Negative Scope For Growth Research Articles

Physiological response to seawater pH of the bivalve Abra alba, a benthic ecosystem engineer, is modulated by low pH

The presence and behaviour of bivalves can affect the functioning of seafloor sediments through the irrigation of deeper strata by feeding and respiring through siphonal channels. Here, we investigated the physiological response and consecutive impact on functioning and body condition of the white furrow shell Abra alba in three pH treatments (pH = 8.2, pH = 7.9 and pH = 7.7). Although no pH effect on survival was found, lowered respiration and calcification rates, decreased energy intake (lower absorption rate) and increased metabolic losses (increased excretion rates) occurred at pH ∼ 7.7. These physiological responses resulted in a negative Scope for Growth and a decreased condition index at this pH. This suggests that the physiological changes may not be sufficient to sustain survival in the long term, which would undoubtedly translate into consequences for ecosystem functioning.

Modeling Atlantic sea scallop (Placopecten magellanicus) scope for growth on the Northeast U.S. Shelf

AbstractAtlantic sea scallops support one of the most valuable fisheries in the eastern United States. The scallop population is susceptible to climate‐related environmental stress. Assessing and projecting climate impacts rely on the fundamental understanding of scallop ecophysiology, including the influences of temperature and food supply on its energy balance and growth potential. In this study, we developed a scope for growth (SFG) model driven by high‐resolution hydrodynamic and biological models to assess the spatial and seasonal variability of scallop energy dynamics. The overall SFG on the Northeast U.S. Shelf is higher in May–June and lower in January–February, with substantial spatial heterogeneity. In the Mid‐Atlantic Bight (MAB), negative SFG occurs from July to October due to strong thermal stress. Particulate organic matter in detrital form is an important food source for scallops, with higher/lower contribution in the cold/warm seasons, respectively. Warming and food deficiency induce a noticeable contraction of suitable scallop habitats in the MAB, while their impacts on Georges Bank are insignificant. Known seasonal spawning patterns and observed growth rates in these regions match the patterns of SFG generated by the model. The sensitivity of SFG to the variations in food and temperature increases with scallop size. Large scallops are more likely to experience low or negative SFGs than smaller ones, implying that the habitats shrink as scallops grow older/bigger. This study provides key information about scallop growth potential and biogeography from the perspective of energy balance, thus helping the development of adaptive fisheries management strategies.

Impact of elevated temperature on physiological energetics of Penaeus monodon post larvae: A mesocosm study.

Global climate change induced warming has profound repercussion on physiological performances of marine animals. The Indian Sundarban is one of the best nursery grounds for various shrimp populations which need estuarine realm to complete their bipartite life cycle. From last couple of decades, a clear indication of temperature escalation has been identified in Sundarban. In the present study, we have assessed the physiological energetics of Penaeus monodon post larvae from Indian Sundarban under different temperatures, 28°C (annual average habitat temperature) and elevated temperatures 30°C, 32°C, 34°C in a month long mesocosm experiment. Significant alterations were observed in growth performances as well as physiological energetics. The length, weight, survival percentages, ingestion rates were reduced, howbeit respiration and ammonia excretion rate had been increased in elevated temperature treatments culminating in a negative Scope for Growth. PERMANOVA results showed a significant (p≤0.05) variation in different physiological performances of shrimp post larvae both in different temperature treatments and days of exposure. The present results clearly highlighted the detrimental effect of elevated temperature on physiological energetics of shrimp larvae that might potentially reduce shrimp population and affect the coastal fishery.

Contrasting responsiveness of four ecologically and economically important bivalves to simulated heat waves

Heat waves are expected to increase in duration and frequency, impacting coastal ecosystems, especially intertidal organisms living near their thermal tolerance limits. Sedentary infaunal species are limited to some extent in escapes from sudden temperature changes, rather modifications to their physiology and behaviour are expected. This may lead to strong ecological and economic impacts on commercial bivalve species, such as Venerupis corrugata, Ruditapes decussatus, the introduced Ruditapes philippinarum and Cerastoderma edule, the most relevant in NW Spain.We investigated lethal and sublethal effects of heat during low tide on these species in the laboratory. Summer temperatures experienced within field, shallow sediments at approximately 2 cm depth i.e. 20 °C (control), 27 °C, 32 °C, and 37 °C, were replicated during four consecutive days and the diffusion of heat at the burrowing depth of each species was estimated; temperature exposure was expressed as degree hours above 22 °C. After two days of tidal exposure, C. edule and V. corrugata suffered significant mortalities, and also the most dramatic decrease in scope for growth (SFG) as well as reduction in burrowing activity. After four days under stress, all species had negative SFG. On recovery, species showed compensation at longer exposures, particularly C. edule. These effects of temperature on mortality, growth potential and burrowing ability may increase the time to achieve commercial size and exposure to predation. Particularly, V. corrugata, with a center of distribution lower in the intertidal and subtidal, and C. edule, shallower in the sediment, may be the most affected. Clearly the intensity and frequency of heat waves will affect these key species in the intertidal sediment flats changing ecosystem functioning and fisheries management strategies.

Unravelling the effects of elevated temperature on the physiological energetics of Bellamya bengalensis.

Temperature is one of the key environmental factors affecting the eco-physiological responses of living organisms and is considered one of the utmost crucial factors in shaping the fundamental niche of a species. The purpose of the present study is to delineate the physiological response and changes in energy allocation strategy of Bellamya bengalensis, a freshwater gastropod in the anticipated summer elevated temperature in the future by measuring the growth, body conditions (change in total weight, change in organ to flesh weight ratio), physiological energetics (ingestion rate, absorption rate, respiration rate, excretion rate and Scope for Growth) and thermal performance, Arrhenius breakpoint temperature (ABT), thermal critical maxima (CTmax), warming tolerance (WT) as well as thermal safety margin (TSM) through a mesocosm experiment. We exposed the animals to three different temperatures, 25°C (average habitat temperature for this animal) and elevated temperatures 30°C, 35°C for 30 days and changes in energy budget were measured twice (on 15th and 30th day). Significant changes were observed in body conditions as well as physiological energetics. The total body weight as well as the organ/flesh weight ratio, ingestion followed by absorption rate decreased whereas, respiration and excretion rate increased with elevated temperature treatments resulting in a negative Scope for Growth in adverse conditions. Though no profound impact was found on ABT/CTmax, the peak of thermal curve was considerably declined for animals that were reared in higher temperature treatments. Our data reflects that thermal stress greatly impact the physiological functioning and growth patterns of B. bengalensis which might jeopardize the freshwater ecosystem functioning in future climate change scenario.

Heatwaves\xa0diminish the survival of a subtidal gastropod through reduction in energy budget and depletion of energy reserves

Extreme climatic events, such as heatwaves, are predicted to be more prevalent in future due to global climate change. The devastating impacts of heatwaves on the survival of marine organisms may be further intensified by ocean acidification. Here, we tested the hypothesis that prolonged exposure to heatwave temperatures (24 °C, +3 °C summer seawater temperature) would diminish energy budget, body condition and ultimately survival of a subtidal gastropod (Thalotia conica) by pushing close to its critical thermal maximum (CTmax). We also tested whether ocean acidification (pCO2: 1000 ppm) affects energy budget, CTmax and hence survival of this gastropod. Following the 8-week experimental period, mortality was markedly higher at 24 °C irrespective of pCO2 level, probably attributed to energy deficit (negative scope for growth) and concomitant depletion of energy reserves (reduced organ weight to flesh weight ratio). CTmax of T. conica appeared at 27 °C and was unaffected by ocean acidification. Our findings imply that prolonged exposure to heatwaves can compromise the survival of marine organisms below CTmax via disruption in energy homeostasis, which possibly explains their mass mortality in the past heatwave events. Therefore, heatwaves would have more profound effects than ocean acidification on future marine ecosystems.

Scope for growth of juvenile Cortez oyster, Crassostrea corteziensis fed isocaloric diets prepared with microalgae and cereal flours

We evaluated the effect of nine isocaloric diets prepared with a mixture of microalgae alone ( Tisochrysis lutea + Chaetoceros calcitrans + Ch. gracilis ; 1:1:1) and different combinations of the algal mixture and cornstarch or wheat flour, on the Scope for Growth (SFG) of Cortez oyster Crassostrea corteziensis juveniles (7.2 ± 1.1 mm shell length). A total 15,000 spat was acclimated to the experimental diets for 30 days at 21oC, prior SFG determinations. Consumed energy, absorbed energy, and SFG decreased with a corresponding decrease in the proportion of live microalgae in the diet. Oysters fed the microalgal diet showed significantly increased physiological activity in all parameters except absorption efficiency. Oysters fed diets containing up to 50% dry feedstuff showed positive SFG values but these were significantly lower than SFG shown in oysters fed microalgae alone. Oysters fed diets containing ≥75% dry feedstuff showed negative SFG. These results suggest that cereals can be used as complementary food or C. corteziensis spat in hatchery operations, provided replacement of microalgae is kept below 50%.

Effects of temperature and body size on the physiological energetics of the stalked sea squirt Styela clava

An assessment of the patterns of thermal acclimation of physiological processes of the stalked sea squirt Styela clava is needed to further understand growth patterns associated with large seasonal fluctuations in ambient temperature. The effects of different exposure temperatures on physiological processes of S. clava were measured at 5, 10, 15, 20, and 25°C. The impact of temperature on energy balance at a whole organism level was then assessed by scope for growth (SFG) and net growth efficiency (K2) measures. Physiological rates of S. clava were well correlated with its dry tissue weight (DW). Positive allometry with DW indicated that all physiological rates investigated were generally higher in the larger individuals regardless of temperature. Some allometric equations (e.g., feces production rate vs. DW and respiration rate vs. DW) exhibited identical estimates of exponents, enabling accurate comparisons of the rates across temperatures. In contrast, disparities across temperatures of the values for the weight exponent were detected in other allometric equations, revealing that thermal effects on these rates have different degrees for large and small individuals. The SFG value in S. clava of different sizes was similar at lower temperatures (5–10°C), peaking at 15°C. The SFG was positive with a relatively constant K2 in this temperature range. This positive energy balance reflects the reduction in metabolic costs at lower temperatures. In contrast, increased metabolic costs at higher temperatures (20–25°C) resulted in lowered SFG and K2 values, indicating that the feeding rate did not increase as greatly in compensation for the increased metabolic costs at higher temperatures. The lowered absorption at 25°C (compared with 20°C) resulted in negative SFG and K2 values. Finally, our results confirm that the absence of compensatory adjustment to warmer conditions leads to energetic disruption at a whole organism level at such temperatures.

Effects of seasonal variations in phytoplankton on the bioenergetic responses of mussels ( Mytilus galloprovincialis ) held on a raft in the proximity of red sea bream ( Pagellus bogaraveo ) net-pens

Abstract The seasonal variability of the physiological components of the Scope for Growth (SFG) of mussels Mytilus galloprovincialis was investigated in a raft adjacent (170 m) to fish net-pens and compared with a raft 550 m distant from the cages in Ria Ares-Betanzos (Galicia, Spain). Chlorophyll and phytoplankton size-classes were determined in the field, simultaneously with SFG. Average chlorophyll- a was 0.65 ± 0.24 μg l − 1 , while nanophytoplankton (2–20 μm) was the most abundant size-class, ranging from 50 to 70% of the total chlorophyll. The temporal pattern found for chlorophyll- a and phytoplankton size-classes reflected the upwelling–downwelling events and were correlated with the feeding, digestive and metabolic rates. Nanophytoplankton and microphytoplankton were preferentially cleared and ingested by mussels. There were no significant differences between the chlorophyll and phytoplankton size-classes among rafts. The lack of any enhancement in food availability resulted in no significant increase in the SFG of mussels beside the fish cages. Maximum SFG corresponded with the autumn (16.60 ± 7.90 J h − 1 ) and spring (12.72 ± 9.32 J h − 1 ) chlorophyll maximums. An abnormally hot summer and reduced chlorophyll levels resulted in lower energy intake, significantly higher metabolic expenditure and a negative SFG (− 34.57 ± 12.55 J h − 1 ). Any particulate wastes and potential fish-derived chlorophyll enhancement would be rapidly diluted by the currents, while the placement of bivalves too distant from the fish farm in an environment with high supplies of natural seston may explain the lack of an augmented SFG of the co-cultured mussels.

Seasonal Comparison of Physiological Adaptation and Growth of Suminoe (Crassostrea ariakensis) and Eastern (Crassostrea virginica) Oysters

Shell growth, survival, and physiology were compared between diploid Suminoe (Crassostrea ariakensis) and eastern oysters (Crassostrea virginica) under conditions simulating a U.S. subtropical estuary. Two age groups (4 mo and 28 mo) of both oyster species were grown for 9 mo (December 2006 through August 2007) in quarantine mesocosms (700 L) supplied with ambient flowing (≥ 10 L/min) water (annual temperature range, 18.6–30.4°C; salinity, 28–37.7). There was no difference in overall rates of shell growth between the 2 oyster species over the 9-mo period. Specific growth rates for C. ariakensis did not differ over time, but they did for C. virginica. The growth rate of C. virginica was slowest in the winter (8.9 × 10-4 mm2/day) and fastest in the spring (43.5 × 10-4 mm2/day). Mortality of both species rose abruptly in April 2007, and all (100%) remaining C. ariakensis were dead by the end of the study. Although 28% of the remaining C. virginica died in April 2007, there was little further mortality in this species before the study was terminated in August 2007. Physiological responses of both species of oysters were compared under seasonal temperate euhaline quarantine conditions to understand better how temperature affects these species without the confounding unexplained mortality encountered in the subtropical mesocosms. The clearance rate of C. ariakensis (1.2 L/g/h) was half that of C. virginica (2.2 L/g/h) during the summer (25°C); however, respiration rates for C. ariakensis (2.6 ml O2/g/h) and C. virginica (2.5 ml O2 g/h) were similar. The low clearance rate of C. ariakensis during the summer resulted in a negative scope for growth (-36.2 J/g/h) during this season. During the winter, C. ariakensis remained physiologically active when water temperatures were as low as 2°C; C. virginica were quiescent during this time. We conclude that the “Oregon” strain of C. ariakensis tested will not thrive in the warm subtropical waters of the U.S. southeastern coast; however, given its native range in Asia, we do not discount the possibility of an adaptation to warmer temperatures over time.

Physiological energetics and gross biochemical composition of the ascidian Styela clava cultured in suspension in a temperate bay of Korea

The ascidian Styela clava has recently become a common species for suspended aquaculture in Korea. Because of the ecological and commercial importance of this species, it is important to understand seasonal variations in its physiological energetics and gross biochemical composition. The purpose of this study was to determine fundamental biological traits for the cultivation of S. clava. Physiological processes (food consumption, feces production, ammonia excretion and respiration), and gross biochemical composition (protein, lipids, and carbohydrates) of the ascidian were measured monthly from April 2008 to April 2009 under in situ environmental conditions in a mariculture region on the temperate coast of Jindong Bay, Korea. Changes in the physiology and somatic growth of S. clava were largely influenced by seasonal variation in water temperatures. The reduced importance of food availability in explaining their physiological adjustments seems to be due to low seston concentrations in water column of the bay. Seasonal variations in the ingestion and respiration rates of the ascidians were mismatched, resulting in an energy imbalance (i.e. an increased metabolic energy cost and lowered ingestion rate; and vice versa). This mismatched activity resulted in negative scope for growth (SFG) values during spring–summer, followed by rapid exhaustion of energy reserves and flesh weight loss. Weight loss during this period was also related to spring spawning. During autumn–winter, the ascidians had a positive SFG as a result of decreased oxygen consumption and elevated ingestion rates, showing accumulation of nutrient reserves and weight gain. The ascidians showed positive SFG, even in cold conditions below 12 °C in the present study. In this respect, an autumn–winter culturing period is recommended to maximize ascidian production in long-line suspended culture under the natural environmental conditions occurring in the study area.

Physiological responses of two sublittoral nassariid gastropods to hypoxia

Anthropogenic hypoxia of coastal bottom waters now affects hundreds of thousands of km 2 worldwide. The present study investigated the physiological responses of 2 sublittoral nassariid gastropods, Nassarius conoidalis and N. siquijorensis, to hypoxia using endpoints, including scope for growth (SfG) and the related energy budget items (i.e. rate of energy intake from food, rate of energy lost to respiration and rate of energy lost to excretion) over a 31-d laboratory experiment. Our results showed that after exposure for ≥8 d, the stronger hypoxia treatment of 1.5 mg O2 l -1 significantly reduced the rate of energy intake for Nassarius siquijorensis, while N. conoidalis stopped feeding in the same treatment. SfG was sig nificantly reduced in N. siquijorensis after exposure to 1.5 mg O2 l -1 during the mid and late exposure period. Exposure to ≤3 mg O2 l -1 also resulted in a negative SfG for N. conoidalis, except for the weaker hypoxia treatment during the late exposure period. Nassariid gastropods occur in great abundance in Hong Kong waters; therefore, any adverse effect on these gastropods may lead to major ecological consequences, including altered trophodynamics and disrupted nutrient recycling processes in coastal ecosystems.

진주조개 Pinctada fucata martensii의 실내월동을 위한 저온노출에 따른 생리적 변화

With the aim of developing and indoor overwintering technique for Pearl oyster, Pinctada fucata martensii, the metabolic rates of young oysters (52.4-83.0 mm in shell length) were measured for 2 weeks at water temperatures of 8, 10, 12, and . The filtration rate (FR) ranged 0 to (mean, to ), with significant changes observed over thme except for the case of a water temperature of . Respiration rate (R) ranged from 0 to (mean, 0 to ), with significant respiratory disorders observed at temperatures below ; in contrast, the rate increased on the day of the experiment in the case of a temperature of 14. No significant difference was observed among the different water temperatures in terms of excretion rate (E) or absorption efficiency (Abs.eff), except for a significant decrease in aerobic metabolism in the case of water temperature of . The estimated scope for growth (SFG) ranged from -9.1 to (mean. to ). A significant energy Joss was found at , with negative SFG observed throughout the experiment and a gradual energy decrease observed over time at water temperatures of and 120C. However. SFG remained positive throughout the experiment in the case of . The estimated minimum energy requirement, assessed from energy expenditure, is (mean, ). In conclusion, the lowest temperature suitable for indoor overwintering is above .

Variation in scope for growth: a test of food limitation among intertidal mussels

Wellington Harbour supports large populations of the mussels Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus that are almost entirely absent from nearby coastal locations in Cook Strait. We calculated scope for growth (SFG) using ambient Cook Strait water over a broad temporal scale and a broad range of seston conditions to determine if negative SFG explains this phenomenon. Although all three mussel species had positive mean SFG values, variation in SFG was high and negative values often occurred: A. maoriana 19.1 J g−1 h−1, 43% of mussels showed negative SFG; M. galloprovincialis 1.26, 52% negative SFG; P. canaliculus 45.6, 27% negative SFG. Negative SFG was most often due to negative absorption efficiency caused by metabolic faecal loss that is characteristic of mussels feeding in environments with low seston quality. From our ecophysiology data we constructed a model to estimate SFG based on physiological responses to the narrow range of seston conditions typical of Cook Strait (Model One), and a model to estimate SFG based on physiological responses of mussels to the broad range of seston conditions typical of Wellington Harbour and Cook Strait (Model Two). We used seston data collected over an 18-month period from sites in Wellington Harbour and Cook Strait to derive 159 estimates of species-specific mussel SFG from both models. Both models produced higher estimates of SFG for mussels in the Harbour compared with those at Cook Strait sites. This was consistent with elevated particulate concentrations in the Harbour than at Cook Strait sites, and in agreement with previous studies. For Cook Strait mussels, both models produced negative estimates of net energy balance for long periods of time (several months), whereas for Harbour mussels negative SFG estimates were generally short in duration. We conclude that our short-term laboratory-based determinations of SFG and our long-term bioenergetics modelling estimates do not conclusively support the hypothesis of food limitation for three coexisting taxa of mussels in the intertidal region of Cook Strait, New Zealand.

Energetic balance of infaunal ( Mulinia edulis King, 1831) and epifaunal ( Mytilus chilensis Hupé, 1854) bivalves in response to wide variations in concentration and quality of seston

Mulinia edulis and Mytilus chilensis are commercially important filter feeding bivalves, which occur in intertidal benthic habitats of southern Chile. These environments are subject to large fluctuations in quantity and quality of suspended particulate matter (seston), due mainly to the resuspension of benthic sediments by the action of winds and tides. In spite of variations in the availability of suspended food, these species demonstrate rapid growth rates during most of the year. Laboratory tests feeding were carried out with different artificially prepared diets in order to understand the physiological compensations effected by these bivalves in response to fluctuations of suspended particulate matter. A total of 48 different combinations of microalgae ( Isochrysis galbana) and natural sediment collected from the intertidal zone were tested, having concentrations ranging from 1.9 to 203 mg l −1, and containing 18–91% organic matter. Absorption, oxygen consumption and excretion were measured in both species for each experimental diet in order to calculate the physiological index scope for growth. Temperature was maintained constant at 14 °C and salinity at 30‰. Both species demonstrated a high degree of physiological plasticity, which allowed them to maintain a positive scope for growth under most dietary conditions. The epifaunal species M. chilensis was better adapted to an environment having intermediate quantities of seston (50–150 mg l −1 and 40–60% organic matter); its scope for growth was high due to its high rate of energy absorption. The infaunal M. edulis was not well adapted with very low concentrations and quality of food (<3 mg l −1 and <25% organic material), and had a negative scope for growth under these conditions due to a low absorption rate and high energy losses through oxygen consumption and excretion. However, this species was well adapted to high concentrations and/or high quality of seston (>100 mg l −1 and 60% organic matter) where it showed a higher scope for growth due to an increase in absorption rate. M. chilensis showed better scope for growth in comparison with M. edulis, explainable by its greater feeding capacity, suggesting that the epifaunal species has an adaptive advantage through being able to better compete for available food, especially when there are low concentrations of seston in the water.

Relationship between bioenergetics responses and organic pollutants in the giant mussel, Choromytilus chorus (Mollusca: Mytilidae)

Samples of Choromytilus chorus (giant mussel) were collected at three sampling stations exposed to different degrees of pollution along the south–central portion of the Chilean coast in spring 1998 and summer 1999. Measurements were carried out on clearance rate, absorption efficiency, and oxygen consumption of the mussels under controlled laboratory conditions, and related to analytical data on organic pollutants in their tissues. Scope for growth (SFG) was employed as a physiological index to evaluate stress produced by pollutants existing at each sampling site. Individuals from San Vicente bay (highly polluted) showed negative SFG values in spring (−4.6 J/h per g) and summer (−3.5 J/h per g). These results indicated severe stress related to the accumulation of toxic compounds in their tissues. Specimens from Corral bay (medium level of pollution) gave a SFG of 15.5 J/h per g in spring and 6.5 J/h per g in summer, while those from Yaldad bay (low pollution) presented an inverse situation was observed with SFG values of 6.2 J/h per g in spring which was lower than the summer value of 25.7 J/h per g. There was a significant negative correlation between the SFG of the different populations of C. chorus and the concentrations of organochlorines (OChs) and polynuclear aromatic hydrocarbons (PAHs) in their tissues.

Naturally low seston concentration and the net energy balance of the greenshell mussel (Perna canaliculus) at Island Bay, Cook Strait, New Zealand

This paper describes an investigation of the physiological energetics of the New Zealand greenshell mussel, Perna canaliculus (Gmelin, 1791), which is aimed at determining if mussels are absent from many shores of Cook Strait as a consequence of a negative energy balance resulting from ambient low seston quantity and quality. Seston characteristics and mussel physiological functions were measured under ambient summer conditions at Island Bay, a site in Cook Strait. Estimates of total particulate matter (TPM in mg litre−1), particulate organic matter (POM in mg litre−1), and particulate organic carbon (POC in |ig litre−1) were low but of a similar magnitude to values reported for many comparable temperate regions. However, seston % organic matter (% OM) values were consistently low (<25%) and resulted in negative net absorption efficiency (AE) values and concomitant negative scope for growth (SFG = net energy balance) values. We suggest that the ongoing costs of energy loss associated with extra‐and intracellular digestion (termed metabolic faecal loss) are greater than the energy derived from seston of such low organic matter. Our data indicate that ambient levels of seston % OM of ≤25% are insufficient to promote a positive net energy balance in P. canaliculus, and as such probably play a major role in explaining the almost complete absence of mussels from many Cook Strait shores.

Physiological responses of the Chilean scallop Argopecten purpuratus to decreasing salinities

Argopecten purpuratus (Chilean scallop) is a filter-feeding bivalve which inhabits sheltered areas of the north and central Chile. Considering that culture of this species has started in the south of Chile, it is of great interest to know the tolerance of this bivalve to conditions of decreasing salinity, which can occur in these locations. For this purpose, different physiological processes related to the acquisition and utilisation of energy (clearance rate, absorption, oxygen uptake and excretion) were measured at different salinities (30, 27, 24, 21 and 18‰) on a wide range of body size after the scallops had been acclimatised to the experimental salinities for a week at 12°C. Clearance rate showed higher and similar values at 30 and 27‰, decreasing significantly at the lower salinities. Oxygen uptake increased with decreasing salinity from 30 to 24‰, showing the lowest value at the extreme condition of 18‰. A similar pattern was presented by the excretion rate, which also increased within the range 30-24‰, to show a reduction with decreasing salinities. The O/N ratio also decreased with reduction in salinity in the 5 and 10 g size classes. A negative relationship was observed between the size of A. purpuratus and the O/N ratio. Scope for growth was highly affected by low salinities, with positive values only between 27 and 30‰. Negative scope for growth was observed at all the other experimental salinities. The data obtained suggest that the selection of sites to cultivate this species must take into consideration the tolerance of this species to the salinity, and positive growth rates can be expected at salinities over 27‰.

Bioenergetic and genetic parameters in relation to susceptibility of blue mussels, Mytilus edulis (L.) to summer mortality

Our study examined whether the differences in susceptibility to summer mass mortality of two stocks of mussels from the Magdalen Islands (Québec, Canada) are related to bioenergetic and/or genetic factors. The relative importance of maintenance and maximal metabolic rates, scope for growth (SFG) and the O:N ratio were followed over time to assess whether the increased incidence of mortality in late summer reflects a decrease in bioenergetic status at this period. The stock of mussels which was more susceptible to summer mortality had higher values of V O 2 . Furthermore this stock had a more negative scope for growth and lower O/N ratio in early August. These parameters are likely to reflect unfavourable environmental conditions, which led the mussels to rely upon protein catabolism. We also observed a negative correlation between multiple-locus heterozygosity and standard V O 2 . The more susceptible stock of mussels had a lower degree of multiple-locus heterozygosity. Thus, we suggest that the periodic, but irregular, outbreaks of summer mortality are the result of a synergistic interaction involving dietary deficiencies, temperature, a possible post-spawning stress and the genetic characteristics of the stock. The higher metabolic demand associated with a reduced degree of heterozygosity will impose a supplementary stress and render such stocks more vulnerable to summer mortality. The results are in agreement with the hypothesis that high levels of heterozygosity are related with lower costs of maintenance.