I think that this qns have been ask many times and answerd too. Suggest you search for older thread and you may find your answer.....
Anyway just FYI........... I try to make it short
Clam commonly sold in the market mainly belong to the tridacna species... A few popular ones are maxima, crocea, derasa, gigas and squamosa........
Crocea- smallest clam species which do not have huge stair case like structure(think they are call scute) on their shell........ They are very colourful and their stretch their mantle out very wide and one of the biggest in the clam family) thus making them prone to predation... Being very colourful, they therefore needs very very strong lighting(>250watts) The shell base is also very wide and that makes it diff from its cousin(maxima) as these 2 species are easily mistaken as each other...... usually lfs owner will try to say that their croceas are maxima because maxima cost more.....
maxima- They have staircase like structure on their shell(some may not have) They are very colourful and therefore lots of lights are needed(some colourful one may need 400watts of light).....The shell base where the byssal is is not as wide as croceas..........
Derasa- This guys have very small and thin base......... When fully close, this clam look like pancake....... When they open, the shell can open to almost 180 degree...... Their colour are not as colourful as maxima or corceas and usually they come in brown, gold or green....... They also have scutes on their shell but the scutes pattern are more unique becos they continue from one end of the shell to the other end........
Squamosa- This clam have shell pattern that resemble maxima... only thing is that maxima scutes are more closely pack while squamosa's are more wide..... Another thing to differentiate maxima from squamosa, is from the mantle pattern, squamosa mantlt tend to be more dull and usually in gold or brown while maximas mantle can be of almost and colour........ Squamosas are very hardy clam and should be recommended for beginners....... They need very little light too(least is many t5s).......
As for gigas and hippophus, i would not talk much abt them as they are seldom see in lfs trade.... Gigas being rare and endangered while hippophus being very ugly thus no lfs owner would want to bring them in.....
From the way you speak, i suggest you read up lots more on clam before buying them, read up on their needs and diseases or predation which may affect them......