In spring, female thick-shelled river mussels (Unio crassus) 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 meter away; squirting cycles lasted 3-6 hours. The jets disturb the river surface and attract fish; mussel larvae in the jets can then attach to the gills of the fish and complete their metamorphosis into adults. This behavior has never been seen in any other mussel species.
The thick-shelled river mussel (Unio crassus) squirts out regular water jets that land in the river water up to a meter away. Image credit: University of Cambridge.
The thick-shelled river mussel is a species of freshwater bivalve mollusk in the family Unionidae (river mussels).
This species occurs in Black Sea region, Iraq, and Europe except Iberian peninsula and British Isles.
It prefers middle sections of clean rivers and smaller running waters.
Until the 1950s the most frequent freshwater bivalve of Europe, it was collected in high numbers in the vicinity of villages and was widely used as food for pigs and chicken.
“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 University of Cambridge’s Professor David Aldridge.
Unlike other mussel species, thick-shelled river mussel has a limited range of suitable host fishes — including minnows and chub. These species were attracted to the falling water jets.
Professor Aldridge and colleagues think the mussels squirt water jets to increase the chances of their larvae attaching to the right host fishes.
By being squirted into the air and not the water, the larvae are propelled greater distances from the parent mussel.
The study was carried out during spring in Biała Tarnowska River in Poland.
Six squirts were collected from each mussel for analysis — which confirmed that they contained viable mussel larvae.
Before now, there was only anecdotal evidence of this behavior.
Some scientists thought the water jets might be a way for the mussels to expel feces.
“This behavior could explain why Unio crassus is an endangered species,” the authors said.
“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.”
“Understanding how this species completes its life cycle is important for its conservation under changing environmental conditions,” they concluded.
A paper on the findings will be published in the journal Ecology.
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David C. Aldridge et al. Fishing for hosts: Larval spurting by the endangered thick-shelled river mussel, Unio crassus. Ecology, published online March 10, 2023; doi: 10.1002/ecy.4026
