The impact of salinity changes associated with size on the wedge clam Donax trunculus Linnaeus, 1758 (Mollusca: Bivalvia): A laboratory assay
Introduction
Estuaries and coastal areas close to river mouths and fluvial discharges are strongly influenced by flooding, eutrophication events, and changes in salinity and temperature (González-Ortegón et al., 2013; Verdelhos et al., 2015). Climate change may induce alterations in these factors (salinity reduction, ocean surface temperature increase and sea level rise) and consequently may produce alterations on benthic communities that inhabit these ecosystems (Verbrugge et al., 2012; Lejeusne et al., 2014; Baptista et al., 2014; Verdelhos et al., 2015; Corte et al., 2017; Peteiro et al., 2018). In recent decades, climate change has given rise to extreme weather events (IPCC et al., 2014) making the average temperature in temperate zones vary (Miranda et al., 2006) and increasing the frequency in which heavy precipitation events occur (Teixeira et al., 2008). Within this scenario of change, with clear human influence, a series of risks are posed for human and natural systems, with the main issues for the latter being the loss of biodiversity and the loss of ecosystem goods and services (IPCC et al., 2014).
Estuaries and tidal areas provide many ecosystem services that benefit human society, including the supply of nursery habitats for species, many of which are of commercial interest. The organisms that inhabit these areas may be exposed to salinity changes in the short (tides) and long term (rainy season), creating an osmotic gradient between the environment and the organism's body which triggers behavioral and physiological responses (Shumway et al., 1977). Consequently, increased stress caused by a sharp drop in seawater salinity can lead to mortality episodes (Matthews and Fairweather, 2004), as was reported after periods of heavy rain (Baptista, 2012). Salinity changes have adverse effects on more than just mortality, also causing sublethal effects such as behavior modification, reproduction, respiration, osmoregulation and feeding. For example, sudden salinity fluctuations result in a decrease of pumping activity, SFG (scope of growth) and burrowing ability (Domínguez et. al., 2020; Woodin et al., 2020) and in loss of 1–24% of the energy acquired (Gosling, 2003). Furthermore, clams under salinity stress might increase their antioxidant defenses, to endure the oxidative stress resulting from hypo and hypersaline conditions, which will result in a substantial modification of their biochemical mechanisms (Carregosa et al., 2014). Bivalves are crucial to estuarine and coastal function, since they stimulate the microphytobenthic productivity and play a key role in the trophic web, transferring energy from primary production to predators such as birds, crabs, fishes and humans (Dumbauld et al., 2009). Bivalves of genus Donax Linnaeus, 1758, are important components of the macrofauna in shallow areas of sandy beaches (Ansell, 1983), where they live buried filtering phytoplankton and suspended particulate organic matter (Wade, 1967). This feeding behavior determines the vertical distribution of this taxon, which normally lives in the swash zone of beaches and shallow sandy bottoms, where eutrophic conditions result in high levels of phytoplankton production (Ansell, 1983) and hydrodynamic forcing promotes resuspension of particles and delays rapid sedimentation of organic matter (Manca Zeichen et al., 2002).
Bivalve shellfisheries play a significant part in the social, economic and cultural well-being of many coastal communities (Gaspar et al., 2012) and they are an important component of the world's fishery production. Europe on its own is responsible for 5.5% of the world production of marine bivalves (Wijsman et al., 2019). Donacidae or wedge shell species are especially important for artisanal and small-scale fishing, and D. trunculus in particular represents an important fishery in Spain, Portugal, France and Italy (da Costa, 2012).
D. trunculus is extended along the Atlantic and Mediterranean French coast, the Iberian Peninsula and North Africa (Ibrahim et al., 2015; Reyes-Martínez et al., 2015). It lives at depths of 0–6 m (Gaspar et al., 2002; La Valle et al., 2011), showing an intraspecific segregation between juvenile and adult specimens. Juveniles are commonly located in shallower areas, whereas the oldest specimens are found at greater depths (Wade, 1967; de la Huz et al., 2002; Gaspar et al., 2002; Manca Zeichen et al., 2002). Potential explanations of this distribution variation refer to strategies for avoiding intraspecific competition (Ansell and Lagardère, 1980), as well as current and hydrodynamic influences since larvae are displaced passively to shallower areas (Gaspar et al., 2002).
On the basis of this pattern of depth size distribution of D. trunculus, it is feasible to consider that young clams will be more vulnerable to sudden and unforeseen salinity variations. These changes could produce high mortality among them, affecting adult cohorts and therefore, the population's reproductive capacity. However, in this study we will test the hypothesis that juvenile individuals will be more tolerant to changes in salinity than adults. With this aim, we will establish the lowest salinity tolerance limit, as well as determine median lethal salinity (LC50) and median lethal time (LT50) both for adults and juveniles of D. trunculus. This information will be very helpful to determine the role of salinity changes on the intraspecific distribution pattern of this species. All the salinities used in this study have been reported using the practical salinity scale.
Section snippets
Species acclimation
400 specimens of D. trunculus were collected in the intertidal zone using rakes on the coast of Isla Cristina (South Western Spain). Before experimentation, wedge clams remained in the laboratory for 7 days for their adaptation to captivity. They were placed in two 40 L bins (one-half of the clams in each bin) with sand on the bottom, continuous water flow, and aeration. Every 24 h 1 μm filtered and UV-treated seawater (FSW) was totally renewed in each bin, and animals were fed ad libitum with
Salinity tolerance
Salinity trials equal to or less than 14.2 resulted in 100% of mortality in both size groups of D. trunculus, whereas the survival rate was 100% at salinity treatments equal to or greater than 26.7 (Fig. 1). Two-way ANOVA results indicated a significant interaction between the factors Salinity and Size class on Survival (Salinity*Size class, F = 4.567, P < 0.05) (Table 1).
The survival values among 16.7 and 21.7 salinity treatments varied between both size classes. Juvenile survival values were
Discussion
D. trunculus is one of the most important bivalve species in the macrobenthic fauna community from the shallowest areas (0.5–2 m) of sandy beaches (Ramón et al., 1995; Escrivá et al., 2020; Gaspar et al., 2002). Furthermore, it has been recently proposed as an adequate sentinel species for the monitoring of environmental status of sandy beaches due to their biological and physiological characteristics (Tlili and Mouneyrac, 2019; Lamine et al., 2020).
One of the effects of the extreme climatic
CRediT authorship contribution statement
M.J. Reyes-Martínez: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. I. Martínez-Pita: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. D. Soler-Navarro: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. F.J. García-García: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original
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.
Acknowledgments
Special thanks to Manolo Prado from AGAPA (Junta de Andalucía) for providing us the wedge clams and to IFAPA Agua del Pino for supplying the filtered seawater and microalgae mixture to feed clams. We are also grateful to Clara Gavira-O'Neill for the English revision of the manuscript and to anonymous reviewers for their helpful comments.
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