Effects of temperature,seawater osmolality and season on oxygen consumption and osmoregulation of the amphipod Gammarus oceanicus

Seasonal variations of oxygen consumption rate, haemolymph osmolality and the concentrations of the inorganic ions potassium and sodium in the haemolymph were measured in the littoral amphipod Gammarus oceanicus collected from the Trondheimsfjorden, Norway in 1987. For each season comparisons were made of amphipods acclimated for 1 wk to 0.5, 4.5, 10.0, 15.0 and 20.0°C, in combination with seawater osmolalities of 100, 500 and 1200 mOsm and to the seawater osmolality corresponding to that of the collecting site. The oxygen consumption rate showed a temperature insensitivity when the amphipods were acclimated to low temperatures in winter and high temperatures in summer. Significant differences were found in oxygen consumption between individuals acclimated to various medium osmolalities, possibly indicating higher energy requirements for osmotic and ionic regulation at low seawater osmolalities. Oxygen consumption rate was significantly higher in summer than in other seasons. Haemolymph osmolality and the concentration of the inorganic ions sodium and potassium were not influenced by temperature or season. Determination of haemolymph osmolalities and concentrations of inorganic ions revealed that G. oceanicus is a strong hyper-osmotic and hyper-ionic regulator in dilute seawater. The concentration of potassium in the haemolymph is less influenced by seawater osmolality than haemolymph osmolality and the haemolymph concentration of sodium.     

Oxygen consumption of the West African blood clamAnadara senilis

The West African blood clamAnadara senilis (L.) is one of a small number of bivalve species which have haemoglobin in their haemolymph. In the investigation reported here, in which the musselMytilus edulis (L.) was also studied for comparative purposes, it was shown thatA. senilis was an oxygen regulator between oxygen tensions corresponding to 50 and 100% air saturation. Oxygen uptake increased by about 128% after 24 h at zero pO₂; although the haemoglobins ofA. senilis were found to be responsible for about 34% of oxygen uptake at 25°C (a proportion unaffected by pO₂), there was clearly insufficient haemoglobin present in the haemolymph to act as an effective oxygen store during prolonged exposure to anoxic conditions.     

Respiratory dynamics of the starry flounder Platichthys stellatus in response to low oxygen and high temperature

Starry flounder (Platichthys stellatus Pallas) were fitted with masks for measurements of ventilation volume and post-gill oxygen tensions, and with arterial and venous cannulae. They were then subjected to periods of low oxygen levels and high temperature. Pre- and post-gill oxygen tension, blood oxygen tension and content, oxygen uptake, and ventilation volume were measured. Effectiveness of gas exchange across the gills, cardiac output, and transfer factor were calculated. A series of blood-oxygen dissociation curves were also constructed at two pH's and temperatures. The data suggest that flounder are able to regulate oxygen uptake down to an environmental oxygen tension of at least 50 mm Hg by increasing ventilation volume and transfer factor while not changing blood flow rate or pattern. Also, the flounder's response to temperature increase is similar to the expected response to exercise, with the exception of an increase in gill diffusion resistance.     

Effects of environmental factors and body size on rates of oxygen consumption in Archaeomysis grebnitzkii and Neomysis awatschensis (Crustacea: Mysidae)

The rates of oxygen consumption in relation to oxygen tension, temperature, salinity and body size were determined for Archaeomysis grebnitzkii Czerniavsky and Neomysis awatschensis (Brandt). Oxygen uptake was regulated by both species down to an oxygen tension of about 50 mm Hg (30% of saturation value); below this level, it was related to the oxygen tension in the medium. Oxygen tension below 20 mm Hg was lethal. A statistically significant interaction was noted for the effects of species, temperature, and salinity. In general, oxygen-uptake rate decreased with decreasing salinity in both species. The effect of size is expressed as a power function of body weight. The regression coefficients of oxygen consumption on body weight, 0.70 for A. grebnitzkii and 0.62 for N. awatschensis, were found to be significantly different from each other.Contribution No. 726 from the Department of Oceanography, University of Washington, Seattle, USA.     

Comparison between the carbon-14 and oxygen consumption method for the determination of the activity of heterotrophic bacterial populations

The activity of the heterotrophic microbial population in the saline Lake Grevelingen (The Netherlands) and the Mediterranean Etang Salses Leucate (France) was determined by measuring the oxygen consumption rate, and the uptake of ¹⁴C-labelled glycollate, pyruvate and an amino acid mixture. The maximum uptake rate of the applied organic compounds in Lake Grevelingen was generally less than 10% of the carbon mineralization rate calculated from the oxygen consumption experiments. Only for pyruvate and glycollate higher values were found of about 30 to 40% with one exceptionally high value for pyruvate of 149%. However, these higher percentages were found in winter, when the activity of the heterotrophic microbial population was very low. In Etang Salses Leucate higher maximum uptake rates of the ¹⁴C-labelled compounds were found, relating this uptake to the oxygen consumption rate. Yet the maximum uptake rate is still always lower than 35% of the carbon mineralization calculated from the oxygen uptake rate. Taking into account that maximum uptake rates were considered, the results demonstrate that the uptake of ¹⁴C-labelled organic compounds represents a serious underestimation of the activity of the bacterial population in situ. The extent of the underestimation depends on the water type. It was concluded that the determination of the heterotrophic activity by measuring oxygen consumption rates offers a better insight into the carbon mineralization process in natural waters than the uptake experiments with ¹⁴C-labelled substrates.Communication no. 228 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

Differential sensitivities to hypoxia by two anoxia-tolerant marine molluscs: A biochemical analysis

The metabolic responses to a series of low oxygen tensions were compared for two species of Mediterranaean bivalves,Mytilus galloprovincialis andScapharca inaequivalvis. Whereas both species have well-developed and similar tolerances of anoxia, the metabolic responses ofS. inaequivalvis to low oxygen tensions indicate a substantially greater tolerance of hypoxia. Compared withM. galloprovincialis, the responses ofS. inaequivalvis included the ability to maintain a constant oxygen consumption down to a much lower pO₂ value (ca. 1.7 vs 3.4 ppm), and a lower critical pO₂ for the recruitment of fermentative pathways of ATP production (ca. 1 vs 3 ppm). Furthermore, a graded increase in the output of anaerobic products (succinate, alanine) occured at oxygen tensions below 3 ppm inM. galloprovincialis and reached a maximum at 1.6 ppm whereas inS. inaequivalvis the net accumulation of anaerobic products at the lowest oxygen tension tested (0.5 ppm) was still substantially less than the level of production output in complete anoxia. This suggests that fermentative pathways are maximally activated at all oxygen tensions below 1.6 ppm inM. galloprovincialis whereas rates of anaerobic pathways are still less than maximum at 0.5 ppm inS. inaequivalvis. These results indicate that in situations of declining oxygen tensions, such as occur due to eutrophication,M. galloprovincialis would not only begin to experience metabolic stress at higher oxygen tensions thanS. inaequivalvis but would experience greater stress at any given pO₂. Such differences in hypoxia tolerances may explain the success of the recently introducedS. inaequivalvis in out-competing the nativeM. galloprovincialis in the Adriatic Sea.This research was conducted at the Consorzio di Studi, Richerche ed Interventi sulle Risorse Marine, Viale Amerigo Vespucci 2, I-47042 Cesenatico (FO), Italy. Delta Institute for Hydrobiological Research, Yerseke, The Netherlands. Publication no. 554     

Effect of dissolved oxygen level on respiratory metabolism, nutritional physiology, and immune condition of southern king crab Lithodes santolla (Molina, 1782) (Decapoda, Lithodidae)

Episodes of hypoxia are common in the marine environment, and their ecological effects depend, in part, on their severity and duration. Many species of decapod crustaceans reside in areas with fluctuating oxygen regimens. Physiological mechanisms enhance the ability of these crustaceans to cope with acute episodes of hypoxia. Southern king crab, Lithodes santolla, fishery is important in the south of South America, and some data describe fishing zones with low dissolved oxygen (DO) levels (3.5 mgO₂ l⁻¹, i.e., 8.3 kPa). Our main objective was to evaluate the effect of dissolved oxygen level on respiratory metabolism, nutritional physiology, and immunological condition of L. santolla juveniles. Individual animals were exposed for 10 days to different oxygen tensions (2.1, 4.2, 8.5, 12.7, and 21.1 kPa) to quantify the oxygen consumption rate; thereafter, blood oxyhemocyanin (Hc), protein concentration, as well as hemocytes, were sampled. Freeze-dried animals were dissected, and digestive gland metabolites (glycogen, protein, glucose, cholesterol, acylglycerol, and lactate) and digestive enzyme activity (general protease, trypsin, and chymotrypsin), as well as gill lactate dehydrogenase (LDH) activity, were quantified. In the present study, Lithodes santolla showed a critical oxygen tension between 4 and 9 kPa, indicating that this crab species is more sensitive to DO than other crustacean species. Protein and Hc concentrations followed a similar pattern to that of oxygen consumption. Digestive gland glycogen and protein concentration did not change after 10 days at different oxygen exposures, but glucose, cholesterol, and acylglycerol concentrations decreased linearly and proportionally to the available oxygen in the water. As in other decapods, chymotrypsin showed over 90% of the total quantified proteases activity. Chymotrypsin activity together with total proteases and trypsin was not affected by the environmental oxygen tension. Gill LDH and digestive gland lactate followed a similar increase at lower environmental oxygen tension but dropped sharply at the lowest tension (2.1 kPa). Dissolved oxygen affected also the immune system through reduction of hemocytes. This could provide a critical window for opportunistic pathogens to become established when crabs are exposed to hypoxic conditions. L. santolla juveniles show a moderate tolerance to low oxygen availability by modifying the concentration of hemolymph proteins, mainly OxyHc, some digestive gland metabolites, and by activating the anaerobic metabolism. This allows L. santolla juveniles to inhabit temporarily low oxygen zones in the deep ocean and suggests an advantage for culture conditions.     

Detection and avoidance of hypoxic water by juvenile Callinectes sapidus and C. similis

Detection and avoidance of hypoxic water by juvenile blue crabs, Callinectes sapidus Rathbun and C. similis Williams were observed under laboratory conditions. Hypoxia avoidance was quantified within an avoidance chamber using response time (time in minutes before a crab's initial entry into the normoxic side of the avoidance chamber after the introduction of hypoxic water into the chamber), total time (amount of time spent by each crab at the hypoxic end of the chamber during the final 50 min of each 60 min treatment), activity (total distance travelled by a crab during a treatment), percent avoidance and preferred oxygen tension as indices of measurement. The mean preferred oxygen tension for C. sapidus was 112 torr oxygen (range = 98 to 125 torr) and for C. similis was 108 torr oxygen (range = 82 to 121 torr). Both species were able to detect hypoxic water and remain at an optimum oxygen tension. When percent avoidance, response time and total time were used as measures of avoidance, C. similis was found to detect and avoid hypoxic water at 0, 25 and 50 torr oxygen tension but not at any higher levels of hypoxia. C. sapidus, on the other hand, did not initiate any significant avoidance behavior even when the water was completely anoxic. Both species of crabs were more active at the higher oxygen tensions than in the more hypoxic levels. Behavioral responses associated with short-term hypoxic exposure included increased activity with the introduction of hypoxic water into the chamber, frequent movement of the eye-stalk, avoidance of hypoxic water by slowly crawling out of the hypoxic zone, restless and erratic movements, and rapid movements of the crabs' antennae. We conclude that C. similis can better detect and avoid hypoxia than C. sapidus.     

Effect of temperature on the microdistribution of the isopod Sphaeroma rugicauda from a saltmarsh habitat

The saltmarsh isopod Sphaeroma rugicauda (Leach) is subjected to widespread diurnal and seasonal temperature fluctuations under natural conditions. Laboratory studies on its activity show that there is a relationship between behaviour activity and exposure temperature between 2.5° and 25°C. Although S. rugicauda has no complex metabolic adaptations, this isopod is able to maintain a rate of aerial oxygen consumption which is similar to that in water within the temperature range 5° to 25°C. The responses of S. rugicauda to changes in environmental temperature are discussed in relation to the seasonal microdistribution of the isopod in the salt-marsh habitat.     

Oxygen uptake and heat production in a metabolic conformer (Littorina irrorata) and a metabolic regulator (Uca pugnax)

The heat production of Littorina irrorata and Uca pugnax in air was measured with a twin calorimeter while oxygen tension was measured with a pO₂ electrode. Both L. irrorata, an oxyconformer, and U. pugnax, a metabolic regulator, showed a rapid decrease in oxygen uptake (below 1.3 mm Hg in L. irrorata and 13.4 mm Hg in U. pugnax) while heat production decreased more slowly. Consequently, during the period of minimum oxygen uptake, the oxycalorific coefficient increased from about 4.8 for both species to an average value of 8.3 in L. irrorata and 19.9 cal ml^-1 O₂ in U. pugnax, indicating the onset of anaerobic metabolism and accumulation of metabolic and products. Above their respective critical pO₂, the oxycalorific coefficient was the same as the commonly used conversion factor of 4.8 cal ml^-1 O₂. From one time interval to the next, however, the coefficient varied from 3.8 to 5.4 in L. irrorata and from 2.9 to 6.0 in U. pugnax, indicating, that the processes of oxygen consumption and heat production are more or less independent of each other and usually not in phase.     

Effect of oxygen and salt on haemolymph oxygen binding in the brine shrimp Artemia franciscana

The oxygen-binding properties of haemolymph from laboratory-reared Artemia franciscana were investigated in vitro. Adult female brine shrimp without eggs were acclimated to seven different combinations of salinity and oxygen. The oxygen affinity (P₅₀) of unprocessed haemolymph rises with acclimation oxygen partial pressure (PO₂) up to normoxic values, after which no influence of oxygen occurs. The increase in P₅₀ with acclimation PO₂ can be explained by the change in proportion of each of three different haemoglobins in the haemolymph. Salinity acclimation has no effect on haemolymph P₅₀. The effects of the different major salts [NaNO₃, NaCl, Ca(NO₃)₂, Mg(NO₃)₂], pH, and the metal-binding ligand EGTA on the oxygen-binding properties of buffered haemolymph (of shrimp acclimated to 10 salinity) were also studied. Little or no effect of these salts could be found. A small Bohr effect (pH 6.5 to 9.1, =-0.11) was noted. Addition of EGTA caused a significant decrease of the oxygen affinity at concentrations up to 50 mmoll^-1.     

Physiological capacity and environmental tolerance in two sandhopper species with contrasting geographical ranges: Talitrus saltator and Talorchestia ugolinii

We investigated the physiological plasticity and environmental tolerance of two phylogenetically closely related, ecologically similar and co-occurring species of supralittoral amphipods differing drastically in the size of their geographical ranges. A series of physiological traits were characterised for the Corsican-endemic Talorchestia ugolinii Bella-Santini and Ruffo and the widespread Talitrus saltator Montagu. The effect of body mass, temperature and salinity on heart rate (used as proxy for metabolic activity and stress), the effect of temperature on oxygen consumption and the tolerance to salinity exposure were investigated in both species, together with the characterisation of haemolymph osmoregulation in T. ugolinii. Our results showed that there is a clear difference in the resting metabolic rates and physiological capacity, as well as environmental tolerance, between T. saltator and T. ugolinii, with T. saltator overall showing a broader physiological niche. Although T. ugolinii showed a relatively good ability to regulate its haemolymph osmotic concentration (similar to that previously described for T. saltator), it demonstrated a lower tolerance to exposure to hypo-osmotic stress. In addition, a consistent picture emerged between the ability to control the cardiac function and the capacity to actively respond to osmotic stress. The physiological findings are discussed in relation to the known ecology and geographical distribution of T. ugolinii.     

How does burrowing by the isopod <Emphasis Type="Italic">Limnoria agrostisa</Emphasis> (Crustacea: Limnoriidae) affect the leaf canopy of the southern Australian seagrass <Emphasis Type="Italic">Amphibolis griffithii</Emphasis>?

In south-western Australia, the isopod Limnoria agrostisa commonly burrows into leaf clusters and immature shoots of Amphibolis griffithii. The isopod also burrows into the sheath and rhizomes of Posidonia species. In A. griffithii, the isopod consumes new tissue within the sheath, damaging or destroying the meristem. This results in malformation of new leaves or destruction of whole leaf clusters with the potential to reduce the photosynthetic area of a shoot. The isopod has been found in all but one meadow of A. griffithii examined over 1,000 km of the Western Australian coastline. It was present throughout the year and showed little variation in abundance. Young were produced year round, but were more abundant in the summer months. Females, 3.5 mm in length or larger, produced 2–5 young that were brooded within the leaf cluster or base of an immature shoot. Within a meadow, 40–70% of shoots and 10–20% of leaf clusters were damaged by isopods. Seasonal trends were not consistent, but damage appeared to be higher in summer when isopod abundance was higher. Approximately 40% of clusters were destroyed by isopod damage. Isopods attack shoots of all ages, but damage was often located on apical clusters. There was no evidence that isopod damage initiated branching or leaf cluster formation. Estimations of clusters damaged or destroyed may be conservative, as only those clusters remaining on a shoot could be counted, and cluster loss could not be quantified. Examination of defoliated stems on upright shoots and horizontal rhizomes indicated that many were destroyed by isopods. The effect of L. agrostisa although substantial appears to be a feature of healthy seagrasses throughout southern Australia.     

Nitrogen uptake kinetics in three year-classes of Laminaria groenlandica (Laminariales: Phaeophyta)

Nitrate and ammonium uptake rates were measured for three year-classes of the perennial macrophyte Laminaria groenlandica Rosenvinge, collected from nitrogen-depleted waters in Barkley Sound, British Columbia, Canada, in summer 1981. A time course of uptake rate revealed that ammonium uptake was high during the first hour and then decreased for all three year-classes; the opposite pattern was exhibited for the time course of nitrate uptake rate. Nitrate uptake rate increased linearly with nitrate concentration up to the highest level tested (60 M). The nitrate uptake rate of first-year plants was three times higher than second- and third-year plants; ammonium uptake rates showed similar patterns to those for nitrate. The interaction between nitrate and ammonium was examined for first-year plants. Nitrate and ammonium were taken up simultaneously and uptake rates were identical and equal to uptake rates when only nitrate or ammonium was present in the medium. Therefore, first-year plants are able to take up twice as much inorganic nitrogen per unit time when both nitrate and ammonium are present. First-year plants showed significant diel periodicity in ammonium uptake rates, whereas second- and third-year plants showed no periodicity in nitrate or ammonium uptake rates.     

Oxygen bioavailability and haemoglobins in the brine shrimp Artemia franciscana

Bioavailability of oxygen for the saline-water invertebrate Artemia franciscana was studied, since both oxygen concentration and oxygen diffusion rate change with salinity. Total haemoglobin concentration and the relative contribution of each of three haemoglobins was measured in specimens acclimated to different salinities and oxygen concentrations. Both haemoglobin concentration and contribution were influenced by salinity and the group observed (males, females with and females without eggs). Multiple regression analysis showed that total haemoglobin concentration was better correlated with oxygen concentration than oxygen partial pressure. The relative proportions of haemoglobins 1, 2 and 3 were better correlated with oxygen partial pressure than oxygen concentration. These results are related to the oxygen carrying capacity and oxygen affinity of the haemolymph. Our results show that oxygen bioavailability in a saline-water environment depends on the response (relative contribution of Hb1, Hb2 and Hb3 or total haemoglobin concentration) that is studied. They also show that oxygen concentration and oxygen partial pressure have a different physiological impact on brine shrimp.     

Flavin-containing monooxygenase activity in the gumboot chitonCryptochiton stelleri

Flavin-containing monooxygenase (FMO) activity was observed in microsomes from the digestive gland of the gumboot chitonCryptochiton stelleri. Digestive-gland microsomal oxygen uptake was initiated after the addition of N,N-dimethylaniline (DMA), methimazole, phenylhydrazine (PH), 2-aminofluorene (2-AF), or 1,1-dimethylhydrazine (DMH) to the incubations. The pH optimum for 2-AF dependent oxygen uptake was 8.4 and required NADPH. The formation of DMA N-oxide and the oxidation of methimazole was measured and confirmed the presence of FMO activity in this tissue.     

Wave action: the environmental trigger for hatching in the California grunion Leuresthes tenuis (Teleostei: Atherinopsidae)

 California grunion Leuresthes tenuis (Teleostei: Atherinopsidae) emerge from the ocean to spawn on beaches of southern California. Grunion eggs do not hatch at a set developmental age, but remain in the sand until turbulent surf at high tide washes them out to sea. In previously studied fishes and amphibians that lay eggs terrestrially, low oxygen is the trigger for hatching in water, and high oxygen tensions inhibit hatching. For the grunion, however, eggs placed in air, seawater and oil did not hatch at any at oxygen tension until they were agitated in fluid. Following agitation in seawater, all eggs hatched within several minutes. Grunion eggs in normoxic or hyperoxic water hatched significantly faster than eggs agitated at the lowest oxygen tensions. Mechanical agitation, not hypoxia, is the environmental trigger for hatching in California grunion eggs. Received: 15 May 1999 / Accepted: 5 April 2000     

Detrimental effects of the isopod, <Emphasis Type="Italic">Edotia doellojuradoi,</Emphasis> on gill morphology and host condition of the mussel, <Emphasis Type="Italic">Mytilus edulis platensis</Emphasis>

There are many reported associations between mussels and other invertebrates, such as pea crabs, polychaetes, turbellarians and copepods, which live in their mantle cavities. The boundary between commensalism and parasitism is often indistinguishable because of insufficient knowledge or because the interaction is variable. Preliminary evidence led to a closer examination of the relationship between the mussel, Mytilus edulis platensis, and an isopod, Edotia doellojuradoi, previously described as commensalism. Monthly intertidal samples of mussels were taken from September 2004 to August 2005 at Caleta Cordova Norte (45°43′S, 67°22′W) in southern Argentina and assessed for the prevalence and abundance levels of isopods. Mussels with and without isopods were measured, examined for evidence of gill damage and their condition (soft tissue dry weight) was determined. The overall isopod prevalence in mussels was 57.9% and infestation varied with mussel length, with maximum occurence at 30.2 mm (medium-sized mussels). Experimental evidence indicated that the position of the isopod inside the mussel depended on the feeding activity of the mussel. Female isopods were observed grasping the ventral food groove of the gill demibranchs and feeding on the mucous food strand produced by the mussel. Juveniles and males were observed clustered together on the dorsal side of the single female in each occupied mussel, suggesting extended maternal care. Gill damage was observed in 58.2% of mussels at the Argentine site and was significantly associated with isopod occurrence. Categorical regression analysis showed that the most important factor associated with the degree of gill lesions was the number of male and juvenile isopods per mussel, followed by the length of female isopods and the developmental stage of juveniles. Conversely, the degree of gill damage decreased with increasing mussel length. Overall, E. doellojuradoi had a significant effect on mussel condition throughout the year, with low flesh weight in mussels with isopods, except during the austral summer and early autumn. In contrast to previous studies, which concluded that the isopod was a commensal, the present study clearly demonstrates that E. doellojuradoi is a parasite of M. edulis platensis. Other symbiotic interactions formerly classified as commensal might not be innocuous on further investigation, especially if samples are taken at multiple sites and at different times of the year.     

Does oxygen deficiency modify the functional response of Saduria entomon (Isopoda) to Bathyporeia pilosa (Amphipoda)?

The functional response of the predatory isopod Saduria entomon to the prey amphipod Bathyporeia pilosa was measured in normoxia (95% O₂ saturation), moderate hypoxia (45% O₂ saturation) and hypoxia (35% O₂ saturation) in aquarium experiments. The prey densities tested ranged from 400 to 8000 ind m⁻². Prey density influenced consumption rates of S. entomon in normoxia and 45% O₂ saturation, but there was no difference between consumption rates at these two oxygen levels. Nevertheless the form of functional response differed. In normoxia S. entomon showed a positively density-dependent functional response to B. pilosa, indicating a potentially stabilizing effect on the prey population. In moderate hypoxia the variance in consumption increased, decreasing the statistical power to distinguish between response models. The functional response of S. entomon in moderate hypoxia was best described with a density-independent response, characterized as destabilizing for the prey population. In hypoxia (35% O₂) predation by S. entomon did not respond to increasing prey density, as almost no amphipods were eaten at this oxygen level. The results are discussed in terms of the usability of theoretical models to examine predator–prey relationships in stressful environments. Received: 26 April 1997 / Accepted: 20 May 1997     

Some aspects of nitrogen metabolism in Mytilus edulis: effects of aerial exposure

The effects of starvation and emersion on the ammonia effluxes of Mytilus edulis L. were studied. Both fed and starved groups showed similar patterns of efflux during re-immersion, indicating no compensation for starvation during anaerobic catabolism; this is taken as evidence that carbohydrate or lipid is the predominant energy source at the outset of anaerobiosis. Instead of an overshoot of ammonia excreted during the 4 h following re-immersion, all groups showed significantly reduced efflux rates independent of emersion duration. Such results suggest some conversion of ammonia during the recovery period, as no significant drop in haemolymph ammonia occurred at this time. The level of ammonia accumulation in haemolymph and mantle cavity fluid decreased exponentially with emersion duration, implying the stabilisation, at a low rate, of ammonia production after a relatively long period of emersion (16 to 24 h). Oxygen levels of the mantle cavity fluid dropped rapidly during emersion but never attained anoxia over 24 h of emersion, which may indicate some oxygen uptake by emersed mussels. It has been concluded that the behaviour and physiological responses shown by M. edulis to emersion appear to reflect a need for respiratory gas exchange more than a response to desiccation – presumably in the interest of energy-conservation. The reductions in ammonia efflux rate during emersion and, to a certain extent, immediately after re-immersion have also been considered here to be energy-saving strategies that illustrate the importance of ammonia in restoring normal levels of some amino acids following re-immersion. Received: 26 August 1997 / Accepted: 27 May 1999