Ecological Indicators 111 (2020) 106031
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Ecological Indicators
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Original Articles
Sublethal responses of four commercially important bivalves to low salinity
a,⁎
a
b
b
a
Rula Domínguez , Elsa Vázquez , Sarah A. Woodin , David S. Wethey , Laura G. Peteiro ,
Gonzalo Machoa, Celia Olabarriaa
T
a
Departamento de Ecoloxía e Bioloxía Animal, Facultade de Ciencias do Mar, Campus As Lagoas-Marcosende s/n and Centro de Investigación Mariña, Universidade de
Vigo, Illa de Toralla s/n, 36331 Vigo, Spain
b
Department of Biological Sciences, University of South Carolina, 715 Sumter Street, Columbia, SC 29208, United States
A R T I C LE I N FO
A B S T R A C T
Keywords:
Bivalves
Small-scale fisheries
Salinity fluctuations
Valve closure
Scope for growth
Burrowing
The abilities of estuarine species to respond to salinity fluctuations by behavioural and physiological responses
can determine the maintenance of populations, particularly in the context of climate change. The native clams
Ruditapes decussatus and Venerupis corrugata, the native cockle Cerastoderma edule and the introduced clam
Ruditapes philippinarum are important resources in Galician (NW Spain) coast. As inhabitants of estuaries, these
species are exposed to frequent salinity fluctuations as a result of heavy rains. This study investigated the shortterm sublethal effects of salinity drops on their physiological (scope for growth, SFG) and behavioural (valve
closure and burrowing activity) responses. Bivalves were exposed to simulated tidal cycles and similar salinities
to the field conditions, i.e., four salinity ramps (5–20, 10–25, 15–30 and 30–30) during six days over three
different periods of the year (autumn, winter and spring). The overall response was the same for all species under
the lower salinities (5, 10 and 15), with a dramatic reduction of pumping activity, SFG and burrowing. Results
differed among species under the higher salinities (20, 25 and 30). While C. edule was the most affected species
in autumn showing no recovery despite having higher SFG compared to the venerids, R. decussatus was more
resistant in all seasons despite having the lowest SFG compared to the rest of species. In winter, V. corrugata was
the most vulnerable due to lower SFG at the lowest salinities. All species showed a compensation pattern in
spring that led to non-recovery of individuals. Burrowing ability had similar patterns to SFG in autumn and
winter but differed in spring, when recovery was the general pattern. The decrease of burrowing ability at lower
salinities during stress seen to some degree in all species can increase vulnerability to predation. Results suggest
that differential responses of lower activity over time could be related to the physiological condition and habitat
preferences of each species and should be taken in consideration for management plans in the context of climate
change. The results drive a discussion of the usefulness of SFG as the metric with which to assess salinity stress in
adult bivalves and the need in future research to increase frequency and duration of stresses in concert with
variables such as food availability.
1. Introduction
Bivalve shellfisheries are an important component of the world’s
fishery production in Europe (FAO, 2018). Mostly because of their
sensitivity to environmental conditions and changes in climate, these
fisheries experience high spatial and temporal variability of catches in
many parts of the world (reviewed by McLachlan and Defeo, 2018) and
particularly in Spain (Juanes et al., 2012; Parada et al., 2012; Morgan
et al., 2013; Aranguren et al., 2014). Most shellfish beds are located in
estuarine areas in the inner part of rias (sensu von Richthofen, 1886)
and thus exposed to large fluctuations of salinity due to heavy rains and
locally managed freshwater releases from river dams, both associated
⁎
with increased river runoffs (Nogueira et al., 1997; Parada and Molares,
2008; Parada et al., 2012). Of special concern are the extreme precipitation events that are expected to increase in frequency in the near
future as estimated by IPCC current predictions of global and regional
climate change (Hoegh-Guldberg et al., 2018). Such episodes of heavy
rainfall occurred often in Galicia, being the most dramatic during autumn-winter 2000–2001, which caused massive mortality of bivalves
(100% mortality in several areas), preventing these resources from
being exploited for 1 year (Parada et al., 2012). Episodes of mass
mortality and recruitment failures in bivalve populations can thus lead
to huge socio-economic costs (Sobral and Fernandes, 2004; Parada
et al., 2012; Verdelhos et al., 2015), as well as ecological effects as
Corresponding author.
E-mail address: ruladominguez@uvigo.es (R. Domínguez).
https://doi.org/10.1016/j.ecolind.2019.106031
Received 12 May 2019; Received in revised form 14 November 2019; Accepted 17 December 2019
1470-160X/ © 2019 Elsevier Ltd. All rights reserved.
Ecological Indicators 111 (2020) 106031
R. Domínguez, et al.
exposures (Molares et al., 2008), although a more recent study using
6 day exposures reported mortalities only at salinities below 7
(Carregosa et al., 2014). In R. philippinarum, mortality was observed at
salinities below 14 after 6 days and rates of mortality were greater than
those for R. decussatus with further salinity reductions. These data are
consistent with field distributions where R. decussatus is more abundant
in fresh water influenced areas than R. philippinarum (Juanes et al.,
2012).
A series of short-term mesocosm experiments simulating field salinity stress were run in sediment with tidal cycles. Exposures to salinities
below 20 occurred during ebb and low tide, periods when river flow
and runoff predominate in the field. Despite previous studies on these
and other bivalve species (Sunoko, 1997; Resgalla et al., 2007; Sará
et al., 2008; Wang et al., 2011; Guzmán-Agüero et al., 2013), the effects
of salinity changes of different magnitude on physiology and behaviour
of these four species are poorly understood (Verdelhos et al., 2015;
Gharbi et al., 2016; Peteiro et al., 2018), especially when considering
realistic scenarios (Nossier, 1986). We tested the cumulative effect of
exposure to low salinities over six consecutive days and two days of
recovery by evaluating scope for growth and activity in terms of valve
closure and burrowing activity of the four species. The hypothesis
tested was that lower salinities would have measurable impacts: increasing valve closure, decreasing scope for growth and decreasing
burrowing activity compared to bivalves in the control salinity. We
expected different responses among species related to their different
physiological tolerances and behavioural strategies. Based on mortalities from continued stress experiments and field data, the tolerance
to low salinity was expected to be lowest for V. corrugata and C. edule,
intermediate for R. philippinarum and highest for R, decussatus (Molares
et al., 2008; Carregosa et al., 2014; Verdelhos et al., 2015). To check for
consistency of responses over time, the experiment was repeated in
autumn, winter and spring, when low salinity events occur in Galician
shellfish beds, as responses are likely to vary depending on the physiological conditions of organisms (Beninger and Lucas 1984; Sará
et al., 2008; Aníbal et al., 2011).
disruptions of food webs, biogeochemical cycles or pelagic-benthic
coupling that result in changes in structure and functioning of communities (Morgan et al., 2013; Haider et al., 2018). Sudden salinity
drops can also occur in combination with temperature changes, parasite
infections or variations in the food supply, enhancing their impact on
species (Aranguren et al., 2014; Villalba et al., 2014; Macho et al.,
2016).
Salinity fluctuations not only cause mortality but also important
sublethal effects, such as altering feeding, respiration, growth, osmoregulation, behaviour, reproduction, and parasite-disease interactions
(Gosling, 2015), even in euryhaline species such as the studied venerids
and cockles (Parada and Molares, 2008; Carregosa et al., 2014). Bivalves have the ability to adapt to small changes in salinity in a few
days, but sudden unforeseen changes produce negative effects, quantified depending on the authors, as loss of 1 to 24% of the energy acquired (Gosling, 2015). Salinity drops can be either regular or episodic
and induce different types of responses in marine organisms; behavioural responses may be the primary strategy under stress (Lockwood,
1976). Among them, valve closure for short periods is common for bivalves frequently exposed to salinity fluctuations over cyclic periods to
avoid osmotic shock (Nossier, 1986; Verdelhos et al., 2015). These
species often can withstand hypoxia for short periods, minimizing the
need for energetic adjustments due to osmotic stress (Shumway, 1977);
if closure is extended in time, filtration activity and therefore energy
acquisiton are reduced, while excretion products are accumulated. In
contrast, if tissues are not isolated by valve closure, changes in salinity
can cause reductions in activity and energy acquisition due to an increase in energy demand to maintain cell volume and avoid osmotic
shock (Burton, 1983; Berger and Kharazova, 1997; Gosling, 2015).
Burrowing behaviour can be also affected by salinity fluctuations because it is an activity of high energetic cost, so after repeated disturbance burrowing may become limited when the energy reserves are
exhausted below a certain threshold, with potential implications for
predation risk (Haider et al., 2018).
Changes in energy balance due to salinity stress can be assesed by
the calculation of scope for growth, SFG, i.e. energy available for
growth (Griffiths and Griffiths, 1987; Sará et al., 2008; Wang et al.,
2011), which is an useful indicator of bivalve physiological responses in
short term laboratory experiments (Widdows, 1978; Bayne, 1998;
Widdows and Staff, 2006; Resgalla et al., 2007; Guzmán-Agüero et al.,
2013). It integrates the difference between the energy absorbed from
the food (clearance rate and absorption efficiency) and the energy loss
via metabolic energy expenditure (respiration and ammonium excretion). In addition, valve closure, monitored during physiological measurements (Stenton-Dozey et al., 1994; Sobral and Fernandes, 2004;
Carregosa et al., 2014) can be considered as a different response variable since, while bivalves remain closed, their activity in terms of
feeding and aerobic respiration is theoretically stopped (Griffiths and
Griffiths, 1987).
The present study investigated the behavioral and physiological
responses of the four most relevant bivalve species for the traditional
fishery in Galicia (northwestern Spain), to salinity decreases of different
intensity over a short-term period. The species selected include the
native venerids Venerupis corrugata (Gmelin 1791) (pullet carpet shell),
Ruditapes decussatus (Linnaeus 1758) (grooved carpet shell), the introduced Ruditapes philippinarum (Adams and Reeve 1850) (Japanese
carpet shell), and the native cockle Cerastoderma edule (Linnaeus 1758)
(common edible cockle). The four species represented 78% of the
landings in 2018 and yielded 9046 tonnes of bivalves worth ~84 million € (own elaboration based on www.pescadegalicia.gal, access
January 2019). They differ in their salinity tolerance ranges and optima. Venerupis corrugata has low tolerance to low salinity (almost
100% mortality at < 10) (Molares et al., 2008) while Cerastoderma
edule shows a similar pattern to V. corrugata with an optimum salinity
range of 20–25 and high mortality at < 10 (Verdelhos et al., 2015).
Salinities below 20 cause mortality in R. decussatus under long
2. Materials and methods
2.1. Clam collection and maintenance
Three experiments were performed in a mesocosm system at
Estación de Ciencias Mariñas de Toralla (ECIMAT) (www.ecimat.uvigo.es) of the Universidade de Vigo (Spain), during autumn 2015
(Dec15), winter and spring 2016 (March16 and May16, respectively).
These are the seasons when heavy rains tend to occur in this region,
with a high impact on the shellfish beds (Parada et al., 2012). The day
before each experiment, adult clams were manually collected in intertidal fishing beds by shellfishers at Ría de Arousa (42° 30′ 55″ N, 08° 48′
53″ W) and adult cockles at Ría de Noia (42° 47′ 0″ N, 8° 53′ 0″ W), and
transported to the laboratory in refrigerated boxes. Both locations are
located at the Rías Baixas, characterized by its temperate and humid
weather, influenced by the upwelling regime typical of this coast due to
their SW- NE orientation (Méndez and Vilas, 2005). Shellfish beds are
influenced by the mesotidal and semidiurnal tidal regime and river
discharges because of their location in the inner part of the rías at
shallow depths around 5–10 m (Alvarez et al., 2005).
Once in the laboratory, bivalves were immediately placed in seawater and afterwards they were individually marked and measured
(mean ± SD for Dec15, March16 and May16 respectively: 40.5 ± 2,
37.5 ± 1.8 and 39.8 ± 1.7 mm for V. corrugata; 42.4 ± 1.3,
45.02 ± 1.5 and 42.6 ± 1.5 mm for R. decussatus; 41.8 ± 1.1,
42.7 ± 1.2 and 41.4 ± 1.4 mm for R. philippinarum; 30.4 ± 1.1,
30 ± 1.2 and 31.3 ± 1.5 mm for C. edule), placed on sediment surfaces in 16 L tanks (17 cm tall × 26 cm width × 36 cm length) and
allowed to burrow. Those that did not burrow within 8 h were discarded and replaced by new individuals before the experiments started.
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The experimental treatment salinities were monitored by AT controllers (AquaMedic) in Dec15 and miniCTDs (Star ODDI) in March16
and May16. Data for a 24 h period in 30 min intervals indicated the
effectiveness of salinity treatments (Fig. 2A and B for Dec15 and
March16, respectively). Tides were simulated only during the day in
Dec15, while in March16 and May16 there were two tides each 24 h.
For each treatment the lower salinity occurred during the ebb and low
tide period and the higher salinity during the flood and high tide
period. Note that maximum water depth was the same during the
periods called ‘low tide’ and ‘high tide’ in this work.
After six days of exposure to these treatments, all tanks were
maintained for two more days at constant salinity (30) to measure bivalve recovery ability (three days for recovery in Dec15). At initiation
28 (Dec15) or 30 (March16 and May16) individuals were placed on the
sediment surface in the small 16 L tanks, one tank per species per larger
tank. Densities were reduced by 6 on each day of measurement including day 0. As of day 0 the densities in the small tanks approximated
those found in the shellfish beds (220 ind m−2).
Fig. 1. Experimental design including the salinity ramps and blocks. 1: water
pumps. 2: head tanks with fresh and salt water. 3: mixing tanks for salinity
treatments (5–20, 10–25, 15–30, 30–30). 4: eight big tanks (two per salinity
ramp). 5: four small tanks, one per species, within a big -tank, each small tank
initially with 30 individuals. 6: Inlet. 7: tank overflow. 8: Burrowing buckets.
2.3. Measured variables
A total of 2880 animals comprising the four species were used in the
three experiments. Oxygen consumption, clearance rate, ammonium
excretion, absorption efficiency, valve closure and burrowing behaviour were measured on 6 individuals per species per block and salinity
treatment. Measurements for each block were taken on day 0 (prestress), day 4 (under stress), day 6 (last day of stress) and day 8 (day 9
in Dec15) (recovery period), during the last half of the flood tide, with
the exception of day 4 in Dec15 when physiological variables were
measured during the lowest salinity of each experimental ramp (ebbtide conditions) in order to test for the physiological limits of individuals. The same individuals were used consecutively for all physiological and behavioural measurements on the same measuring day
and then removed from the experiment.
During physiological measurements the activity of organisms,
measured as valve closure, was checked every 5 min (every 20 min
while individuals were in ammonium excretion beakers) and scored as
follows: 0 valves closed, 1 valves open with or without siphons visible.
Individual oxygen consumption rate (mgO2 h−1) was estimated by
measuring oxygen fluxes every 60 s inside cylindrical respirometers
(125 ml) connected to luminescent dissolved oxygen probes (Hach
Lange HQ40D). Respirometers were filled with aerated 50 µm-filtered
seawater at ambient temperature, 15–18 °C, depending on the season,
using seawater at the treatment salinity, mixed gently with a magnetic
stirrer during the measuring period. Dissolved oxygen concentration
was recorded until it declined ≈ 20% from the initial value or after
20 min. Estimates were normalized by the volume of the seawater inside the respirometer. In addition, controls with only shells of each
species maintained in each experimental treatment served as blanks to
correct for respiration rates of bacterial film in the shell.
Individual clearance rate (CR; L h−1) was measured after oxygen
consumption measurements. Each individual was placed in a glass
beaker with 500 ml of 50 µm-filtered seawater with aeration at each
salinity treatment. After 5 min of acclimation, each individual was fed
with Isochrysis galbana (≈ 50 000 cells ml−1 to avoid the production of
pseudofeces, Sobral and Widdows, 1997). Water samples were taken at
5 and 30 min and preserved with lugol. Concentration of particles was
measured in an electronic particle Coulter Counter (Beckman Coulter
Multisizer 3). Two glass beakers without individuals, but with the same
microalgal concentration were also sampled in each run of measurements to correct for particle loss due to sedimentation.
Individual ammonium excretion rate (mg NH4-N h−1) was calculated following clearance rate measurements. Each individual was
gently transferred to a new beaker with 200 ml of 50 µm-filtered sea
water of each salinity treatment, and left for 2.5 h without aeration. The
feces were collected by pipetting and all water was stored and frozen
The 16 L tanks were placed inside eight 480 L (50 cm tall × 80 cm
width × 120 cm length) experimental tanks with running 50 μm-filtered seawater (salinity approximately from 35 to 37 depending on the
season) in a room with temperature set at 18 °C. Then animals were fed
and kept overnight at those conditions until the experiment started the
following day. Animals were fed in the evenings during the experiments
with a microalgae mixture of Isochrysis galbana, Tetraselmis suecica,
Chaetoceros gracilis and Rhodomonas lens. Several liters (volume varied
daily depending on the concentration of the phytoplankton cultures) of
the algal mix were added to each tank to reach a 1% maintenance diet
for the clams’ density and size (mean individual dry weight of 0.68 g).
2.2. Experimental setup
Four small plastic tanks, one per species, (16 L, 17 cm tall × 26 cm
width × 36 cm length) were placed inside each of 8 big tanks (480 L,
50 cm tall × 80 cm width × 120 cm length) with bottom drains (Fig. 1)
that were randomly located in the experimental room.
There were two 480 L tanks for each salinity treatment (four salinity
ramps, see below), one per block. Blocks 1 and 2 were measured on
consecutive days due to time constraints for physiological measurements. The 16 L tanks had four 2 cm bottom orifices covered with
80 μm mesh to avoid sediment loss but to allow water flux through the
column of sediment. They were filled to the top with sediment collected
from the intertidal at Canido (42° 11.68′ N; 8° 47.81′ W) where clams
live (median grain size of 0.19 mm). To produce salinity profiles similar
to those experienced by bivalves in the field (www.intecmar.gal),
treatments consisted of four salinity ramps in which salinity varied from
5 to 20, 10 to 25, 15 to 30 and 30 to 30, i.e. control treatment. Salinity
ramps were created automatically through the use of timers controlling
dual bellows pumps that mixed dechlorinated fresh water and 50 μmfiltered sea water at ambient temperature at different proportions (see
Fig. 1). Water entered via inlets in the bottom of the large tanks and
exited via ~ 30 cm tall standpipes. Total water exchange took ~90 min
at 1 L min−1. During daylight hours the ebb tides were simulated every
day in each tank by drainage followed by the automatic change in water
source and thus salinity resulting in a rapid salinity change. During
night tides the automatic change in water source occurred but was not
preceded by drainage so salinity change occurred more slowly (Fig. 2).
This was particularly true for salinity changes from a higher to a lower
salinity because the incoming water was of lower density than the water
in the tank.
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R. Domínguez, et al.
Fig. 2. Salinity profiles for each treatment during
experiments. A) data for December 2015 (AT controller), B) data for March 2016 (miniCTD). Solid
line represents the S30-30 treatment, dashed line
represents the S15-30 treatment and dotted-dashed
line and dotted line represent the S10-25 and S5-20
treatments, respectively. Data for May 16 are
omitted because they followed a similar pattern to
March16. Missing values in March16 are due to the
period that the miniCTD was out of the water
during the daytime tide.
the difference between the energy absorbed from the food (food consumption × absorption efficiency) and energy loss via metabolic energy
expenditure was determined following the equation given by Warren
and Davis (1967):
for analysis by electrometric determination (Carpenter and Capone,
1983). Afterwards, feces together with three replicates of food samples
(20 ml) previously taken were rinsed with 0.5 M ammonium formiate to
dissolve salt crystals and filtered throughout 25-mm prewashed and
weighed Whatman GF/C glass filters. Those filters were dried for 24 h
at 60 °C, weighed, then left in a muffle furnace for 2 h at 450 °C to
determine ash free weight. Content of organic matter assimilated was
calculated for each bivalve as the difference between ash free dry
weight and dry weight.
After physiological measurements were finished, individuals were
carefully placed on the surface of buckets previously filled with clean
sediment from the same source as that used in treatments to measure
their burrowing activity. The sediment was prepared by exchanging the
porewater several times followed by aeration to ensure that both the
porewater and the overlying water were at the corresponding salinity
treatment. Burrowing activity was recorded every 15 min during 2 h.
Animals were scored as burrowed if ≥75% of the body was within the
sediment.
Finally, bivalve tissues were removed from their shells and dried to
obtain their dry weight for the standardization of all estimates to 1 g
dry weight.
Scope for Growth P = A − (R + U), where
A (energy absorbed) = (C) × food absorption efficiency.
C (energy consumed or ingested) = [maximum clearance rate: L
g−1h−1] × [mg POM L−1] × [18.74 J mg−1 POM]
R (energy respired) = (µmoles O2 g−1 h−1) × 0.456
U (energy excreted) = (µmoles NH4-N g−1 h−1) × 0.349
Energy equivalents used to convert rates of oxygen uptake, clearance rate and excretion to joules were 1 µmol O2 = 0.456 J (Gnaiger,
1983), 1 mg algal cells or particulate organic matter = 18.74 J (Whyte,
1987) and 1 µmol NH4-N = 0.349 J (Elliott and Davidson, 1975). All
the values of the energy components were expressed in Joules per hour
per gram of dry weight (J h−1 gdw−1).
2.5. Statistical analysis
All analyses were performed for each species and experiment separately due to intrinsic differences among species and seasons, as revealed by preliminary analyses. Generalized Linear Models (GLMs)
were used when exploratory analysis showed a linear pattern in data,
and Generalized Additive Models (GAMs) for non-linear patterns.
Changes in valve closure activity of organisms (0 = inactive,
1 = active) and burrowing activity (0 = not burrowed, 1 = burrowed)
were analysed through regression analyses fitting GLMs with a binomial
distribution of the error term and a logit link function, i.e. logistic regression, with Salinity treatment as a fixed factor (S, 4 levels: 5–20,
10–25, 15–30 and 30–30). Two orthogonal a priori contrasts, i.e. S30-30
vs all others, and S5-20 vs S10-25 and S15-30) were made to evaluate
differences among low salinity treatments and control for valve closure
analysis.
The relative importance of each variable (i.e. clearance rate, respiration, absorption efficiency and ammonium excretion) to the scope
for growth was also calculated based on a bootstrapping analysis with
95% confidence intervals of a linear model including all variables.
Changes in SFG were analysed through GAMs with Salinity treatment as
2.4. Data calculation
Valve closure activity during the physiological measurements was
identified for each individual and data subsequently analysed.
Individuals with 0 activity, with valves closed during the clearance rate
measurement were classified as non-active.
The physiological rates of oxygen consumption, clearance rate and
absorption efficiency were calculated according to Widdows and Staff
(2006). Oxygen consumption was estimated by regressing oxygen
concentration (mg L-1) in the respirometer over time using only the
steepest part of the regression slope. To correct for activity of individual
animals, clearance rate (CR; L h−1) was multiplied by the average
activity based on the 6 observations of valve closure during filtration,
i.e. a score of 0 was given to animals with valves closed at the beginning
and end of each 5 min interval, 0.5 to animals with open valves at the
beginning and closed at the end or the opposite behaviour, and 1 was
given if valves were opened both at the beginning and the end.
The SFG formula that integrates all these calculations and represents
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Ecological Indicators 111 (2020) 106031
R. Domínguez, et al.
smoothed fixed factor. Prior to analyses, normality and homogeneity of
variances were checked by visual inspection of Q-Q and residual plots
as well as by Shapiro-Wilk and Levene’s tests, respectively. For data
collected on day 4 in the Dec15 experiment, analyses were carried out
with untransformed data because no transformation removed nonnormality and heterogeneity of data.
Analyses were made using the car package (Fox and Weisberg,
2011) and modEvA package (Barbosa et al., 2013) for GLM and mgcv
(Wood 2011) for GAM in R version 3.6.1 (R Development Core Team,
2010). Package multcomp (Hothorn et al., 2008) was used to do a priori
contrasts and package relaimpo (Grömping, 2006) was used to determine the relative importance of each variable to the SFG. Package
ggplot2 (Wickham, 2016) was used to produce graphs with 95% confidence intervals fitting a GLM or GAM function. Average data are reported as means ± standard deviation.
Table 1
Total number (N) and proportion of active individuals (PA) in the experiments.
The significant effects of a priori contrasts of salinity treatments are also shown
(Sg): * represents the contrast S30 vs all others, + represents the contrast S5-20
vs S10-25 and S15-30. D0, D4, D6, D8/9 represent the days of the measurement. Note that measurements on day 4 of the Dec15 experiment were done
under different conditions, i.e. at the lowest salinity of each ramp.
Species
V. corrugata
Day
D0
D4
D6
3. Results
D8/9
As expected with short term exposures to low salinity, no mortality
was observed for any of the species after the acclimatization period or
during the low salinity treatments.
R. decussatus
3.1. Activity (valve closure)
Most individuals remained active during all measurements with
some remarkable exceptions, particularly when the measurements were
made at the lowest salinity of each experimental ramp on day 4 of the
Dec15 experiment, when significant differences were found for all
species (Table 1). Individuals were significantly more likely to be inactive in the S5-20, S10-25 and S15-30 treatments than in the control.
Both V. corrugata and C. edule however had active individuals at salinity
5 while no individuals of R. decussatus and R. philippinarum were active.
Only V. corrugata had active individuals in all salinity treatments below
30. In March16 and May16 measurements were made at the highest
salinity of each ramp on both day 4 and 6. In all measurements on day 4
with R. decussatus there was significantly less activity in the S5-20
treatment than in the S10-25 and S15-30 treatments (Table 1). Surprisingly in two cases there were fewer active individuals in the control
compared to the others: for R. decussatus on day 8 in May16, and for R.
philippinarum on day 4 in March16, marked by statistically significant
differences.
D0
D4
D6
D8/9
R. philippinarum
D0
D4
D6
D8/9
3.2. Scope for growth
C. edule
3.2.1. Responses under ebb-tide conditions
Results of measurements under the lowest salinities of each ramp,
i.e. S5, S10 and S15 after 4 days of stress in Dec15 indicated lower
activity of bivalves at the lowest salinities relative to controls (Fig. 3-A,
B, C, D, Table 2). The SFG models explained a high percentage of deviance (between 36% and 81%) and showed significant differences
among treatments for all species (Table 2) with lower values in all reduced salinity treatments compared to the controls. This pattern was
driven by a dramatic reduction in clearance rate which was not compensated by a reduction on respiration in the lower salinity treatments
(Fig. 3-A,B,C,D).
D0
D4
D6
D8/9
Salinity
Dec15
N
PA
30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
17
12
12
11
12
11
11
11
12
12
10
12
12
0.88
0.33
0.25
0.27
1
1
1
1
1
0.92
1
0.92
0.92
30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
16
12
12
13
13
11
12
12
12
12
12
12
12
0.69
0
0
0.46
0.92
1
1
1
1
1
1
1
1
30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
15
12
12
12
13
12
12
12
12
12
12
12
12
0.94
0
0.08
0
0.92
0.92
1
0.75
1
0.75
1
1
0.83
30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
5–20
10–25
15–30
30–30
16
12
11
12
13
12
12
12
12
12
12
12
12
0.88
0.5
0
0.33
0.92
1
1
1
1
1
1
1
1
Mar16
Sg
*
+
*
*
+
*
N
PA
16
12
12
12
12
12
12
12
12
12
12
12
12
1
1
1
1
1
1
1
1
1
0.92
1
1
0.92
16
10
12
12
12
12
12
12
11
12
12
12
12
1
0.83
1
1
1
1
1
1
0.92
1
1
1
1
16
12
11
12
9
12
12
11
11
12
12
12
11
1
1
0.92
1
0.75
1
1
0.92
0.92
1
1
1
0.92
15
12
12
12
12
12
12
11
12
11
11
12
12
0.94
1
1
0.92
0.92
1
1
1
1
0.92
0.92
1
1
May16
Sg
+
*
N
PA
15
11
12
12
12
11
12
12
12
12
12
12
12
0.94
0.92
1
1
1
0.92
1
1
1
1
1
1
1
16
10
12
12
12
12
12
12
12
12
12
12
7
1
0.83
1
1
1
1
1
1
1
1
1
1
0.78
14
12
12
11
12
10
12
12
12
12
12
12
12
0.88
1
1
0.92
1
1
1
1
1
1
1
1
1
14
12
12
12
12
12
12
12
12
12
11
12
12
0.88
1
1
1
1
1
1
1
1
1
0.92
1
1
Sg
+
*
March16, models fitted the data better, with ~20% of deviance explained. The SFG varied significantly among treatments all days, although only marginally on day 8 (recovery). However, while on day 4,
the SFG increased almost linearly with salinity, on days 6 and 8 the SFG
followed a similar pattern with the lowest values at S5-20 and the
highest at S10-25 (Fig. 4B). The SFG curve on day 8 was above those of
days 4 and 6 in the reduced salinity treatments, which indicated a recovery of individuals after the stress. Differences in SFG were mainly
driven by clearance rate that showed strong differential patterns among
treatments and days compared to respiration. In May16, the models
3.2.2. Responses under flood-tide conditions
The examined species showed different patterns of variation in SFG
after exposure to the salinity treatments and responses differed across
experiments, i.e. Dec15, March16 and May16, in most cases.
Venerupis corrugata had the lowest mean basal SFG of all species in
Dec15 (2.8 ± 12.2 Jh−1g−1), it was higher in March16 (17.7 ± 20.3
Jh−1g−1) and then negative in May16 (-10.1 ± 13.1 Jh−1g−1)
(Fig. 4A-C, Table 2). In Dec15, no significant differences were found on
days 6 and 9 (Table 2), and no patterns were identified (Fig. 4A) with a
very low percentage of deviance explained by the models (< 2%). In
5
Ecological Indicators 111 (2020) 106031
R. Domínguez, et al.
Fig. 3. A,B,C,D. Scope for growth, clearance rate and respiration rate (Jh−1g−1) of A: Venerupis corrugata, B: Ruditapes decussatus, C: Ruditapes philippinarum and D:
Cerastoderma edule measured on day 4 of the Dec15 experiment under low tide conditions. Line is the predicted line of the salinity model with 95% confidence band.
The dots are the raw data for each salinity treatment. Note the different scales used for R. philippinarum and C. edule in Clearance rate and SFG.
25 and S15-30 below those of days 4 and 6 indicated no recovery of
individuals at those treatments. Patterns were again clearly driven by
clearance rates that differed strongly from respiration.
Ruditapes decussatus had a mean basal SFG of 8.2 ± 17.5 Jh−1g−1
in Dec15, 3.7 ± 6.4 Jh−1g−1 in March 16, and 7.1 ± 17.1 Jh−1g−1
in May 16 (Fig. 5A-C, Table 2). In Dec15, the model explained 22% of
explained the greatest percentage of deviance (> 39% on all days) and
showed again significant differences among treatments all days. In this
case, a similar parabolic pattern was seen for day 4 and 6 with lowest
values at S5-20, increasing at S10-25 and S15-30 and decreasing for the
controls (Fig. 4C). On day 8, the SFG increased almost linearly from the
lowest values at S5-20 to the highest at control. The SFG curve at S106
Ecological Indicators 111 (2020) 106031
R. Domínguez, et al.
Table 2
Summary of results of the GAM analyses to test the effect of salinity treatments on the Scope for Growth (SFG). Values in bold are statistically significant (p < 0.05)
and for those graphical patterns are presented by: + when the expected pattern of lower SFG for lower salinities was apparent in the graphical analysis, − when the
inverse pattern was apparent and 0 when neither of these patterns was apparent. Ref. df: reference degrees of freedom; DE: % of deviance explained by the model. a:
Note that measurements on day 4 of the Dec15 experiment were done under different conditions, i.e. at the lowest salinity of each ramp.
Species
V. corrugata
Exp
Dec 15
Mar 16
May 16
R. decussatus
Dec 15
Mar 16
May 16
R. philippinarum
Dec 15
Mar 16
May 16
C. edule
Dec 15
Mar 16
May 16
Day
Ref.df
F, pvalue
% DE
Patterns in SFG
5–20 lower
30–30 higher
0
+
+
+
+
+
+
0
+
–
–
–
–
+
4a
6
9
4
6
8
4
6
8
2.709
1
1.318
1
2.947
2.37
2.707
2.798
2.237
7.955,
0.028,
0.619,
8.278,
4.608,
2.426,
11.01,
13.72,
12.32,
4a
6
9
4
6
8
4
6
8
2.561
2.99
1
2.951
1.042
1.209
2.705
2.408
1
9.998, p = 6.46e-05***
3.914, p = 0.0163*
0.272, p = 0.604
4.495, p = 0.00493**
0, p = 0.98
2.096, p = 0.173
4.031, p = 0.0101*
2.267, p = 0.143
10.85, p = 0.00194**
38.2
22.1
0.589
27.5
0.092
5.1
24.4
12.1
20.2
0
0
+
0
+
–
+
–
+
+
4a
6
9
4
6
8
4
6
8
2.996
1
1.511
2.99
2.243
2.458
2.141
2.939
2.398
62.48, p < 2e-16***
0.266, p = 0.609
0.886, p = 0.498
5.606, p = 0.00191**
1.54, p = 0.208
3.593, p = 0.0393*
1.168, p = 0.362
7.561, p = 0.00117***
6.507, p = 0.00263**
81.3
0.574
3.08
29.7
9.48
17.7
6.57
33.6
26.8
0
+
0
0
+
–
+
0
–
+
4a
6
9
4
6
8
4
6
8
2.982
1
1
2.16
2.383
2.96
1
2.998
2.352
26.73, p = 9.88e-12***
0.092, p = 0.763
4.933, p = 0.0312*
2.94, p = 0.0621.
0.934, p = 0.536
2.379, p = 0.0615.
1.13, p = 0.293
9.519, p = 5.41e-05***
2.686, p = 0.0592.
65.5
0.199
9.68
13.8
5.72
17.2
2.4
40
15.2
0
+
+
+
0
–
0
+
0
+
p
p
p
p
p
p
p
p
p
=
=
=
=
=
=
=
=
=
0.000269***
0.867
0.619
0.00601**
0.0159*
0.0939.
5.45e-05 ***
5.37e-06 ***
2.69 e-05 ***
36.2
0.0612
1.7
15.3
23
13.6
40.6
46.8
39.1
salinity treatments was above those of the other days indicating recovery of individuals. In May 16, the models for days 6 and 8 explained
34% and 27% of deviance, respectively, and indicated highly significant
differences among treatments, but with contrasting patterns driven by
clearance rates (Fig. 6C). While on day 6, there was a parabolic curve
with the highest values at S10-25, similar to those of the other venerids,
on day 8 there was an increase of SFG at S15-30 and control. The SFG
curve of that day was below those of the other days except for control,
indicating no recovery of individuals.
Cerastoderma edule had the highest basal values compared to the
other species with a mean value of 89.4 ± 70.3 Jh−1g−1 in Dec15,
then lower in March16 and May16 48.2 ± 36.5 Jh−1g−1 and
5.8 ± 33.69 Jh−1g−1, respectively (Fig. 7A-C, Table 2). In Dec15
there were significant differences in SFG among treatments on day 9,
with models explaining around 10% of deviance and the SFG increasing
almost linearly with salinity treatments (Fig. 7A). The pattern was
driven by clearance rate. In March16, the SFG varied among treatments
with marginally lower values at S5-20. On day 8, the model explained
17% of deviance and indicated significant differences with the highest
values at S15-30 compared to the rest of treatments (Fig. 7B). Such
patterns were driven by both clearance rate and respiration. In May16,
the SFG varied significantly among treatments on days 6 and 8, with
models explaining 40% and 15% of deviance, respectively. Such patterns were mainly driven by clearance rate (Fig. 7C). On day 6, the SFG
showed higher values at S5-20 and a peak at S10-25 perhaps reflecting
deviance on day 6, and showed significant differences among treatments due to higher values at S10-25. On day 9, the model was not
significant (Fig. 5A). In March16 only on day 4, the model showed
significant variation in SFG among treatments and explained ~ 27% of
deviance, with the highest values at S15-30 (Fig. 5B). In May16, the
response was similar to that of V. corrugata although less pronounced
(Fig. 5C). The SFG varied significantly among salinity treatments on
day 4 and 8, both models explaining higher deviance (~24%) compared
to the model for day 6 (12%). On days 4 and 6, the SFG followed a
parabolic pattern with highest values at S10-25 and S15-30, whereas on
day 8 the SFG increased almost linearly with salinity, but the curve was
below those of days 4 and 6 indicating no recovery of individuals. The
pattern of SFG resembled that of clearance rate.
Ruditapes philippinarum had a mean basal SFG of 24.1 ± 26.5
Jh−1g−1 in Dec15, 10.4 ± 14.4 Jh−1g−1 in March16 and then in
May16 mean basal SFG was negative −8.6 ± 6.4 Jh−1g−1 (Fig. 6A-C,
Table 2). On days 6 and 9 in Dec 15, models explained a very low
percentage of deviance (< 4%) and did not show significant differences
among treatments (Fig. 6A). In March16, the models for days 4 and 8
explained 30% and 18% of deviance, respectively, and showed significant differences among treatments, but with very different patterns
mainly driven by clearance rate. While on day 4 the SFG decreased at
S10-25 and then increased at S15-30 following a sinusoidal pattern, on
day 8 there was a parabolic response with values increasing at the intermediate salinities (Fig. 6B). The SFG curve on day 8 in the reduced
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Fig. 4. Scope for growth, clearance rate and respiration rate (Jh-1g−1) of Venerupis corrugata in A: Dec15, B: March16 and C: May16 experiments, measured under
flood tide conditions. Lines are the predicted lines of the salinity model with 95% confidence bands. The symbols are the raw data for each salinity treatment. Yellow
solid circles: day 4, red rhombus: day 6, blue crosses: day 8/9.
compensation activity apparent both in clearance rates and respiration.
On day 8, the SFG was similar at all treatments except for controls and
values were below those on day 6, indicating no recovery of individuals.
decussatus.
3.4. Burrowing activity
3.4.1. Responses under ebb-tide conditions
The four species were unable to burrow after four days of stress at
the lowest salinities of each treatment, i.e. 5, 10 and 15 compared to the
control (χ2 values all p < 0.0001; Fig. 8-A,B,C,D, Table 3). The models
explained between 40% and 70% of deviance. Only a few individuals of
C. edule burrowed at salinities of 10 and 15; none burrowed at 5.
3.3. The importance of variables in SFG
The relative importance analysis confirmed clearance rate as the
main relevant variable, with adjusted R2 values from 0.6 to 0.95 across
all experiments and species. Respiration was the second variable in
importance ranging from 0.25 to < 0.1 followed by absorption efficiency and ammonia, which were never above 0.1 (see Supplementary
material, S.1). The highest values of relative importance for clearance
rate were for C. edule and R. philippinarum in all seasons (> 0.8). The
maximum relative importance of respiration for any of the four species
was 0.25, found in V. corrugata in Dec15 and in March16 for R.
3.4.2. Responses under flood-tide conditions
Individuals of V. corrugata showed significant differences in burrowing probability among treatments on day 6 in Dec15 and March16
(Fig. 9A, Table 3) due to a marked increase in the probability of burrowing with increasing salinity in the reduced salinity treatments and
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Fig. 5. Scope for growth, clearance rate and respiration rate (Jh-1g−1) of Ruditapes decussatus in A: Dec15, B: March16 and C: May16 experiments, measured under
flood tide conditions. Lines are the predicted lines of the salinity model with 95% confidence bands. The symbols are the raw data for each salinity treatment. Yellow
solid circles: day 4, red rhombus: day 6, blue crosses: day 8/9.
salinities. In March16, on day 6 highly significant differences were
found with the model explaining 32% of deviance. The lowest probability of individuals to burrow was at S5-20 and S10-25 and then it
increased markedly with higher salinity treatments. In May16 marginal
differences were detected only on day 8 and the model explaining 11%
of deviance although the pattern was the opposite to the previous days,
with highest probability of burrowing at S5-20, and then decreased
markedly with salinity treatments, reaching the lowest probability at
control.
Cerastoderma edule in Dec15 had a similar pattern all days, with
significant differences on day 9 due to lower probability of individuals
to burrow in all reduced salinity treatments compared to the control
and 16% of deviance explained (Fig. 9D, Table 3). In March and May16
only marginally significant differences among salinity treatments were
found. the model on day 4 in March16 explained 24% of the deviance
showed higher probability to burrow with increasing salinity. In May16
the model on day 6 with very little deviance explained (10%) and lower
models explaining 19% and 39% of deviance, respectively. After recovery on day 8/9, no significant differences were found among
treatments. In May16, marginally significant differences were found on
day 4 (Fig. 9A, Table 3) with the model explaining 13% of the deviance
and showing the same pattern of higher probability of burrowing for
individuals at salinities above S5-20.
On day 6 in Dec15 no differences were found for R. decussatus
(Fig. 9B, Table 3), as on day 9, when a clear recovery was observed.
Both models explained little of the deviance (< 10%). In March16 the
models explained very little of the deviance (< 5%) and did not show
significant differences among treatments. In May16 significant differences were found only on day 4, explaining 14% of deviance. The
lowest probability of individuals to burrow was at S5-20 and it increased markedly from S10-25 to higher salinities.
Ruditapes philippinarum showed differences on day 6 in Dec15 and
the model explained 21% of deviance (Fig. 9C, Table 3). The lowest
probability was at S5-20 and then it increased smoothly with increasing
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Fig. 6. Scope for growth, clearance rate and respiration rate (Jh-1g−1) of Ruditapes philippinarum in A: Dec15, B: March16 and C: May16 experiments, measured under
flood tide conditions. Lines are the predicted lines of the salinity model with 95% confidence bands. The symbols are the raw data for each salinity treatment. Yellow
solid circles: day 4, red rhombus: day 6, blue crosses: day 8/9.
treatments, but on day 6 neither exhibited higher SFG in the controls
(Table 2). In terms of burrowing ability, the venerids V. corrugata and R.
philippinarum, were the most affected species at lower salinity treatments although they recovered by day 8. In spring (May16), there were
two types of compensation response of SFG that led to an inability of
individuals to recover: the venerids V. corrugata, R. philippinarum and to
lower extent R. decussatus showed parabolic curves under stress on Day
6 with higher values for treatments S10-25 and S15-30. The cockle
showed a pronounced sinusoidal response with higher values at treatment S10-25 on day 6. In terms of burrowing ability, V. corrugata and R.
decussatus were the most impacted species at the lowest salinity treatments although they recovered after the stress (Table 3).
Valve closure was the strategy used by all species under the higher
stress (salinity 15 and below), as expected from previous findings
(Carregosa et al., 2014; Gharbi et al., 2016), though with remarkable
differences (Table 1). One third of V. corrugata individuals opened in all
treatments, which could explain its higher abundance in the low
burrowing probabilities at lower salinities were found.
4. Discussion
The overall response to stress was the same for all species at the
lower salinities (5, 10, 15), with almost no activity (valves closed and
no burrowing) and SFG near 0, supporting the general hypothesis of
salinity drops tapering physiological activity (Fig. 3A-D). Under the
higher salinities (20, 25, 30), species reacted diversely. In autumn
(Dec15) all species resisted generally well in terms of SFG, except for C.
edule which showed no recovery with lower values for the reduced
salinity treatments compared to controls (Table 2: Dec15: Day 9). All
species recovered burrowing ability although the responses differed.
Whereas the effects of low salinity treatments were still evident in V.
corrugata and C. edule, in the other two species they were not (Fig. 9,
Day 9). In winter (March16), both V. corrugata and R. decussatus exhibited reduced SFG on day 4 as expected in the reduced salinity
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Fig. 7. Scope for growth, clearance rate and respiration rate (Jh-1g−1) of Cerastoderma edule in A: Dec15, B: March16 and C: May16 experiments, measured under
flood tide conditions. Lines are the predicted lines of the salinity model with 95% confidence bands. The symbols are the raw data for each salinity treatment. Yellow
solid circles: day 4, red rhombus: day 6, blue crosses: day 8/9.
Widdows (1997) imposed temperature stress on R. decussatus and obtained similar values for respiration or ammonia but the clearance rates
were higher. Similar values to ours have been reported for SFG following stress due to ambient ammonia after low tide in estuaries
(Sobral and Fernandes, 2004). Albentosa et al. (2007) reported lower
respiration rates given food limitation for V. corrugata and R. decussatus.
The values of SFG found in this study were comparable to those found
in the literature considering that the experiments reported here were
designed to be sublethal with intermittent exposure to stress, while the
majority of those in the literature were designed for a continuous exposure to low salinity and often animals suffer significant mortality
(Kim et al., 2001).
Mean negative values of SFG were found in the controls of V. corrugata and R. philippinarum at the end of the experiment in Dec15 and in
March16. Moreover, these species had negative values of SFG in the
basal measurements before the imposition of stress in May16. The
highly energy-demanding process of reproduction (Griffiths and
intertidal and subtidal, where it is less exposed to long periods of low
salinity and presumed higher mortality associated with valve opening
at low salinities (Akberali and Trueman, 1985; McFarland et al., 2013;
Carregosa et al., 2014). Valve closure under lower salinities was more
extended in R. decussatus and R. philippinarum (Gharbi et al., 2016; Kim
et al., 2001), as in other intertidal species (Shumway, 1977). Similarly
to present results, adults of C. edule have low activity below 10
(Verdelhos et al., 2015), or recruits at 15 (Peteiro et al., 2018). However, a variable percentage remained open, as expected in a species
with intermitent gaping under periodic salinity fluctuations (Nossier,
1986; Malham et al., 2012).
Once salinity rose above 20, all four species opened valves to resume respiration, feeding and excretion of waste products, in agreement with previous findings (Kim et al., 2001; Verdelhos et al., 2015).
The values of the components of SFG reported here are similar to other
studies of salinity stress when clams were exposed continuously to
salinities of 20 or below (Kim et al., 2001; Nie et al., 2016). Sobral and
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Ecological Indicators 111 (2020) 106031
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Fig. 8. A,B,C,D. Burrowing probabilities for A:
Venerupis corrugata, B: Ruditapes decussatus, C:
Ruditapes philippinarum and D: Cerastoderma edule
on day 4 of the Dec15 experiment at the lowest
salinity of each ramp. Lines are the predicted lines
of the salinity model with 95% confidence bands.
The dots are the raw data for each salinity treatment.
Montaudouin and Bachelet, 1996; Solidoro et al., 2000; Garcia et al.,
2001; Bidegain et al., 2013).
Failure of burrowing may be an indicator of high physiological
stress and compromise individual survival (Haider et al., 2018). With
salinities ≤15 on day 4 of Dec15 no individuals buried at salinity 5 and
only a few C. edule at 10 and 15 (Fig. 8, Table 3). Under less extreme
stresses such as salinities ≥20, behaviour can show different patterns
due to energy trade-offs with basal maintenance and reproduction
(Ansell and Trueman, 1973). Generally, in the three experiments during
the stress the ability to burrow decreased under lower salinity treatments, increasing during the recovery period as expected (Verdelhos
et al., 2015).
Our results indicated that clearance rate was a good proxy to assess
salinity stress of adult bivalves in the laboratory when a complex experimental design to measure many variables is not feasible.
Nevertheless, respiration rate gives information that is useful to understand underlying processes such as physiological compensation (Kim
et al., 2001) and it is an easy variable to measure directly without much
manipulation. Finally, ammonia excretion is already avoided in many
studies since its weight in the calculation of SFG is small (Widdows and
Staff, 2006). For two of the species reported here, R. philippinarum and
C. edule, clearance rate had a relative importance of ≥0.8 in all experiments and ≥0.9 in both March16 and May16. In contrast, for V.
corrugata and R. decussatus, clearance rate was ≥0.6 in all experiments,
but respiration was ≥0.15 in all but one case. For the first two species
therefore, one might be able to measure just clearance rate to estimate
SFG, but not in the other two (see Supplementary material, Table S.1).
According to these results, ≤15 was the salinity threshold for high
frequencies of valve closure and sharp decrease of activity for most
individuals. The data on SFG and valve closure activity resulting from
these experiments together with field data from a salinity monitoring
system in the field (own data not published) allowed us to make some
Griffiths, 1987) is often associated with an increase in respiration rates
in bivalves (Widdows, 1978; Rueda and Smaal, 2004), as seen on V.
corrugata, R. philippinarum and C. edule, which are reproductive in May.
During late autumn and winter V. corrugata and R. philippinarum are
mostly in the gametogenic stage, progressing to a ripening period before spawning in spring (Joaquim et al., 2010; Rodríguez-Moscoso
et al., 1992) while gametogenesis in C. edule occurs somewhat earlier
(Martínez-Castro and Vázquez, 2012). Conservative species such as
those in this study (Pérez-Camacho et al., 2003; Karray et al., 2015)
mainly rely on energy obtained from exogenous feeding for reproduction. In the current study, clams received 1% of the mean dry weight
clams−1 day−1 of algae, a diet chosen to allow detection of stress. The
clams were not fed continuously, instead having their entire daily ration delivered over an approximate time span of 4–6 h. The decrease of
SFG of animals in controls in Dec15 and March16 may indicate that
clams received insufficient food to afford energetic costs associated
with gametogenesis and they were relying on internal energy reserves.
Negative basal values in May16 may be related to the exhaustion of
species after a spawning event.
SFG values can be classified into three categories (Widdows et al.,
2002): low growth potential/high stress (< 5 Jh−1g−1), moderate
growth potential/ moderate stress (5–15 Jh−1g−1), and high growth
potential/low stress (> 15 Jh−1g−1). Low growth potential was found
for all species at salinities ≤15, when many animals stayed closed or
siphons were mostly retracted temporarily reducing pumping activity
(Table 1: Dec15, D4; Woodin et al., 2020). For salinities > 15, R. decussatus had the lowest growth potential, regardless of season, while V.
corrugata ranged between low in autumn and moderate in winter and
spring, R. philippinarum between low in autumn and moderate to high in
winter and spring, and C. edule often had the highest growth potential,
particularly in spring (Figs. 3–6). Those results are in agreement with
field growth data of these species (Stenton-Dozey et al., 1994;
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Ecological Indicators 111 (2020) 106031
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Table 3
Summary of results of the GLM analyses to test the effect of salinity treatments on the burrowing activity. Values in bold are statistically significant (p < 0.05) and
for those graphical patterns are presented by: + when the expected pattern of lower SFG for lower salinities was apparent in the graphical analysis, − when the
iverse pattern was apparent and 0 when neither of these patterns was apparent. a: Note that measurements on day 4 of the Dec15 experiment were done under
different conditions, i.e. at the lowest salinity of each ramp.
Species
V. corrugata
Exp
Dec 15
Mar 16
May 16
R. decussatus
Dec 15
Mar 16
May 16
R. philippinarum
Dec 15
Mar 16
May 16
C. edule
Dec 15
Mar 16
May 16
Day
Df
Chisq, pvalue
% DE
Patterns in Burrowing
5–20 lower
30–30 higher
4a
6
9
4
6
8
4
6
8
3
3
3
3
3
3
3
3
3
27.977, p = 3.672e−06***
10.162, p = 0.01724*
3.3425, p = 0.3418
0.94212, p = 0.8153
23.968, p = 2.537e−05***
1.512, p = 0.6795
7.807, p = 0.05017.
5.0851, p = 0.1657
3.721, p = 0.2932
64.7
18.8
5.4
1.4
38.9
2.5
12.9
7.6
6.4
0
+
+
+
+
+
+
+
4a
6
9
4
6
8
4
6
8
3
3
3
3
3
3
3
3
3
23.891, p = 2.633e−05***
5.6528, p = 0.1298
0.50802, p = 0.9171
2.2805, p = 0.5163
2.8539, p = 0.4147
3.0698, p = 0.381
8.8016, p = 0.03205*
2.8683, p = 0.4124
2.4924, p = 0.4767
59.4
9.09
1.03
3.4
4.4
4.7
14.3
5.1
4.1
0
+
+
+
4a
6
9
4
6
8
4
6
8
3
3
3
3
3
3
3
3
3
32.831, p = 3.495e−07***
13.612, p = 0.003483**
1.0455, p = 0.7902
3.0545, p = 0.3833
20.199, p = 0.0001544***
0.94212, p = 0.8153
4.612, p = 0.2025
0.98551, p = 0.8048
7.2211, p = 0.06517
70.7
20.9
7.9
4.6
32.3
1.4
7.2
1.5
10.9
0
+
+
+
+
+
–
–
4a
6
9
4
6
8
4
6
8
3
3
3
3
3
3
3
3
3
22.196, p = 5.938e−05***
3.5895, p = 0.3093
10.322, p = 0.01602*
7.7682, p = 0.05105.
0.88275, p = 0.8296
4.1795, p = 0.2427
2.3875, p = 0.496
6.7767, p = 0.07937.
4.1442, p = 0.2463
39.6
5.7
15.6
24.2
2.1
8.1
3.6
10.5
6.2
+
+
+
0
+
0
+
+
projections of seasonal fitness for each species to define management
strategies. During high tide, seawater salinity in one of the most productive shellfish beds (Carril, Ria de Arousa) during autumn 2015, was
≤15 during 9.5% of the time, while in winter and spring salinities ≤15
occurred 21% and 23% of the time, respectively. The SFG values
multiplied by the proportion of active animals at reduced salinities
(Table 1) and the amount of time when salinity was ≤15 yield SFG in
the field (f-SFG) for each species and season (see Supplementary material, Table S.2). These data allow us to address the question about the
impact of low salinities in the field on energetics and behavioral responses. A striking result is the gross reduction in SFG by applying the
field salinity adjustment. The loss of > 20% of the original SFG value
due to short-term exposures to salinities ≤15 is impressive. For time
periods when SFG without stress is already low such as March16 for R.
decussatus and May16 for V. corrugata, reducing SFG due to low salinities in the field with the behavioral response of valve closure increases the probability of organisms in energy deficit.
The incidence of extreme events such as heavy rainfall is expected to
increase according to future climate change scenarios. As these results
indicate, consequences for growth of tidally fluctuating salinities can be
significant even though mass mortality does not occur, and managers
need to consider the consequences in their management plans. In this
sense we present a conceptual model (Fig. 10) that can serve as a
planning tool for managers and it can be used to develop understanding
among researchers, managers and policy makers.
CRediT authorship contribution statement
Rula Domínguez: Conceptualization, Methodology, Investigation,
Formal analysis, Writing - original draft, Visualization. Elsa Vázquez:
Conceptualization, Methodology, Investigation, Formal analysis,
Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Sarah A. Woodin: Conceptualization,
Methodology, Investigation, Formal analysis, Resources, Writing - review & editing, Funding acquisition. David S. Wethey:
Conceptualization, Methodology, Investigation, Formal analysis,
Resources, Funding acquisition. Laura G. Peteiro: Methodology,
Investigation, Writing - review & editing. Gonzalo Macho:
Methodology, Investigation, Writing - review & editing. Celia
Olabarria: Conceptualization, Methodology, Investigation, Formal
analysis, Resources, Writing - review & editing, Supervision, Project
administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence the work reported in this paper.
13
Ecological Indicators 111 (2020) 106031
R. Domínguez, et al.
Fig. 9. A,B,C,D. Burrowing probabilities for A: Venerupis corrugata, B: Ruditapes decussatus, C: Ruditapes philippinarum and D: Cerastoderma edule of the three experiments at the highest salinity of each ramp. From left to right column: Dec15, March16 and May16. Yellow solid circles: day 4, red rhombus: day 6, blue crosses:
days 8/9.
14
Ecological Indicators 111 (2020) 106031
R. Domínguez, et al.
Fig. 10. Conceptual model applied to the fishery management. The main effects and consequences of salinity changes on SFG and burrowing activity on the four
species over time.
and Noia respectively, for providing the clams and cockles and valuable
comments for the experiment.
Acknowledgments
This research was supported by grants CTM2014-51935-R from the
Spanish Ministerio de Economıa y Competitividad to the project
MARISCO, a pre-doctoral grant for RD (CTM2014-51935-R), and the
Autonomous government Xunta de Galicia-FEDER (project GRC2013004) and grants NNX11AP77G from the US National Aeronautics and
Space Administration (NASA) and OCE1129401 from the US National
Science Foundation to DSW and SAW. Facilities were kindly provided
by the Estacion de Ciencias Mariñas de Toralla (ECIMAT) of the
University of Vigo. We want to thank Esther Pérez and all the staff at
ECIMAT for their technical support. We also thank J.C. Mariño and L.
Solís, technical Assistants of the Cofradía de Pescadores of Cambados
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ecolind.2019.106031.
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