Post-capture hyperbaric simulations to study the mechanism of shell regeneration of the deep-sea hydrothermal vent mussel Bathymodiolus azoricus (Bivalvia: Mytilidae)

This study describes the influence of hydrostatic pressure (HP) on regeneration of the artificially perforated shell in the vent mytilid Bathymodiolus azoricus. Pressure-dependent and size-dependent shell repair mechanism and haemocyte response was detected following post-capture hyperbaric simulations at environmentally relevant pressures at 1, 85, 175 and 230 bars as compared to control situation, i.e. prior to shell perforation. We provide evidence for the following: (1) shell wound repair is influenced by both hydrostatic pressure and depositional substrate: newly forming crystals emerge 10 days after induction, but only in animals maintained under HP similar to their site of provenience, in spite of the fact that B. azoricus is known to be euribarofilic as indicated by its biomass-dominance at various sites along a bathymetric gradient; shell repair also commenced at 1 bar, but only when the regeneration substrate was periostracum; (2) wound recovery, i.e. closure of the shell notch in B. azoricus is slower than in non-vent bivalves, i.e. it was not completed in 120 days at 1 bar; (3) shell repair involves the Ca bearing haemocytes, referred here as granulocytes, but this may not be automatically considered as the mechanism for (non-induced) shell calcification; blood cell counts showed that the raise in haemocyte number, where statistically significant, was reversible to pre-induction levels by day 10, which may indicate immune response rather than an active calcification process; (4) blood cells of the freshly collected animals contained bacteria resembling sulphur oxidiser endosymbionts from the gill, which may have profound immunological implications, and may be considered as an additional mechanism for environmental transmission of symbiosis.     

Infection with the protozoan parasite Bonamia ostreae modifies in vitro haemocyte activities of flat oyster Ostrea edulis

Bonamia ostreae is an intracellular protozoan parasite, infecting haemocytes of the European flat oyster Ostrea edulis. Oyster defence mechanisms mainly rely on haemocytes. In the present study in vitro interactions between parasites and flat oyster haemocytes were investigated using flow cytometry and light microscopy.Haemocyte parameters including: non specific esterase activity, reactive oxygen species (ROS) production and phagocytosis were monitored using flow cytometry after 2 h cell incubation with live and dead B. ostreae. Two ratios of parasites per haemocyte were tested (5:1 and 10:1), haemocytes alone were used as controls and the experiment was carried out three times. Flow cytometry revealed a decrease of non specific esterase activities and ROS production by haemocytes after incubation with live parasites, while there was little difference in phagocytosis activity when compared with controls. Similarly, dead parasites induced a decrease in haemocyte activities but to a lesser extent compared to live parasites. These results suggest that B. ostreae actively contributes to the modification of haemocyte activities in order to ensure its own intracellular survival.     

Shell nacre ultrastructure and depressurisation dissolution in the deep-sea hydrothermal vent mussel Bathymodiolus azoricus

This study describes the micro-morphological features of the shell nacre in the vent mytilid Bathymodiolus azoricus collected along a bathymetric gradient of deep-sea hydrothermal vents of the mid-Atlantic ridge (MAR). Pressure-dependent crystallisation patterns were detected in animals subjected to post-capture hydrostatic simulations. We provide evidence for the following: (1) shell micro morphology in B. azoricus is similar to that of several vent and cold-seep species, but the prismatic shell layers may vary among bathymodiolids; (2) nacre micro-morphology of mussels from three vent sites of the MAR did not differ significantly; minor differences do not appear to be related to hydrostatic pressure, but rather to calcium ion availability; (3) decompression stress may cause drop off in pH of the pallial fluid that damages nascent crystals, and in a more advanced phase, the aragonite tablets as well as the continuous layer of mature nacre; and (4) adverse effects of decompression on calcium salt deposition in shells was diminished by re-pressurisation of specimens. The implications of the putative influence of hydrostatic pressure on biomineralisation processes in molluscs are discussed. An erratum to this article can be found at     

A comparative study on the influence of manganese on the bactericidal response of marine invertebrates

Manganese, Mn, is a naturally abundant metal in marine sediments. During hypoxic conditions the metal converts into a bioavailable state and can reach levels that have been shown immunotoxic to the crustacean Nephrops norvegicus. For this species it has previously been shown that exposure to 15 mg L⁻¹ of Mn decreased the number of circulating haemocytes while it for the echinoderm Asterias rubens increased the number of coelomocytes. Here, we compared if five days of exposure to the same concentration of Mn affects the bactericidal capacity of these two species and the mollusc Mytilus edulis when inoculated with the bacterium Vibrio parahaemolyticus. Viable counts of the bacteria were investigated at a time-course post-injection in the blood and the digestive glands of Mn-exposed and un-exposed (controls) animals. Accumulation of Mn was also analyzed in these tissues. When exposed to Mn the haemocyte numbers were significantly reduced in M. edulis and it was shown that the bactericidal capacity was impaired in the mussels as well as in N. norvegicus. This was most obvious in the digestive glands. These two species also showed the highest accumulation of the metal. In A. rubens the bactericidal capacity was not affected and the metal concentration was similar to the exposure concentration. After a recovery period of three days the concentration of Mn was significantly reduced in all three species. However, in M. edulis and N. norvegicus it was still double that of A. rubens which could explain the remaining bactericidal suppression observed in N. norvegicus. This study pointed out that exposure to such Mn-levels that are realistic to find in nature could have effects on the whole organism level, in terms of susceptibility to infections. The effect seemed associated to the accumulated concentration of Mn which differed on species level.     

Annual spawning of the hydrothermal vent mussel, Bathymodiolus azoricus, under controlled aquarium, conditions at atmospheric pressure

Dense macrofaunal communities of modioliform mussels are a major component of many hydrothermal vent and cold seep ecosystems. The hydrothermal vent mussel Bathymodiolus azoricus, that dominates hydrothermal vent communities near the Azores Triple Junction, can be maintained in aquaria at atmospheric pressure. Cages containing these mussels were placed over diffuse vent outlets and recovered at different times. Cages recovered in January 2003 contained mussels with ripe gonads while those recovered in July, August and November 2001 and in April 2003 did not. Mussels collected post-spawning in April 2003 spawned in the aquaria in January 2004. Young mussels recruited to the cages in April 2003. The data support a main single period of spawning and of juvenile recruitment for B. azoricus.     

Regeneration of excised shell by the invasive apple snail Pomacea canaliculata

After excision of shell from Pomacea canaliculata, survival rates, repair process, haemocyte response, and calcium content were observed and quantified for three weeks. The following experimental treatments were used: black-colored individuals with large (BL-group) or small (BS-group) shell heights, yellow-colored individuals with large (YL-group) or small (YS-group) shell heights. The survival rates in BL-, YL-, BS-, and YS-groups after making partial excisions of snail shells were, respectively, 93.59, 94.87, 92.31, and 96.15%. Freshly regenerated shell could be seen at 1 day among the four groups after induction of shell regeneration. Regeneration shell filled the wound after 5–10 days. The area of regenerated shell in yellow-colored individuals was larger than that of black-colored individuals, but the thickness of regenerated shell was not significantly different. Multiple (four times) inductions of shell regeneration significantly enhanced the thickness of newly formed shells. Two types of circulating haemocytes (hyalinocytes and granulocytes) were identified under light microscopy based on Wright’s staining. Artificial induction of shell regeneration rendered a significant increase in the number of total circulating haemocytes which was followed by a decrease to pre-wounding levels. This peak value of total circulating haemocytes was 2.86, 2.43, 1.69, and 1.71-fold higher than pre-wounding levels in group BL, YL, BS, and YS, respectively. The number of total circulating haemocytes in yellow-colored individuals was significantly higher than in black-colored. Calcium concentrations in the mantles of snails are favorable for calcium nucleation. It is postulated that calcium content increased in the mantle increase at the start of the experiment and then returned to pre-wounding levels at the end of the experiment. This study shows that apple snails have effective shell regeneration abilities and that haemocytes and calcium transportation appear to play an important role in shell growth and regeneration.     

Multiple I-Type Lysozymes in the Hydrothermal Vent Mussel Bathymodiolus azoricus and Their Role in Symbiotic Plasticity

The aim of this study was first to identify lysozymes paralogs in the deep sea mussel Bathymodiolus azoricus then to measure their relative expression or activity in different tissue or conditions. B. azoricus is a bivalve that lives close to hydrothermal chimney in the Mid-Atlantic Ridge (MAR). They harbour in specialized gill cells two types of endosymbiont (gram—bacteria): sulphide oxidizing bacteria (SOX) and methanotrophic bacteria (MOX). This association is thought to be ruled by specific mechanism or actors of regulation to deal with the presence of symbiont but these mechanisms are still poorly understood. Here, we focused on the implication of lysozyme, a bactericidal enzyme, in this endosymbiosis. The relative expression of Ba-lysozymes paralogs and the global anti-microbial activity, were measured in natural population (Lucky Strike -1700m, Mid-Atlantic Ridge), and in in situ experimental conditions. B. azoricus individuals were moved away from the hydrothermal fluid to induce a loss of symbiont. Then after 6 days some mussels were brought back to the mussel bed to induce a re-acquisition of symbiotic bacteria. Results show the presence of 6 paralogs in B. azoricus. In absence of symbionts, 3 paralogs are up-regulated while others are not differentially expressed. Moreover the global activity of lysozyme is increasing with the loss of symbiont. All together these results suggest that lysozyme may play a crucial role in symbiont regulation.     

Variability of haemocyte and haemolymph parameters in European flat oyster Ostrea edulis families obtained from brood stocks of different geographical origins and relation with infection by the protozoan Bonamia ostreae

A research project to compare productive traits (growth and mortality), disease susceptibility and immune capability between Ostrea edulis stocks was performed. This article reports the results on the immune capability and its relation with infection by the intrahaemocytic protozoan Bonamia ostreae. Four to five oyster spat families were produced from each of four European flat oyster populations (one from Ireland, one from Greece and two from Galicia, Spain) in a hatchery. The spat were transferred to a raft in the Ría de Arousa (Galicia) for on growing for 2 years. Total haemocyte count (THC) and differential haemocyte count (DHC) were estimated monthly through the second year of growing-out. Three types of haemocytes were distinguished: granulocytes (GH), large hyalinocytes (LHH) and small hyalinocytes (SHH). Significant correlations between the mean relative abundance of GH and SHH of the families and the mean prevalence of B. ostreae, the overall incidence of pathological conditions and the cumulative mortality of the families were found; these correlations supported the hypothesis that high %GH and low %SHH would enhance oyster immune ability and, consequently, would contribute to lower susceptibility to disease and longer lifespan. Infection by B. ostreae involved a significant increase of circulating haemocytes, which affected more markedly the LHH type. The higher the infection intensity the higher the %LHH. This illustrates the ability of B. ostreae to modulate the immune responses of the O. edulis to favour its own multiplication. A significant reduction of the phenoloxidase activity in the haemolymph of oysters O. edulis infected by B. ostreae was observed. Nineteen enzymatic activities in the haemolymph of O. edulis and Crassostrea gigas (used as a B. ostreae resistant reference) were measured using the kit api ZYM^®, Biomerieux. Qualitative and quantitative differences in enzyme activities in both haemocyte and plasma fractions between B. ostreae noninfected O. edulis from different origins were recorded. However, no clear positive association between enzyme activity and susceptibility to bonamiosis was found. The only enzyme detected in the resistant species C. gigas that was not found in the susceptible one O. edulis was β-glucosidase (in plasma). B. ostreae infected O. edulis showed significant increase of some enzyme activities and the occurrence of enzymes that were not detected in noninfected oysters. These changes could be due to infection-induced enzyme synthesis by the host or to enzyme synthesis by the parasite.     

The complete mitogenome of the hydrothermal vent crab Xenograpsus testudinatus (Decapoda, Brachyura) and comparison with brachyuran crabs

In this study, we analyzed the complete mitochondrial (mt) genome of a hydrothermal vent crab Xenograpsus testudinatus (Decapoda: Brachyura) obtained from the hydrothermal vents off Kueishantao Island, Taiwan, which extend from the deep sea Okinawa Trench. The mitogenome of X. testudinatus was 15,796 bp in length and contained the same 37 genes (e.g. 2 rRNAs, 22 tRNAs, and 13 PCGs) found in other metazoan mitogenomes. Analysis of the structural mt gene order in X. testudinatus revealed that the 13 PCGs, excluding a translocation of ND6-Cyt b cluster, were similarly ordered when compared to the pancrustacean ground pattern; however the tRNAs were severely rearranged. Phylogenetic analysis of decapod mitogenomes showed that the molecular taxonomy of the vent crab was in accordance with its morphological systematics. Together, these findings suggest that the vent crab studied here has little mitochondrial genetic variation when compared with morphologically defined conspecifics from other marine habitats.     

Geochemical investigation of gametogenic calcite addition in the planktonic foraminifera Orbulina universa

Several species of planktonic foraminifera precipitate a final layer of calcite onto the shell surface immediately prior to gamete release at the end of the foraminifera life cycle. Here, we present the results of carbon-13, oxygen-18 and thermal labeling experiments conducted under high (HL) and low light (LL) regimes that vary symbiont photosynthetic activity. Mean experimental group data show that gametogenic (gam) calcite contributes between 4–17% and 14–20% to final shell mass for high and low light experiments respectively. These data indicate that past studies of gam calcite addition have overestimated the amount of gam calcite on foraminifera shells by ~ 30–55%. Calculations indicate that the mass of gam calcite added to the O. universa shell, 4.2 ± 2.0 μg and 4.0 ± 2.4 μg, is constant in the HL and LL groups respectively. We propose that the production of gam calcite may be the result of the discharge of a relatively constant-volume cytoplasmic pool of either Ca²⁺ or alkalinity (carbon pool) that increases the calcite saturation state in the microenvironment adjacent to the foraminifera shell just prior to gamete formation and release. Results from these experiments indicate that the geochemistry of thin-walled O. universa from deep sea sediments is composed of > 80% ontogenetic calcite that was precipitated in the primary, near surface habitat of this species.     

Comparative analysis of symbiont ratios and gene expression in natural populations of two Bathymodiolus mussel species

Bathymodiolus mussels associated with deep-sea hydrothermal vents and cold seeps harbor chemosynthetic endosymbiotic bacteria in bacteriocytes located in the gill epithelium. Two distinct morphotypes of γ-proteobacteria, sulfur- and methane-oxidizing, have been identified and form a dual symbiosis in B. azoricus mussels from the Mid-Atlantic Ridge and in B. aff. boomerang from cold seeps in the Gulf of Guinea. Thiotrophic bacteria (SOX) are capable of fixing CO2 in the presence of sulfide or thiosulfate and methanotrophic bacteria (MOX) use methane both as a carbon and an energy source. In this study we used quantitative real-time PCR to test whether symbiont abundance and gene expression varied between the two mussel species. Results showed that B. azoricus from two hydrothermal sites had similar ratios and gene expression pattern for both symbiont types. In B. aff. boomerang, SOX ratio and ATP sulfurylase gene expression show differences between specimens collected on the different sites. Analysis of symbiont ratios in both species indicated a clear dominance of MOX symbionts in B. aff. boomerang and SOX symbionts in B. azoricus. We also evidenced that the species from the deeper sites (B. aff. boomerang) and mussels collected from sulfur and methane rich habitats showed higher symbiont ratio suggesting that environmental parameters may have significant impacts on the symbiont ratios in Bathymodiolus mussels.     

Global depression in gene expression as a response to rapid thermal changes in vent mussels

Hydrothermal vent mussels belonging to the genus Bathymodiolus are distributed worldwide and dominate communities at shallow Atlantic hydrothermal sites. While organisms inhabiting coastal ecosystems are subjected to predictable oscillations of physical and chemical variables owing to tidal cycles, the vent mussels sustain pronounced temperature changes over short periods of time, correlated to the alternation of oxic/anoxic phases. In this context, we focused on the short-term adaptive response of mussels to temperature change at a molecular level. The mRNA expression of 23 genes involved in various cell functions of the vent mussel Bathymodiolus azoricus was followed after heat shocks for either 30 or 120 min, at 25 and 30°C over a 48 h recovery period at 5°C. Mussels were genotyped at 10 enzyme loci to explore a relationship between natural genetic variation, gene expression and temperature adaptation. Results indicate that the mussel response to increasing temperature is a depression in gene expression, such a response being genotypically correlated at least for the Pgm-1 locus. This suggests that an increase in temperature could be a signal triggering anaerobiosis for B. azoricus or this latter alternatively behaves more like a ‘cold’ stenotherm species, an attribute more related to its phylogenetic history, a cold seeps/wood fall origin.     

Haemocyte apoptosis as a general cellular immune response of the snail, <Emphasis Type="BoldItalic">Lymnaea stagnalis</Emphasis>, to a toxicant

The effects of a xenobiotic on the circulating haemocytes of Lymnaea stagnalis were investigated after short-term (24 h, 96 h) and long-term (504 h) exposure of snails to environmental concentrations. Fomesafen, a pro-oxidant generator led to the activation of the haemocyte apoptotic program by promoting reactive oxygen species (ROS). Cells entering apoptosis underwent a series of events, both on the plasma membrane and in the mitochondria; these events were quantified by flow cytofluorometry. The data showed a loss of mitochondrial transmembrane potential (Δψm), which was dose-dependent and time-dependent and related to an increased release of superoxide anions. The phosphatidylserine that was exposed at the outer plasma membrane was not related to the disruption of either ROS or Δψm but was strongly correlated with the haemocyte concentration (total haemocyte count). This cascade of apoptotic processes occurred in a dose-independent manner and was not strengthened over time. The increase of circulating haemocytes depended upon the life span of the cells and might have reflected either facilitated cell turn-over or the accompanying presence of haemocytes phagocytosing apoptotic cells.     

亚硝酸盐胁迫对罗氏沼虾血细胞及其抗氧化酶活力的影响

【背景】亚硝酸盐是虾类集约化养殖过程中最常见的毒性污染物之一,研究亚硝酸盐胁迫对罗氏沼虾血细胞的毒性以及抗氧化酶在抗胁迫防御中的作用,能够为罗氏沼虾养殖过程中的亚硝酸盐中毒防治提供理论参考。【方法】以不同浓度(0、1、5和10 mg·L~(-1))的亚硝态氮(NO_2~--N)对罗氏沼虾进行胁迫,于胁迫后的0、6、12、24和48 h取样,应用流式细胞术检测血细胞活性氧(ROS)含量和细胞凋亡率,同时测定血细胞总数(THC)和胞内抗氧化酶活力。【结果】1 mg·L~(-1)NO_2~--N在48 h内对血细胞ROS含量、凋亡率和THC均无显著影响。5 mg·L~(-1)NO_2~--N胁迫24 h,血细胞ROS含量显著上升,THC显著下降,胁迫48 h凋亡率显著提高。10 mg·L~(-1)NO_2~--N胁迫6 h,血细胞ROS含量和凋亡率均显著上升,胁迫12 h THC显著下降。血细胞的超氧化物歧化酶(SOD)、过氧化氢酶(CAT)和谷胱甘肽过氧化物酶(GPx)的活力均不同程度地被NO_2~--N胁迫所诱导,CAT活力主要在胁迫前期提高,而GPx活力在胁迫后期提高。【结论与意义】亚硝酸盐存在浓度和时间毒性效应,一定浓度的亚硝酸盐会诱导虾血细胞产生ROS,这些ROS的过量产生诱导了血细胞发生凋亡,继而导致THC下降,这一氧化胁迫过程可能是亚硝酸盐对罗氏沼虾产生细胞毒性的重要机制之一。抗氧化酶活力的诱导表明抗氧化酶在亚硝酸盐胁迫过程中发挥防御作用。     

Speciation in the Deep Sea: Multi-Locus Analysis of Divergence and Gene Flow between Two Hybridizing Species of Hydrothermal Vent Mussels

Background

Reconstructing the history of divergence and gene flow between closely-related organisms has long been a difficult task of evolutionary genetics. Recently, new approaches based on the coalescence theory have been developed to test the existence of gene flow during the process of divergence. The deep sea is a motivating place to apply these new approaches. Differentiation by adaptation can be driven by the heterogeneity of the hydrothermal environment while populations should not have been strongly perturbed by climatic oscillations, the main cause of geographic isolation at the surface.

Methodology/Principal Finding

Samples of DNA sequences were obtained for seven nuclear loci and a mitochondrial locus in order to conduct a multi-locus analysis of divergence and gene flow between two closely related and hybridizing species of hydrothermal vent mussels, Bathymodiolus azoricus and B. puteoserpentis. The analysis revealed that (i) the two species have started to diverge approximately 0.760 million years ago, (ii) the B. azoricus population size was 2 to 5 time greater than the B. puteoserpentis and the ancestral population and (iii) gene flow between the two species occurred over the complete species range and was mainly asymmetric, at least for the chromosomal regions studied.

Conclusions/Significance

A long history of gene flow has been detected between the two Bathymodiolus species. However, it proved very difficult to conclusively distinguish secondary introgression from ongoing parapatric differentiation. As powerful as coalescence approaches could be, we are left by the fact that natural populations often deviates from standard assumptions of the underlying model. A more direct observation of the history of recombination at one of the seven loci studied suggests an initial period of allopatric differentiation during which recombination was blocked between lineages. Even in the deep sea, geographic isolation may well be a crucial promoter of speciation.     

Production of superoxides and nitric oxide generation in haemocytes of mussel Mytilus galloprovincialis (Lmk.) after exposure to cadmium: A possible involvement of Na/H exchanger in the induction of cadmium toxic effects

The present study investigates cadmium (Cd) ability to enhance superoxides (O₂⁻) and nitric oxide (NO) production (as nitrites) in haemocytes of mussel Mytilus galloprovincialis as well as the possible involvement of Na⁺/H⁺ exchanger (NHE) in the induction of NADPH oxidase and NO synthase activity. PMA, a well-known PKC-mediated NADPH oxidase as well as NO synthase stimulator was also used, in order to verify Cd effects on both O₂⁻ and NO generation. According to the results of the present study, micromolar concentrations of Cd (0.05, 5, 10 and 50 μM) seemed to enhance O₂⁻ and NO generation in haemocytes of mussels. Moreover, O₂⁻ and NO generation in haemocytes exposed to Cd could be enhanced by its ability to induce reactive oxygen species (ROS) but respiratory burst activation as well. Inhibition of NO synthase with 10 μM l-NAME, significantly attenuated Cd ability to enhance O₂⁻ production and diminished NO generation, thus leading to the suggestion that Cd toxic effects, started at concentration of 50 μM, could enhance NADPH oxidase and NO synthase stimulation in haemocytes of mussels. NHE seems to play a regulatory role in the induction of either O₂⁻ or NO generation in haemocytes exposed to the metal, since its inhibition with the use of 10 μM EIPA significantly decrease both O₂⁻ and NO production. The involvement of NHE in the induction of O₂⁻ and NO generation, probably via PKC-mediated NADPH oxidase and NO synthase activation, is likely to be crucial to haemocytes exposed to heavy metals, such as Cd.     

Behaviour in vitro of separated fractions of haemocytes of the locust Schistocerca gregaria

Summary The haemocytes of locusts (Schistocerca gregaria) have been partially separated, by centrifugation on discontinuous Percoll gradients, into 3 subpopulations of plasmatocytes of which the most dense (band 5) displays more than 95% morphological homogeneity. The cells of band 5 are very granular, adhere and spread on glass, and contain nearly all the phenoloxidase activity of the total haemocyte population, as shown by 1,3-glucan (laminarin)-stimulation of both haemocyte lysate supernatant and monolayers of living cells. Cells from band 5 show negligible endocytosis of fluorescent latex beads, whereas those of band 4, which are less granular, plasmatocyte-like cells, are actively endocytic in vitro. The majority of the haemocytes are found in band 3, which is a mixture of coagulocytes and agranular plasmatocytes, with negligible phenoloxidase activity. The in vitro locomotory behaviour of adherent cells from bands 3 and 5 was compared, and addition of laminarin-activated haemocyte lysate supernatant, in which the prophenoloxidase activation sequence had been initiated, stimulated chemokinesis in cells of band 5 but not band 3. The separated fractions thus show marked behavioural and biochemical differences.     

Commensals of Mytilus galloprovincialis in the Black Sea: Urastoma cyprinae (Turbellaria) and Polydora ciliata (Polychaeta)

The intensity of infection of the mussel Mytilus galloprovincialis Lamarck in the Black Sea by the turbellarian Urastoma cyprinae (Graff), which lives on its gills, was found to be higher in larger hosts, reaching a maximum in mussels of 50–70 mm length. Greater numbers occured in mussels inhabiting a silty bottom than in cultivated mussels suspended above the bottom. Over the period 1982–1987, U. cyprinae was most numerous in winter and especially so in years that were colder. The spionid polychaete Polydora ciliata (Johnston) also infects M. galloprovincialis, burrowing into the shell. Young spionids of up to 1 mm length occured in mussels with a shell length of 35 mm. Numbers of this commensal reached a maximum in mussels of intermediate size.     

Shell nacre ultrastructure and depressurisation dissolution in the deep-sea hydrothermal vent mussel <Emphasis Type="Italic">Bathymodiolus azoricus</Emphasis>

This study describes the micro-morphological features of the shell nacre in the vent mytilid Bathymodiolus azoricus collected along a bathymetric gradient of deep-sea hydrothermal vents of the mid-Atlantic ridge (MAR). Pressure-dependent crystallisation patterns were detected in animals subjected to post-capture hydrostatic simulations. We provide evidence for the following: (1) shell micro morphology in B. azoricus is similar to that of several vent and cold-seep species, but the prismatic shell layers may vary among bathymodiolids; (2) nacre micro-morphology of mussels from three vent sites of the MAR did not differ significantly; minor differences do not appear to be related to hydrostatic pressure, but rather to calcium ion availability; (3) decompression stress may cause drop off in pH of the pallial fluid that damages nascent crystals, and in a more advanced phase, the aragonite tablets as well as the continuous layer of mature nacre; and (4) adverse effects of decompression on calcium salt deposition in shells was diminished by re-pressurisation of specimens. The implications of the putative influence of hydrostatic pressure on biomineralisation processes in molluscs are discussed.