With rapid efficiency, a mysterious parasite is seeking out and killing a giant species of clam found only in the Mediterranean Sea. Unless scientists can find a way of stopping it soon, they say the mollusk could go extinct.
A dead pen shell stands open in a seagrass meadow in the Aegean Sea's Saronic Gulf. /AP Photo
For thousands of years, the emblematic noble pen shell has been intrinsically connected to human civilization. The largest bivalve in the Mediterranean can grow to more than a meter long and has provided food and one of the world's rarest materials: sea silk spun from fibers it uses to secure itself to the seabed. The mollusk also contributes to clear water by filtering out organic particulates.
The pen shells, which have a lifespan of several decades and take years to reach reproductive age, were already dying faster than they could be replaced. So the spread of the microscopic parasite, which first appeared in the western Mediterranean in late 2016 and was identified just this year as a new species, has alarmed experts.
A diver observes a pen shell on the seabed in the Aegean Sea. /AP Photo
Exactly how the parasite kills isn't completely clear, although scientists have found it attacks the pen shell's digestive system. The infected animal is also unable to close its shell, incapacitating its defense against predators. Once infected, death is almost certain.
“In less than a year it wiped out (the pen shell population of) the Spanish coast,” said Maria del Mar Otero of the International Union for Conservation of Nature.
Soon parts of France, Malta, Tunisia, and Italy were affected. In recent weeks, tests confirmed the same parasite, Haplosporidium pinnae, is responsible for pen shell die-offs in parts of Greece, and researchers have reported mass mortality as far east as Turkey and Cyprus.
A pen shell stands on the seabed of the Aegean Sea. /AP Photo
Scientists are now racing to understand how the parasite spreads and its life cycle — essential information for a successful rescue program. One theory is that it could be spreading through phytoplankton, the bivalve's food source, but nobody knows for certain.
“We cannot be sure of anything at this point,” said Pantelis Katharios, senior researcher at the Institute of Marine Biology, Biotechnology, and Aquaculture of the Hellenic Center for Marine Research, or HCMR.
“What we know now is that the Pinnas are dying, that the cause is this parasite, and we know that it's spreading very, very rapidly,” he said. “And that is going to be a huge problem (for) the ecology and the balance of the ecosystem in the Mediterranean.”
Yiannis Issaris, marine ecologist at the HCMR's Institute of Oceanography, initially noticed widespread pen shell death off the coast of Anavyssos, southeast of Athens, in mid-summer. He suspected the culprit could be the same one causing mortality in Spain — and testing proved him right.
Now the question is how to tackle the outbreak.
“This is very fresh for the scientific community,” Issaris said. “We're still at the stage of recording where it has spread to.”
One thing's clear: the parasite is very particular in its choice of victim. A smaller, related species, the Pinna rudis, which also exists outside the Mediterranean, is unaffected.
“Normally parasites in nature do not have any benefit from harming the host, because they depend on the host,” Katharios explained. “But once in a while, we may come across incidents like this, where we have massive mortalities.”
This could just be a natural phenomenon in which the parasite will eventually be wiped out along with its host, he said. Another possibility is that it originated in a different species and for some reason jumped to the pen shell. A third is that the pen shell's immune system has been compromised by factors such as pollution, climate change or water temperature fluctuations.