They don't reproduce asexually, but that doesn't mean they're not hermaphrodites. I beleive that the term is "protandric hermaphrodite" (and somebody with a biology degree can feel free to step in here and correct me 8) ). Basically when they hit sexual maturity, they are first males only, later on becoming hermaphrodites (both male and female), although, when spawning, they cannot release both sperm and eggs simultaneously; thus (hopefully) preventing self-fertilization.
Now ... I beleive I have read of experiments where several species of clams have been induced to self-fertilize in a laboratory, but this is not natural (nor desireable, of course). I should mention I have not read any studies where this was performed on
Tridacna sp. or
Hippopus sp., however.
Here is some online reading for your interest (I just did some random google searches):
http://www.animalnetwork.com/fish2/a.../1/default.asp
http://www.richard-seaman.com/Underw...ia/GiantClams/
http://www.zibycom.com/members/00223...2/index11.html
http://www.exotictropicals.com/encyc...99/fnc0799.htm
The definitive answers to your questions can be found in Daniel Knop's book about Giant Clams and I highly recommend this book to anyone serious about keeping clams and wanting to understand them.