Remarkable reprodutive spurting behaviour of endangered thick shelled river mussel, Unio crassus
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abstract
Freshwater mussels (Unionida) dominate the biomass and are important keystone
organisms in many rivers, yet they are declining precipitously worldwide. Their
dispersal is facilitated by possession of parasitic larvae (glochidia) which typically
encyst and metamorphose on the gills and fins of host fishes. Long eo-evolutionary
histories in some North American mussels has resulted in specificity towards single
host fish species that share the same microhabitat as the mussel and has led to the
development of lures and behaviours that dramatically increase the likelihood of
attachment and successful transmission. Elsewhere in the world, mussels are
typically more generalist in host use and thought to release glochidia freely into the
water column without using specific lures and attractants. Here we show that the
endangered European thick shelled river mussel, Unio crassus, displays a
remarkable spurting behaviour where females migrate to river margins and project
jets of water up to 1m back into the channel. Spurted material carries glochidia and
attracts larval host fishes thus increasing the likelihood of successful transmission.
Mature glochidia remain viable for up to 48 hours and carry long larval threads,
which can wrap around fixed and floating debris thus keeping the glochidia within
the water column. This unique spurting behaviour may explain the disappearance
of U. crassus from regulated rivers, where margins have been lost through
impoundment, or where increased sporadic discharges displace gravid females from
shallow water. The reproductive behaviour of many endangered freshwater mussels
is largely unknown but may be central to explaining the dramatic decline in these
important ecosystem engineers.
