated that the infestation thresholds were 2-17 ind clam(-1) for adult hard clams, whereas 4 ind clam(-1) for nursery hard clams. The average colonization thresholds were estimated to be 81-89% for cultivated and nursery hard clam farms, respectively. Our results indicated that false mussel density and infestation, which caused 50% hard clam mortality, were estimated to be 2,812 ind m(-2) and 31 ind clam(-1), respectively. This study further indicated that hard clam farms that are close to the coastal area have at least 50% probability for 43% mortality caused by infestation. This study highlighted that a probabilistic risk-based framework characterized by probability distributions and risk curves is an effective representation of scientific assessments for farmed hard clam in response to the nonnative false mussel invasion.

To understand the integrated effects of organic matter loading on the benthic chemical properties, microbial communities, and benthos in an estuarine system, two field studies and a set of laboratory experiments were conducted. In the Tanshui River, the horizontal characteristics of the estuarine sediments were revealed in three spatial settings from the river mouth to a mid-reach section: a nitrate-abundance type, a phosphate-release type, and a sulfide-generation type; the vertical characteristics in four layers of Guandu sediment were also elucidated. With the exception of the mixed layer, a variety of metabolic processes by denitrifier strains occur in different vertical layers including nitrification-denitrification coupling processes in the nitrate-concentrated layer, denitrification or dissimilatory nitrate reduction to ammonium in the denitrifier-aggregation layer, and fermentation or spore formation in the ammonium-enriched layer. In addition, two laboratory experiments indicated that environmental condition gradually became worse, which leads to decreasing of dissolved oxygen, increasing of ammonium, nitrate and sulfide concentrate. However, high levels of treatment with the photosynthetic bacterium (PSB, Ectothiorhodospira sp.) decreased sulfide and nitrogenous nutrients in both aerated and hypoxic conditions, and then significantly improved the survival rate of the hard clam (Meretrix lusoria). In summary, the degree of organic matter degradation could be defined by the chemical properties in both the horizontal and vertical sediment profiles. A variety of metabolic processes by denitrifiers, which occur in the comparative sediment profile, indirectly correspond to these different stages. The use of the PSB, Ectothiorhodospira sp., actively removed some chemicals and mediated the sediment properties (from polluted to a transitory stage). Benthic hard clams can serve as a good indicator of the shift in benthic chemical processes toxic to the benthos, which are incapable of being detected by environmental measurements.

After 1 min aerial exposure, the hemolymph pO2 of hard clams (Meretrix lusoria) was reduced from 23.8 mmHg initially to 18.3 mmHg at 15℃, while it significantly decreased from 27 mmHg initially to 14 mmHg at 30℃. This rapid decline in the hemolymph pO2 of hard clams may have been due to a low concentration of oxygen carriers in the hemolymph. Indeed, the hemoglobin in hemolymph of hard clams was not detectable, while a value of 0.075 mM hemocyanin in the hemolymph was obtained. After clams were exposed to Mg(superscript +2)-free artificial seawater for 10 h, the hemolymph pO2 of significantly dropped from 24 to 13 mmHg. However, the hemolymph pCO2 and HCO(superscript -)3increased from 4.8 mmHg and 4.3 mmol/l initially to 10 mmHg and 9.9 mmol/l at 10 h of exposure and then decreased to 6.5 mmHg and 7.6 mmol/l at 48 h of exposure, respectively. Potassium reduced the hemolymph pO2 and increased the hemolymph pCO2, while it had no influence on hemolymph pH or HCO(superscript -)3 during the experiment. KCN persistently decreased the hemolymph pO2, while its only effect on the hemolymph pH and HCO(superscript -)3 occurred in the early phase of exposure.