Mysterious microscopic parasites are massacring the Mediterranean’s giant clams

  • Scientists are racing to understand how the parasite spreads in time to save the giant clam species, which is already endangered
PUBLISHED : Tuesday, 04 December, 2018, 11:12pm
UPDATED : Wednesday, 05 December, 2018, 12:42am

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 mollusc could go extinct.

For thousands of years, the noble pen shell, an emblematic species of mollusc, has been intrinsically connected to human civilisation. The largest bivalve in the Mediterranean can grow more than a metre (three feet) long and has provided food and one of the world’s rarest materials: sea silk spun from fibres it uses to secure itself to the seabed.


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; the species has been on the European Union’s protected species list for decades. So the spread of the parasite, which first appeared in the western Mediterranean in late 2016 and was identified just this year as a new species, has alarmed experts.

Exactly how the parasite kills is not clear, although scientists have found that it attacks the digestive system. The infected animal is unable to close its shell, and thus unable to defend itself from 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 that 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.

Scientists are racing to understand how the parasite spreads and its life cycle – essential information for a successful rescue programme. One theory is it could be spreading through phytoplankton, the clam’s food source.

“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 Centre for Marine Research (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. 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 midsummer. He determined that the culprit was the same one causing mortality in Spain.

“This is very fresh for the scientific community,” Issaris said. “We’re still at the stage of recording where it has spread to.”

Parts of Greece still have healthy pen shell populations, while they have been wiped out in other areas.

Protecting the pen shell in its natural habitat appears “difficult to impossible”, Issaris said, without knowing how the parasite spreads.

One thing is clear: the parasite is very particular in its choice of victim. A smaller, related mollusc species, the Pinna rudis, which also exists outside the Mediterranean, is unaffected.

“We don’t know how it has appeared in the Mediterranean … We only know that it causes mortality only in the Pinna nobilis,” Issaris said.

In the clear, shallow waters south of the Greek capital, dozens of dead pen shells lie scattered in a seagrass meadow, testament to the devastation.

Peering through a microscope at the tissue of an infected individual in his office in Crete, Katharios points out the culprit: small, oval-shaped parasites spread throughout the sample.

Scientists are puzzling over why an organism would be so lethal to a species it depends on for survival.

“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 the parasite originated in a different species and for some reason jumped to the pen shell. A third is the pen shell’s immune system has been compromised by factors such as pollution.

“It’s extremely, extremely difficult to find the truth,” Katharios said.