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re: freshwater clams

James wrote:

<Hello all,
<I was wondering if anyone here has any freshwater clams in their tanks.  If
<so, are they helpful?  Are there any special concerning or negatives to having
<some in a planted aquarium?  The ones I am thinking about are Freshwater Pygmy
<Clams which are larger than an eraser head, but smaller than a dime in size.

	Aaah, inverts, one of my favorite topics.  If searching for freshwater bivalves one would be hard-pressed to find a permanent body of water without some specie.  From large 25cm unionids which depend on fish for their parasitic young stage to the tiny <15mm fingernail clams, bivalves are an interesting topic.  Except for a few species of mussels, the nearly 2000 freshwater bivalves are clams.  Nearly, if not all freshwater bivalves are suspension feeders, and are considered the most advanced suspension feeders at that.  
	Water enters the incurrent siphon and passes over and through the perforated and thickly ciliated gills.   Here bacteria, organic solids, diatoms and phytoplankton are sieved and passed along cilia to the palps and the mouth to be digested.  Their specialized gill also sorts the mostly smaller food from the mainly larger sediment particles.  Take for instance the zebra mussel, an exotic specie, and the topic of many studies in my part of the country.  It's been published that individuals filter 5-7 liters of water a day!  Considering the population density and the size of the specie thats an incredible amount of filtration.  Over the past decade the St. Lawrence river has taken a dramatic change in character due to just this reason.  The mussels increased the clarity many times over and the plant life has just exploded.  There are many consequences of their introduction but thats entirely a different story.
	Bivalves also have an interesting reproductive cycle.  Males release sperm into the water column which enters the female and fertilization takes place near the interior of the gills, the zygotes are then expelled into the water.  In some species the female actually waits for a fish to pass over, as the fish is a host for the parasitic zygotes (glochidia).  The zygotes must either attatch to a fish's gills or fins, if not they die within a few days.  This accounts for some of the "black spots" seen on fish at certain times of the year.  Then the glochidia parasitizes the fish until reaching a juvenile clam stage, then falls to the substrate and lives the normal clam life.  
In other species the zygotes develop in the female and by-pass the parasitic stage.  I've never had reproduction in the aquarium but if anyone knows a triggering method, or more specific information please join in.
	I've kept several species over the years from zebra mussels to the fingernail clams you mention.  My main interest in aquatic plants lies with local species, and my tanks are set up to accommodate this.  If you'd like specifics, e-mail me.  But I've seen nothing harmful by having bivalves in the aquarium.  I've kept individuals for up to 3 years.  I do water changes with "hard" well water, ph 7.9 and use CO2 injection.  One thing about the delicate shell of tiny clams, fish love em, especially those with pharyngeal teeth. If they can fit em in their mouth they'll crunch 'em up. 

Canton NY