Steve-the super condensed version:  The first mollusks in space were veligers of Mytilus edulis our common blue mussel which flew of STS 77 in May 1996 as part of the Canadian Aquatic Research Facility (ARF) payload along with two separate experiments on seastars and sea urchins.  The Mytilus researchers from the University of Dalhousie in Nova Scotia measured calcium deposition on shell formation in space.  Space travelers lose bone mass during extended periods in the microgravity of space.  I had the opportunity to have ARF as one of my assigned payloads during payload processing at Kennedy Space Center.  Cassette sized aquaria in triple containment housed the experiment (seawater leaking out would not be a good thing for the electronics of the space station in microgravity).  One of the challenges in using Canadian animals, the aquaria had to be kept at a cold 60 degrees F., hard to do in the heat of Florida.  I gave a program on this in 1998 at the Orlando COA.
The second shuttle flight used the freshwater snail Biomphalaria glabrata on the European Space Agency (ESA) Neurolab flight experiment which included sending fish into space on STS 90 in 1998.  I did not work on that payload but did visit the processing facility to see the racks of large aquaria full of fish.  The ESA researcher was somewhat surprised but pleased I showed an interest in the snails to fly as everyone else were more interested in the fish.  I asked her the purpose of the snails- and the answer was the same as with your home aquarium - to eat the algae forming on the tanks.
And Paul- no Liguus live this far north, unfortunately.  The worst potential hitchhiker to the shuttle was to the external foam tank  when flickers decided to peck away at the foam in order to make a nest.  Since we are in a wildlife refuge, the birds were gently and humanely relocated and the hole patched.
Alan Gettleman
Merritt Island, Florida

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