Population scarcity of an estuarine crab (Chiromantes dehaani) caused by intertidal reclamation from the perspective of reduced salinity
Introduction
In recent decades, many coastal cities have had to address the pressing problem of sustaining an increasing human population with limited land resources. Wetland reclamation is widely considered as providing an effective way to solve this problem of land shortage. However, such reclamation may interfere with ecosystems processes and undermine the ecological integrity of intertidal wetlands (Yim et al., 2018). On a global scale, intertidal wetlands, namely 50% of salt marshes, 35% of mangroves, 30% of coral reefs, and 29% of seagrasses, have been heavily degraded or lost entirely due to reclamation projects and other human activities (Barbier et al., 2011). Moreover, reclamation activities may weaken or impair the ecological functions and services that intertidal wetlands provide for many aquatic animals and migrating birds.
Macrobenthos are major secondary producers in intertidal wetlands (Lv et al., 2018). Due to their sedentary life history and high environmental sensitivity, macrobenthos are considered as potentially powerful bio-indicators for the reliable monitoring of environmental change (Zhou et al., 2019, Vassallo et al., 2020). Previous studies have shown that reclamation projects may cause species loss and community degradation of macrobenthos, because of the changes of abiotic (elevation, sediment characteristics and water salinity) and biotic (vegetation) factors (Yuan and Lu, 2001, Yang et al., 2018). However, the knowledge on why this community degradation occurs is rather sparse, because too few studies have investigated the altered physiological traits of macrobenthos caused by environmental changes that occur when wetlands are reclaimed.
According to many researchers, reduced water salinity is a major environmental characteristic that is affected after wetlands are enclosed by seawalls (Lv et al., 2016, Matsuda and Kokubu, 2016, Liu et al., 2018). The irregular change of water salinity may significantly disturb the growth and reproductive performance of macrobenthos, because water salinity is usually characterized by interannual periodic changes in estuarine area, due to the complex interaction between freshwater flow and tidal currents (Chen et al., 2012, Hu et al., 2014). Macrobenthos usually complete their life cycle in the process of water salinity changes. In the laboratory, much research suggests that osmoregulation (Romano and Zeng, 2006, Gomes et al., 2019, Lu et al., 2019), energy metabolism (Chen et al., 2014, Huang et al., 2019), gonadal development (Long et al., 2017), mating behavior (Wu et al., 2013, Ikhwanuddin et al., 2014) and embryonic development (Rotllant et al., 2015, Urzúa et al., 2018, Wang et al., 2019) of economically important macrobenthos are easily affected by changes to salinity.
Chiromantes dehaani is a typical crab that mainly lives in estuarine areas of East Asia. This crab has similar ecological importance to other intertidal crabs. Many studies have demonstrated that the bioturbation by crabs, e.g., ingestion and excretion, can promote biogeochemical cycling in wetlands (Thongtham et al., 2008, Chandler et al., 2015, Abdullah and Lee, 2016). Because it figures prominently in ecosystem dynamics, C. dehaani is often called an “ecosystem engineer” (Lee, 1998). Moreover, crabs and their larvae are one of the most important food resources for birds and fishes. In Yangtze estuary, C. dehaani is often used to indicate environmental degradation, because it is easy to collect and sensitive to environmental changes. Hence, to better understand the reasons behind the degradation of macrobenthic communities in reclaimed wetlands, it is pertinent to not only consider population-level changes but also how the growth and reproductive measures of C. dehaani are affected under different levels of water salinity.
In this study, we assessed the number of C. dehaani in two reclaimed wetlands of the Yangtze estuary to detect the effect of reclamation on C. dehaani populations. Then, in the laboratory, we simulated the changes in water salinity characterizing reclaimed wetlands and measured the concentrations of Na and K in the hemolymph and the activity of Na/K ATPase (NKA) in the gills of C. dehaani. We also analyzed the levels of triglyceride (TG) and total cholesterol (TCHO) in hemolymph, as well as lipid contents in the ovary and hepatopancreas. Going further, we examined the impact of different salinity levels on the crab’s reproductive metrics, including gonadal development and embryonic development. The primary objective of this study was to detect the mechanism(s) of salinity decrease in reclaimed wetlands on the C. dehaani population.
Section snippets
Experiment 1
In experiment 1, we compared the water salinity and number and weight of C. dehaani between a fully-enclosed wetland (i.e., east shoal of Hengsha Island) and semi-enclosed wetland (i.e., the east shoal of Chongming Island).
The east shoal of Hengsha Island was fully enclosed from the sea by dikes in 2012. Thus, natural seawater cannot enter into its enclosed area. The vegetation types here include Phragmites australis, Scirpus mariqueter, as well as emerging terrestrial vegetation. The east
Results
Effects of reduced salinity on C. dehaani population in reclaimed wetlands
In this investigation, the fully-enclosed wetland in the Yangtze estuary had an extremely low level of water salinity (0.2–0.5)in its reclaimed zone in which no C. dehaani crabs were found. By contrast, the crab’s number and weight were similar between reclaimed and natural zones in the semi-enclosed wetland. Moreover, we found a significant positive correlation between the number of C. dehaani and water salinity. These above results suggest reduced salinity may cause the decrement or
Conclusion
Our comprehensive study used both field and laboratory data to explore the reasons for the scarcity of C. dehaani population under lowered salinity conditions in reclaimed wetlands. In the investigation, no crabs were found in the fully-enclosed wetland, which had an extremely low water salinity (0.2–0.5). Female C. dehaani were exposed to freshwater and salinity conditions in a laboratory setting. Results showed that the crabs possessed the ability of hypertonic regulation in a freshwater
CRediT authorship contribution statement
Weiwei Lv: Writing - original draft, Methodology, Software. Qiang Chen: Data curation. Quan Yuan: Visualization, Investigation. Wenzong Zhou: Supervision. Yunlong Zhao: Conceptualization, Validation, Writing - review & editing.
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
This study was supported by the Shanghai Municipal Science and Technology Commission, China [grant number 19ZR1436900]; and the SAAS Program for Excellent Research Team, China [nong ke chuang 2017 (A-03)].
References (39)
- et al.
Meiofauna and crabs in mangroves and adjoining sandflats: Is the interaction physical or trophic?
J. Exp. Mar. Biol. Ecol.
(2016) - et al.
Spatial and temporal variations in nitrogen and phosphorous nutrients in the Yangtze River Estuary
Mar. Pollut. Bull.
(2012) - et al.
A simple method for the isolation and purification of total lipides from animal tissues
J. Biol. Chem.
(1957) Variations de la composition lipidique tissulaire au cours de la vitellogenese chez la crevette Penaeus indicus Milne Edwards
J. Exp. Mar. Biol. Ecol.
(1984)- et al.
Variability in soil microbial community and activity between coastal and riparian wetlands in the Yangtze River estuary – Potential impacts on carbon sequestration
Soil Biol. Biochem.
(2014) - et al.
Effects of salinity on growth, fatty acid synthesis, and expression of stress response genes in the cyclopoid copepod Paracyclopina nana
Aquaculture
(2017) - et al.
Different effects of reclamation methods on macrobenthos community structure in the Yangtze Estuary, China
Mar. Pollut. Bull.
(2018) - et al.
Loss and self-restoration of macrobenthic diversity in reclamation habitats of estuarine islands in Yangtze Estuary, China
Mar. Pollut. Bull.
(2016) - et al.
Macrobenthos functional groups as indicators of ecological restoration in reclaimed intertidal wetlands of China’s Yangtze Estuary
Reg. Stud. Mar. Sci.
(2018) - et al.
Recent coastal environmental management based on new concept of Satoumi which promotes land-ocean interaction: A case study in Japan
Estuar. Coast. Shelf S
(2016)