Kallumakka or Kadukka, known as Asian green mussels or Perna viridis, may be among the most sought-after non-vegetarian dishes in Kerala cuisine. While it’s typically known as a delicacy from the north of the state, it has found favour across Kerala. However, it now appears that a foreign invasive species is threatening to replace the Asian green mussels in the backwaters of the state.
Rajan, who makes a living by collecting clams in the backwaters of Kochi, notes that the invasive mussels have doubled in numbers in the last few years.
"We find them in large quantities, clinging in clusters to the wall around the backwaters, on the bottom of boats, on rocks or whatever hard objects they find in the water. They are black in color and smaller in size compared to our green or brown mussels," Rajan says.
These observations from fishermen were validated in a study published recently by the Department of Aquatic Biology and Fisheries, University of Kerala.
"The invasive bivalve mussel Mytella strigata is spreading in many estuarine ecosystems and brackishwater lakes in Kerala coast. Its rapid spread across the state might have been triggered by Ockhi, a strong tropical cyclone in the Arabian sea that struck the Kerala coast in 2017," the study states. It was conducted by the head of the department professor Biju Kumar A, research scholars Ravinish R, Oliver PG, Tan SK and Sadasivan K.
The invasive species of the Mytella genus are commonly known as American Brackish water mussels or Charru mussels. They can cling to any hard surface, including man made structures, and are native to the South and Central American coasts.
The Indian native green mussel species lives partially buried under mud or gravel in water, according to the study. Researchers were able to identify the presence of the invasive mussels in Kadinamkulam, Paravur, Edava-Nadayara, Ashtamudi, Kayamkulam, Vembanad, Chettuvai, and the Ponnani estuaries/backwaters of Kerala.
"Among the various estuaries and backwaters of Kerala, invasion of M. strigata is most intensive in the Ashtamudi Lake. The invasive mussel has established breeding populations in the last two years (2018 and 2019)," the study notes.
It also says that the American brackish water mussels have effectively replaced the native green mussel, which had earlier been present in abundant quantities in the Ashtamudi lake in Kollam.
"They were first found in 2018 and by 2019, they were spread everywhere in Vembanad and Ashtamudi lake. They form like carpets, even in the small creeks attached to these lakes," said KK Appukuttan, former principal scientist with the Central Marine Fisheries Research Institute (CMFRI).
"The invasion of this species has caused the decline of the native mussels as the former is more dominant. They spread very fast and multiply in the mussel beds, where native species cannot survive. It has also affected the availability of clams in the area. The stock of short-neck clams, which was used for export, has decreased considerably this year due to the Mytella mussel," he added. The short-necked clam or Paphia malabarica was also given the eco-label from the international organisation Marine Stewardship Council (MSC), the first in India to receive the label.
He also notes that the species have even affected the black clam beds, causing concern for those who depend on the clams.
The rapid invasion of the species poses a major threat for the livelihood of local fishermen who rely on the Vembanad and Ashtamudi lakes, where the available resources are fast depleting, the study notes.
However, eradication of the new species is not easy. The study suggests that it will be necessary to take precautionary measures in other coastal areas of the country. And since the meat content of the invasive mussels is very low in quantity (unlike the native green mussels), people are unlikely to use it commonly.
“The meat is edible. Earlier, people had doubts about whether it's good to eat. But now some are using it. But they are very small in size and the meat content is very little. You will have to collect a large quantity to get enough meat. Moreover, its shell is not hard enough to use in industries. Otherwise they would have taken it," says Appukuttan. Clam shells are often used as soil neutralizers.