The mention of bivalves in the initial question shows that Galeommatoidea was in mind.  Some galeommatoideans seem to think that they are bivalved gastropods and crawl around actively.  Courtship has been observed in these; I think the reference is Mikkelsen and Bieler, for Divariscintilla yoyo, but may be mistaken.  Courtship generally requires both at least a minimal amount of mobility and localized fertilization.  Most bivalves and many other mollusks simply release gametes into the water and hope for the best.

As might be expected, cephalopods are generally most impressive.  Their control of body coloring allows for complex visual displays.  I recall an AMS talk on reef squid behavior.  Once they observed a male between a female and another male.  On the side of his body towards the female, he was showing off all his fancy courtship display patterns, while the side towards his potential rival was just showing the ho-hum everyday pattern.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!