Subtidal Mussels Research Articles

Longevity of subtidal mussel beds (Mytilus edulis) in eutrophic coastal areas

Mussel populations (Mytilus edulis) around the coasts of Europe and the North Atlantic are often ephemeral and have notably experienced a large decline in abundance. Since 1993, annual blue mussel stock surveys have been carried out in the Limfjorden, Denmark. We used the stock survey data combined with electronic monitoring fishing data and Mechanistic Models for the Limfjorden, providing environmental data to investigate the impact of various stressors. Multiple factors were found to affect the longevity of subtidal mussel beds in Limfjorden. Predation by starfish, fishing activities, shell length of the mussels, amplitude in summer temperature and consecutive days of oxygen depletion decreased the longevity of the beds. Conversely, increased biomass, multiple cohorts and increasing water depth demonstrate stabilising effects. Water column stratification had both a negative and positive impact on bed longevity depending on the duration. These analyses can help inform environmental, conservation, and fisheries managers on the long-term trends of population dynamics and gain a deeper understanding of what factors can affect mussel bed longevity in the context of declining stocks.

Dislodgement and mortality challenges when restoring shallow mussel beds (Mytilus edulis) in a Danish estuary

In recent decades, mussel beds in the northern Atlantic and Scandinavia have declined rapidly in extent due to anthropogenic impacts, similar to many other marine habitats. In this study, a large‐scale restoration experiment was conducted to identify major challenges that arise during restoration efforts on shallow subtidal mussel beds. Suspension‐grown mussels (Mytilus edulis) were relayed in two different treatments either directly on bare bottom sandy sediments, or on coir nets (Net), used as a proxy for suitable byssal attachment substrate. The treatments were monitored for 1.5 years and coverage (%), biomass (WW), and population dynamics were quantified. Two main challenges of shallow bed restoration were identified: (1) Lack of suitable attachment substrate resulting in dislodgment of individuals during storm events. The Net treatment had significantly higher coverage and biomass of Mytilus at the end of the monitoring period, clearly demonstrating the importance of suitable substrate at physically exposed locations. (2) High mortality of juvenile mussels. Population dynamics revealed a high mortality of juvenile Mytilus, which resulted in almost complete loss of relayed Mytilus individuals less than 30 mm within the first season. This was most likely due to high meso‐predator densities, as a result of declining top‐predator populations. The high mortality of juvenile Mytilus prevented successful annual recruitment, thereby making the population unsustainable long‐term. Both challenges need to be addressed to create stable beds during restoration. Additionally, the experiment demonstrated the viability of using suspension‐grown Mytilus as a seed‐source when restoring mussel beds.

Nature-based solutions for coastal protection in sheltered and exposed coastal waters: integrated monitoring program for baseline ecological structure and functioning assessment

Nature-based solutions, such as shellfish reefs, can support natural coastal defence and be a potential solution for climate-resilient shorelines in the future. In the Belgian Part of the North Sea, the “Coastbusters” projects aim to develop nature-based coastal protection by favouring subtidal mussel bed establishment on the seafloor through typical longline aquaculture techniques. Mussel beds are dependent on environmental conditions, and both influence the physical and biogeochemical features in a soft-sediment environment. Therefore, a comprehensive ecological monitoring program is essential to assess the success of future mussel bed development and its influence on the surrounding ecosystem. For establishing a monitoring baseline of the two experimental areas, a combination of conventional benthic assessment methods (grab sampling and granulometry) and non-invasive techniques (sediment profile imaging and transect diving video surveys) were utilised. Although mussel reefs did not yet develop by the time of this study, clear differences in ecological and sedimentological characteristics were found between two experimental areas (sheltered and exposed), subjected to slightly different hydrodynamic conditions. The one sheltered by coastal sandbanks was dominated by fine-muddy sand, higher species richness, biomass, and higher biological activity (burrows, fauna, and biological beds) as observed by all methods in one or another way. Moreover, functional diversity indices revealed a higher partitioning of the total available resources, suggesting more complex ecological processes in the sheltered area. Conversely, the area more exposed to the open sea was dominated by more sandy sediments, and fewer organisms were found. The combination of those different monitoring tools provides an integrated, complementary view, from different perspectives, on the biological, physical and functional characteristics of the study areas.

Metabolic scaling variation as a constitutive adaptation to tide level in Mytilus galloprovincialis

Understanding how allometric exponents vary in the different biologically determined patterns turns out to be fundamental for the development of a unifying hypothesis that intends to explain most of the variation among taxa and physiological states. The aims of this study were (i) to analyze the scaling exponents of oxygen consumption at different metabolic rates in Mytilus galloprovincialis according to different seasons, habitat, and acclimation to laboratory conditions and (ii) to examine the variation in shell morphology depending on habitat or seasonal environmental hazards. The allometric exponent for standard metabolic rate (b value) did not vary across seasons or tide level, presenting a consistent value of 0.644. However, the mass-specific standard oxygen consumption (a value), i.e. metabolic level, was lower in intertidal mussels (subtidal mussels: a = - 1.364; intertidal mussels: a = - 1.634). The allometric exponent for routine metabolic rate changed significantly with tide level: lower allometric exponents for intertidal mussels (b = 0.673) than for subtidal mussels (b = 0.871). This differential response did not change for at least two months after the environmental cue was removed. We suggest that this is the result of intertidal mussels investing fundamentally in surface-dependent organs (gill and shell), with the exception of the slightly higher values obtained in May as a likely consequence of gonadal tissue development. Subtidal mussels, on the contrary, are probably in constant demand for volume-related resources, which makes them consistently obtain an allometric exponent of around 0.87.

Out of their depth: The successful use of cultured subtidal mussels for intertidal restoration

AbstractEcosystem restoration has proliferated across the globe to combat widespread ecosystem decline. Translocations of viable individuals into degraded habitats form a core component of restoration efforts, but the selection of source populations poses challenges because phenotypic differences between source populations and depleted populations result in low post‐translocation survival. Intertidal mussel restoration is typically reliant on subtidal source populations despite their weaker shells, lighter weights, and poorer respiratory abilities compared to intertidal mussels. Intertidal mussel restoration using subtidal mussels has recorded very low survival, leading to suggestions that subtidal mussels are unfit for the intertidal zone, effectively eliminating the possibility of translocation‐based intertidal mussel restoration. In this study, small‐scale intertidal mussel restoration was tested using subtidal cultured mussels transplanted to paired plots, half in the lower intertidal and half in the upper subtidal. The results demonstrate the first scientific evidence of the successful restoration of intertidal mussel reefs using subtidal mussels at this scale. Specifically, one location recorded very high survival after 12 months (95.1 ± 0.6%; mean ± SE), comparable to the survival recorded in the subtidal zone (97.6 ± 0.3%). Translocations to the intertidal at the other two locations experienced mortality during the Austral summer (final survival: 81.3 ± 1.6% and 56.6 ± 7.4%), which directly correlated with exposure to extreme temperatures (>40°C). These results reveal that despite phenotypic differences, subtidal cultured mussels are potential sources for successful intertidal restoration. However, care should also be taken to avoid locations prone to high temperatures, particularly in light of a warming global environment. Ultimately, to achieve the massive upscaling in restoration called for by the UN Decade on Ecosystem Restoration, we encourage restoration managers to think broadly about potential source populations, avoid rejections based solely on phenotypic differences, and explore every potential avenue for restoration success.

Nature-based solutions in a sandy foreshore: A biological assessment of a longline mussel aquaculture technique to establish subtidal reefs

Climate-induced natural disasters such as floods and storms are expected to become more frequent and intense in the coming years. From 2017 till 2020, a pilot project was set up where mussel reefs were established as a nature-based solution to mitigate coastal erosion off the Belgian Coast. These subtidal mussel reefs were created by applying a longline mussel aquaculture technique consisting of filamentous and non-filamentous dropper lines of different materials. In this study, several biological aspects of the Nature-based Solution (NbS) set-up were examined: the suitability of the set-up as a whole, the suitability of the seven dropper line types, distribution of biofouling organism on the dropper lines and the spatio-temporal evolution of the mussel reef and its impact on the underlying benthic communities. Only during the summer months, a mussel reef could be established under the dropper line structure. Filamentous and non-filamentous dropper lines harbored different mussel densities and sizes, leading to a possible different effect on the underlying mussel reef. The biofouling community on the dropper lines was also defined in this study with amphipod species such as Jassa sp. and Monocorophium sp. as the dominant species. The impact of the NbS set-up on the macrofaunal communities remained low for the study period. First indications of a shift towards a less diverse and oligochaete dominated community was observed, although longer time series and an optimized sampling protocol will be needed. This pilot study showed the application of using a mussel longline technique in order to establish a subtidal mussel reef in a soft-sediment environment.

Enhancing multiple scales of seafloor biodiversity with mussel restoration

Restoration projects are underway internationally in response to global declines in shellfish beds. As diverse biological assemblages underpin a variety of ecosystem services, understanding broader changes in biodiversity associated with mussel restoration becomes increasingly valuable to scientists and restoration practitioners. Studies generally show bivalve beds increase species richness and abundance, but results are scale-dependent and conditional on the mobility of specific communities observed. We examined biodiversity at multiple scales to determine how communities with varying levels of mobility are influenced by subtidal mussel restoration. Significant changes in assemblage structure were observed in both mobile fish and epifaunal communities, with enhanced species richness and total abundance of associated individuals. In contrast, we observed site-dependent effects of bivalve restoration on macrofaunal community structure and composition, with sheltered, harbour mussel bed communities numerically dominated by detritivores accustomed to organically enriched, muddy sediments. Sediment organic matter significantly increased within mussel beds, and distance-based linear models showed that sediment organic matter was an important predictor of macrofaunal assemblage structure on mussel beds, highlighting the significance of benthic-pelagic coupling and biodeposition to soft-sediment communities. This study contributes novel methods and ecological insights on the role of species mobility and site selection in structuring restoration outcomes, better informing future mussel restoration efforts aimed at emphasising functionally-driven ecosystem services.

The longevity of subtidal mussel beds in the Dutch Wadden Sea

Soft-bottom beds of the blue mussel (Mytilus edulis) are of high ecological importance in intertidal and subtidal habitats. They create habitat, shelter and food for other organisms, and play a dominant role in energy flow and nutrient cycling. Intertidal beds are much better studied than subtidal beds. Though it is often assumed that subtidal mussel beds resemble their intertidal counterparts, major differences in factors driving recruitment, growth and survival can be expected. The aim of our study was to estimate survival chances of the mussel beds in the subtidal parts of the Dutch Wadden Sea in relation to environmental variables, and to compare the results with those obtained previously from the intertidal areas. We used data from a long-term annual survey, resulting in a survival analysis of 365 individual subtidal mussel beds. The average life span of subtidal mussel beds, once they have survived their first winter, was estimated at 2.3 years. This is lower than what was found in the intertidal (3.4 years) in a previous study. However, the survival of subtidal mussel beds in less-saline areas is comparable to survival of intertidal mussel beds, whereas survival of subtidal mussel beds in more-saline areas is significantly lower. The strong, significant effect of salinity is most likely an indication of an effect from starfish predation, since starfish (Asterias rubens) are virtually absent from the intertidal and their abundance is strongly reduced in the subtidal at lower salinities. Furthermore, the survival of individual beds is positively correlated with their size. This may be a direct effect of the bed size itself, or also an indirect effect of environmental factors that can affect the size of newly settled beds. A secondary aim was to compare two methods, based on different types of field data: 1) empirical point data and 2) estimated bed contours based on the point data and additional sources of information. Both methods give similar results. Advantages and disadvantages of both methods are discussed.

Variability in strength of byssus attachment and index condition of subtidal mussels during the maximum growth stage

Local environmental conditions can have strong effects on ecophysiological responses in bivalve molluscs. Molluscs must allocate the energetic resources acquired in their habitats to survival and fitness. In this study, we investigated the spatial and temporal patterns of response in relation to byssus strength and soft tissue weight during the maximum growth stage in the subtidal mussel Mytilus galloprovincialis between March and July 2015 in the Ría de Arousa (NW Spain). Mussels growing in high-energy environments secreted stronger byssus than those inhabiting low-energy sites. The integrated values of wave height and food availability for the previous 15 days showed a positive relationship explaining 45% and 9% of total variability in tenacity of the byssus threads. Differences in condition index were greatest between the northern (highest values) and southern sites at the mouth of the ria owing to differences in food availability. Despite similar hydrodynamic energy values, the high food availability in the exposed northern zones supplied sufficient energetic resources for both strong byssus secretion and high CI. Despite lower food availability, high CI values were also reached in mussels in the innermost, sheltered sites, because of the low need for survival with strong attachment. The condition index was positively correlated with temperature and food availability integrated during 45 previous days prior explained a 57% and 13% of total variance, respectively, while wave height explained 2% but with an inverse relationship. Environmental differences between the geographical zones of the estuary modulated byssus secretion functionality affecting the soft tissues status of mussels.

Environmental predictors of sediment denitrification rates within restored green-lipped mussel Perna canaliculus beds

Benthic processes in coastal marine environments can enhance the natural removal of bioreactive nitrogen through denitrification (DNF), a valuable ecosystem service as nutrient over-enrichment intensifies globally. Enhancing ecosystem services is an important justification for restoring coastal ecosystems, and while it is known that epifaunal bivalves (oysters or mussels) are capable of influencing nitrogen cycling, empirical measurement of the role of particular species across a range of environmental conditions is missing. Bivalves within shellfish beds are not uniformly distributed and thus clumps and empty patches within restored beds may differentially impact DNF given the importance of local biogeochemistry. This study reports DNF and respiration rates in 4 restored beds that vary in sediment composition, while identifying which ecological and biogeochemical factors best explain the observed variability in measured fluxes. We deployed benthic chambers in sediments with and without mussels at these sites, and measured net N2 fluxes through membrane inlet mass spectroscopy. Sediment organic matter proved to be the most significant predictor of DNF rates in regression tree and random forest models, suggesting that biodeposition by green-lipped mussels enhances nitrogen removal at these sites and that these effects occur across beds despite differences in mussel density. Greatest DNF rates corresponded to lower sediment chlorophyll a concentrations and higher nitrate/nitrite effluxes measured within chambers. Determining the influence of subtidal mussel restoration on significant sediment processes informs future restoration efforts aiming to maximise this nitrogen removal service, while providing insights on underrepresented oligotrophic systems of the southern hemisphere.

Considering the use of subadult and juvenile mussels for mussel reef restoration

Widespread resource extraction and habitat degradation have severely reduced functionally important subtidal mussel reefs globally. While methods for restoring oyster reefs are becoming increasingly well‐established, the development of techniques for the effective restoration of mussel reefs remain in their infancy and face biological and logistical challenges. This study investigated the potential use of subadult and juvenile green‐lipped mussels (Perna canaliculus) for mussel reef restoration with the aim of understanding factors related to subadult and juvenile mussel survival after transfer to the seafloor. Small‐scale (m2) field experiments were conducted subtidally in a New Zealand harbor to assess subadult and juvenile mussel survival after translocation to soft‐sediment seafloor, the efficacy of biodegradable substrate to support reef development, and whether juvenile mussel survival was related to changes in seeding density. Results demonstrated survival of cultured subadult and juvenile mussels after transfer to soft‐sediment seafloor only when completely protected from mobile predators. Attachment to biodegradable substrate alone was insufficient to prevent the loss of cultured juvenile mussels, while ~80% of wild subadult mussels survived translocation to the seafloor without predator protection—indicating a certain level of resilience. Changes in seeding density failed to prevent loss of cultured juvenile mussels. This study supports further consideration for incorporating cultured subadult and juvenile mussels into restoration, provided subadult and juvenile mussels can be protected until they become established as adults.

The impact of mussel seed fishery on the dynamics of wild subtidal mussel beds in the western Wadden Sea, The Netherlands

For the cultivation of mussels, wild stocks of juveniles are harvested to collect mussel seed as starting material for the culture. These wild stocks are found in the sublittoral western Wadden Sea (NL). After summer spat fall, fisheries in Autumn on newly formed beds is carried out in areas that have the risk of washing away due to storms, or are vulnerable for starfish predation. These wild beds are considered as relatively unstable. On remaining more stable wild beds, seed fishery is carried out in next Spring. As the Wadden Sea is a nature conservation area, mussel seed fisheries is only allowed if no negative impacts on the nature management objectives can be expected. Seed fishery impacts were addressed in an extensive study including effects on sediment composition, macrobenthos and epifauna. In this paper we describe the effects of mussel seed fisheries on the development of the mussel stocks with and without fisheries in 39 pairwise studied impact and control plots.Stocks on seed beds in areas of the sublittoral Western Wadden Sea that are known as unstable, show a large decline within one year after settlement, also when there is no seed fishery. Harvesting seed on more stable beds in Spring results in a statistical significant reduction in stock size, which lasts for a period of two years after the first fishery. For the longer term, there is a gradual decline of the mussel stocks on all studied plots. On three out of the 39 plots, mussel biomass showed a large increase, both on control and impact parts. Also these mussel beds declined and eventually disappeared. A difference in life expectancy of fished and unfished beds was not demonstrated.It is concluded that sublittoral beds gradually disappear, also without fisheries. As a consequence, new recruitment is of critical importance for the long-term survival of sublittoral mussel beds. As we found no significant difference between recruitment on fished and control parts, there are no indications for negative impacts of seed fishery on new recruitment.

How do life-history traits influence the fate of intertidal and subtidal Mytilus galloprovincialis in a changing climate?

Coastal organisms (i.e. intertidal or upper subtidal species) live in between the terrestrial and aquatic realms, making them particularly vulnerable to climate change. In this context, intertidal organisms may suffer from the predicted sea level rise (increasing their submerged time) while subtidal organisms may suffer from anthropically-induced hypoxia and its consequences. Although there is some knowledge on how coastal organisms adapt to environmental changes, the biochemical and physiological consequences of prolonged submergence periods have not yet been well characterized. Thus, the present study aimed to assess the biochemical alterations experienced by intertidal organisms maintained always under tidal exposure (IT); intertidal organisms maintained submersed (IS); subtidal organisms maintained always submersed (SS); subtidal organisms under tidal exposure (ST). For this, Mytilus galloprovincialis specimens from contiguous intertidal and subtidal populations were exposed to the above mentioned conditions for twenty-eight days. Results indicated that both intertidal and subtidal mussels are adapted to the oxidative stress pressure caused by tidal and submerged conditions tested. Intertidal mussels did not seem to be negatively affected by submergence while ST specimens were energetically challenged by tidal exposure. Both IT and ST mussels consumed glycogen to fuel up mechanisms aiming to maintain redox homeostasis. Overall, both intertidal and subtidal populations were capable of coping with tidal exposure, although the strategies employed differed between them. These findings indicate that although IT mussels may not significantly suffer from the longer-term submergence, hypoxic events occurring in the context of global warming and other anthropogenic impacts may have consequences on both IT and ST populations. Altogether, it is important to highlight that tides may act as a confounding factor in experiments concerning coastal organisms, as it causes additional physiological and biochemical perturbations.

Occurrence, stability, and associated species of subtidal mussel beds in the North Frisian Wadden Sea (German North Sea Coast)

According to historical literature, subtidal blue mussel (Mytilus edulis) beds were relatively widespread in the North Frisian Wadden Sea in the first decades of the 20th century. However, natural subtidal mussel beds subsequently suffered severe losses, partly due to natural causes but also as a consequence of uncontrolled fishing. Especially after the Second World War blue mussels could only be reliably found at a few natural locations by the 2000s. Notably, past sampling techniques using grabs and dredges mean that our knowledge of the areal dimensions and characters of these blue mussel habitats remained limited. Since 2009 broad side scan sonar surveys have been applied to map sediments and to detect subtidal mussel beds or their potential habitats. Apart from several locations with hints of mussels, only two mussel beds were recognized as biogenic reefs according to the EU Habitats Directive. These beds have been monitored annually since 2012. Although the beds have remained relatively stable in terms of area, both beds showed an increasing degradation of their accuracy of topographical discrimination from the surrounding seabed over the years. In line with this degradation, we observed a decline in blue mussel abundance and a strong increase in the dominance of the Pacific oyster (Magallana gigas). However, the final year of the investigation (2018) was characterised by a strong decline in both Mytilus and Magallana populations at both sites.

Physiological responses to heat stress in an invasive mussel Mytilus galloprovincialis depend on tidal habitat

Mussels are ecologically important organisms that can survive in subtidal and intertidal zones where they experience thermal stress. We know little about how mussels from different tidal habitats respond to thermal stress. We used the mussel Mytilus galloprovincialis from separate subtidal and intertidal populations to test whether heart rate and indicators of potential aerobic (citrate synthase activity) and anaerobic (cytosolic malate dehydrogenase activity) metabolic capacity are affected by increased temperatures while exposed to air or submerged in water. Subtidal mussels were affected by warming when submerged in water (decreased heart rate) but showed no effect in air. In contrast, intertidal mussels were affected by exposure to air (increased anaerobic capacity) but not by warming. Overall, physiological responses of mussels to thermal stress were dependent on their tidal habitat. These results highlight the importance of considering the natural habitat of mussels when assessing their responses to environmental challenges.

The prevalence of the pea crab Afropinnotheres monodi in mussels depending on the degree of habitat exposure: Implications for mussel culture

The African parasitic pea crab Afropinnotheres monodi lives in association with a high number of different bivalve species, although in its reproductive phase (soft females) a high prevalence has been observed infesting mainly intertidal Mytilus galloprovincialis. In recent years this pea crab has been recorded in the south of Iberian Peninsula, with well-established populations in the Gulf of Cadiz, and it has been observed that infested mussels showed a decrease of their condition index of 14.7%, on average. Until now, a high prevalence of A. monodi has been observed only in intertidal mussels inhabiting sheltered and relatively shallow habitats, but it has not yet been found in aquaculture farms located offshore in the surrounding zone. Considering the commercially-important mussel farms located in the Galician Rias and Ebro Delta (sheltered systems), the aim of this study was to test whether the prevalence of this pea crab in subtidal mussels depends on the degree of exposure/shallowness of their habitats and consequently on the coexistence of the primary hosts of all crab demographic categories in a reduced area. For this, we have assessed the prevalence of A. monodi in mussels attached to the submerged chains of the navigational buoys of the navigable channel of the Bay of Cadiz. The results show that A. monodi prevalence is correlated with the distance of mussels from the nearest inshore areas, where bivalves host the initial phases of this species (r = −0.92, p < .01). On average, the highest prevalence corresponded to B10 (the nearest inshore buoy sampled), with 74.5% of mussels hosting crabs and the lowest to B2 (the furthest buoy sampled) where only 17.0% of mussels hosted crabs. These results confirm that A. monodi infects subtidal mussels, in a condition similar to the cultured mussels in rafts and long-lines. Therefore, if the pea crab reaches sheltered areas where mussels are cultivated (i.e. the Galician Rias and Ebro Delta bays), it could become successfully established, with negative commercial consequences for these aquaculture farms.

The prevalence of parasitic pea crab Afropinnotheres monodi in mussels depending on the degree of exposure of habitats: implications for mussel culture

XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.

Population dynamics of subtidal blue mussels Mytilus edulis and the impact of cultivation

Fishery on subtidal mussel beds and subsequent relying on culture plots in the same system is a common practice in bottom mussel culture. We address factors that determine the population dynamics of subtidal blue mussels Mytilus edulis L. and to what extent total (natural plus cultured) subtidal mussel biomass in the system is affected by fishery practices. Mussel size and density of spatially segregated natural and cultured subtidal mussel populations in the western Wadden Sea were measured over time in 2 field studies. Spatial patterns in survival rates showed better spat survival in areas with lower salinity and lower density of sea stars Asterias rubens. This suggests that mussel survival is negatively related to sea star distribution which is to a large extent controlled by salinity. The Asterias−Mytilus relation in the Wadden Sea is an example of the concept that environmental stress determines the successes of the prey by affecting the prey− predator relationship. Natural beds that escape predation are found at lower salinities, and mussels on these beds showed low growth rates, also because of lower food quality in these areas. Mussel culture is strongly affecting the population dynamics of the subtidal mussel population, through relay of mussels from natural mussel beds to culture plots. This activity increases mussel growth and survival, because food quality on culture plots is high and predation is controlled. Despite harvesting, mussel biomass production on culture plots was higher than on natural mussel beds, enhancing total subtidal mussel stock.

Allometric size-scaling of biometric growth parameters and metabolic and excretion rates. A comparative study of intertidal and subtidal populations of mussels (Mytilus galloprovincialis)

Allometric relationships between biometric parameters (i.e., soft body and shell weights and shell organic content vs. shell length) as well as for routine and standard metabolic and ammonia excretion rates related to flesh weight and shell length were estimated and compared for subtidal and intertidal populations of Mytilus galloprovincialis in Galicia (NW Spain). This is the first report on allometric size-scaling of excretion and metabolic (both routine and standard) rates in this species. No evidences of differences in size exponent were found between physiological rates or between both populations for any physiological rate. Intercepts of regression lines were significantly higher in subtidal than in intertidal mussels, indicating greater levels of energy expenditure in the former. However, metabolic scope for feeding and growth was about two-fold in intertidal mussels, pointing to a reduced growth efficiency compared with subtidal mussels. Evolution of biometric parameters of body components with size indicated that subtidal mussels allocated energy resources preferably into flesh growth, achieving higher condition indices, while intertidal mussels put more effort on shell calcification and thickening which resulted in heavier shells of reduced organic content. These differentiated growth “strategies” of both populations could be related to their differences in growth efficiencies.

Flexibility of Physiological Traits Underlying Inter-Individual Growth Differences in Intertidal and Subtidal Mussels Mytilusgalloprovincialis.

Mussel seed (Mytilusgalloprovincialis) gathered from the intertidal and subtidal environments of a Galician embayment (NW, Spain) were maintained in the laboratory during five months to select fast (F) and slow (S) growing mussels. The physiological basis underlying inter-individual growth variations were compared for F and S mussels from both origins. Fast growing seemed to be a consequence of greater energy intake (20% higher clearance and ingestion rate) and higher food absorption rate coupled with low metabolic costs. The enhanced energy absorption (around 65% higher) resulted in 3 times higher Scope for Growth in F mussels (20.5±4.9 J h−1) than S individuals (7.3±1.1 J h−1). The higher clearance rate of F mussels appears to be linked with larger gill filtration surface compared to S mussels. Intertidal mussels showed higher food acquisition and absorption per mg of organic weight (i.e. mass-specific standardization) than subtidal mussels under the optimal feeding conditions of the laboratory. However, the enhanced feeding and digestive rates were not enough to compensate for the initial differences in tissue weight between mussels of similar shell length collected from the intertidal and subtidal environments. At the end of the experiment, subtidal individuals had higher gill efficiency, which probably lead to higher total feeding and absorption rates relative to intertidal individuals.