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Theme report

A Key Indicator of Mudflat Health – The Cyclina sinensis (Black Clam)
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Treasure that connects wetlands and folk’s daily live
Walk through the streets and alleys of Taijiang, under the shades of roadside trees and behind the restaurant banners you might chance upon sizzling flames on stoves, accompanied by the fragrance of basil leaves, leading you in with their savory smells. With the frantic stirring of woks by busy chefs and the irresistible scent of red pepper mixed with oyster sauce, passersby cannot help themselves stopping by to have a look. Abundant fresh seafood awaits their fate in the water tanks and freezers of the restaurants. Black clams, common orient clams, oysters, mud crabs, sand shrimps, milkfish, gracilaria plants, and plate after plate of fresh-caught sea fish are on offer. Local ingredients form an integral part of the local food culture, and the rich biodiversity of the wetlands is transferred through the food chain, ultimately gracing our tables with the abundant gifts of nature.
This delicious plate of black clams (Cyclina sinensis) comes from the Taijiang Wetlands. Several streams flow through the Taijiang National Park, including Qigu River, Zengwen River, Yanshui River, Luermen River, Zhufagang River, Jianan Canal, etc. The rivers and water networks are interconnected, flowing through the Qigu Lagoon, Zengwen River Wetlands, Sicao Wetlands, and the mangroves along the coastal estuaries and mudflats before finally entering the sea. This wetland environment formed by land, the sea, and rivers has created an excellent habitat for aquatic animals. People have lived near the water since ancient times, and have integrated into coastal living over the generations into a part of the wetland biosphere. Various methods of "surviving off the sea" by the local populace have been developed, including the consumption of fish, shrimp, crabs, snails, and other biological resources of the wetlands. This form of livelihood has formed a “network” of marine ecosystems and the human settlement culture of the Taijiang Wetlands. Therefore, wetlands have nourished human life and played an extremely important role in the development of human civilization in Taijiang. For this reason, harvesting black clams and mud crabs, laying fishing nets and placing cages, and other forms of fisheries, as well as the development of aquaculture in the wetlands and lagoons of the coastal areas have become a common memory of coastal residents in the Taijiang area and has formed into a distinctive marine culture.
The Many Faces of the Black Clam
Among the many seafood delicacies in the Taijiang Wetlands, what locals refer to as “Chi Zui,",” literally red mouth, is the Cyclina sinensis (black clam). This clam is a clam species in the venus clam family Veneridae under the class Bivalvia. Black clams grow in warm waters and have a wide distribution along the coastal areas and mudflats of East Asia from North Korea, Japan, Ryukyu Islands, China, Taiwan, to Hainan and even to parts of Southeast Asia. They are known by different names in different places. In Taiwan, they are commonly known as “red-mouthed clams.".” In China, they are often called green clams, sea clams, black clams, iron clams, and bull-eye clams. In Japan, they are called オキシジミ (big clams).
Shell and anatomy of black clams
Cyclina sinensis shells are nearly round in shape, with a length of less than 4 cm. The shell length is generally slightly larger than or equal to the shell height. When the shell length exceeds 4 cm, the shell height is generally slightly larger than the shell length, and the maximum shell length can reach 6-7 cm. The shell surface is round and features a convex, with a thin but firm texture. The clam has two shells of equal size connected by two adductor muscles, and feature concentric growth lines. The color of the shell surface is either white, pink, purple, brown, light yellow, and black. Clams under 1 cm are usually purple; the subsequent color of the shell is closely related to the material of the seabed. A muddy seabed will result in black-colored shells. A sandy seabed will usually result in white-colored shells. The inner edge of the shell has a row of small hinge teeth, with 3 main teeth on each side of the shell. The clam meat is delicious and rich in nutrients, and the shell, which contains calcium carbonate, also has traditional Chinese medicinal values such as clearing away heat and reducing phlegm, softening and relieving lumps, reducing soreness, and relieving pain.
Habitat of black clams
The habitats of the black clams are mostly near the estuary where freshwater flows into the ocean. At low tide, they will be exposed on the water surface or silt during high tide and mid-tide areas of the intertidal zone, or the beaches of a lagoon. Therefore Taijiang is an ideal habitat for black clams. The clams are highly adaptable and can grow in coarse sand, soft powdery sand, and muddy areas. It likes waters that are clean, preferably with flowing tidal water, and that is rich in benthic diatoms. The clam has longer siphons than the common orient clam, and the fully extended siphon is about 2-3 times the body length. When burrowing in the sand, the front end of the body is downward and the back end is upward, and the foot is used to burrow the sand to bury its body. After low tide, the surface of the mudflat will have small oval holes formed by the extension of clam siphons. The burial depth varies with the season, individual size, and sediment type, but generally 9-16 cm in depth. Their burrowing depth is shallower in summer and deeper in winter; in the same season, their burrowing depth in fine silt is deeper than in sandy and muddy sediment. When burrowing in the sand, the two shells open slightly, with the feet and siphons extended. Once external stimulation is felt, the siphons quickly retract into the shell to prevent external threats. Unlike the common Manila clam (also known as Japanese cockle), black clams seldom crawl out of the mud and "drift with the current,” so their ability to move and migrate is very small.
Diet of black clams
Clams mainly feed on planktonic food. It lives in muddy environments and stretches out a long siphon on the bottom of the water surface to draw seawater in. The seawater is introduced through the mantle cavity inside the shell. After the gill filaments on the pair of gill flaps filter the water, the cilia on the gills will transport the algae filtered from the water layer to the mouth. The lip of the mouth collects the food and mucus together into the stomach. From analyzing stomach content, we can observe that clams mainly feed on diatoms, including Nitzschia crescentia, Cyclosporidium, Plume, Platymonas, Navicula, and sometimes many copepod stumps and organic debris. Juvenile and adult clams alike are not selective in food choice. Clams and other mudflat shellfish are a key part of the mudflat ecosystem. In addition to filtering phytoplankton in the water, they play the role of a first-level consumer, and they also filter organic debris such as organic particles in the water to become detritus. At the same time, they are also an important source of food for other marine life. Their predators on wetlands include moon snails, mud crabs, and Nereis worms. In addition, there has been recorded evidence of black clams becoming an intermediate host for parasitic flatworms.
Reproduction and growth of black clams
The reproductive season of the ring clam in the Taijiang area is about August to September. The ring clam is dioecious, and no difference can be observed from the shell. If the closed double shell is opened, the gonads of sexually mature females over 2-3cm in size are pink. The male gonads are pale yellow. When a small piece of the gonads are retrieved with tweezers and dripped with seawater, the sperm appears white and cloudy. The eggs come in the form of particles, spherical or elliptical in shape, with an egg diameter of 93.8μm. The sperm swims with a flagellum and attaches to the egg cell. After fertilization, the embryo starts growth and development. After multiple cell divisions into 8 cells, 16 cells, 32 cells, etc., the embryo develops into a multicellular stage with no cavity inside the embryo, called a morula stage. At this time, the cell division continues and the embryo develops into the blastocyst stage. In the blastocyst stage, cilia grow densely around the embryo, and the embryo begins to rotate. As part of the cells invaginate, the gastrulation stage is formed. Then the embryo grows a ciliary ring with a flagella bundle in the center, which moves straight in the water and becomes a basal larva. The embryonic body develops to the formation of an embryonic disk, with a D-shaped shell covering both sides, and the outline becomes D-shaped. Under normal environmental conditions, it will develop to the D-shaped larval stage in about 16 hours. At this stage, the intestinal tract is fully developed and starts to eat, excrete, and float on the water surface. When the larva develops into the umbo stage, the seeding stage ensues. At around the 3rd day of fertilization, the nascent foot will extend and stretch. At this time, the embryonic disk is still present, with the nascent foot and the disk alternately moving until gradually the disk shrinks and degenerates. During the transition into the creeping larva stage, the foot becomes the only organ for movement, and the siphon begins to form. The outlet siphon is extended on the 7th day of fertilization and the inlet siphon is extended on the 20th day. The single siphon then develops into a double siphon. During this period, the shell shape and internal structure are similar to adult clams, and they likewise exhibit similar behavior. They then enter the juvenile stage and become a member of the wetland ecosystem.
With sufficient food, juvenile clams can grow to about 0.3-0.5 mm in length within one to three months. As the shell grows larger, the growth ring and level of translucence on the shell surface can exhibit light and dark bands. With survey samples of the clam population, the age of the clams can be estimated from the size of the shell. The length of the first-year shell is about 1.4-2.4 cm, the second-year shell is about 2.5-3.6 cm, and the third-year shell is about 2.5-3.6 cm. The third-year shell length is about 3.6-3.8 cm, the fourth year is about 4.0-4.5 cm, the fifth-year is around 4.5-5.0 cm, the sixth-year around 5.0-5.8 cm, and the seventh to eighth-year at more than 6 cm. The growth of juvenile shells to one-year-old shells is relatively fast. It is estimated that the growth of wild clams will be slower as the shell age gradually ages.
The tug-of-war between wetland evolution and management
The entire lifecycle of the black clam revolves around wetland environments. The bottom of the wetland is the space where benthic animals such as shellfish live on and is an important place for foraging and habitat. Therefore, the quality of the seabed environment is directly related to the survival of the clams. Clams are filter-feeding animals with a strong water filtering ability. It drives the biological sedimentation in the water environment and guides the organic suspended particles in the water to accelerate the sedimentation and accumulation to the bottom. The accumulation of organic matter will also ensure the active growth of sulfate-reducing microorganisms. The increase in the content of sulfide in the sediment will cause the deterioration of the ecological environment and even the death of benthic marine life.
During the evolution process of wetland, sedimentation will accelerate the aging of wetlands, and with the change of the composition of wetland clusters, the management of the habitats and environment of black clams must be considered as a whole. At present, the Shifen Black-faced Spoonbill Ecological Reserve in Taijiang National Park, like other natural habitats of clams along the coastline, is affected by the cultivation and utilization of adjacent land, as well as the upstream nutrients and suspended solids brought in by river water. This results in varying degrees of accumulation of nutrients. The accumulation of nutrients will lead to the eutrophication of water quality and the formation of an anoxic environment. The sulfide in the substrate will produce toxic substances through the process of dissimilation and reduction of sulfate or the process of microbial decomposition of sulfur-containing amino acids in the organic matter. Under certain conditions, the sulfide in these substrates enters the water body through release, water convection, and other means. This, combined with the hemoglobin in the blood of aquatic organisms such as fish, forms sulfur hemoglobin, reducing the oxygen-carrying capacity of the blood in the organism. In addition, sulfide has a strong stimulating and corrosive effect on the gill tissue of aquatic organisms, which can cause coagulation and necrosis of the tissue, and even cause respiratory disorders, and eventually suffocation. Sulfide has a strong toxic effect on benthic animals such as black clams and may result in a large number of deaths of low-sensitivity black clams, so the population of black clams is an important indicator to assess the quality of the environment.
Science-based management strategies
The change of wetland seabed environment is a risk factor faced by organisms in coastal and mudflat wetlands, which leads to potential threats to wetland ecology. Therefore, in the face of the aging of the seabed bottom along the coastal regions, it is imperative to carry out environmental restoration work based on scientific rigor and cautious operations and to improve the problem of substrate aging through the research and application of multiple restoration methods such as physics and biology to maintain the ecological function of wetlands.