A Parasitic Life in the River
In the freshwater of the Nakdong River lives a unique population of mussels whose genetic story is linked to the river's ecosystem. Species like Nodularia breviconcha, which is endemic to the Korean Peninsula, have a complex life cycle that depends entirely on a parasitic relationship with specific fish. In early summer, male mussels release sperm into the water, which is then inhaled by the females to fertilize eggs. These eggs develop into microscopic larvae, known as glochidia, within the female's gills.
To survive, these glochidia must be released and attach themselves to the gills or fins of a suitable host fish. For some Korean species, a primary host is the Amur bitterling (Rhodeus sericeus). The larvae live as parasites on the fish for several months, drawing nutrients and developing into juvenile mussels. This stage allows the otherwise sedentary mussels achieve dispersal throughout the river system. Once the metamorphosis is complete, the young mussels, now about 0.4 mm in size, detach from their host and drop to the riverbed, where they burrow into the sand and gravel to begin their long lives, which can exceed a century.
A Genetic and Environmental Record
DNA analysis reveals that freshwater mussel populations in Korea are distinct from their European counterparts. One Korean species, Nodularia breviconcha, is a full species, elevated from a subspecies of Nodularia douglasiae. Time-calibrated phylogeny based on mitochondrial DNA suggests that different genetic lineages of this mussel diverged during the late Miocene, between 6 and 8 million years ago. This ancient divergence is linked to the formation of the Taebaek and Sobaek-Noryeong mountain ranges, which isolated river systems and set these mussels on a separate evolutionary path.
The survival of these genetically unique mussels indicates of the Nakdong River's health. Their populations have declined due to habitat loss and pollution. A major event was the Four Major Rivers Restoration Project, announced in June 2009, which involved dredging and the construction of numerous dams and weirs on the Nakdong and other rivers. The Nakdong River barrage, built between 1983 and 1987, had already altered the natural flow and habitat. The additional weirs turned flowing river sections into standing water, creating ideal conditions for harmful algal blooms and altering the sediment composition on which the mussels depend. This degradation of their habitat, combined with the disruption of their host fish populations, threatens the continued existence of these ancient lineages.