Gametes and glochidia

Most pearly mussels have separate sexes.  The males release sperm aggregates (spermatozeugmata or "spermballs") and downstream females filter these from the water.  The eggs of the female are fertilized and then enter spaces in her outer pair of gills (marsupial gills).  There they develop into embryos and eventually glochidia.  The female may hold or "brood" the eggs over the winter (long term brooders) or for only a few weeks in spring or summer (short term brooders). 

Click on the thumbnails for larger views...

These are spermatozeugmata of the flat floater (Anodonta suborbiculata).  Several thousand sperm are attached to a transparent sphere, with their flagella projecting outward.   Eventually the sperm aggregate on one side, perhaps as the structure swells after entering the water.  The flagella beat and the whole assembly swims through the water!  Here is a video clip (1.6 MB WMV).
These immature glochidia of Elliptio dilatata are still developing within the eggs.  Development in mussels takes weeks.  This is very slow compared to development in many marine bivalves, which can be completed within a few days.
gloch'.jpg (74271 bytes) Glochidia come in various sizes- the larger ones (1/4 mm) are fatmucket- the smaller are spectaclecase.  Glochidia are unable to swim or crawl.  Their only behavior is to close at an opportune moment when they come into contact with the gills or fins of the host fish.  
16'.jpg (124352 bytes) The tissue of the fish reacts by forming a capsule or cyst around the glochidium.   Does it hurt the fish?  Not much. Usually there are so few attached that the effect on the fish must be almost nil. Yes, the glochidia are parasites- but aren't all kids?  We love them anyway.  
17'.jpg (108733 bytes) Most mussels are host-specific, meaning that they can only use one or a few particular species of fish as hosts.  In some cases this relationship is so fine-tuned that a mussel population can only use fish from the same river system as hosts.  If glochidia attach to the wrong species, the cysts are soon lost.  This glochidium of Simpsonaias ambigua is on the wrong host and is about to fall off.
63.jpg (73175 bytes) 64.jpg (117756 bytes) Technical tip- it's difficult to see glochidia on preserved gills because the gill tissue becomes opaque (photo on far left).  You can render the tissue transparent by treating it with ~2% KOH (second photo, same specimen).
juv-1.jpg (73053 bytes) After metamorphosis the juvenile mussels drop off the fish and begin feeding on algae (object at lower right in the photo is a paper staple).  These early juvenile mussels are very active and crawl about, appearing more like snails than bivalves.  Both the foot and mantle are covered with cilia which move vigorous currents of water.  
juv-3.jpg (61011 bytes) Juvenile mussels are rarely encountered in nature.  As a result we know little of their habits or requirements. 

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