Friday Feature: parasitic pearls

Its been a while since I've posted a Friday Feature on larval related news.  Finally, I'm back at home with a break in traveling and the holidays over (though my daughter wishes otherwise)... so its time.

Larvae can develop under many different conditions - in the open ocean (planktonic), brooded within/on a parent, and snuggled in an egg capsule.  These types of development have very different consequences for maternal investment (how much energy the mother spends on reproduction rather than her own metabolism and growth), survival, and dispersal.  Broadcast spawning eggs and sperm into the ocean results in 'wastage' due to the high mortality rates for larvae in the open water.  Brooding and encapsulating the larvae protects the larvae and thus reduces mortality, but this usually comes at an energetic cost and reduces dispersal potential.  Brooding entails parental care during development. Parents often leave egg capsules once laid, reducing the cost of actively caring for them, but there is still a cost associated with making the egg capsules themselves and providing sufficient nutrients to fuel development within the capsules (rather than from feeding in the water column).  There are always trade offs.

Freshwater pearl mussels (not marine, I know, but pretty cool... so we'll go with it) seem to have found way around some of the costs associated with brooding or encapsulation.  Mussel larvae are released into the water but instead of drifting in the streams for months, they attach themselves to fish gills.  The arvae will develop attached to trout or salmon gills until they are old enough to drop off into the gravel where they will spend the rest of their lives. 

A parasitic strategy provides some of the benefits of brooding and of planktonic development.  The fish provide protection for the larvae, but at a lower/no cost to the mussel parent.  Movements of the fish will contribute to larval dispersal, but will prevent being washed away to inhospitable locations. 

Despite this ingenious strategy for larval development, the freshwater pearl mussel is endangered.  Overfishing of the mussel to obtain freshwater pearls and river pollution have drastically diminished it abundance and distribution.  Fishing of the fish hosts also poses a threat.


Breeding programs are finally becoming successful.  The complex life cycle posed challenges, but it appears that there may be new hope for restocking freshwater pearl mussels and preventing their extinction.

BBC news: Freshwater pearl mussels breeding in Northumberland http://www.bbc.co.uk/news/uk-england-tyne-12446161