ATHENS – A fan mussel found only in the Mediterranean was officially added to the list of critically endangered species, after scientists recorded a dramatic decline in its population numbers caused by a newly discovered pathogen.

The International Union for Conservation of Nature said in a press release this week that its updated Red List includes the noble pen shell, or Pinna nobilis, the Mediterranean’s largest mollusk. The pen shell, which can grow to more than a meter (3 feet) tall, contributes to clear water by filtering out organic particulates. For centuries it provided humans with food and one of the world’s rarest materials: sea silk spun from the fibers with which it secures itself to the seabed.

A protected species, the pen shell was already under pressure from pollution, illegal fishing, habitat loss, invasive species and climate change. “However, none of these threats have led to the extremely widespread and rapid population declines caused by the ongoing disease,” the IUCN said.

The new pathogen first appeared in 2016 along the Spanish coast and has since spread across the Mediterranean, with an 80-100% death rate. Last year, when scientists discovered it had spread to Greek waters, they told The Associated Press the species was at risk of extinction.

The IUCN said there was an “urgent need” to set up a regional monitoring program for the pen shell, and that further research was needed to understand the disease and find ways of limiting its spread.

The IUCN’s updated Red List, which includes threatened species worldwide, showed conservation efforts had led to an improvement in the status of two freshwater fish species and eight bird species, including a bird previously listed extinct in the wild, the flightless Guam Rail.

The Red List now includes 112,432 species, of which 30,178 are threatened with extinction.