Endangered freshwater mussels squirt water to help larvae survive

An endangered freshwater mussel squirts water up to a metre away in order to give its young the best chance of survival, new research suggests.

Researchers were amazed to see the behaviour in the brainless creature captured on film.

In spring, females of the Unio crassus mussels were seen moving to the water’s edge and anchoring into the riverbed, with their back ends raised above the waterline.

Then they squirted out regular water jets, which landed in the water up to a metre away. Squirting cycles lasted three to six hours.

These jets disturb the river surface and attract fish, and mussel larvae in the jets can then attach to the gills of the fish and complete their metamorphosis into adults.

This squirting has never been seen in any other mussel species, the University of Cambridge researchers say.

‘Who’d have thought that a mussel, that doesn’t even have a head or a brain, knows to move to the river margin and squirt jets of water back into the river during springtime? It’s amazing,’ said Professor David Aldridge in the university’s department of zoology, lead author of the report.

Unlike other mussel species, Unio crassus has a limited range of suitable host fishes – including minnows and chub.

According to the study, these species were attracted to the falling water jets.

The researchers think the mussels squirt water jets to increase the chances of their larvae attaching to the right host fishes.

Squirting into the air and not the water, means the larvae are propelled greater distances from the parent mussel.

The study was carried out during spring in the Biala Tarnowska River, Poland.

Samples of the water jets were collected from each mussel for analysis – which confirmed they contained viable mussel larvae.

Before now, there was only anecdotal evidence of this behaviour, and some scientists thought the water jets might be a way for the mussels to expel faeces.

Researchers suggest their findings may help explain why Unio crassus is an endangered species.

Climbing out of the water to squirt makes it vulnerable to floods, destruction of river margins, and predators like mink. And its need for specific host fishes links its survival to theirs.

The research, published in the journal Ecology, was funded by the Woolf Fisher Trust.