Invasive mussel spreads in backwaters

Charru mussel has in many places replaced the Asian green mussel

An invasive mussel native to the South and Central American coasts is spreading quickly in the backwaters of Kerala, elbowing out other mussel and clam species and threatening the livelihoods of fishermen engaged in molluscan fisheries.

The rapid spread of the Charru mussel (Mytella strigata) may have been triggered by Cyclone Ockhi which struck the region in 2017, according to a paper published in the Journal of Aquatic Biology and Fisheries.

Worst-hit

Surveys show the presence of the Charru mussel in the Kadinamkulam, Paravur, Edava-Nadayara, Ashtamudi, Kayamkulam, Vembanad, Chettuva and Ponnani estuaries/backwaters. Ashtamudi Lake, a Ramsar site in Kollam district, remains the worst-hit. With a population as high as 11,384 per sq metre here, it has replaced the Asian green mussel (Perna viridis) and the edible oyster Magallana bilineata (known locally as muringa).

Externally, the Charru mussel resembles the green and brown mussels (kallummekka in Malayalam), but is much smaller in size. Its colour varies from black to brown, purple or dark green.

In many areas, this invasive species has smothered beds of the short-neck clam (manja kakka), according to the paper authored by A. Biju Kumar and Ravinesh R. of the Department of Aquatic Biology and Fisheries, University of Kerala, Graham Oliver of the National Museum of Wales, S. K. Tan of the National University of Singapore, and Kalesh Sadasivan of the Travancore Natural History Society.

In Ashtamudi Lake, the Charru mussel had established breeding populations in 2018 and 2019, acquiring the moniker ‘varathan kakka’ (alien mollusc).

The short-necked clam fisheries in the lake had obtained an eco label from the Marine Stewardship Council and about 3,000 people are dependent on fisheries here. In such a scenario, the fast-breeding Charru mussel could be seen as a ‘pest,’ the authors state.

“The potential of Mytella strigata to outcompete the lucrative clam fishery is a serious concern that urgently needs to be addressed,” they add. Though this smaller mussel is edible, the overall economic loss and impact on biodiversity is much bigger, it is pointed out.

In all probability, the mussel reached the Indian shores attached to ship hulls or as larval forms in ballast water discharges.

Cyclone Ockhi may have simply sped up their invasion of inland waters. In this scenario, there is an urgent need to identify the presence of the Charru mussel in other parts of India by locating the pathways of introduction, said Biju Kumar of the University of Kerala.

Invasion biology

There is also a need to promote studies on invasion biology and strengthen awareness on marine invasive species, he said.